A densitometer is used to accurately measure the photographic (optical) density at any spot on a radiographic film. For most types of densitometers the size of the measured area is approx. 1 mm2 . The measuring range runs from density 0 to 4.
Since it is a logarithmic scale, this equals a factor 10,000 (104 ) in density. It is very important to regularly recalibrate these instruments, particularly around values 2 and 2.3, since those are the minimum densities (depending on class: A or B) which a film must have in accordance with standard EN-444, to allow it to be interpreted.
Densitometers are supplied with reference material (density strips) to re-calibrate them. Regular recalibration, at least once a year according to code, is essential. The most commonly used density strips deteriorate quickly as a result of scratching and disintegration of the sealed transparent wrapping in which they are usually kept. Their service-life, depending on use, is usually not much longer than six months. Agfa has developed the “Denstep” density step wedge film and has succeeded in considerably extending the service-life of these strips by supplying them in special wear proof wrapping. These density strips are certified and have a guaranteed minimum service life of four years. The 15 steps of the “Denstep” comprise a density range from 0.3 to 4.