No types of filters are used almost exclusively in the photographic darkroom. They are safelight filters and printing filters. The printing filters include variable contrast filters for printing black-and-white materials and color printing filters for printing color materials.

SAFELIGHT FILTERS

The word safelight in photography is used to describe filtered tungsten illumination or direct illumination from a sodium-vapor lamp. The color of a sodium-vapor lamp does not affect (expose) light-sensitive materials under prescribed darkroom conditions. The word safe is misleading since light-sensitive materials are never completely safe from safelight illumination. The use of a safelight with some types of light- sensitive materials is not recommended Compatible safelight filters for use with certain light-sensitive materials should be selected on the basis of color sensitivity and emulsion speed of the material. The best method of selecting a darkroom safelight filter is to use the filter recommended by the manufacturer of the light-sensitive material. Safelight filters absorb that portion of the visible spectrum produced by a tungsten lamp that would affect the light-sensitive material being handled.

Sodium-vapor lamp safelights use sodium gas to provide safelight illumination. Incandescent sodium gas produces a very narrow band of visible light in the yellow-orange portion of the spectrum. Colorblind printing papers are not sensitive to this monochromatic (one color) band of light, whereas the human eye is very sensitive to it. Therefore, a brighter print room is possible without the light affecting the printing paper. By using specially designed filters that further reduce the narrow band of sodium-vapor light, black-and-white materials sensitive to green and red light can be handled under this illumination. Table 3-7 provides some examples for the application of safelight filters. Always consult the Photo-Lab-Index to determine the best safelight for use with various light-sensitive materials.

VARIABLE CONTRAST PAPER PRINTING FILTERS

To obtain various degrees of contrast using variable contrast printing papers, use a series of magenta and yellow filters. The magenta filters are used to print black-and-white negatives that are low in contrast.

Table 3-7. Safelight Filters

Yellow filters are used to print black-and-white negatives that are high in contrast. Variable contrast printing filters are discussed in chapter 10 of this TRAMAN.