This is the measurement of density for the purposes of establishing the characteristics of a film or print material. Density is the logarithm of the reciprocal of the transmittance (for a print, reflectance). Transmittance is the measured transmitted light intensity divided by the incident light intensity. Its numerical value lies between 0 (totally opaque) and 1 (totally transparent). Reflectance is defined similarly, for reflected light. Both are often quoted as percentage values. A densitometer can be a simple gadget which gives a direct reading of density from a negative on a light table, or as complicated as the devices used in large processing houses, which measure and automatically analyse colour negative or transparency tests, and suggest modifications to the routines or the processing solutions. Tests for emulsion characteristics are carried out using a neutral grey step tablet, which is a strip of film having a series of graded density steps, usually 0-3 in steps of 0.15. The reflection density of a print uses a densitometer head that has a built-in light source (oblique, to avoid specular reflections). Where accurate colour and/or tonal reproduction is essential in a final print, a calibrated grey scale is included in the original photograph.
— Graham Saxby
See also sensitometry.
