Not to be confused with hydrometer which measures mass density of liquids.
Heiland Densitometer TRDZ 1
A densitometer is a device that measures the degree of darkness (the optical density) of a photographic or semitransparent material or of a reflecting surface. The densitometer is basically a light source aimed at a photoelectric cell. It determines the density of a sample placed between the light source and the photoeectric cell from differences in the readings. Modern densitometers have the same components, but also have electronic integrated circuitry for better reading.
Transmission densitometers that measure transparent materials
Reflection densitometers that measure light reflected from a surface.
Applications in Photography
Some modern types of German manufacture are capable of both types of measurements selectable by a switch. They are used in film photography to measure densities of negatives with the switch in the "T" (Transmission) position and the saturation of a resulting print in the "R" position. Such measurements enable the photographer to choose the right photo paper and the correct exposure, obviating experiments with test strips. Once the papers and darkroom have been calibrated, the first print from a previously measured negative is a success at once.
Densitometers are used for measuring color saturation by print professionals