Densitometry is the
quantitative measurement of optic density in light-sensitive materials, such as
photographic film, due to exposure to light. Optic 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.
Principle of spot light densitometry
source:Wikipedia