Previously referred to as "extinction E
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%