The measured absorbance, A, can be used to determine the concentration of a coloured material in solution. Absorbance is a function of three factors:
     - the pathlength, b, of the light passing through the solution
     - the absorptivity, a, which is characteristic of the material
     - the concentration, C, of the material

The relationship of these three parameters is expressed in Beer's Law:
                              A = abC

If the concentration of the material measured is expressed in units of molarity, the absorptivity, a, is referred to as the molar absorptivity and given the symbol ?. In the older literature, ? is called the extinction coefficient. The units of the molar absorptivity are ?, with the pathlength in cm.

Absorbance, A, is the logarithm of the ratio of the radiant power of the incident light falling on the solution, Po, relative to the power of the light emerging from that solution P.
                              A = log (Po / P)

Transmittance, T, of a solution is defined as the fraction of the original light that passes through the sample:
                              T = P / Po