For example, an absorbance of 3 means that the optical power is attenuated by the factor 103 = 1000.
That would correspond to an attenuation by 30 decibels and a transmittance of 10−3.
It is usually assume that any optical power losses are caused by absorption and not e.g. by scattering.
Otherwise, should use the term attenuance.
Absorbance should not be confused with absorptance, which is a dimensionless quantity.
If several absorbing devices are used in series, their absorbance values can simply be added.
The absorbance of a homogeneously doped laser crystal, for example, is proportional to its length and the doping concentration.
The absorption per unit length is often quantified with an absorption coefficientα.
The power transmission factor (transmittance) for a propagation length z is then exp(−αz).
Therefore, the absorbance can be calculated as
In some cases, one uses a decadic absorption coefficient, which is smaller by the factor ln 10, so that the absorbance is simply that coefficient times the optical path length.
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