HOLOGRAPHY A BEGINNER'S GUIDE Douglas E. Tyler

© 1999 Douglas E. Tyler No part of this publication may be reproduced in any manner or by any means without the prior written permission of the author. CAUTION: The use of lasers, photochemistry and other materials involved in the creation of holograms may be hazardous. Participants in this workshop should exercise caution in dealing with all materials as instructed by staff of the workshop. In particular, participants should be aware of the following cautions: GENERAL SAFETY PRECAUTIONS FOR WORKSHOP PARTICIPANTS A. Never stare directly into a beam of laser light or at it's reflection off of shiny surfaces. Laser light is a concentrated form of energy which can cause damage to the human eye. B. Avoid direct contact with those photochemicals used in processing holograms. Use protective gloves or photo instruments while handling these materials. Use photochemicals only in well ventilated areas and avoid prolonged breathing of vapors. C. In instances where elecrical instruments, including timers, dryers and so forth are used near wet processing areas participants must be certain that their hands are dry before handling these instruments.

HOLOGRAPHYHOLOGRAPHY

A BEGINNER'S GUIDE

HOLO / GRAPHY WHOLE MESSAGE

The essential theoretical principles of holography were proposed by Dr. Dennis Gabor in the late 1940's. Gabor did not originate the term holography, however, instead, in his early papers, he referred to it by the rather complex, descriptive name "wave-front reconstruction." Although it is not absolutely clear who first introduced the term holography to refer to Gabor's discovery there is reason to believe that Gordon Rogers, an early pioneer in the field, deserves this credit. Rogers, of Dundee, Scotland, apparently used this term because the theory of "wave-front reconstruction" provided a technique for a more complete form of image recording. Over time, a simple vocabulary has developed within the medium wherein the general process of wavefront reconstruction is referred to as holography. HOLOGRAMS (not holographs): In contrast to the process of holography, a single image created through holographic means is referred to as a hologram ! This is obviously simple terminology, but its proper use helps to clarify meaning while discussing the medium. The term "holograph" has been used to describe a singular image as well and in fact is considered proper usage according to the Oxford English Dictionary. However, a "holograph" is also defined as a written document such as a will, contract, etc. and because this presents some confusion it seems more practical to refer the singular image as a hologram. A DEFINITION OF HOLOGRAPHY:

Although it is difficult, if not presumptuous, to try to define sophisticated scientific principles and natural phenomena of the holographic process in a few short sentences, the following gives an adequate overview of the primary components in this technology. "Holography is the recording (construction) of wavefront inter- ference patterns created by coherent (laser) light sources and the replay (reconstruction) of these patterns as visual images or other forms of wave media." In some instances of definition, it is as important to clarify what you are not describing as what you are. This is certainly the case in holography. The popular awareness of three-dimensionality in the holographic image has led to many confusing associations of the medium with other, less sophisticated, technologies. Among these are the following:

A. Lenticular Devices - There are a great variety of these and most of us have encountered them in post card form at some time. These are the popular cards which either have a three-dimensional appearance or that jump rapidly from one flat image to another as the card is moved in the hand. Often "lenticulars" can by distinguished by their highly textured surface (these are the lenses which form the image you see). In recent times the "NIMSLO" camera has offered a related process which allows you to create these sorts of images by using their special

1 camera and processing procedures. B. Film Animation - Fictional, future-oriented, projections of holography have often gone far beyond the real capabilities of the medium. Wanting to satisfy the dreams of movie audiences however, filmmakers have resorted to special animation techniques and trick photography to simulate holography as it might be at some point in the future. The most popular examples of this are the scenes from such films as "Star Wars." Many of us wish that holography was at such an advanced stage in its development but, sadly, it isn't. Real holograms have been used in films such as "Logan's Run" and "The Man Who Fell Earth," but because film is a two-dimensional medium it is incapable of carrying the same level of information as a hologram (i.e. a 2-D image of a 3-D image is still 2-D).

C. The Floating Penny and Other Optical I l lusions - Long before the development of holography, physicists and artists used special mirror and optical configurations to project images into and through space. Various novelties sold today still rely upon this, as do some of the popular diversions in the haunted house at Disneyworld and other theme parks. These are not holograms, simply sophisticated visual tricks for your viewing pleasure !

D. Stereo Photography (anaglyphs, stereo pairs, et al) – Arguably the earliest

form of 3-D imaging. A pair of images created from slightly separated vantage points are reconstructed with the aid of a viewing device.

E. Random Dot Stereograms – a popular visual diversion in which a seemingly endless

series of small images (or dots) when viewed properly reconstructs for the eye/brain as a 3-D form.

F. Virtual reality – A sophisticated form of digital stereo imaging in which with the aid

of a stereo viewing device one may navigate seemingly real 3-D space. The more sophisticated forms incorporate body sensing elements that permit the participant to interact with this “virtual space.” FEATURES OF A HOLOGRAM: Because of the many varieties of holograms which are possible with the present technology, it is difficulty to describe what a hologram is. The following features may indicate that what you are looking at is a hologram. Keep in mind that these are just a few – there are many others.

A. There is some surface into which you are looking. The physical portion of a hologram is a photographic emulsion and it needs some support surface. Generally when looking at holograms the image you see is related to a tangible material (glass plate or piece of film). B. The image is "vaporous" and does not appear to have a physical presence. You may be able to touch the image itself, but you feel nothing when you do. C. Some special lighting is involved. A hologram is an image comprised of light, created by the holograms ability to modulate (bend, that is) light in a highly sophisticated and controlled manner. In order for the proper replay of the holographic image a special form and arrangement of illumination may be required. In certain instances this may involve cumbersome and noticeable paraphernalia. 2

OPTICS: SOME BASIC BACKGROUND: The dictionary defines Optics as "the science of light . . . that deals with its genesis and propagation and the effects that it undergoes and produces." Essentially, the study of optics has to do with understanding the properties and actions of light as it moves through or comes in contact with various materials. Within the field of optics there are essentially three primary divisions which focus upon these specific properties of light. In ascending order of complexity they are: Geometrical optics - this area of optics operates on the primary assumption that light travels in a straight line. Although this is an abstraction of reality, it nevertheless permits rather direct and accurate explanation and prediction of certain optical phenomena such as the focusing of light by lenses. In this text, the use of geometrical optics relies heavily upon the principle of refraction to explain the interaction of light with various substances. Wave optics - is also sometimes referred to as physical optics because it seeks to explain the action and effects of light through the mechanism of waves, a more physical phenomenon. Although less abstracted than geometrical optics, it nevertheless relies more upon mathematical models, and thus is still distanced from the physical realities involved in the actions of light. For our purposes the primary value of wave optics will be in explaining the concept of diffraction. Quantum optics - this field of optics concerns itself with the interaction of light and matter on the atomic level. Although such information may be useful to scientists in explaining certain events these effects are not easy to demonstrate in a visible form and therefore don't recieve significant attention in these notes.

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GOMETRICAL OPTICS AND REFRACTION: As stated previously, geometrical optics assumes that light travels in a straight line and in this form is referred to as a ray. There is no concern here with the substance of the light itself; however, there is considerable interest in this field of optics with the effects which differing substances will have upon a ray of light as it interacts with them. In this regard there are essentially three common and well-known properties of physical materials that describe their effect upon light. They are: Opaque - light striking these materials may be reflected in greater or lesser degree but usually the material absorbs a certain portion of it. Critical to concern in optics is the fact that such materials do not permit light to pass through them. Translucent - the physical structure of these materials does permit rays of light to pass through them, but often distorts the direction of their movement so drastically that images may not be recognizable through it. Examples of such a material are ground glass, milk glass and so forth. Transparent - such materials as these permit the passage of rays of light in a relatively unhindered fashion. In most instances the movement of light rays through transparent materials is also accompanied by distortions and changes in the directions of the rays movement. Of great importance here, however, is the fact that such movement and distortion can often be controlled in order to produce very specific optical effects.

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REFRACTION:

The effect of transparent materials upon rays of light moving in a given direction is usually described in geometrical optics by the principle of refraction. This phenomena is widely known and often demonstrated and observed by the effect which water has upon the appearance of objects, a pencil is the popular example, placed in a glass of water. The dynamic effects of refraction are more dramatically demonstrated in the "floating penny" demonstration. In this demonstration a coin is placed in the bottom of a dish and the observer asked to sight along the top of the dish. As water gradually fills the bowl, light reflected off the coin on the bottom is bent toward the direction of the observer and the coin thus appears to them, as if floating on the surface of the water.

EMPTY GLASS WATER REFRACTSNO REFRACTION LIGHT OF PENCIL

The significance of this refraction demonstration lies not only in its evidencing the ability of certain materials to bend light, but also the spatial positioning of images in optical systems. If the observer in this situation attempts to touch or locate the coin in the image they are viewing, they will discover that it appears to come from the surface of the water. Although this location

5 may be correct to the eye, it obviously does not correspond to the physical position of the coin. Thus, it must be understood from the outset that the image of an object does not always correspond to its physical location in space. This simple coin experiment may help us to understand what eventually occurs in a holographic image. Light, modulated by the recording emulsion of the hologram reconstructs waves of light which appear to come from a specific object located at a specific point in space. Thus, the images which we see are mere patterns of light and if through some technique we are able to recreate those patterns we are able to make images appear without their commonly associated object.

6 Understanding this, we must develop some understanding of the mechanism of refraction. In the simplest terms it may be stated that when a ray of light moving through space passes into a medium of different density the direction of its movement will be altered so long as it does not enter the medium at a perpendicular (90) angle. A physical explanation for this may be that light seeks the shortest time/distance between two points. Thus, it may be that the path of a ray of light may be more efficient if it assumes the "bent" path than if it continues in a straight line.

The degree to which a ray of light is bent while passing through a transparent material is dependent upon the density of the material which it enters. The denser the material the more drastically the ray will be bent or diverted from its original course. This fact is often expressed in the refractive index of the material which is a ratio of the speed of light through a vacuum compared to the speed of light through any more dense material. The refractive index is usually symbolized by the letter "n" and the refractive index of a vacuum as base is referred to as 1.0 or n = 1.0. In comparison, water has a refractive index of about n = 1.33 and glass of about n = 1.50. 7

The direction in which light bends when passing between materials of differing densities is easily predicted by its relationship to the surface which it enters. If a ray of light enters at 90˚ or perpendicular to the surface (Fig. 1) there is no change brought about in the direction of its movement. This perpendicular line is often referred to as the NORMAL and is used as a basis

for expressing other directional relationships. If a ray passes from a medium of lesser density to a medium of greater density its path or direction is bent toward the normal (Fig. 2).

Obviously, if a beam passes from a more dense to a less dense material the opposite is true and the path of the ray is diverted away from the normal (Fig. 3). Generally speaking, the sharper the angle of entry (the greater the angle in relationship to the normal) the more sharply a ray will be bent when entering the new material.

8 The relationship here can be described in a simple mathematical model or equation: n x sine θ = n' x sine θ'

Thus if we know the refractive indices of two materials, say air and water, we can predict the bending which will occur when a ray of light moves between these two materials. Following are some sample equations to demonstrate how simply this formula may be worked. 1. a ray of light traveling through air (n = 1.0) is incident at an angle of 45˚ on a smooth water surface (n = 1.3). Given this information determine the angle of refraction of this light ray. n X sine θ = n' X sine θ' so 1 X sine 45˚ = 1.3 X sine θ' Then sine 45˚ = 1.3 X sine θ' and sine 45 = sine θ' (note: the sine of a 45˚ angle equals .7071) 1.3 Then sine θ' = .5439 the corresponding angle for this sine is 32.9514. Then the angle O' = 32.9514 or 33˚ . The diagram below illustrates how these various angle measurement relate.

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This simple equation is known as Snell's Law and may be worked on an infinite variety of examples. Try working this example. A ray of light traveling through air is incident on a smooth glass surface at an angle of 13˚. If the refractive index of this glass is 1.5358 what will the angle be at which the ray is refracted within the glass? n x sine θ = n' x sine θ' so 1 x sine 13˚ = 1.5258 x sine θ' then sine 13˚ = 1.5258 x sine θ' (note: the sin of a 13˚ angle = .2246) and .2246 = sine θ' 1.5258 then sine θ' = .1472 the corresponding angle for this sine is 8.46 then the angle θ' = 8.46 or 9˚ The diagram below illustrates how these various angle measurements relate.

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REFRACTION AND COLOR DISPERSION: It might be appropriate at this stage to point out that not all wavelengths of light are refracted in the same manner as each other. In examining the refractive response of different colors of light it becomes clear that light sources with shorter wavelengths, and thus higher energy levels, are refracted more radically than are those with longer wavelengths. Therefore, if white light passes through various materials at the appropriate angles, the various wavelengths of light which comprise the white-light source will be diverted to specific points in space, a phenomena which we experience when viewing a prism spectrum or rainbow.

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Having observed the effect of transparent materials upon the movement of a ray of light it might be of interest to inquire about the opposing action i.e. what happens to a ray of light as it passes from a material of greater density to one of lesser density. By backtracking through our example we discover that the opposite effect is observed. When a ray of light passes from glass into air it bends away from the normal. In examing this phenomena we soon realize that this must be the case, what occurs with light moving in one direction must be repeated when the direction of the light is reversed. In optics this is referred to as the "principle of reversibil ity" and we may observe it by reworking the formula previously given for Snell's law. A ray of light is passing through a glass of water and is incident upon an air surface at an angle of 32.9154. At what angle or direction (relative to the normal) will the beam travel in air? Knowing the refractive index figures for air and glass, we may plug them into the formula for the answer. n x sine θ = n' x sin θ' so: 1 x sine θ = 1.3 x sine 32.9514 then: sine θ = 1.3 x .5439 (if the sine of 32.9514˚ is .5439) then sine θ = .7071057 and 45˚ is the corresponding angle for this sine. then the angle θ = 45˚ The diagram below illustrates how these various angle measurements relate.

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The principle of reversibility is important to understand because it accounts for some optical phenomena that may plague the holographer. The phenomena in question is referred to as "TOTAL INTERNAL REFLECTION" and occurs when a beam of light moving from a dense material into a less dense material is incident on this interface at too steep an angle. The ray of light in this case cannot exit the denser material and is 100 % totally reflected internally. For this simple reason this is referred to as total internal refection and the angle at which rays of light begin to be totally internally reflected is referred to as the "CRITICAL ANGLE". In glass the critical angle is approximately 41˚. Thus any ray of light incident on a glass air interface at an angle greater than 41˚ (relative to the normal) will be reflected back into the glass. Again the formula expressing Snell's law reflects this phenomena, however, for brevity a diagram is included to demonstrate the concept.

The phenomena of Total Internal Reflection has practical implications when we you create holograms. The support materials for holographic emulsions are usually glass or some form of plastic. Frequently the laser light being used to construct a hologram will enter the supporting glass or plastic from the side of the material. Those rays that enter your cover glass from the side are often subject to total internal reflection. The result is, in the case of single beam reflection holograms, a series of progressively stepped bands of color in the image. This undesirable effect can be prevented by blocking the edges of the cover glass during the exposure of the actual hologram.

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The principle of fiber optics is reliant on the concept of total internal reflection. As may be observed in the diagram below, in constructing fiber optic materials, glass or plastic materials are heated and drawn into very fine fibers. In the process of drawing out these fibers a glass of a lower index of refraction is layered on the outside of a glass fiber of a higher index of refraction. So long as the rays of light which pass through these fibers do not fall below the critical angle, they will always be totally internally reflected and thus light rays entering one end of the fiber stream will continue along it without leaving the fiber It should be noted here that fiber optics represent a powerful tool to aid the holographer in creating the special interference patterns that establish her images. It is possible that in the near future, holographers will employ fiber optics to direct laser beams in their recording apparatus. This may ultimately be more effecient and less costly than the present system of mirrors and lenses commonly in use today.

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REFRACTION OPTICS & LENSES

15 REFRACTION, OPTICS, AND LENSES: Perhaps the most common and practical application of the priniples and laws of refraction is with respect to lenses used in various optical systems (eyeglasses, camera lenses and so forth). Relying on simply the basic laws of refraction we may predict the response of light rays as they pass through various types of lenses. In order to do this we might look at the two most common lenses (Fig. 1 & Fig. 2):

The positive lens (Fig.1) is typified by an expansive curvature and is therefore thicker in the mid region. This type of lens is also referred to as convex and is the type commonly found in magnifying glasses. The effect which such a lens shape has upon parallel rays of light which enter it is easily understood if we refer back to the model of a normal. Those rays which enter close to the center of the lens are nearly perpendicular to the normal and thus pass through almost directly. Those on the periphery, however, strike the surface at increasingly large angles to the normal and thus are increasingly bent toward it. The result of this is that the various rays which enter parallel in direction to one another are forced to converge at some point on the opposite side of the lens. The distance from the center of the lens to the point at which these rays are focused is referred to as the FOCAL LENGTH (F.L.) of the lens. Because the rays

of light are focused a measurable distance from the center of the lens, this focal length is expressed in positive terms, and thus it is referred to as a positive (or convex) lens.

16 A simple means for relating the focusing action of the positive lens is to relate it to the refracting action of a prism. The positive lens is similar in effect to two prisms stacked together (see Fig. 2) base to base. Parallel rays of light directed at such a construct would converge.

17 The second major type of lens is a negative (or concave) lens whose profile reveals an inward curvature in the center. This type of lens is most frequently used in viewing devices and is also employed in various compound lenses. The effects which such a lens has upon parallel rays of light is understandably the reverse of the effect of a positive lens. Parallel rays which strike the center of the lens at a perpendicular to the surface pass through almost directly. Those rays which strike the periphery however are now forced to diverge rather than converge as in the

case of the positive lens. The interesting result of this is that such a lens does not have a focusing capability. Interestingly though, it does have a focal length which is expressed in negative terms. This focal length is derived by reversing the direction of the diverged rays to determine an imaginary point of convergence. The focal length in this instance being expressed as F.L. -13 mm. NEGATIVE LENS

PARALELL LIGHT RAYS

F. L. = FOCAL LENGTH

LIGHT PATH

18 REAL AND VIRTUAL - IMAGES IN SPACE: Perhaps the most fascinating possibility afforded by the focusing capability of lenses is the ability to project images to various points in space so that they appear either in front of or behind the imaging lens. Most instructive in this regard is the positive lens because of its

versatility. If we examine the focusing pattern of a positive lens (Fig. 1) we notice that the image of the object is focused in space at a location on the opposite side of the lens as the object which is being imaged. This will be the case if the object's distance from the lens is greater than the focal length of the lens only. It is also important to notice that the image will appear to be both upside down and laterally reversed (flipped from side to side.). An image such as this is referred to as a REAL image since it exists in the "real" space of the viewer and actually is focused in space (it may be observed easily with a sheet of ground glass or waxed/greased paper). In addition to real, this type of image is also referred to a "pseudoscopic". If, unlike the previous example, the object is located closer to the lens than the focal length of the lens, then the image of the object will appear in virtual format. In this form, the image of the object will be upright rather than inverted as with the real image.

19 In the case of a negative lens, only one type of image may be created and this is the type commonly referred to as a virtual image. Because the negative lens is incapable of converging light rays to focus them in space the images which it creates appear to be focused behind the lens surface. These images are never inverted or laterally reversed, but instead appear upright

and in their proper lateral orientation. In addition, these images are characteristically reduced in size, appearing to present a large field of view from a tiny point in space. Because the image which we see is not focused in space, but originates from an imaginary focal point, this type of image is called virtual or "orthoscopic".

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An awareness of real and virtual images often produces questions about where the various images formed by these lenses exist in space. With real images it is possible to demonstrate their presence by viewing the projected image on ground glass (as was done with some earlier

cameras), thin tissue papers or onto a white card. The images created by virtual formation may not be viewed in thi way and therefore are more difficult to visualize through physical demonstrations. Despite the nature of a lens and its image forming properties, predicting where images focused by a lens will exist in space is made relatively easy by the application of a simple mathematical equation known as the "LENS FORMULA". In its simplest form the equation states 1 1 1 ________ + _______ = _________ d1 d2 F. L.

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that the sum of inverses of the focal points for a lens will be equal to the inverse of the focal length for that lens. Armed with this equation we can determine a great deal about a lens, among which, the following information may prove useful. 1. If supplied with a source of collimated light in which the rays are parallel, we may determine the focal length of a simple lens by applying the formula given above. To do this we must note that since the rays comprising the distance d in our diagram are all parallel, they are originating at INFINITY (represented as a lazy or sideways "8") which is equivalent to zero in our equation. As a result the equation may be simplified to the form: 1 = 1 d2 F.L. Given this simplified formula it is obvious that the F.L. will equal the distance required for parallel rays entering the lens to converge to a point, thus d and F.L. are equivalent!

22 2. Using the same "LENS FORMULA", we may predict the position of an objects image in space given only the distance of the object (d) from the lens and the focal length of the

lens. To make the calculation easier ,however, we may rearrange or transpose the equation given earlier to the following form: d2 = d1 X F.L. d1 - F.L. Therefore, if we place an object 15cm from a positive lens with a focal length of 10 cm, the location of the real image created by this lens may be easily calculated. d2= 15 X 10 = +30cm 15 - 10 Because this image exists on the opposite side of the lens as the object, it is known as a real image. Note here that d is a positive number. If d had been a negative number this would indicate that the image was virtual or orthoscopic and that its true focal point was an imaginary one located on the same side of the lens as the object.

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3. Here is a sample problem for you to solve. Its easy since you only need to plug the appropriate figures in the equation you've been given and calculate the answer. An object (in this case a small model of a tree) is located 15 cm from a lens with a focal length of 20 cm. Using the lens formula below, place the appropriate figures in the spaces and complete the calculation of the equation. d2 = d1 X F.L. d1 - F.L. Notice that the answer you have calculated has a negative sign before it. This indicates that the image will not be a real one but instead will be a virtual orthoscopic image. Thus, the image of the object will appear to be on the same side fo the lens as the object itself, located at a distance 60 cm behind the lens in the diagram. It should be noted here that when an object is placed between a lens and the focal point, a virtual image is created rather than a real image. Try completing the diagram below illustrating the direction of rays as they pass through the lens.

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MAGNIFICATION: When either a positive or negative lens refracts the light of an object, it does so in a manner that affects the scale of the image in relation to the object itself. These changes in size from that of the original object size are known as lateral magnifications and may be either enhancements or reductions in size of the original objects, like the lens formula presented earlier, the formula for lateral magnification is quite simple: m = - d2 d1 If we relate this back to our first example, we could easily compute that the magnification for this lens would be - 2.0. The minus sign in this calculation indicates that this is a real image that would be both inverted and laterally reversed. The value two indicates that the image would be approximately twice the size of the object.

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WAVE OPTICS AND DIFFRACTION

26 WAVE OPTICS AND DIFFRACTION: As was pointed out earlier, many of the basic laws of lenses and their effect upon light are based upon the concept of refraction and of light as a "ray" phenomena. In wave optics, light is conceptualized as a wave of electromagnetic energy propogating through space at tremendous speed. According to this model, light is a disturbance of the electromagnetic fields which exist in our environment. These disturbances are much like the effect seen when a pebble is dropped in a pond. As the pebble penetrates the surface of still water, it sends out spherical waves in all directions. When static fields of electromagnetic energy are disturbed, they likewise send out waves that continue through the electromagnetic field.

What is important about this different model is that it permits us to explain and predict certain phenomena that are incongruous with the geometrical model of light. In particular wave optics more

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more accurately predicts the effects of light when it passes by the edges of objects. If we were to use the "ray" concept it is easy to believe that an object (Fig. 1), placed between a point light source and a fixed wall surface, would cast a sharp and distinctive shadow on the wall behind it. In reality however, we seldom find this to be the case. Although some shadows may appear sharp, on closer inspection we recognize that light is blurred on the edges of the shadow, indicating that in passing the object's edge, the rays of light are somehow bent. The geometrical model of light cannot predict or explain this behavior.

OPAQUE OBJECT

HARD EDGED SHADOW

WALL SURFACE

GEOMETRICAL (RAY) THEORY

REALITY

POINT SOURCE(LASER)

RAYS BENDING AT EDGEOF OBJECT

28 LIGHT - THE WAVE MODEL: Although light is obviously essential for our survival, many individuals know little about it. In past centuries, light was important commercially for its role in illuminating certain activities, whereas in our own times it is becoming important as a tool in its own right, performing some of the very activities it previously made visible to us. In performing these functions light often does so at vastly increased speeds and with far greater economy. Holography is an excellent example of this gradual evolution toward the use of light as a tool in modern technology. Therefore, in studying holography it is important to develop a basic understanding of light, the mechanisms and theories which explain its behavior, and its significance in relation to the theory and applied principles of holography. To discuss light we might begin by recognizing that in the sciences there are essentially two models used to explain the actions of light. One of these is the QUANTUM model which essentially describes the actions of light as resulting from the effect of discrete packets of energy with clearly defined properties. These packets of energy are referred to as photons. In the second conceptualization of light, the form is defined as a wave of electro-magnetic energy which is propagated through space in a cyclical fashion, its energy level rising and falling in a fashion most often visualized as a sine wave (fig.1). Wave theory is of more practical use in explaining some basic phenomena in holography and therefore we focus upon it here.

In describing light as a wave, several terms are commonly used to denote parts of the wave: CREST - the highest point of each wave cycle. TROUGH - the lowest point of each wave cycle AMPLITUDE -the intensity of the wave. It is usually measured from the center line to the top of a crest.

29 Generally the sine wave has two important features which are used in defining its character and its effects: WAVELENGTH -The distance between two consecutive and similar points on the wave. In holography and optics, the common measure for wavelengths is the nanometer. A nanomter is equivalent to one billionth of a meter, - 10-9 meters or .000000001 meters. In red light for instance, the length of waves would be around 600 nanometers. The greek symbol lambda “λ” is often used to designate wavelength. FREQUENCY -Is the number of cycles (ups and downs) a wave goes through during a fixed period of time. This period of time is commonly set as one second, and the number of cycles per second are referred to as Hertz (abbreviated Hz). Light waves oscillate at about 10-8 Hz – 10-4 Hz. c = Hz X λ This small formula expresses the relationship between frequency and wavelength and equates them to the constant c, a universal symbol for the speed of light (3 X 10 meters/second, 300,000,000 meters/second or 186,000 miles per second).

30 The sine wave can be somewhat deceiving. Not all waves of electromagnetic radiation occur in such a precise form in nature. Rather, various forces, collisions with objects, the affects of other forms of energy and interference with other waves may modify the original wave. When waves are altered by such forces we refer to them as modulated. It is important to note before proceeding further that many phenomena, besides visible light, are the result of waves of electromagnetic radiation. Varying wavelengths of such energy account for such phenomena as radio waves, ultraviolet light, x-rays and so forth. The various types, their wavelengths and their common applications. Although holograms can be recorded using many types of waves, including sound waves, our interest will be limited to waves in the visible spectrum, or from about 400-700 nanometers (violet to red that is!). The chart below shows the wavelengths for visible colors.

AMPLITUDE MODULATED

FREQUENCY MODULATED

AMPLITUDE AND FREQUENCY MODULATED 31

WAVELENGTH AND COLOR: Early on, in Junior High School, we learn that the colors we perceive are the result of various frequencies (or wavelengths) of electromagnetic energy (waves) stimulating certain nerves in the eye. These various colors and their relationship to certain wavelength or frequencies can be useful to remember in holography. Essentially, red wavelengths are longer and therefore have a correspondingly lower frequency and energy level. Wavelengths in the violet and ultraviolet range are much shorter with a much higher frequency and energy level.

3 seconds (one cycle)

------------------------------------------------------------------ 3 seconds (three cycles) ------------------------------------------------------------------ 3 seconds (ten cycles) ------------------------------------------------------------------ 3 seconds (30 cycles) ------------------------------------------------------------------ 3 seconds (100 cycles) ------------------------------------------------------------------

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LIGHT AND INTERFERENCE

The coherent light of lasers is quite useful in itself for industrial research, communications and various other applications. For holography, however, the coherent character of a light source is essential. This is so because the technology of holography is made possible by the nature of light waves and the manner in which they interact with one another. The effect of light waves upon one another is generally described in a relationship known as interference.

If we think of light waves as water waves (there are some great similarities between them) we can demonstrate some of their interactive effects. If two wave sources collide with one another (Fig.1) the phase conditions of these waves (the relation of their crests and the phase troughs) mutually interact with one another. The rising action of the crests may be reduced if they coincide with the trough segment of another wave and vice versa, or two crests coinciding may create an amplified crest and trough in the wave. The results constitute a combined effect. In the illustration, wave A and B combine in such a manner that both waves reach a cresting stage at the same time. The result of this is that the two waves magnify each others intensity (amplitude) as a result of their additive relationship. The wave which results from this combination is represented in C. Although the wavelength of the original waves has not changed, the height of its crest and depth of its trough has, as if the energy in the separate waves had been combined. This additive effect is referred to as CONSTRUCTIVE INTERFERENCE.

WAVE A

WAVE B WAVE A & B COMBINED

33 In contrast to this situation, it is possible for another event to occur in which two waves of opposite phase collide with one another. In this case, something very curious occurs. Because the energy states of the two waves oppose each other (one is cresting while the other is troughing) the two waves process to cancel each other out. Because of destruction or nullification of the wave's energy at this point, this phenomena is referred to as DESTRUCTIVE INTERFERENCE.

In reality, it is somewhat unlikely that two waves would meet so neatly to form a new wave of such a consistent configuration. Instead, it is more likely for them to be slightly out of phase with one another and therefore forming somewhat irregular patterns of constructive and destructive interference. In describing the interference mechanism of light it is important for us to realize that this effect is essentially characteristic of light, whether the source is coherent or incoherent. Thus interference effects can be demonstrated using ordinary incandescent sources of light (this is what Thomas Young did in his famous double slit experiment). However, as we now know, incoherent light is not so clearly organized and structured as coherent light and this makes it difficult to create highly ordered patterns of interference from incoherent light. Since the mechanism of holography relies upon the creation and recording of the interference relationships a coherent light source provides a more orderly pattern and thus a more highly resolved image.

34

DIFFRACTION AND INTERFERENCE: It is this phenomena of light bending at the edge of objects which is rather clearly explained in the wave theory of light. According to this theory, light is transmitted as a wave and as explained in the description of constructive and destructive interference, these waves have the ability to turn each other on and off depending upon how they interact with one another. This

interaction has been the object of scrutiny by many scientists over the past several centuries. Huygens claimed that light was a pulse phenomenon, a concept which laid the foundation for a wave theory of light. However, it was the work of the English scientist Thomas Young and his now famous "double slit" experiment near the beginning of the 19th Century that lent great credibility to the theory. In Young's ingenious experiment, a light source was first passed through a narrow slit to generate a single source effect. Close behind this slit was located a barrier containing an additional two slits. Given the design of this experiment, it appeared to those supporting the ray optics model of light that , since light traveled in a straight line, no light would be observed beyond the barrier with the double slit. In reality however, light was visible, but with a very peculiar pattern present, later known as "Young's fringes".

PLANE WAVESAPPROACHING SLIT

SPHERICAL WAVES FORMED BY SLIT

SPHERICAL WAVES INTERFERING

35

The pattern created was a very distinctive series of light and dark bands whose position and intensity could not be accounted for by the "rays" of light employed in geometrical optics. The pattern could, however, be accounted for by a new priciple termed "diffraction". This theory postulated that light moves as waves and that a wave leaving slit A will travel the same

distance as a wave leaving slit B when they meet at the centerpoint C (between the two slit position). Because they travel the same distance these waves woud meet "in phase" with one another. The constructive interference which would result would cause this location to be well illuminated. Further, if we select yet another point, say C', we can see that the distance from slit A to this point is different than the distance of slit B to this point. However, if the distance (d) is exactly one wavelength longer, then these two waves will also interfere constructively and form a bright spot. In this case, because the difference in distance is only one wavelength greater it is known as the firest order of diffraction.

36

Likewise, we can appreciate that any waves meeting in such fashion that the differences in their distances (path lengths) were more or less than one wavelength would be out of phase, thus canceling one another out and leaving a dark spot of limited or no illumination. We can understand that any waves meeting here would be out of phase and their destructive interference would leave a dark area at this location. These various bright and dark areas created in Young's double slit experiment are known as diffraction pattern and the areas of dark and light represent the various orders of diffraction which are numbered from the center area out, beginning with the central zone being referred to as the zeroeth order, followed by the first order, second and so forth, symmetrically about the axis. The illustration below describes this pattern.

C

C'

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C = ZERO ORDER OF DIFFRACTION

C' = FIRST ORDER OF DIFFRACTION

C'

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A

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WAVE "A" IN FIRST ORDER TRAVELSONE WAVELENGTH FURTHER THANWAVE "B" (LENGTH NOTED AS "D") S0WAVES INTERFERE CONSTRUCTIVELYFORMING A BRIGHT AREA AT C'.

D

37

From this it could be observed that light possessed a wave-like property which was useful in explaining its behavior. Whatsmore, it would be seen that light, even if thought to be propagating in all directions, could have a straight-line nature. Light emitted from a source at each point in its propagation acts as a source emitting still other waves. However, if it is considered that light interferes with light it may be understood that these various expanding wave forms do interfere with each other as well and in fact their interference is so complete as to eradicate, by destructive interference, any opposing waves coming from other sources. Therefore, although light is being propagated in all directions, only those portions of the waves advancing directly forward remain since all others are cancelled out by destructive interference effects.

WAVEFRONTS CANCELONE ANOTHER

WAVEFRONT MOVING STRAIGHTFORWARD IS NOT CANCELLED

38

The complementary wave cancellation described above does not occur in situations where a portion of the light source has been eliminated. This is true since any disruption of the accompanying waves eliminates the cancellation source for many other waves. Without their necessary cancellation counterparts, waves begin to propagate at various angles from their straing path.

Because these waves continue to interfere with other waves which were not blocked by the original object, a certain amount of cancellation continues to occur but not the total cancellation experienced prior to the objects blocking the light path. The cancellation which does occur has a very special pattern which can be referred to as a diffraction pattern in which light is brightest in the proximity of the edge of the objects shadow and dimmer at further distances from it. The effect of this is a finely graded shadow with edges which we experience when light strikes and object and casts its shadow onto a surface. 39

The diffraction pattern which Thomas Young first observed can be explained using the wave model of light, and it is these same diffraction graduations which we observe join the edges of shadows, being very minute, however, and disguised by many wavelengths mixing together the effect is less distict. The diffraction pattern created by Young's double slit device is dependent upon the spacing between the two slits. The greater this spacing, the more narrow or compressed the pattern will appear to be and thus light will appear to have been bent less (Fig. 4). A simple formula expresses this relationship and may be used to predict the physical separation between light and dark fringes and angle at which the various orders are bent out from the central axis: Fringe spacing = λ x D d Therefore, if we have light from a helium Neon laser at .6328 mm incident on a slit with a slit separation of 1 mm and a distance to a reading surface of 1 decimeter (or 100 mm) then the equation is easy to solve: Fringe spacing = .6328 x 100 1 Then Fringe spacing = 63.28 mm

40 Another formula which predicts the angle at which the various orders of diffraction will occur is just as useful and equally as simple to employ. It requires the following elements for calculation. m = the order of diffraction λ = the wavelength of light employed (helium-Neon laser 632.8 nM) d = the fringe separation distance θ = the angle at which light will be diffracted from the normal represented in the zeroeth order. In its primary form the equation is usually written as: M x λ = d x sine θ Thus if light from a helium helium-neon laser at 632.8 nM is incident on a slit with d = 1_m (or 1000 nM) then the degree of diffraction of the first order (M = 1) would be: 1 x 6328 = 1000 x Sin θ Then Sin θ = 632.8 and Sin θ = .6328 1000 Then θ = _____ The slits used in this example may be replaced by line elements whose edges act as the diffracting elements. The more lines which occur (the higher the frequency) the smaller the spacing between lines and the greater the bending effect of the various orders. In the sciences special glass plates are prepared in which these lines are carefully etched. These devices are referred to as Diffraction gratings. Many materials have this fine grating property as an inherent characteristic of their structure. The fine fabrics used in serigraphic printing (also known as silkscreen printing) create startling diffraction patterns, as do the surfaces of the popular new compact discs! Today, these diffraction gratings can be generated optically by a holographic process.

41 It should be noted here that the degree of shift in a diffracted wavefront is related to it's wavelength. Thus, different colors of light will be diffracted to greater or lesser degrees depending upon their wavelength. Blue light had a shorter wavelength than red. As a result of this, two blue sources of light travel a shorter distance than two sources of red light in order to create an area of constructive interference. The result of this is that the 1st order diffraction for a blue wave of light may be located closer to the zeroeth order of diffraction. What this means, is that blue light is bent less radically than red light. This is the opposite of the effect observed in refraction and can help in distinguishing the means being employed to manipulate light.

So it is apparent from this wave model, that light may be manipulated in a variety of ways depending upon its wave-like properties. By the use of diffraction gratings light may be focused to points in space as with a common glass lens, or broken into its spectral hues as with a prism. For the application of the concept of diffraction to the optical manipulation of light perhaps no other technology is more promising than that of holography.

42 CREATING LIGHT: The light we are commonly familiar with , from incandescent bulbs, is created when energy is transferred from one source (household electricity for instance) to a particular material (tungsten filament). When this occurs, the atoms which comprise the tungsten filament are affected by the electrical current. Essentially the electrons which orbit the atoms in this material absorb the extra electrical energy and are in turn excited to a higher level of activity. In the atom itself these electrons may move to higher orbits at a further distance from the nucleus of the atom. Over time however, the electron seeks to return to its previous (ground) state and gives off the energy it acquired in the form of a photon (packet of light energy) just prior to its return to ground state. The release of the electron's acquired energy in the form of a photon is referred to a spontaneous emission, because there is no regulation over how much energy the various electrons absorb or emit spontaneously. Those photons which are released may vibrate at varying frequencies, thus creating variable wavelengths and sundry colors. The simultaneous exposure of the eye to these various frequencies of electromagnetic radiation (light) creates the visual sensation of white light coming from the light bulb.

It is important to note here that the various waves are not only different in their wavelength, but also in how their movements are synchronized with each other. Because white light from

sources such as the sun is random in both the frequencies it emits and their relations to one another, these sources of light are referred to as INCOHERENT. 43 COHERENT AND INCOHERENT LIGHT: As stated in the discussion of light, because photons are emitted spontaneously from sources such as light bulbs, they are not emitted in an orderly or structured fashion. Firstly, the waves which are emitted consist of a myriad of differing wavelengths. If we compared them in a diagram:

LIGHTSOURCE

we would say that such waves are not TEMPORALLY COHERENT with one another. Essentially, we would perceive their wavelength variations as differing color sensations. White light is temporally incoherent since it lacks a singular, specified wavelength. It is the quality of coherent/incoherent light which helps to distinguish the LASER from common light sources such as neon tubes, florescent tubes, and the like. Unlike these incoherent light sources the laser generates (in theory at least) one very specific wavelenghth of light. In the case of the helium-Neon lasers commonly used in holography the specified wavelength is 632.8 nanometers or .0000006328 meters. Because the wavelength is so specific the light generated by a laser appears very pure and intense, in the case of the helium-neon laser the wavelength for lasing creates a brilliant red. Finally, because all the wavelengths emitted are the same, we say that laser light is temporally coherent.

LASER SOURCE

ALL WAVELENGTHS EQUAL

44

If we continue to examine the character of light generated by the laser we find that there is yet another pattern to it. Not only are the waves of energy emitted all of the same length, but they are also perfectly in step with one another. Thus, all the waves of light which the laser creates are in perfect synchronization with each other - as one of the waves rises, they all rise and so forth. This second, highly ordered characteristic of laser light, is known as SPATIAL COHERENCE and when waves of light are synchronized in such a manner we say that they are IN-PHASE (conversely, if these wave movements are not synchronized they are said to be "out of phase.")

ALL WAVELENGTHS OUT-OF-PHASE

The special coherent properties of laser light (both temporal and spatial) are very important because it is these properties of laser light which make it ideal for creating the interference effects which are the essence of the holographic recording process.

LASER SOURCE

ALL WAVELENGTHS IN-PHASE

45 CREATING COHERENT LIGHT: L.A.S.E.R. Laser is an acronym which stands for Light Amplification by the Stimulated Emission of Radiation. Lasers produce a very pure source of light in which all the wavelengths are uniform and move in step (in phase) with one another. Because the laser's light has only one wavelength it is of one color or monochromatic (the beam of a Helium-Neon laser is a very pure red). The synchronized relation of the waves adds to this highly structured pattern of laser light and it is therefore referred to as coherent. There are many types of lasers and these are usually classified by the material which supports their lasing action. To produce laser light the electrons in the atoms of the lasing material are excited by absorbing added energy from a special source (A) (in the popular helium-neon laser the energy source is electricity). As certain of these electrons are excited, they move to higher level orbits within the atom for brief periods of time. In their higher level orbits these electrons are relatively unstable and are inclined to return to their original orbit levels. Before returning to their lower level orbits however, these electrons must release the energy acquired during their excitation. The emission of this energy takes the form of light (B) the electron emitting a photon as it returns to its lower energy orbit.

The emission of light energy and return of an electron to a lower orbit may occur spontaneously and is therefore referred to as "spontaneous emission". The photon released however may also stimulate emission of other photons from similarly excited electrons in nearby atoms, causing the "stimulated emission" described in the laser acronym. Those photons whose emission is

46

stimulated by other photons are of the same wavelength and "in phase" with the photons which caused their emission originally. By placing a mirror at one end of the laser cavity and a partially silvered mirror at the opposite end, a small portion of these photons are formed into a highly directional beam. Thus in turn, by exciting a great many electrons in the atoms of a lasing material and setting off a large chain reaction, the laser creates a highly structured or coherent beam of light energy in which all the wavelengths are identical and in phase with one another.

A more sophisticated diagram of a helium-neon laser is presented here. This diagram outlines the critical working components of a low-power laser. Typically a low-power laser tube of this design might actually be from 10” – 14” in length. This diagram reveals the Capillary Tube set inside the center of the larger Plasma Tube which contains the ballast for the laser. In addition the diagram indicates a typical positioning for the electrical connections to the laser tube itself (the anode and cathode connections). These electrical connections provide the input of electrical energy which is transformed into the emission of laser light. As the text of the diagram indicates, the actual lasing action of the instrument occurs in the very small space defined as the capillary tube. The special mirrors which allow for the oscillation of photons and the consequent lasing of gases (stimulated emission of radiation) are seen on the ends of the capillary tube. Today holographers also have at their disposal a range of increasingly sophisticated Solid State Lasers. These lasers are constructed using principles and materials similar to those employed in the creation of solid state transistors and other modern solid state electronic circuitry. These lasers are currently being employed in electronics and optical research and will certainly have a great effect on the evolution, practice and application of holography in the future.

47

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48

HOLOGRAPHY AND DIFFRACTION:

To understand the application of diffraction concepts to holography we may begin by visualizing two coherent plane wave sources interfering with one another. If such interference created by two sources (labeled A & B in the diagram below) were recorded on a light sensitive emulsion placed at “C”, the processed film would contain a series of thin striated lines (fringes) which simulate a mechanically generated diffraction grating. The dark lines in this pattern would represent exposed areas of the film where constructive interference occurred; the clear areas of the film would represent those areas where destructive interference occurred and thus the emulsion was not exposed.

EXPOSED FRINGES

ZONES OF CONSTRUCTIVEINTERFERENCE

WAVELENGTH

WAVE CREST

In such a holographic diffraction grating, the spaces between the black lines serve the role of the slits described in Thomas Young’s early experiment into diffraction. If a coherent plane wave source is now shown onto this holographically generated diffraction grating, the slits, as in any other hand or machine made grating, will diffract the incoming waves into their various orders of diffraction. Using these simple diagrams and our knowledge of diffraction we can thereby understand that a hologram (in this case a hologram of a light source) can be employed to bend light.

49 The holographic grating described above may be varied in a number of ways. Perhaps the simplest modification is to change one of the sets of waves used to construct the slit elements in the grating. In the diagram below, the set of waves generated from “B” has been altered to the form of a spherical wave instead of a plane wave. For purposes of demonstration, we will treat this wave as if it is the light being reflected from the point on an object which we are recording holographically. For our purposes we will use a point on the tip of a tree from which the waves are being reflected. This is in fact much closer to the reality of a holographic recording in which spherical waves are reflected off points on the surfaces of objects.

SOURCE "A"PLANE WAVES

SLIT

FRINGESOURCE "B"IMAGE OF APOINT

The difference in effect in the grating which is formed by such an arrangement is dramatic. Now, because of the introduction of the spherical wave pattern from the point on the object, the grating which is formed by interference of the two wave sources “A&B” is itself

50

spherical in character. If greatly enlarged and viewed straight on the pattern of the spherical diffraction grating would look very much like a series of concentric rings. In this way it emulates an optical device known as a zone plate and also appears similar to a fresnel lens (the type you see in gift shop windows that often show a virtual image of the interior space). In this spherical form grating, however, the fringes (or slit spaces) are no longer equal in dimension. Instead the slits at the center of the grating are larger than those at its perimeter.

DEVELOPED FRINGE PATTERN VIEWED STRAIGHT-ON

DEVELOPED FRINGE PATTERN CUT-AWAY VIEW FROM SIDE

51

Knowing that such a new form of grating may be created we might want to ask what the effect would be if the source of plane waves only was projected onto the processed holographic grating. The result is relatively simple to see. The wavefronts of light are diffracted in such a fashion that one set of wavefronts emerging from the grating is deviated upwards in the form of diverging rays of waves of light.

SOURCE "A"PLANE WAVES

SLIT

FRINGESOURCE "B"IMAGE OF APOINT

RECONSTRUCTED OBJECT WAVES

To the viewer, peering into such a holographic diffraction grating from the side opposite of its illumination, it would appear that these diverging rays of light originated from the point on the object from which they were originally reflected, the object however in our diagram has been removed and cannot account for these rays of light. They are strictly recreated, or as holographers term it “reconstructed”. It is easy to see from this that if the spherical waves from many points on the object were recorded that all of these points would be reconstructed for our viewer and they would see an image of the object..

52

In looking at the image reconstructed by the deviated rays of the holographic grating in our example, it is important to note that in the image described, the rays of light are diverging. For this reason we must note that the image is of a “virtual” nature. Therefore, the image reconstructed by the diffraction grating appears beyond the plane of the diffraction grating.

SOURCE "A"PLANE WAVES

SLIT

FRINGE

REAL IMAGE IMAGE SOURCE / ORIGINAL OBJECT

PROJECTED IMAGEPOINT

In examining the effects of the spherical diffraction grating upon a set of plane waves it must also be observed that a set of waves is deflected downward by the grating, but in such a fashion that the waves and the rays used to represent them are forced to converge rather than diverge. The effect of this is that the converging rays form a complementary image point in space on the same side of the holographic diffraction grating as the viewer, but opposite from the side on which the original object was placed. This point would represent a REAL image

53 and while difficult for the eye to view, could be seen by placing a piece of ground glass (or waxed paper) in the position of the point. It is easy then to imagine, as with the virtual image, that if many of the points from this object were originally recorded, the image of the object could be reconstructed in space.

REFERENCE BEAMPLANE WAVES RECONSTRUCTED

IMAGE WAVES

REAL IMAGE (INVERTED ANDLATERALLY REVERSED)

From this we realize that holographically generated diffraction gratings may produce both real and virtual images of point sources (or points on an object). If we then stop to consider that the objects which we observe in our environment are a complex array of such points (point sources) of light, we may imagine that a very complex image such as our object, with its myriad of reflecting points, could be recorded holographically. Now, for each point, the spherical waves coming from the object (object beam) interfere with a plane wave from the reference source (reference beam) to construct a spherical holographic diffraction grating on the recording film. In an actual recording, millions of these holographic diffraction gratings are formed, overlapping and integrating with one another, to form a very complex and unintelligible pattern which is unrelated to the appearance of the object being recorded. These gratings however, which are the essence of the hologram, when properly illuminated allow us to reconstruct the waves of light as if they came from the object itself.

54 HOLOGRAPHY: CONSTRUCTION AND RECONSTRUCTION STAGES CONSTRUCTION STAGE: When an artist sets out to create or record holograms we refer to this as the “construction stage” of holography. This is the stage in which the interference patterns (involving the constructive and destructive interference phenomena) of wavefronts of light will be initially created using the special properties of laser light. It is also the stage in which these interference patterns will be recorded on light sensitive photographic emulsions. The use of the interference mechanism is crucial here because this pattern contains information about the subject elements being recorded in the hologram. The basic phenomenon of constructive and destructive interference are fairly universal with light; however, the manner in which sources of light energy interfere with one another may create differing forms of interference patterns. The classical example in holography is the laser transmission hologram developed by Leith and Upatnieks at the University of Michigan in Ann Arbor in the early 1960s. This type of hologram is designated laser transmission because a laser is used both to construct (record) and reconstruct (view/playback) this type of hologram.

In a laser transmission hologram, as diagrammed here, the beam of coherent light generated by a laser (A) is split into two beams by an optical element called a “beam

splitter” (B) (this beam splitter device may be as simple as a piece of window glass or as complex as a precision coated optic). Each of the two beams which results is then diffused by a lens (C, C’). For this task negative lenses are frequently

55 employed, in certain instances spatial filters might be substituted for these lenses. At this point both the sources remain coherent with all their wavefronts synchronized with one another. Eventually, one of these diffused beams is reflected off a mirror and toward an object. As the coherent light source strikes this object however, its regularity (or spatial coherence) is partially disturbed. Although the wavelength of the light (or temporal coherence) is maintained, the phase

RECORDING FILM

OBJECT

OBJECT WAVES REFLECTINGFROM A SINGLE POINT

relationship between the waves is modulated by the object, as is their amplitude (fig. 2). This beam, having been modulated by the object, is referred to as the OBJECT BEAM. The light energy from this beam is now scattered in all directions from the object, and a certain portion proceeds in the direction of the recording material (film) placed near the top of your diagram. The remaining beam does not collide with any objects; instead, after being diffused, it is reflected off another mirror and passes

directly to the recording material. Because it is unaffected by an object the REFERENCE BEAM maintains its coherent structure. It is referred to as a reference beam because it is in relationship to this that the object beams patterns are known and may be reconstructed.

56

When the waves of light from the OBJECT and REFERENCE beam sources collide with one another they create varying patterns of constructive and destructive interference depending on their amplitude and phase relationships. Those areas of constructive interference which occur in coincidence with the photosensitive emulsion of the recording material are then exposed, and conversely the areas coinciding with the destructive commingling of waves are not. When traditional silver based photographic emulsions are the holographic recording material, these areas of exposure and non-exposure create latent images which must be chemically “developed”. After this development, certain areas of the recording material may have a greater density than other areas in correspondence with their degree of exposure. Traditionally, these denser areas of the emulsion are referred to as FRINGES (fig. 3). It is these fringe areas which in turn manipulate or modulate the light passing by them to reconstruct a holographic image.

RECORDING FILM

OBJECT WAVES REFLECTINGFROM A SINGLE POINT

REFERENCE BEAM WAVES (PLANE)

57 RECONSTRUCTION STAGE: After processing, the hologram may be placed before a diffused beam of laser light for viewing. In order for the holographic image to be reconstructed, however, the laser light must pass through the hologram from precisely the same orientation as the original reference beam. For this reason, the hologram is generally returned to its original recording position (Fig. 4), and the reference

beam is passed through it. The original object and object beam are no longer essential for viewing the object and are removed from the diagram. Now, the laser light is passed through the hologram from the same orientation as the original reference beam and the object, though no longer present, may be seen. At this point, the fringes recorded during the CONSTRUCTION stage serve to bend the light passing through the hologram. In one manner of representation, these fringes act like very small prisms or lenses, which when combined in a minute and complex pattern permit the hologram to bend the reference beam’s light so that it replicates those waves of light which originally constituted the object beam. The manner in which the light is bent is dependent upon the size, shape, and orientation of these fringes which was in turn determined by the modulated waves coming from the object during the recording of the original hologram. Now the hologram simply reconstructs those object waves, making it appear as though the object is still there. NOTE: The wave interference patterns which create holograms may be generated by many different wavelengths from the electromagnetic spectrum or from other wave media. Thus X-ray, infrared, microwave and acoustical (sound wave) holography are all possibilities.

REFLECTION HOLOGRAPHY: The recording of reflection holograms relies on the same principles of constructive and destructive interference described in the case of laser transmission holography. The method employed to create the necessary interference is quite different however and requires a radically different form of reconstruction in the final image. Reflection holograms are illuminated from the same side as they are viewed, the light frequently comes from overhead. That light which enters the hologram is partly reflected back toward the viewer with information about the object(s) introduced into the reflected waves by the fringes of the hologram.

In constructing a reflection hologram, the reference and object beams (A) are made to interfere by passing through the recording film from opposite sides. For this purpose the recording material being used must be clear, for exposure on both surfaces. The effect of such a construction technique is the formation (B) of interference fringes within the thickness of the recording material. These fringes are like flat reflecting surfaces or planes and occur in multiple layers.

55 If, after processing, a white light is cast onto the hologram from the proper angle (C), these thin planes will reflect light back to the viewer.

The relationship of those waves reflected back is dependent upon the spacing formed between the fringes of the hologram during recording. For light of the correct wavelength the reflected waves interfere constructively during illumination of the

hologram and thus create a visible image. Those wavelengths which are longer and shorter are reflected in such a manner as to interfere destructively and therefore are negated in the final image. As a result of this process, one primary wavelength is selected from the many wavelengths (colors) in the white light source. The holographic image which appears to the viewer is therefore of only one primary color, or monochromatic. The multicolor reflection hologram is created by using several separate exposures, between which the recording film is swollen so that the distance between fringes varies for each exposure.

56 TRANSMISSION HOLOGRAPHY: Laser transmission holograms of the off-axis variety were first created by Leith and Upatnieks at the University of Michigan in Ann Arbor during the early 1960's. Laser transmission and other transmission holograms are easily distinguishable, since for reconstruction, they must be illuminated by passing light through the hologram from the side opposite that from which they are to be viewed. Early transmission holograms required lasers for viewing as well as recording the holographic image. Because these holograms were viewed with the aid of lasers, their images were of a single color (that of the laser). In the late 1960's Dr. Stephen Benton of the Polaroid Corporation developed a technique for creating holograms which were viewable under white light, thus permitting some limited use of color in holographic images of the transmission type. A white light transmission hologram begins with the creation of a laser transmission image, often referred to as the "master" hologram. If this hologram is inverted for replay, its image will project in front of it (A)

into space (this is referred to as a real image). By passing the beam of reconstructing laser light through a special cylindrical lens, this image may be recreated from a thin strip in the center of the hologram. Now, if a new recording plate is placed near this real image and a reference beam made to interfere with it, a second image will be

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formed - a hologram of a hologram! When, after processing, this second stage hologram is viewed using a white light source, the thin slit used to create it will project out an image as if coming from a prism (B). The various wavelengths of light will form images for the viewer at different points in space. While moving up and down before such an image these spectral colors demonstrate why this is often referred to as a "rainbow hologram".

58 EMBOSSED HOLOGRAPHY: Holographic images can be created in great quantity and at relatively low cost through the embossing process. The flexibility of embossed holograms permits them to be used in a wide variety of applications, from magazine displays to anti-fraud devices on popular credit cards. Because holograms contain a great deal of information in a very discrete form, they are difficult to forge. This enhanced level of information storage also makes holograms potentially useful for educational, as well as data storage purposes.

The making of an embossed hologram involves several distinct physical stages, some of which are similar to those in the other categories of holography. To begin, a laser transmission hologram is recorded (A) onto a photo-resist material. After exposure, this hologram is processed leaving behind a distinct surface pattern (B) caused by the interference effects and fringe formation. Although the photo-resist in which the hologram is originally recorded is fairly durable, it alone could not stand up to the strain of producing thousands of impressions. In order to create a more durable surface a metallic mold of the photo-resist is made by plating (C) the original photo-resist material with a fine layer of nickel. After this stage, the metal mold (D) is used to stamp out impressions of the original interference pattern (E) in great numbers. The material most popular for such impressions is a mirrorized plastic. The final hologram is seen when the embossed image is properly illuminated (F).

59 INTEGRAL HOLOGRAPHY:

The integral, or "multiplex" hologram is created from cinema/computer imagery and is a hybrid process that combines the visual technologies of holography and film/digital imaging. In creating an integral hologram, original cine footage (A) is made of the desired subject. After this film has been processed it is then placed in a

special camera mechanism which is linked to holographic recording equipment. Through this camera each frame of the processed cine film is projected onto a holographic recor-ding material. With the aid of cylindrical lenses, (B) these camera images and the reference beam which interferes with them are formed into thin vertical lines on the film (the striated vertical line pattern on the hologram is easily observed with the unaided eye). By placing the separate frames of the film in consecutive proximity to one another, they may be viewed simultaneously. As we move about the hologram, each of our eyes sees separate frames but fuses them stereoscopically, thus creating dramatic spatial sensations and effects. Because of its reliance on this stereo concept, the integral hologram is sometimes referred to as a `'holographic stereogram."

Integral holograms are often easy to distinguish by the cylindrical displays used in their reconstruction. Integral holograms are, however, also created in a flat format. The light source in many cases is built into the integral display and often simply consists of a 100 watt clear light bulb. Because of the dispersion of white light caused by the hologram, these images, like white light transmission holograms, also have a distinctive rainbow effect.

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MAKING IT - Single beam reflection hologram Materials required: 5 mW Laser Stabilization system 1 spatial filter 1 4 x 5 front surface mirror glass plates or film support device assorted rods and clamps laser safety goggles shutter mechanism Optical set-up: The diagram on the following page indicates that single-beam reflection holograms are produced by passing the diffused laser beam through the holographic recording material onto the object below it. From here the laser light is reflected back up into the film where it interferes with later portions of the wave train causing a fringe pattern to be recorded. It is improtant to remember that the angle at which the laser beam strikes the glass determines your reference angle. When you reconstruct your hologram it will need to be illuminated from the same angle for proper reconstruction. PROCESSING: time agitation Developer - Stop - Fix - Rinse - Bleach - Rinse - NOTE: All processing should be completed at 70 F 47

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Presentation pointers: A. To recreate a clear sharp image of your hologram you will need a point source of point source of illumination. Some useful examples are sunlight, slide projectors, some flashlights, automobile headlights or clear light bulbs. Without a point source for illumination your image will appear fuzzy and unclear - try it under a flourescent lamp or frosted light bulb to observe this effect. B. Your hologram's fringes may be used to reconstruct either a virtual or a real image. Try flipping your hologram over from its virtual image position and observing its real image. Elements which were close in the virtual image will be far away and vice-versa. The entire visual field of your image has been turned inside out. C. Your hologram will appear brighter if placed against a dark ground such as black matboard, black plexiglass, or black fabric. Some holographers use black spray paint to coat the backs of their holograms in a more fixed manner, others cover them with a black contact paper. D. Because your hologram was processed using a rehalogenating bleach, it is still sensitive to light. Overexposure to the sun may cause some darkening of the film. Photographers refer to this as "printing out." The hologram may be rebleached if this effect is undesirable otherwise you must be careful of where the hologram is displayed and stored. Avoid prolonged exposure to the sun. E. Because of the character of reflection friges in your hologram, the image which you have created here is sensitive to humidity as well as sunlight. Although humidity will not necessarily damage your image, it may cause subtle or dramatic color shifts in the image. In many instances the image will return to its "original" color when the humidity conditions resemmble those under which it was made. F. As with most standard photographic materials being stored archivally, limit the exposure of your hologram to contaminants of all types. Wrapping the film or plate in an acidic paper for instance may cause a deterioration in the emulsion. Some holographers attempt to seal there work into glass or plexiglas using various tapes, glues (epoxy) or optical cements.

48 HOLOGENICS: When making photographs, people used to describe good subjects as being "photogenic." Holography, as a new medium, is not yet so formalized. There are however, for technical reasons, some qualities which may be desirable in the subjects which you employ in your holograms. Among these are the following: 1. Stability: In general, your subject matter should be of a stable nature, unlikely to move (even slightly) during the time period during which you are recording it. Because you are recording the relationships between waves of light, even slight movements may cause distortions or "blackholes" in you images. Therefore, it may be helpful to secure moving or unstable elements in the objects you use in your hologram by gluing them, mounting them to a more sturdy surface or transferring their image to a more stable material (i.e. a plaster cast of your subject). Because of the super sensitivity of holographic recording processes to even the slightest movements, objects that might normally seem stable are in an active state of decay, growth or some dynamic process which makes them difficult to record holographically (plants are a good example). 2. Lightness or Darkness: Although holography is theoretically advanced, it still relies heavily on the 19th Century technology of photosensitve emulsions for recording the interference fringes which constitute the hologram. Therefore, just as in photography, if your subject is too dark, it will not reflect enough of the laser light to be recorded on your holographic film. If the materials you choose to record are not bright enough you paint them white, or a metallic silver or gold, or lighten them in some other manner. Metallic materials, and translucent or transparent subjects may create special glare problems whcih we will discusss in lab. 3. Size: When you look through a hologram, it's like looking through a window. If an object is close to that window and larger than the window then you may not see all of it. Likewise, if its small and far away, it may be hard to make out. The depth or thickness of your subjects will depend on the type of hologram you are making. Single beam reflection holograms permit only a few inches of depth to be recorded, but single beam (deep scene) transmission holograms viewed with a laser may permit up to several feet of depth to be recorded. In preparing your subject it may be useful to its study in relationship to some framework which simulates the window size opening through which you will be viewing your final hologram.

49 THE BIG "T" SYSTEM In order to record the interference of wavefronts of light with a continuous wave laser it is necessary to stabilize all the components used in the recording process (lenses, mirrors, film, etc.). This is often termed "vibration isolation" by holographer, and in many laboratories this is achieved by using state of the art equipment costing many thousands of dollars. In response to this high cost, the vibration isolation system described here was developed by Chicago artist/holographer Edward Wesly in the early 1980's and is intended to provide an economical means by which to make hologram, thus aiding artists and educators who are unable to invest large sums of money into equipment. Essentially, the BIG "T" operates like an optical rail, but because of its size and physical features it provides for flexible adaptation to numerous holographic geometries. Building a BIG "T" is quiet simple and the following directions may be modified in any manner whatsoever to accomodate this equipment to your working environment, existing materials, or special needs.

MATERIALS NEEDED 1. A beam of wood, length and thickness negotiable, but you'll probably want at least a three foot length of 4 x 4 lumber. When buying you beam, try to select dry wood which is straight and has few cracks (try to get a good grade of a harder wood if it's affordable). 2. One or two eight foot 2 x 4's. 3. Black paint (spray and flat latex, and some brushes) and some type of sealer such as a polyurthane varnish. 4. Three inner-tubes. For smaller systems, go-cart inner tubes are fine, though automobile inner tubes are usually cheaper. 5. Long carriage bolts (size depending on size of wook stock). 6. Concrete blocks (optional). 8. About 5 standard laboratory clamps, capable of holding up to 1/2" rods.

ASSEMBLY INSTRUCTION A.If the wood you buy is not dry, let it dry out for a few weeks then fill in any splits or cracks it might have with some type of wood filler. After these materials have dryed, paint the beam with liberal quantities of flat black paint. After this paint has thoroughly dried, coat the wood components with a sealer such as a polyurethane varnish. B.The 2 x 4's should be cut so that they may form an "X" which extends over the top of the inflated inner tubes. After cutting these to size, they should be notched so that they lie

together flush. Another piece of 2 x 4 or heavier wood should be cut ot form the top of your BIG "T". This section of wood will stretch from the center of one of the inner tube X's to the center of the other. At the point at which the top of the BIG "T" meets the X's the three pieces of wood may be drilled, and a carriage bolt inserted to secure them to one another. For stability the length of the top of the BIG "T" should be about 4/10ths as long as the main beam. 50 C. Your beam may be assembled on the floor or, to reduce back strain it may be elevated off the floor on columns of concrete blocks. Try to locate your system in an environment where there is not excessive movement, but it is not necessary to work there. Remember that the ultimate test of the correctness of your systemn is the results it achieves - no the way it looks. D. Holes may be bored along the length of the main beam, or on sections of the crossmember at the top. These holes are used to support the 1/2" rods which are inserted into the beam and are used for holding optics and other devices. Because their is no phycical mechanism to secure these rods, the holes hould be bored slightly smaller than the diametr of the rod so that the rods will fit snugly and securely int the wood hole. Both these rods and the lab clamps you purchased should be spray painted flat black to reduce the possibility of unwanted reflections when you are recording your holograms. 1. A slightly neater system but not much costlier, involves setting 1/4 x 20 T-nuts into your beam and crossmember where you wish to place rods. T-nuts are available at most hardware stores and the beam must be pre-drilled to accept them. In order for the rod to be set into the T-nut, it must be bored and tapped to accept a 1/4 x 20 set screw. This set screw should be expoxyed into the threaded hole in the rod so that approximately 1/2 its length projects out. The rod may then be screwed into the T-nuts on your beam. 2. A more elaborate and costlier adaptation of this idea is to have a 1" x 1 1/2" piece of aluminum bar stock bored and tapped on 1 1/2" intervals to accept the 1/4 x 20 threaded rods described above. This bar or rail may then be screwed onto the side of your beam so that it allows you to secure rods at any location along the side of the beam.

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PROCESSING HOLOGRAMS

HANDLING TECHNIQUES: EXPOSURE CONSIDERATIONS - The high resolution films employed in holgraphy have some very unique aspects although they remain similar to many common B/W photographic films. In general, because of the very fine grain structure required for recording the ultra-fine fringes of laser light interference patterns, most holographic films are relatively insensitive to light. In regard to the traditonal ISO/ASA film speed ratings it is probable that films from most manufacturers would have ISO/ASA ratings of approximately 1/2 or lower. Thus, even in situations with considerable laser power levels and small film areas, it is not uncommon to find holographers using exposures of 2, 5 and 10 seconds or longer. Although most manufacturers of holographic films can provide information on the amount of energy required to produce adequate exposure of their photo-sensitive emulsions, in many cases such information cannot be easily translated into an exposure guideline. In certain instances holographers rely on creating exposures of a given density to insure proper exposure and related development of their emulsion. By exposing and developing your holographic film so that it matches the density of certain neutral density filters, you may be able to achieve agreeable results. For the image oriented holographer the choice of exposure and development may require finer adjustment than that simply described by matching certain density characteristics in the processed emulsion. Essentially this means that the desired holographic exposure and processing be determined by simple test exposure techniques. The most rigorous testing involves a "ring-around test" wherein exposures are made on individual pieces of film and then processed for varying times. The selection of proper exposure and development are then based upon the visual result much as in traditional photography. In a simplified version of this test, a single piece of film is exposed for varying ime by covering all but the desired area with an opaque material such as a dark mat board or the like. The several exposures may all then be processed simultaneously. INTERMODULATION NOISE - One of the reasons that it is difficult to rely entiely on energy readings or density matching to determine exposure and development of holographic materials is the presence of noise components in the holographic recording process. One of the most

critical of these noise components is known as "Intermodulation Noise" and results when light from various points on the objects being recorded interferes with itself rather than the reference beam. The result of this is that fringes are formed which will diffract light but do not contain information about the object being recorded. Often this noise appears as a haze throughout the image and while it does not obstruct the presence of the image this noise is distracting in that it reduces the overall brightness of the image and limits the usefulness of the hologram for generating other secondary images. LATENT IMAGE DECAY - As with all photographic materials, holographic emulsions suffer from an effect known as latent image decay. After the exposure of a photo-sensitive emuslions to light the chemical structure containing the image information begins to disintegrate or decay causing an eventual degrading of image quality. This effect begins to occur immediately after exposure and thereforre it is best to process your exposed film as quickly as possible. SAFELIGHT - Holographic films are generally monochromatic and sensitive to light from a relatively small portion of the visible spectrum. This spectral sensitivity has been discussed and it is important to relate to the selection of safelights. Generally, the holographic films fall into one of two categories of sensitivity (A) red/orange sensitive or (B) blue/green sensitive. The proper safelight must be opposite in color to the film's color sensitivity. Thus if using a red-sensitive film such as Agfa 8E75, then a green or blue/green safelight is indicated. FILM STORAGE - Unlike many commercial photographic materials, holographic films traditionally have relatively short shelf lives, especially when stored in room temperature conditions. Generally speaking holographic films should be stored in a cool, dry environment. If properly packaged a film may be refigerated or frozen to promote logevity. In instances where the film is frozen or refigerated it is important to warm the film slowly prior to usage. MAKER EMULSION THICKNESS SENSITIVITY RESOLUTION SPECTRAL TYPE (MICRONS) (ERGS/CM2) (LINE/mm) (SENSI) KODAK 120 6 400 2000+ 600-750 173 6 400 2000+ 550-750 649F (17P)(7f) 900 2000+ 450-650 125 3 50 1250 442-515 131 7 4-8 1500 600-650 253 9 4-8 1500 600-650 AGFA 8E56 7 250 5000 350-560 8E75 * 7 100-200 5000 600-750 10E56 10 3000 350-560 10E75 20 3000 600-750

ILFORD SP672 7 750 7000 350-560 SP673 7 800 7000 525-750 PROCESSING GUIDELINES: (All processing at 70 / photo-flo and squeegee after final rinse) AGFA 8E75 NAH REFLECTION HOLOGRAMS : PYRO-DEVELOPER RINSE DICHRO-BLEACH RINSE 1-2 MINUTES 5' clear + 1 minute 10' D-19 DEVELOPER 5' clear + 1 minute 10' 1-2 minutes CWC2 5' clear + 1 minute 10' 1-2 minutes EDTA bleach may be substituted for the Dichro bleach TRANSMISSION HOLOGRAMS: (without an anti-halo backing, transmission holograms may contain considerable noise) PYRO-DEVELOPER RINSE DICHRO-BLEACH RINSE 4 MINUTES 5' clear + 1 minute 10' D-19 DEVELOPER 5' clear + 1 minute 10' 4 minutes

CWC2 5' clear + 1 minute 10' 4 minutes EDTA bleach may be substituted for the Dichro bleach KODAK SO-173 REFLECTION HOLOGRAMS - because SO-173 is only manufactured with a anti-halo backing it may not be used for reflection holograms (unless this backing is removed). TRANSMISSION HOLOGRAMS PYRO-DEVELOPER (Pyro developer is not recommended for use with so-173) D-19 DEVELOPER Stop Fix Rinse 2-5 minutes 30 sec. 5 minutes 5' Bleach (Ferric-Nitrate) Rinse 1-2 minutes 10' D-8 DEVELOPER Stop Fix Rinse 2-5 minutes 30 sec. 5 minutes 5' Bleach (Ferric-Nitrate) Rinse 1-2 minutes 10'

DRYING TECHNIQUES: Holograms may be dried using several techniques: Squeegee technique - In this process, holograms are given a final rinse with photo-flo and then squeegee the residual water off both sides of the hologram. At this point the hologram may be air-dried or dried with a forced air. Air drying - In this technique the freshly rinsed print is not squeegeed but simply leaned against a surface to dry by allowing the surface water to run off an evaporate. Alcohol drying - Using this technique the hologram is progressively dryed in solutions of water and alcohol.