||During the development of photographic papers in a darkroom, certain wavelenghts to which the paper is not sensitive can be used for general illumination. Ordinary incandescent lamps equipped with narrow-band filter coatings are traditionally employed, and are available in a variety of colours to suit various types of photographic paper. Owing to the low luminous efficacy and the fact that only a few narrow wavelength ranges can be transmitted, the total luminous flux emitted can be extremely low, in many cases less than one lumen. While such lamps may be adequate for the amateur photographer, a greater quantity of light is required in larger commercial darkrooms.
In such situations an ideal source of light is the low pressure sodium lamp, as many types of paper are insensitive to its deep yellow monochromatic radiation. However the output of these lamps is not absolutely monochromatic - there are some very weak lines radiated in the blue and green. While their presence is completely insignificant in general lighting applications, they can cause fogging of photographic papers at high light levels. For this reason, darkroom luminaires containing sodium lamps are equipped with additional filters.
Before such specialised luminaires became popular, Philips marketed for a brief period this highly unusual sodium lamp whose outer bulb is painted with an orange filter. The integral filter cuts out almost all wavelenghts other than the sodium resonance lines. The rest of the construction is identical to a standard 45W SOI/H type lamp. No reference to this lamp has been found - perhaps it was only marketed for a brief period of time until luminaires with integral filters were developed around standard sodium lamps.