The Chemistry of Photography Peter Jantz Chemistry 496 February 27, 2004

Black and White Film/Developing Color Film/Developing Overview History Black and White Film/Developing Color Film/Developing

1824, Joseph-Nicephore Niepce First Permanent Image “Heliogravure” Asphalt on a Pewter Plate Exposure took 5 days

1839, Louis Daguerre “Daguerreotypes” Sensitized Silver-plated Copper Sheet Mercury-vapor developer

1841, William Henry Fox Talbot “Calotypes” Salt Solution with Silver nitrate Quicker Development/Lower Image Resolution

The Gelatine Emulsion 1881, George Eastman 1887, Hannibal Goodwin Gelatin Dispersion of Silver-Halide 1887, Hannibal Goodwin Plastic-backed “modern film”

The Chemistry of Black and White Film

AgI >> AgBr >> AgCl silver halide salts AgI >> AgBr >> AgCl Silver halides form imperfect crystals in the Dispersion A photon of light may hit the crystal, exciting the electron on the negatively charged halide The electron is transferred into the conduction band of the silver halide microcrystal at the sensitivity speck

Chemical engineer’s resource page www.cheresources.com/photochem Image Formation Chemical engineer’s resource page www.cheresources.com/photochem

The emulsion layer is ~12% silver halide Grain size affects contrast and light sensitivity 0.1 ~ 3 microns Anti-halation backing prevents reflections More photons = more silver

Developer: Amplifies the Image A reducing agent (Hydroquinone) Sodium hydroxide and sodium carbonate added as accelerators Potassium bromide added as restrainer Reaction must be timed

Stop Bath Developers work at high pH A weak acid (acetic acid) is used to halt the reaction

Fixer Sodium thiosulfate makes water soluble silver thiosulfate Removes undeveloped silver halide crystals Sodium thiosulfate makes water soluble silver thiosulfate Photo paper is rinsed and dried

1912, Rudolph Fischer's Dye Coupler Process Color Film 1912, Rudolph Fischer's Dye Coupler Process

Color Film Silver halides only sensitive up to 525 nm Coupling to dyes expands sensitivity range Lower resolution than Black and White film Latent images are black and white

Color Development paraphenylendiamine Silver halide crystals are reduced by developer Oxidized developer reacts with the dye couplers, activating the dye color Printing paper has dye-coupled layers

History of Silver halide Chemistry Summary History of Silver halide Chemistry B&W vs. Color Film

References [1] Ian R. Gould et al. 2000. Two-Electron Sensitization: A New Concept for Silver Halide Photography. J. Am. Chem. Soc., 122 (48), pp 11934 -11943. [2] Tao Deng et al. 2000. Fabrication of Metallic Microstructures Using Exposed, Developed Silver Halide-Based Photographic Film. Anal. Chem., 72 (4), pp 645 -651. [3] Current, Ira. Photographic Color Printing: Theory and Technique. Copyright 1987, Butterworth Publishers. [4] C. Swedlund. Photography. Copyright 1981. Holt, Rinehart, and Winston. [5] The Chemical Engineer’s Resource Page. http://www.cheresources.com/photochem.shtml retrieved 2-19-2004. [6] T. Naugler. University of Nova Scotia http://www.nsac.ns.ca/envsci/staff/jhoyle/students/tnaugler2/Index.html#bib retrieved 2-19-2004