Absorbance is defined as , with as absorbance ( Beer-Lambert law ) . It is a measure of a given substance 's transparency ( , the ratio of light intensity passing through sample to that of the incident light ) to a given wavelength of light . Absorbance is defined as , with as absorbance ( Beer-Lambert law ) and indicates degree of transparency ( , the ratio of light intensity exiting the sample to that of the incident light ) to a given wavelength of light .