The term "absorbance" with the symbol A is a measure of the ability of a substance or a solution to absorb electromagnetic radiation or light at a specific wavelength.
It is equal to the logarithm of the recioprocal of the transmittance.
It is equal to the logarithm of the ratio of the radiant power of the incident radiation, P0, to the radiant power of the transmitted radiation, 0.
For a solution, absorbance is expressed as the logarithm of the ratio of the radiant power of light transmitted through the reference sample to that of the light transmitted through the solution.
Usually the observations is made using identical cells or quartz cuvettes.