optical densitometer

optical densitometer, device that measures the quantity of light that passes through a material or is reflected from a surface. Two types of densitometers, known as transmission densitometers and reflection densitometers, respectively, provide numeric values representing the optical density of the material or surface under investigation.

Optical densitometers are equipped with a light source, which can be aimed at a surface or material, and a photoelectric cell; the material to be tested is placed between these two components. In visual methods, two light beams of equal intensity are used, one of which is incident on the material, and the other is used as the reference. The intensity of the reference beam is adjusted until the two beams have equal intensity, judged by the eye or by the photoelectric cell. Depending on the device, reference beam intensity can be manipulated by an optical wedge, which slightly alters the direction of the light beam; by an iris diaphragm, which employs a circular aperture to control the amount of light that passes through; or by moving the source. The photoelectric cell records the intensity of the light that is transmitted through or reflected from the material and converts the light energy to an electric signal, which determines the optical density. With proper calibration, the density can be read directly using a digital display. Other densitometer methods employ photoelectric cells to measure the intensity of the same beam with and without the material inserted in the path, the difference in intensity being a measure of optical density.

Optical densitometers have various applications. For example, in the printing industry, they are used to determine the optical density of printing inks or films and to calibrate printing equipment. In the manufacturing and healthcare industries, densitometers serve a key function in quality control—for example, inspecting paper and film in manufacturing and analyzing medical images in healthcare. Optical densitometers are also used in photography to measure the optical densities of negatives.