In photography, irradiation refers to the scattering of light rays as they pass through the photographic emulsion. This effect is caused by the physical structure of the emulsion and the distribution of silver halide grains, which can influence how light exposes the film or paper.
Understanding irradiation is important for photographers and darkroom technicians, as it affects image sharpness, contrast, and tonal rendering. By accounting for irradiation, photographers can better predict exposure results and achieve more precise control over the final quality of black-and-white or colour images.