In photography, the term “safelight” describes darkroom illumination that does not fog or otherwise cause a visible change to light-sensitive material when it is correctly handled and processed. The word “safe” is relative. Most sensitized materials will become fogged if you expose them to safelight illumination for an extended period of time. Because photographic materials vary in speed and sensitivity to different colors of light, the recommended bulb wattage and colors of safelight filters also vary.
Ideally, safelight filters should transmit only what is outside the color-sensitivity (wavelength) range of the photographic materials for which they are recommended. The safelight filters recommended by Kodak provide maximum transmission of colors to which the film or paper emulsion is relatively insensitive. However, the color sensitivity of most emulsions does not end abruptly at a particular wavelength in the spectrum. Most emulsions are somewhat sensitive to colors outside their intended range. This means that most films and papers have some sensitivity even to the colors of light transmitted by the recommended safelight filters. Therefore, always minimize the exposure of photographic materials to safelight illuminations.The apparent color of a safelight filter is only a partial indication of its transmission characteristics. Colored bulbs or other improvised safelights may appear to be the right color, but they may actually emit light (or other forms of radiant energy) that will fog a photographic emulsion. Kodak Safelight Filters are made to precise light transmission and absorption standards that relate to the spectral sensitivities of photographic materials.Safelight filters gradually fade with use. This means that they transmit more and more light of the colors that they absorb when they are new. You should plan to change safelight filters periodically. For example, if you use safelight lamps for 8 to 12 hours a day, you may need to change the filters every three months.Bulbs eventually blacken and produce less light. We recommend that you change bulbs before running safelight tests. To keep the illumination level consistent, change bulbs periodically too. Noting the replacement dates on a sticker on the safelight housing will help you keep track of bulb and filter changes.
Many factors can cause unsafe illumination: an incorrect safelight filter, a faded or cracked filter, incorrect bulb wattage (too high), safelight location, or too many safelights. You can also experience light fogging from other sources, such as light escaping from an enlarger head, lighted dials on equipment controls, or non-opaque darkroom construction materials. For example, pinholes between the darkroom space and lighted areas can admit visible light, or plywood that appears opaque may admit infrared illumination. Even when you use the correct safelight filter and bulb, and observe the recommended safelight distance for the product, you should still test your darkroom conditions to be certain that they are safe for the length of time that the photographic material will be under the safelight. Using an exposure test series that is described in Kodak Publication K-4, How Safe is Your Safelight?, you can determine a safe time for your darkroom conditions, and limit your safelight exposure to that time. This publication is available by writing to Eastman Kodak Company, Department 412L, Rochester, NY 14650- 0532. Excessive exposure to safelight illumination may show up only in the image area of your negative or print, because that area receives additional exposure from TEM or enlarger. This means that you may not recognize a change in image quality caused by excessive safelight exposure unless you perform a safelight test.
Note: Safelight tests that involve partially covering a piece of photographic material with an opaque object (such as a coin) and then exposing the material to safelight illumination can be misleading. They test only for fog - not for the added effects of safelight, TEM and enlarger exposures.Always keep safelight exposure to a minimum. Store film and paper in light proof containers, and make a habit of handling paper with the emulsion side down (away from the safelights). Place your enlarger so that the easel area receives very little safelight illumination. When you develop prints, insert the paper into the developer with the emulsion side down; turn the paper emulsion side up when experience tells you that the image has become visible. Let the results of your safelight test and your own practical experience guide you in determining how much time you have in handling photographic materials.
Courtesy of Eastman Kodak