Densitometry is the quantitative measurement of optical density in light-sensitive materials, such as photographic paper or photographic film, due to exposure to light.

Overview

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 Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

. 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 decadic (base-10)

logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...

of the reciprocal of the

transmittance Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is th ...

is called the absorbance or ''density''. ''DMax'' and ''DMin'' refer to the maximum and minimum density that can be produced by the material. The difference between the two is the ''density range''. 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 the Hurter–Driffield curve. In the context of photography, the dynamic range is often measured in "stops", which is the

binary logarithm In mathematics, the binary logarithm () is the power to which the number must be raised to obtain the value . That is, for any real number , :x=\log_2 n \quad\Longleftrightarrow\quad 2^x=n. For example, the binary logarithm of is , the ...

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.

Uses

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 Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

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 Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engin ...

, called ''bone densitometers''.

References

{{reflist

External links

''Fundamentals of Densitometry''
by Mark Vivino. Optical metrology

Overview

Uses

See also

References

External links