Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
TraditionalTraditionalImagingImaging
Imaging Science Fundamentals
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
““Grain” of Film and PaperGrain” of Film and Paper
Electron Photomicrographs of Emulsion Grains
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
What is Silver Halide?What is Silver Halide?
Silver (Ag)
Halide group
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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™!
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Silver Halide GrainsSilver Halide Grains
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
*
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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).
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
What determines how dark film What determines how dark film becomes?becomes?
THE GRAINS! Size Shape Chemical composition Distribution
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
““Grain” of Film and PaperGrain” of Film and Paper
Electron Photomicrographs of Emulsion Grains (n.b. Measurement Bars
indicate scale)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Contact PrintingContact Printing
Light
Negative
PhotographicPaper
A CONTACT print:The negative is in direct contact with the photographic paper --essentially creating a shadow-gram.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Enlarger / ReducerEnlarger / Reducer
LightNegative
PhotographicPaper
Optics are used to produce an image of the negative on photographic paper.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Typical imaging chain for pinhole cameraTypical imaging chain for pinhole camera
Visible light source
Object
Dark box
pinhole
Film (capture)
processing
Image
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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).
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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