Only a 21-step sensitometer can be used for mammographic QC.
Sensitometry is a system of processor monitoring that measures the relationship between the amount of light exposure a film receives from a sensitometer and the optical density that is recorded on the film.
This verification, called the Sensitometer Technique for the Evaluation of Processing or STEP test, is a comparison of the sensitometric values from an FDA control film processed in the facility's processor using the facility's normal processing cycle to values from the same type of film run in a processor known to be operating at manufacturer's specifications.
Developing a master sensitometer has proven to be more difficult than expected, but manufacturers have produced "reference sensitometers" that are matched within 1% of an ideal reading and within 2% of each other.
Equipment such as sensitometers and densitometers must have a tight range of repeatability and must provide information on long-term stability.
Medical physicists, government regulators and others who check the medical facility's QA/QC programs must use calibrated sensitometers that are recertified every 2 years by a licensed laboratory.