Originally, all photographic papers and films were orthochromatic. That meant that they were partially blind to red light. Consequently, these materials could be used in a darkroom with dim red safelights provided that the exposure was limited in both time and intensity.

Later, films and papers were made sensitive to all, or nearly all, colors of light. These modern materials sensitive to all colors are called panchromatic. Panchromatic papers are somewhat insensitive to the amber-colored light passed by a Kodak OC safelight filter.

The Kodak OC filter is the correct filter to use with panchromatic black-and-white papers. Black-and-white panchromatic films are too sensitive to use under any safelight in most cases.

Just about all printing papers are now panchromatic and should be used under a relatively weak OC-filtered safelight. There are still a few orthochromatic papers from Europe, such as Foma that require a dim red safelight filter.

In general, panchromatic papers are not safe under red filters and simple red bulbs generally donít have the accurate filtering properties of a safelight and a Kodak OC filter.