Colour sensitising: orthochromatic and panchromatic emulsions

How it began

Silver halides respond mostly to blue and ultraviolet light, which Scheele had seen. Skies came out white and red lips black. In 1873 Hermann Wilhelm Vogel in Berlin noticed that some plates, stained with dye to stop halation, had become sensitive to green.

How it works

A dye that absorbs, say, green light sits on the surface of the silver halide crystal and passes the absorbed energy to it. Adding dyes for green made orthochromatic emulsions; adding dyes for red made panchromatic ones, sensitive to the whole visible spectrum.

Why it mattered

Black-and-white pictures could show tones as the eye sees them, with filters to adjust them. Colour photography needs separate records of red, green and blue, so panchromatic emulsions made the autochrome and every later colour film possible.

Limits and what replaced it

Panchromatic material must be handled in total darkness instead of under a red safelight. Ortho film stayed in use for graphic arts, where that convenience mattered more.

Today

Every film, black and white or colour, uses sensitising dyes to respond across the spectrum.

Sources

• Wikipedia: Orthochromasia ↗
• Eder, ch. LXIV (p. 457)