Densitometry is the quantitativemeasurement ofoptical density in light-sensitive materials, such asphotographic paper or photographic film, due to exposure to light.
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 ascale.[citation needed]
Since density is usually measured by the decrease in the amount of light which shines through a transparent film, it is also calledabsorptiometry, the measure of lightabsorption through the medium. The corresponding measuring device is called adensitometer (absorptiometer). The decadic (base-10)logarithm of the reciprocal of thetransmittance is called theabsorbance ordensity.[1]
DMax andDMin refer to the maximum and minimum density that can be produced by the material. The difference between the two is thedensity range.[1] The density range is related to the exposure range (dynamic range), which is the range of light intensity that is represented by the recording, via theHurter–Driffield curve. In the context of photography, the dynamic range is often measured in "stops", which is thebinary logarithm of the ratio of highest and lowest distinguishable exposures; in an engineering context, the dynamic range is usually given by itsdecadic logarithm expressed indecibels.[citation needed]
According to the principle of operation of the densitometer, one can have:
Dual-energy X-ray absorptiometry is used in medicine to evaluatecalciumbone density, which is altered in several diseases such asosteopenia andosteoporosis. Special devices have been developed and are in current use for clinicaldiagnosis, calledbone densitometers.
