# Optical density

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In spectroscopy, the absorbance A (also called optical density) is defined as:[2]

where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and I0 is the intensity of the light before it enters the sample or incident light intensity (or power). Absorbance measurements are often carried out in analytical chemistry, since the absorbance of a sample is proportional to the thickness of the sample and the concentration of the absorbing species in the sample, in contrast to the transmittance I / I0 of a sample, which varies logarithmically with thickness and concentration.

Absorptance[3] (not absorbance) is defined as: The ratio of the radiant flux absorbed by a body to that incident upon it. Also called [absorption] factor. Compare absorptivity. Total absorptance refers to absorptance measured over all wavelengths.Spectral absorptance refers to absorptance measured at a specified wavelength.

Absorptance is explained, as it relates to absorbance, on the Color and Vision Research Laboratories, Institute of Ophthalmology, UCL, in this way:

Absorbance spectra are typically used to define photopigment spectra because their shape, when normalized (i.e., plotted as a fraction of the maximum absorbance), is independent of pigment optical density (pigment concentration). In contrast, the absorptance spectra, like the spectral sensitivity of the human subject, broadens as the optical density increases.[4]

Outside the field of analytical chemistry, e.g. when used with the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique, the absorbance is often defined using the natural logarithm instead of the common logarithm, i.e. as

See the Beer-Lambert law for a more complete discussion.

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### Explanation

The term absorption refers to the physical process of absorbing light, while absorbance refers to the mathematical quantity. Also, absorbance does not always measure absorption: if a given sample is, for example, a dispersion, part of the incident light will in fact be scattered by the dispersed particles, and not really absorbed. However, in such cases, it is recommended that the term "attenuance" (formerly called "extinction") be used, which accounts for losses due to scattering and luminescence.[5]

Although absorbance does not have true units, it is quite often reported in "Absorbance Units" or AU (not to be confused with the Astronomical unit).