Traditional%20Imaging

1
TraditionalImaging

• Imaging Science Fundamentals

2
(No Transcript)
3
(No Transcript)
4
Grain of Film and Paper

• Electron Photomicrographs of Emulsion Grains

5
What is Silver Halide?
Silver (Ag)
Halide group
6
Structure of a Typical BW Film

• Film base
• Plastic

• Antihalation backing
• Prevents light from reflecting back.

• Emulsion

Silver Halide Crystals
Suspended in gelatin, like fruits in Jell-O!
7
Exposed 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
After Exposure
8
Silver Halide Process Chain
Exposure
Processing
Develop
Stop
Fix
Visible (Stable) Image
Latent Image

• A latent image is formed after exposure
  (invisible to human eye).
• After processing, the latent image is turned into
  a visible, stable image.

9
Processing 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.

10
Silver Halide Grains
11
Light 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.

12
Point Source Casting Rays
Object

• 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.

13
Three 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.

14
Need 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.

15
Ideal 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).
16
Realistic Pinhole Camera
Since the pinhole cannot be infinitely small,
more than one ray actually gets through, blurring
the image formed at the image plane.
17
Capture

• The image can now be captured using a detection
  system, such as photographic film.
• Film must be processed to yield a permanent,
  visible image.

18
Limitations of Pinhole Camera

• Finite pinhole size and diffraction degrade final
  image.
• Light collection is poor - aperture must be small
  - so image acquisition is slow.

19
Image Formation

• For a camera to be efficient, the pinhole is
  replaced by a lens.
• The lens redirects light rays emanating from the
  object.

20
Why does processed film look 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.

21
What determines how dark film becomes?

• THE GRAINS!
• Size
• Shape
• Chemical composition
• Distribution

22
Grain of Film and Paper

• Electron Photomicrographs of Emulsion Grains
• (n.b. Measurement Bars indicate scale)

23
(No Transcript)
24
Photographic 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

25
Contact Printing
A CONTACT print The negative is in direct
contact with the photographic paper
--essentially creating a shadow-gram.
26
Enlarger / Reducer
Light
Optics are used to produce an image of the
negative on photographic paper.
27
(No Transcript)
28
(No Transcript)
29
Typical imaging chain for pinhole camera
Visible light source
Film (capture)
pinhole
processing
Object
Image
Dark box
30
Problems 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.

31
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).

32
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.

33
Contact Printing vs. Enlarger
Enlarger
Contact Printing

• Flexible image size.
• Some loss in resolution due to enlargement.
• Additional optics may degrade final image
  quality.
• Again, flexibility!

• No loss of signal by the optics.
• No reduction in resolution.
• Simpler system.
• Fixed image size.

34
What determines how dark film becomes?
Darker
Lighter
Less Exposure
More Exposure

• 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.

35
D-Log H Curve and Contrast
More contrast
Less contrast
Image
Film response