A safelight exists to let paper be handled while it cannot yet be exposed by the print itself. The compromise is real: no safelight is indefinitely safe. Kodak’s publication K-4, How Safe Is Your Safelight?, states the principle plainly, that most sensitised materials are affected if exposed to safelight illumination long enough. Choosing the right filter and confirming it works are two separate tasks, and the second is the one usually skipped.

Matching colour and brightness to the emulsion

A safelight filter should transmit only wavelengths to which the emulsion is largely insensitive. Conventional black and white enlarging papers are primarily blue sensitive, so their safelights are amber, orange or light brown. Ilford’s safelight datasheet recommends the orange SL1 and the light-brown 902 filter for its enlarging papers, including Multigrade RC and FB, Ilfospeed RC DeLuxe and Ilfobrom Galerie FB. The same sheet specifies a 15 W bulb in the SL1 lamp and a minimum working distance of 1.2 m (4 ft) between safelight and paper.

Variable-contrast papers carry green-sensitive emulsion as well, so a brighter or greener safelight erodes their highlight separation faster than it fogs a graded paper. Apparent colour alone is unreliable, since a tinted bulb may still emit wavelengths that fog the emulsion. Filters also fade with use and transmit more unwanted light over time, so under heavy use they are replaced periodically.

Running a fog test that reveals more than fog

The familiar test places an opaque object, such as a coin, on a sheet of paper, exposes it to the safelight, then develops it; any outline means the safelight is fogging. K-4 notes the limitation: this detects only outright fog, not the loss of contrast and density that appears earlier. A more revealing test first gives the paper a faint enlarger exposure to a light grey tone, density roughly 0.25 to 0.50, then progressively uncovers strips for cumulative safelight exposures of one to several minutes. Degradation shows in the pre-exposed grey long before a coin would leave a mark.

Defining a safe working time

K-4 defines safe time as no more than half the exposure needed to produce a detectable change. If a strip first shows degradation at six minutes, three minutes is the working limit. The test is run with a fresh bulb, after checking for enlarger and door light leaks, since fog blamed on the safelight often originates elsewhere.