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.
An alternative term, which however is ambiguous, is optical density.
Absorbance values often depend on the optical wavelength.
Relation to the Absorption Coefficient
The absorption per unit length is often quantified with an absorption coefficient <$\alpha$>. The power transmission factor (transmittance) for a propagation length <$z$> is then <$\exp(-\alpha z)$>. Therefore, the absorbance can be calculated as$$A = \lg \left( \exp (\alpha \;z) \right) = \alpha \;z/\ln 10 \approx \alpha \;z/2.303$$
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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