de Glossar

Absorbance, spectral

A(l)

Previously referred to as "extinction El".

The absorbance is the logarithm to base ten of the reciprocal of the spectral internal transmittance  ti(l).

A(l) = lg (l/ti) = lg ( flin / flex )

The reading displayed by most commercially available photometers is the absorbance, because it is proportional to the concentration according to the Lambert-Beer Law. But what the photometer actually measures is the transmittance. As the Figure shows, the logarithmic conversion produces a variably high resolution of the absorbance for different transmittance values.

Absorbance and transmittance

The resolution is particularly good in the high transmittance range. For this reason, the range of 0..0.6 A (100..25% t) is used wherever possible. The lowest range evaluated by Sigrist photometers is 0..0.3 A.

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