are Prerequisites
There at least five conditions that need to be fulfilled in order for Beers law to be valid. These are:
1. 2.
The absorbers must act independently of each other; The absorbing medium must be homogeneously distributed in the interaction volume and
must not scatter the radiation;
3.
The incident radiation must consist of parallel rays, each traversing the same length in the
absorbing medium;
4.
The incident radiation should preferably be monochromatic, or have at least a width that
is narrow more than the absorbing transition; and
5.
The incident flux must not influence the atoms or molecules; it should only act as a non-
invasive probe of the species under study. In particular, this implies that the light should not cause optical saturation or optical pumping, since such effects will deplete the lower level and possibly give rise to stimulated emission. If any of these conditions are not fulfilled, there will be deviations from Beers law.
http://en.wikipedia.org/wiki/BeerLambert_law