Contrast viewing filters,

Balancing lighting by eye is a matter of experience. Decisions can be made easier by using contrast viewing filters.

These are designed to limit the eye, with its much wider range of apparent densities, to resemble the range of different types of film. Use contrast viewers to judge the relative densities of highlights and shadows.

There are viewers for black and white films, as well as different density viewers for colour films. A darker viewer is used for slower films, where you would tend to use brighter lighting.

A faster film, which can be used in darker environments, would require a lighter viewer.
The Tiffen #1 viewer is for black and white imaging. Viewer #2 is for film speeds up to 100; #3 is for faster films. They can also be used for video, with #3 being better suited to lower light levels. The green #4 viewer is for process photography. The blue #5 is for blue screen work, as well as for adjusting colour video monitors.