Optical density is a result of the darkness of a developed picture and can be expressed absolutely as the number of dark spots (i.e., silver grains in developed films) in a given area, but usually it is a relative value, expressed in a scale.
[1] DMax and DMin refer to the maximum and minimum density that can be produced by the material.
In the context of photography, the dynamic range is often measured in "stops", which is the binary logarithm of the ratio of highest and lowest distinguishable exposures; in an engineering context, the dynamic range is usually given by its decadic logarithm expressed in decibels.
[citation needed] According to the principle of operation of the densitometer, one can have: 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.