Since density is usually measured by the decrease in the amount of light which shines through a transparent film, it is also called absorptiometry, the measure of light absorption through the medium. The corresponding measuring device is called a densitometer (absorptiometer). The logarithm of the reciprocal of the transmittance is called the absorbance or density.
DMax and DMin refer to the maximum and minimum density that can be recorded on the material. The difference between the two is the density range. This related to the exposure range (dynamic range), which is the range of light intensity that is represented by the recording. The dynamic range is rated in "stops", which is the binary logarithm of the terminal exposures.
Dual energy X-ray absorptiometry is used in medicine to evaluate calcium bone density, which is altered in several diseases such as osteopenia and osteoporosis. Special devices have been developed and are in current use for clinical diagnosis, called bone densitometers.
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