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 they are safe for the length of time that the photographic material will be under the safelight. Use the safelight tests described in
Excessive exposure to safelight illumination may show up only in the image area of your print, because that area receives additional exposure from the 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 and enlarger exposures.
Always keep safelight exposure to a minimum. Store paper in lighttight 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.
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