According to Beer–Lambert law,

log ( I_o / I _t ) = A= ε c l

where I _o and I _t are the incident and transmitted intensities, A = absorbance and ε , a constant = absorptivity (formerly called the extinction coefficient ). If the concentration is measured in mol.L −1 , the absorptivity is called the molar absorptivity . where I _o and I _t are the incident and transmitted intensities, A = absorbance and ε, a constant = absorptivity (formerly called the extinction coefficient). If the concentration is measured in mol.L ⁻¹ , the absorptivity is called the molar absorptivity.

In the above equation, c can be determined if other quantities are known. Since this is a straight line equation with slope = (ε.l) and zero intercept, ε is determined from the slope of a linear plot of absorbance versus concentration, called a calibration curve or calibration plot. Calibration graph is constructed by plotting absorbance at a given wavelength versus concentration for a series of standard solutions whose concentrations are accurately known. Since the absorptivity is a function of wavelength, often its value at the highest absorbance, λ _max (at the peak position i.e., at the wavelength of maximum absorbance) is determined. One should note that the Beer–Lambert law is obeyed by many substances mainly at low to moderate concentrations; therefore, dilute concentrations of the absorbing species should be measured. In practice it is advisable to measure absorbances in the range 0.1< A <1.0. Care must be taken to avoid any kind of chemical associations/dissociations of the absorbing species. Once the calibration plot is obtained, the concentration of the unknown solution can be determined graphically or mathematically from the equation of the calibration curve if the absorbance of the unknown sample at the λ _max is measured. Therefore, to find an unknown concentration for a sample mathematically, subtract the intercept from the absorbance reading and divide the result by the slope for a sample holder of unit path length. Here is the equation:

Unknown Concentration = (Absorbance – Intercept)/Slope .