Silver chloride, bromide, and iodide, known collectively as silver halides, are used in various proportions in photographic emulsions. In these, the silver halide is formed as a dispersion of microscopic crystals in gelatin. Strictly speaking, this is not an emulsion at all, but a solid suspension, though the term has now been accepted in photographic lore too long to be changed. When coated in liquid form onto a substrate of glass or lm and allowed to dry, it becomes a thin, tough, exible layer that is sensitive to light at wavelengths less than about 500 nm. By the addition of appropriate sensitizing dyes, this basic sensitivity can be extended to cover the green part of the spectrum (orthochromatic) or the whole visible spectrum (panchromatic), and even infrared. e addition of these dyes also increases the overall sensitivity of the silver halide crystals at least twofold.