What the Exposure Actually Does
A black-and-white emulsion is a layer of gelatin holding crystals of silver halide. Each crystal carries sensitivity specks of silver sulfide on its surface.
The Gurney-Mott mechanism
Published by Gurney and Mott in 1938, the mechanism runs in alternating stages:
- An absorbed photon frees an electron.
- The electron is trapped at a sensitivity speck.
- An interstitial silver ion moves to the speck and is neutralised into a silver atom.
- Repeating this builds the speck.
A speck becomes developable once it holds roughly three or four silver atoms. The developer reduces the entire crystal only if it carries such a speck. This yields an amplification of roughly ten to the eighth or ninth power.
Colour Sensitivity
Plain silver halide absorbs only blue and ultraviolet light. In 1873, Hermann Vogel found that adding dyes extends sensitivity into green light, producing orthochromatic emulsions. By 1906, Wratten and Wainwright sold the first panchromatic plates, sensitive across the entire visible spectrum.
| Emulsion | Sensitive to | Darkroom handling |
|---|---|---|
| Blue-sensitive | up to ~500 nm | Bright yellow or red safelight |
| Orthochromatic | up to ~590 nm | Deep red safelight |
| Panchromatic | up to ~690 nm | Total darkness |
Common pitfall: thinking of the latent image as a faint version of the picture. It is merely a pattern of developable specks acting as binary switches.