The densitometer is an instrument t

The densitometer is an instrument that measures film density (Figs. 7 and 8). Before the invention of portable densitometers, densities were estimated by comparing the radiographic density to a comparator strip. The strip contained a series of densities established by cumbersome and unwieldy early densitometers. Many of these early radiographic density determinations were simple, visual estimates. The operation of modern densitometers is quite simple. After calibration, using a density strip with known values for a number of different densities, the radiograph is positioned between the light source, usually located at the base of the densitometer, and the head, which contains a photomultiplier. Because the transmitted light intensity decreases as radiographic film density increases, less light reaches the photosensitive surface in the head and the voltage output from the photomultiplier (to the meter or digital display) will indicate a higher density reading. Conversely, as more light passes through a lower density region of the radiographic film and interacts with the photosensitive surface in the head, a lower density is indicated on the meter or digital display. An aperture is installed near the light source to establish the precise region of the film that is being measured. Changing apertures requires recalibration.