Optical density is a measure of the amount of light a sample absorbs. Not indicating the optical nature of their analysis, the instruments that measure this property are known as densitometers. Typical samples are printed graphics, and photographic and radiological films.

A densitometer contains a light source—usually of white light, but UV can also be used—to provide illumination and a photocell to detect the light’s intensity after it interacts with the sample. In transmission densitometry, the sample is between the light source and the detector, and the amount of light transmitted is measured. This is often used with partly transparent samples, such as film negatives. For opaque samples, such as printed matter, the light detector is arranged to measure the light reflected from the sample’s surface. Many commercial densitometers can be operated in both modes. The standard measurement of density is based on the logarithm of the ratio of transmitted to incident light. For example, if a sample transmits 10% of the light, it has a density of 1; if it transmits 1% of the light, it has a density of 2. Although density is dimensionless, it is commonly cited with the “unit” D. While many densitometers simply make measurements of the total white-light density, there are color densitometers (usually used in reflectance mode to analyze color printing) that provide data on color balance and accuracy.

Since densitometry provides a reliable and standardized method for checking the quality and accuracy of printing, the printing and graphics industry is one of the major sources of demand. A related application area is photography. Densitometers are used not only to check photographic prints but also to directly analyze film negatives. The resulting data about the darkroom and development process can be used for quality control. Another application is the examination of radiological films. The film’s optical density can be related to the physical density of tissues and bones in mammograms and x-rays. Optical densitometers are not to be confused with medical densitometers, which use x-rays to directly measure bone density. Optical densitometers can also be used in the life sciences as radiation detectors. For example, if radioactive nucleotides are present in an electrophoresis gel, the gel can be “imaged” with photographic film, and the resulting autoradiograph can be analyzed by a densitometer, which provides information about the concentration of the radioactive compounds at different parts of the gel.

The 2013 market demand for densitometry was roughly $44 million. There are few significant vendors, and all also offer other forms of color-measurement systems. X-Rite (Danaher) is the largest competitor, offering a suite of systems for various applications. Konica Minolta and Techkon both focus more on color densitometry for printing and graphics. Other competitors include Barbieri, Fluke Biomedical (Danaher), IBA Dosimetry, Optel and Tobias Associates.

Densitometry at a Glance:

Leading Suppliers

• X-Rite (Danaher)

• Konica Minolta

• Techkon

Largest Markets

• Printing

• Photography

• Medicine

Instrument Cost

• $1,000–$6,000