Orthochromatic describes a photographic emulsion that is sensitive to blue and green light but insensitive to red light. This selective sensitivity affects how colours are rendered in black-and-white photography, with reds appearing very dark and blues and greens appearing lighter.
Orthochromatic emulsions were widely used before the advent of panchromatic film and remain important for understanding historical photographic processes. They are useful for achieving specific tonal effects, reproducing halftone images, or working under red safelights in the darkroom.