Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Traditional Imaging...

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

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Imaging Science Fundamentals


““Grain” of Film and PaperGrain” of Film and Paper

Electron Photomicrographs of Emulsion Grains


What is Silver Halide?What is Silver Halide?

Silver (Ag)

Halide group


Structure of a Typical B&W FilmStructure of a Typical B&W Film

Film base Plastic

Antihalation backing Prevents light from reflecting back.

EmulsionSilver Halide Crystals

Suspended in gelatin, likefruits in Jell-O™!


Exposed AgX CrystalsExposed AgX Crystals

When a silver halide crystal is exposed to light, some of the AgX molecules break up into their constituents, one of which is metallic silver (“pure” Ag).

Exposure

AfterExposure


Silver Halide Process ChainSilver Halide Process Chain

A latent image is formed after exposure (invisible to human eye).

After processing, the latent image is turned into a visible, stable image.

Exposure Processing

Develop Stop FixLatentImage

Visible(Stable)Image


Processing Photographic FilmProcessing Photographic Film

Developer “amplifies” the atomic silver to visible silver strands.

Stop Bath stops the development process.

Fix dissolves the unexposed AgX crystals, making the film safe to expose to light.

Wash with water to rinse fix chemicals away.


Silver Halide GrainsSilver Halide Grains


Light from a Point SourceLight from a Point Source

A point source is a source in which the light appears to be emanating from one point in space.

A point source casts energy in all directions, shown above as rays.

*


Point Source Casting RaysPoint Source Casting Rays

Take a look at 7 of the rays cast from one position (the tip) of the object.

With no collection element, all of these rays fall on the “image plane.” There is no way to determine the source of the rays.

Object


Three Points on the ObjectThree Points on the Object

This becomes a problem when three points on the object are considered.

The energy can be collected at the image plane, but all information about the object is lost.


Need for Collection ElementNeed for Collection Element

A crucial element in most imaging systems is the component responsible for collecting the energy emerging from a given point on the object, such that it is brought to a single point in the image.


Ideal Pinhole CameraIdeal Pinhole Camera

An ideal pinhole allows only a single ray from each point on the object to pass through. This preserves the spatial distribution of energy, creating a sharp image.

An ideal pinhole camera has an infinitely small aperture (opening).


Realistic Pinhole CameraRealistic Pinhole Camera

Since the pinhole cannot be infinitely small, more than one ray actually gets through, blurring the image formed at the image plane.


CaptureCapture

The image can now be captured using a detection system, such as photographic film.

Film must be processed to yield a permanent, visible image.


Limitations of Pinhole CameraLimitations of Pinhole Camera

Finite pinhole size and diffraction degrade final image.

Light collection is poor - aperture must be small - so image acquisition is slow.


Image FormationImage Formation

For a camera to be efficient, the pinhole is replaced by a lens.

The lens redirects light rays emanating from the object.


Why does processed film look Why does processed film look “negative”?“negative”?

Silver strands formed by exposure of photographic film to light actually appear dark (they are NOT shiny).

So, where light hits the film during exposure, it turns darker.


What determines how dark film What determines how dark film becomes?becomes?

THE GRAINS! Size Shape Chemical composition Distribution


““Grain” of Film and PaperGrain” of Film and Paper

Electron Photomicrographs of Emulsion Grains (n.b. Measurement Bars

indicate scale)


Photographic FinishingPhotographic Finishing

In order to get a “positive” final print, the negative must be projected onto photographic paper.

Negative * negative = positive! 2 basic “finishing” methods: CONTACT and

ENLARGEMENT/REDUCTION


Contact PrintingContact Printing

Light

Negative

PhotographicPaper

A CONTACT print:The negative is in direct contact with the photographic paper --essentially creating a shadow-gram.


Enlarger / ReducerEnlarger / Reducer

LightNegative

PhotographicPaper

Optics are used to produce an image of the negative on photographic paper.


Typical imaging chain for pinhole cameraTypical imaging chain for pinhole camera

Visible light source

Object

Dark box

pinhole

Film (capture)

processing

Image


Problems with Ideal Pinhole CameraProblems with Ideal Pinhole Camera

An infinitely small aperture allows an infinitely small amount of light (zero) to pass through.

Diffraction (which is ignored in geometric optics) blurs the image when the pinhole is very small.


Latent Image Formation (Ex.: shadowgram)Latent Image Formation (Ex.: shadowgram)

• Group of AgX Crystals

• Mask (object) prevents AgX crystals underneath to be exposed.

• “Unmasked” AgX is exposed to photons.

• Exposed crystals have different material property (I.e. some AgX bonds have been broken).


Latent Image Formation (with optics)Latent Image Formation (with optics)

• Group of AgX Crystals

• Optics used to image object onto the film.

•Illuminated AgX is exposed to photons; but not all AgX is illuminated.

• Exposed crystals have a different material property.


Contact Printing vs. EnlargerContact Printing vs. Enlarger

No loss of signal by the optics.

No reduction in resolution.

Simpler system. Fixed image size.

Flexible image size. Some loss in

resolution due to enlargement.

Additional optics may degrade final image quality.

Again, flexibility!

Contact Printing Enlarger


What determines how dark film What determines how dark film becomes?becomes?

Consider the so-called “D-Log H” curve. Describes how film responds to light: Density (D) is how dark the film is. Log H is the exposure (H) in logarithmic scale.

Log H

D

More ExposureLess Exposure

Lighter

Darker


D-Log H Curve and ContrastD-Log H Curve and Contrast

Log H

D

Log H

D

More contrast Less contrast

Film response

Image

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Traditional Imaging...

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