Silver Halide (AgX) Imaging

1. Silver Halide (AgX)Imaging Imaging Science Fundamentals

2. What is “Silver Halide”? • Silver Halide is a compound made of silver (Ag) atoms and atoms from the halide group (Cl,Br,I) of the periodic table. • The material property of a silver halide crystal changes after absorbing photons.

3. Silver and halide atoms within the periodic table of the elements Silver (Ag) Halide group

4. Origins of Modern Photography • Silver halide was first used as an imaging material in the 1830’s by W. H. Fox Talbot, J. N. Niepce, and J. M. Daguerre. • Today’s black and white film derives from Talbot’s negative-positive process.

5. 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™!

6. 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). hn Exposure After Exposure

7. 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).

8. 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.

9. Processing Photographic Film • Turns latent image into visible image. • Basic steps: • Developer • Stop bath • Fix bath • All chemicals used in the process are in liquid form.

10. 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.

11. 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.

12. “Grain” of Film and Paper • Electron Photomicrographs of Emulsion Grains • (n.b. Measurement Bars indicate scale)

13. 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.

14. 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

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

16. 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

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

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

19. 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