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Thermal Recording and InfraredPhotography of Hot Objects

For a long time there has been considerable interest inthe photography of objects by reflected infrared radiationusing photographic emulsions sensitized to respond tonear-infrared radiation (infrared waves that are near thered end of the visible spectrum). The source of thereflected radiation is usually the sun or some otherincandescent source such as a tungsten-filament lamp.You have probably seen the unusual effect in infraredphotographs taken with sunlight as the source of infraredradiation (IR). These photographs are possible becausemost objects reflect not only radiation in the visiblespectrum but also infrared radiation, and the sun providesvirtually as much near-infrared as visible radiation. Thesephotographs do not indicate the temperature of theobjects, but only their ability to reflect infrared radiationfrom the sun.

More recently there has been increasing interest inrecording infrared radiation directly emitted from objectsthat are hot or even incandescent. Infrared photography

can be used for such recording, but it is practical overonly a limited range of temperatures. Unfortunately, thisrange of temperatures does not include that of objects atthe ambient temperatures found near the earth's surface(- 50 to 50°Cor - 60 to 120°F).Therefore, suchapplications as recording the heat loss from buildingscannot be accomplished directly with infraredphotography.

A method called electronic thermography detects long-wave or far-infrared radiation and subsequently displaysit at visible wavelengths. Sophisticated electro-opticalscanning devices convert temperature differences in anobject into brightness or color differences on a closed-circuit television screen. This display may then berecorded on conventional black-and-white or color film inan ordinary camera. While the electronically generatedimages are frequently referred to as infrared photographs,they are actually only conventional photographs ofelectronic displays.

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Infrared RadiationInfrared radiation differs from visible radiation mainlywith regard to the length of the electromagnetic waves.Red light has a longer wavelength than blue light, andinfrared radiation has a longer wavelength than either. Inall other respects, these radiations behave similarly. Theytravel through space in the same wave-frontconfiguration and at the same speed. Appropriate mirrorscan reflect them, and the proper refractive elements orlenses can bend their paths and focus them. The efficiencywith which various materials transmit or reflect visibleand infrared radiations can vary considerably.

At all temperatures above absolute zero (- 273°Cor- 460°F),every object emits energy from its surface in theform of a spectrum of differing wavelengths andintensities. The range of wavelengths emitted and theintensity of emission.depend on the absolute temperatureand the emissivity of the surface.* As the object becomeshotter, the amount of energy emitted at each wavelengthincreases. Also, the range in which radiation occursbroadens to include shorter wavelengths. The peak moves

from the long-wavelength end of the infrared spectrumtoward the short-wavelength end as the temperatureincreases. An object below 525 K (250°Cor 480°F)radiateswavelengths that lie roughly between 1400and 100,000nanometres (nm). For example, an object whosetemperature is roughly that of the earth's surface (- 50 to50°Cor - 60 to 120°F)will emit radiation at peakwavelengths ranging from 9000to 13,000nm. The humanbody radiates infrared energy at wavelengths between4000and 20,000nm. These wavelengths are not "actinic";that is, they will not expose photographic materials nomatter how sensitive the materials are.

*The emissivity of an object is the ratio of the amount ofradiant energy it emits to that emitted by a perfect radiator atthe same temperature. A perfect radiator is known as a"blackbody" and has an emissivity of 1.0. Emissivity values forvarious materials appear in most physics handbooks.

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At about 525 K, enough near-infrared radiation ispresent to expose infrared-sensitive photographicmaterials. The amount of near-infrared radiationincreases as the temperature rises until, at about 775 K(5000e or 930°F),the object begins to glow with a dull redcolor, and will show up on films with full panchromaticsensitivity as well as on IR-sensitive films. With furthertemperature increases, the spectral shift continues andthe color changes through orange and yellow to white. Atthis point, the object is emitting all visible wavelengthsalong with some shorter wavelengths (ultraviolet) and anabundance of "invisible" infrared radiation. When thetemperature reaches approximately 5000K, the emissionis highest in the visible region of the spectrum.

Photographic SensitivityThe spectral sensitivity of an emulsion describes itsphotographic response to radiation of variouswavelengths. Human vision has a spectral range of 400 to700 nm. Photographic materials can be sensitized toprovide spectral response from 250to 1200nm.Photographic emulsions that contain silver halides areinherently sensitive to blue and ultraviolet wavelengths.This inherent sensitivity is usually extended to longerwavelengths in manufacturing. An emulsion that issensitive to most visible wavelengths is calledpanchromatic. With further special treatment, it ispossible to extend photographic sensitivity beyond thevisible and into the near-infrared.

Although the infrared spectrum extends from 700 nm toapproximately 1,000,000nm, very little of it can berecorded photographically. Production of silver-halidefilms with sensitivity beyond about 1300nm is consideredimpossible. At longer wavelengths, the energy of a photonis insufficient to contribute to the formation of a latentimage except under some extreme conditions that are notapplicable here. Furthermore, films that might respond tolonger radiations would present serious problems asthermal sensitivity increased. Even warmthapproximating body temperature in the camera andsurroundings would quickly fog the film. Such specializedfilm, even if it could be made, would have to be stored andused at very low temperatures.

Most infrared films have little sensitivity beyond900 nm, although certain specialized materials forastronomy extend to about 1250nm. For example,KODAK High Speed Infrared Film 2481is sensitivethrough the visible regions of the spectrum and in theinfrared to approximately 900 nm, with maximum IRsensitivity from 700 to 880 nm.

Infrared Photography of Hot ObjectsMeasuring temperatures photographically is based on twopremises: (1) For a given surface, radiation increases withtemperature; and (2) the greater the amount of radiationstriking the film, the greater the density produced. Areasthat produce equal densities in the negative are at thesame temperature, provided that their emissivities areequal. (The amount of IR an object emits, and thereforethe amount of exposure required, is proportional to itsemissivity. While a blackbody has an emissivity of 1.0,polished metal objects may have a rating as low as 0.02,which would require a 50Xincrease in exposure.)

When an object reaches incandescent temperaturesabove 5000e (932°F),it becomes self-luminous. It emitsactinic radiation and can be photographed without anyother source of illumination. Its light can be recorded onpanchromatic or color film, and its infrared radiation canbe recorded on infrared-sensitive film.

An object at a temperature below 5000e emits no visibleradiation (light), and therefore cannot be photographedwith panchromatic or color film unless it is illuminatedby an external source. However, at temperatures between250and 5000e (482and 932°F),it does emit enough actinicenergy to produce an image on infrared film if theexposure time is long enough.

Infrared-sensitive film records the tonal gradationsproduced by emitted infrared radiation that can't berecorded with normal panchromatic photographicemulsions. The technique of using such films to recordthe temperature profiles of bodies at temperatures above2500e (482°F)is much like pictorial photography. Acamera with a large-aperture lens «/2 or greater), aKODAK WRATTEN Gelatin Filter No. 87,a sturdytripod, and a means of calibrating exposures will be

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adequate for most studies. Exposures may be extremelylong-in some cases, hours.

Since exposures of hot objects depend on the objects'temperature and emissivity (both usually unknown),experimentation is required. Try this as an example.Photograph an electric-stove heating element(approximately a blackbody) in the dark, heated tosubincandescence (no visual sensation to a dark-adaptedeye). Use an exposure of 1 second at fll.4 with KODAKHigh Speed Infrared Film. Even though you're makingthe exposure in the dark, use a KODAKWRATTENGelatin Filter No. 87C over the lens to exclude anypossibility of visible-light exposure. Because you will takethe exposure solely with infrared radiation, use the IRfocus mark on the lens. Colder bodies near the heatingelement may also show up on the film if they reflect someof the infrared radiation emitted by the element.

When you want to study the infrared radiation emittedby an object, you must not allow infrared radiation fromother sources (e.g., the sun, lamps, or other hot objects) tofall on that object. Otherwise, the reflected radiation willcombine with the emitted infrared radiation to produce agreater exposure on the film. This will indicate that theobject is hotter than it actually is. Another reason fordoing most heat-loss studies in the dark is that infraredfilms are sensitive to all wavelengths in the visible regionof the spectrum as well as being sensitive to infraredradiation. A visually opaque, infrared-transmitting filter,such as the KODAK WRATTENGelatin Filter No. 88Aor 87C, will also prevent visible radiation (400to 700nm)from reaching the film. However, it will not preventreflected infrared radiation from reaching the film-only

darkness or filtration of the source of radiation willdo this.

Unfortunately, infrared films are not sensitive enoughto record sources of heat loss in buildings, or radiationfrom overheated machinery. Generally, this is possibleonly if the object is shielded from other sources ofinfrared radiation (usually in the dark) and is heated to atemperature greater than 250°C(482°F).At thistemperature there is barely enough infrared radiationfrom the object to record on film; however, it would behot enough for a wood-framebuilding to ignite!

To allow quantitative interpretation of infraredexposures of hot bodies, a calibration of some sort isnecessary. One way of calibrating is to include a series ofexposures of a standard hot object made at progressivelyhigher known temperatures within a particular range.You should make these exposures on the same roll of filmyou use to record the object under investigation. You canthen make a calibration curve by plotting density versustemperature.

The temperature range that can be covered by anyoneexposure is about 60 to 120°C(110to 220°F),and theactual position of this range on the temperature scaledetermines the exposure required. If the temperaturevariations over the surface of the body are greater than120°C(220°F),the density range on the film does notaccurately translate into temperature measurements. Inthis case, two exposures are a better choice-a longexposure to record the cooler parts and a shorterexposure for the hotter areas. If the range is confined to60°C(nO°F) or less, temperature differences of 15°C(60°F)are easily distinguishable.

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Electronic ThermographyAs we have indicated, objects at temperatures below250°C(482°F)cannot be photographed directly by usingself-emitted energy as the source of exposure. Althoughsuch bodies emit infrared radiation, the wavelength ofthis radiation lies well beyond the sensitivity range ofphotographic emulsions. This emitted radiation can onlybe imaged by using electronic thermographic processes.These are infrared imaging systems that employelectronic detection to translate variations in infraredradiation into a visual display that can then bephotographed by conventional methods. These systemscan record the images of objects with temperatures thatfall well below 250°Cby recording the longer-wavelengthinfrared that is emitted. To avoid recording reflectedinfrared from other sources, the operation is often carriedout in complete darkness.

In one type of instrument, the optics scan the objectpoint by point. Infrared radiation from the object isdirected onto a super-cooled photodetector, whichconverts the incoming energy into a proportionateelectric signal and then amplifies it. This amplified signalis often recorded on magnetic tape for use in a variety ofways for interpretive purposes, or the signal may godirectly to a cathode-ray tube (CRT).The image on theface of the CRT shows temperature variations as changesin image brightness. Usually, the brighter the image, thehotter the object. This image can be observed andinterpreted directly; frequently the CRT display isrecorded with conventional photographic equipment.

Liquid CrystalsIn many applications, high instrumentation cost makeselectronic thermography impractical. Under certainconditions, liquid-crystal thermography offers a relativelysimple, and much less expensive, means of detectingnonactinic heat patterns. Certain optically active organicmaterials are capable of forming a phase that is

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intermediate between the liquid and the solid state. Thisliquid-crystal phase is very sensitive to changes intemperature. When illuminated with white light, it showscolor changes as the temperature fluctuates provided thetemperature profile falls within the active range ofthe crystals.

Color patterns formed by liquid crystals are bestobserved by reflection rather than by transmission. Thesurface on which the thermal pattern is to be observed isusually blackened before the liquid-crystal solution isapplied. A black, washable, water-base paint, brushed orsprayed on, is often suitable for this purpose. The blacklayer absorbs the light that is not scattered by thecrystals, thus enhancing the color saturation.

The colors result from selective reflection of circularlypolarized light; the hue is determined by the intermediatestructure of the liquid crystal. Above and below theusable temperature range, the substance is colorless. Atthe lower end of the temperature range, the materialbecomes red and progresses through yellows and greensto blue at the upper limit as the temperature rises.

The progressive representation of all wavelengths inthe visible spectrum can result from a temperaturechange as small as lOC(2°F)with some crystal structures,making detection of very minute temperature differencespossible.

Permanent records of the displayed colors can be madewith ordinary color film in an ordinary camera. However,accurate temperature measurement requires carefulplacement of cameras and lights. The photographer mustexperiment with the placement of the equipment. Also,the light source and camera lens must be equipped withpolarizing filters.

Eastman Kodak Company manufactures liquid crystalcomponents and intermediates.

For more information on liquid crystals, write to theLiquid Crystal Institute, Kent State University, Kent,OH 44240.

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Some Suppliers of CholestricLiquid Crystal MaterialsEM Laboratories, Inc.5 Skyline DriveHawthorne, New York 10532914-592-4660

Thermal Recording and Infrared Photography of Hot ObjectsSome Suppliers of ThermographicEquipment and ServicesAGA Corporation550 County AvenueSecaucus, New Jersey 07094201-867-5390

Barnes Engineering CompanyP.O. Box 5344 Commerce RoadStamford, Connecticut 06904203-348-5381

Daedalus Enterprises, Inc.Box 1869Ann Arbor, Michigan 48106313-769-5649

Hughes Aircraft CompanyProbeye Marketing6155El Camino RealCarlsbad, California 92008619-438-9191ext. 624

Inframetrics, Inc.12 Oak Park DriveBedford, Massachusetts 01730617-275-8990

Texas Instruments, Inc.Advanced Infrared Systems DivisionP.O. Box 6015Dallas, Texas 75222214-283-3333

UTi325 North Mathilda AvenueSunnyvale, California 94086408-738-3301

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Halcrest Products, Inc.1820Pickwick LaneGlenview, Illinois 60025312-998-8580

Liquid Crystal Applications990 Raritan RoadClark, New Jersey 07066201-574-1496

Pressure Chemical Co.3419Smallman StreetPittsburgh, Pennsylvania 15201412-682-5882

More InformationKodak publishes hundreds of books on photographic subjects. These are listed inthe Index to KODAK Information, KODAK Publication No. L-5. To obtain acomplimentary copy of the Index, write to Eastman Kodak Company,Department 412-L,Rochester, NY 14650.Ask for the Index by name and codenumber. You will find descriptions of several publications dealing with infraredphotography in the Index. These include Applied Infrared Photography, KODAKPublication No. M-28,and KODAK Infrared Films, KODAK Publication No. N-17.

Photographic Products Group

EASTMAN KODAK COMPANY. ROCHESTER, NEW YORK 14650Thermal Recording and Infrared

Photography of Hot ObjectsKODAK Publication No. P-570 Kodak and Wratten are trademarks.

M5K006 Minor Revision 12·85·CPrinted in the U.S.A.