Beer specifically looked at solutions. Solutions are homogeneous and do not scatter light (Ultraviolet, visible, Infrared) of wavelengths commonly used in analytical spectroscopy (except upon entry and exit). The attenuation of a beam of light within a solution is assumed to be only due to absorption. In order to approximate the conditions required for the Beer Lambert law to hold, often the intensity of transmitted light through a reference sample ( I R ) consisting of pure solvent is measured, and compared to the intensity of light transmitted through a sample ( I S ) , with the absorbance of the sample taken as: log 10 ⁡ ⁡ ( I R / I S ) . It is for this case that the common mathematical formulation (see below) applies: log 10 ⁡ ⁡ ( I R / I S ) = A = ε ε ℓ ℓ c