densitometer

A densitometer is an instrument that quantifies the optical density of a material by measuring transmitted or reflected light, providing a numerical value that correlates with the degree of darkness or concentration of a substance. It is used to assess the density of photographic negatives, prints, color inks, and stained biological specimens.

There are several variants: transmission densitometers measure light transmitted through a sample; reflectance densitometers measure light reflected from a sample; microdensitometers operate on small fields in microscopy or autoradiography; spectrophotometric densitometers measure absorption at selected wavelengths and can be integrated with color management.

Most densitometers use a controlled light source, a detector, and calibration standards. They report units such as optical density (log10 of the inverse transmittance), or percent reflectance for reflectance densitometers. In color work, measurements are often reported for specific color channels (red, green, blue) and related ink densities.

Applications include quality control in printing and reprography (ink density, gray balance), film processing and archival preservation, gel and blot densitometry in biology, and clinical bone densitometry using X-ray techniques (DEXA) to assess mineral content. The term “densitometry” covers any technique quantifying density of light-absorbing material or tissue.

In practice, densitometers require careful calibration with reference standards to ensure repeatability, and results are used to guide process control, standardization, and diagnostic assessment.