Beer-Lambert Law is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:

A = a() * b * c

where A is the measured absorbance, a() is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration. When working in concentration units of molarity, the Beer-Lambert law is written as:

A = * b * c

Where is the wavelength-dependent molar absorptivity coefficient with units of M^-1 cm^-1. Data are frequently reported in percent transmission (I/I₀ * 100) or in absorbannce [A = log (I/I₀)].

Experimental measurements are usually made in terms of transmittance (T), which is defined as:

T = I / I_o

where I is the light intensity after it passes through the sample and I_o is the initial light intensity. The relation between A and T is:

A = -log T = - log (I / I_o).