Monochrome films. Film emulsions are sensitized during manufacture to some or all the colors of the spectrum. This means that they respond to light of all colors as well as to shorter UV wavelengths. In fact, this response does not quite match the human eye’s concept of light and dark colors. Pan film ‘sees’ (reproduces on the print) violet, blue and orange red as somewhat lighter in tone and greens darker in tone than we would generally judge them to be. The difference is generally accepted and is of some value in allowing slow pan materials to be handled under very deep green darkroom safe lighting. For a more exact match you can shoot using a pale-yellow camera filter.
A few black and white films are made insensitive to the red end of the spectrum beyond about 590 nm and are known as orthochromatic (‘ortho’). These films reproduce red as black on the final print, and orange as very dark.
Ortho materials – mostly sheet film – are useful for copying black and white prints or drawings not involving colors. You can conveniently handle them in the darkroom under red safelight illumination. They are also used for some forms of medical, forensic, and scientific photography. Ortho film speed rating is lower when the subject is lit by tungsten light rather than daylight or flash, because the former contains a higher proportion of red wavelengths.
One or two films, very slow and intended for the printing darkroom rather than camera purposes, are made with blue (and UV) sensitivity only. These are used for making copy negatives, usually in large formats for special printing processes. You can also buy special films for camera use with special ‘black’ filters to become insensitive to almost the whole visual spectrum but respond to infrared and UV;
Color films, both negative and slide types, often have a stack of six or more emulsion layers. They use three different kinds of color sensitization. The top emulsion is sensitive to blue only; others respond to blue and green; and the remainder primarily to red.
A yellow filter may be incorporated below the blue-sensitive layer so that blue light cannot proceed farther into the film. Therefore, you effectively have a multi-layered emulsion responding to blue only, green only, and red only – the three thirds of the spectrum. (Later each of these three emulsion types will form its image in a different colored dye, to reproduce the final image in full color;