Glossary Terms
Isochromatic emulsion is a black and white photographic emulsion with color sensitivity range laying between orthochromatic and panchromatic emulsions.

Isochromatic emulsion is excessively sensitive to violet and blue, has generally correct sensitivity to yelow, slightly low sensitivity to green and registers wavelenghts up to 620-650 nm (orange/red), so it's not sensitive to deep red color.
Yellow and orange filters can be used when photographing with isochromatic films, mid yellow filter giving generally good reproduction of color brightness. Isochromatic films can be handled and processed under dark red light.