Traditional Imaging

1. TraditionalImaging Imaging Science Fundamentals

4. “Grain” of Film and Paper • Electron Photomicrographs of Emulsion Grains

5. What is Silver Halide? Silver (Ag) Halide group

6. Film base Plastic Structure of a Typical B&W Film • 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. Developer “amplifies” the atomic silver to visible silver strands. Processing Photographic Film • 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)

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. Light Negative Photographic Paper Contact Printing A CONTACT print: The negative is in direct contact with the photographic paper --essentially creating a shadow-gram.

26. Negative Photographic Paper Enlarger / Reducer Light Optics are used to produce an image of the negative on photographic paper.

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. 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 vs. Enlarger Enlarger Contact Printing

34. D Log H What determines how dark film becomes? Darker Lighter • 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. Less Exposure More Exposure

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