Safelights

All film stocks are sensitive to light, this being fundamental to the photographic process. However, different stocks have different colour sensitivities depending on their purpose.

There are three broad sensitivities of Black and White film in use.

1. Blue Sensitive films are, as the name implies, only sensitive to blue light. This enables a yellow safelight to be used. Blue sensitive films are used to print from Black and White originals.
2. Orthochromatic films are sensitive to Blue and Green light. Some Sound Recording films are orthochromatic in order to give increased emulsion speed. These require a red safelight.
3. Panchromatic films are sensitive to Blue, Green and Red light, the full range of the visible spectrum. This is used for stocks used to duplicate from a colour original and colour stocks themselves. The most common safelight for these materials is dark green or brown [dark yellow] as the human eye is particularly sensitive to this central area of the visible spectrum. In general, there are no true safelights for panchromatic and colour films, as all light will fog the film given sufficient time.

As a rough "rule of thumb" if film is exposed to a safelight 2 m away for 5 minute and no additional density occurs over the base level the system is safe. Safelight filters fade in time or are burnt by heat and eventually allow through other wavelengths, so they have to be checked periodically.