An instrument for measuring the opacity of translucent materials such as photographic negatives, optical philtres and liquids. It passes a beam of light through the material and measures the ratio of the intensity of the light emerging from the material to the intensity of the light entering the material. This ratio is the transmissivity; the (optical) density is the logarithm of the reciprocal of the transmissivity. Density and transmissivity depend not only on the material but also on the spectral constitution of the light, the diameter and direction of the beam, and other factors. If the beam is of very small diameter at its place of entry into the material, the instrument is called a micro-densitometer.