Photographic density is the common logarithm of the opacitance of a specific part of a negative or transparency; this is the reciprocal of its transmittance. There are several reasons for choosing this expression rather than transmittance itself. First, photographers tend to think of a negative in terms of darkness rather than lightness; secondly, our perceptual processes follow an approximately logarithmic response (the Weber-Fechner law); thirdly, densities add together in a simple way, whereas opacitances and transmittances have to be multiplied together. Thus two densities of 1.0 together give a density of 2.0, whereas the corresponding transmittances of 0.1 together give a transmittance of 0.01. A density of 0 represents total transparency. Density is measured using a device called a densitometer.
Reflection density is the logarithm of the reciprocal of the reflectance. A reflection density of 0 is standardized on the reflectance of a freshly prepared surface of magnesium carbonate.
In a colour photograph the densities are measured three times, through red, green, and blue filters, measuring respectively the cyan, magenta, and yellow layers.
— Graham Saxby
See also densitometry.
