Densitometry

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Principle of spot light densitometry

Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as 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 nitrate grains in developed films) in a given area, but usually it is a relative value, expressed in a scale. Density is also the logarithm of the inverse of transparency.

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 or an absorptiometer.

According to the principle of operation of the densitometer, one can have:

spot densitometry: the value of light absorption is measured at a single spot
line densitometry: the values of successive spots along a dimension are expressed as a graph
bidimensional densitometry: the values of light absorption are expressed as a 2D synthetic image, usually using false-color shading

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.