A densitometer is a piece of scientific equipment which is designed to measure optical density. Densitometers are used in a variety of scientific applications and also by photographers and printers who need to check on the quality of their work. Numerous companies manufacture densitometers, including specialty equipment which has been customized for particular applications.
Some densitometers use a transmission technique, in which light is transmitted through the substance being measured, and a photoelectric cell reads the light as it passes through. The optical density is determined by seeing how much light made it through the object being read. This type of densitometer is often used to measure optical density in film, with the data being used in a variety of ways during the film developing process.
A reflection process can be used with solid objects which cannot be used with a transmission densitometer. In this case, the amount of light reflected by the object is measured. For example, a photographer could use the transmission method to read film, and the reflection method to read a developed photograph. Some densitometers are capable of both functions, allowing people to adjust the setting for various uses.
Printers can use densitometers to check color saturation and to confirm that a print run is consistent. The densitometer is also used during press checks to confirm that material is printing properly, and to compare the appearance of the films used to prepare print plates with the resulting printed product.
Densitometry is used in a wide variety of ways. In a specialty type of densitometry used to measure bone density, density is measured by passing x-rays through the body. In this case, bone density can be determined by seeing how many x-rays pass through. Extreme variations in bone density can often be identified just by looking at x-ray film, as more porous bone will show up darker on the x-ray, while denser bone will appear whiter, indicating that those areas of the film were not as heavily exposed to x-rays.
Densitometers usually need to be allowed to warm up before they can be used. It is also important to calibrate them to ensure that the readings are accurate and consistent. Calibration can be accomplished with calibration cards provided by the manufacturer; each card has a number of sample areas of density which can be run through the densitometer, with the device readings being compared against the printed level of density on the card.