Beer-Lambert’s law has a wide range of applications, spanning the fields of pure sciences and engineering, and is commonly used for the determination of the concentration of an optically sensitive compound, at "low" concentrations. In this report it is shown that a log-log expression, log(T/T0) = K log(C/0), accurately models experimental data (+/-3%) for four different absorbing species, at "high" concentrations. The dimensionless constant K is shown to be only a characteristic of the absorbing species, and independent of the spectrophotometer/cuvette system used. The new equation is used for on-line monitoring of dynamic biomass concentration in a continuous bioreactor (CSTBR) for a step change in dilution rate.