[blt]

The magic word here is "orthochromatic". Orthochromatic photo emulsions (the light-sensitive part of film or photo paper) are only sensitive to short wavelengths of light. The first photo emulsions were all orthochromatic, which makes skin look weird. Later we developed Panchromatic film which is equally sensitive to all colors. It replaced ortho in the camera, but ortho continued to be very useful in the darkroom and in compositing because it allows the red safelight and tricks like non-photo blue.

[stan_rogers]

Not necessarily. Orthochromatic ("correct colour"), or ortho, materials were actually improved-spectrum materials that were sensitive well into the yellow-green. Prior to that, film and paper were really only significantly sensitive to blue/ultraviolet or "actinic" light. Getting to panchromatic ("all colours") was indeed significant, but ortho was advanced technology at the time. (And yes, being able to see what you were doing in the process room was a Good Thing™. Also, rubylith for masking.)

[blt]

Whoa, didn't know that. Very interesting. Thanks!