  In a medium where a photoreactive species produces N equally photoreactive
radicals, by combining the Lambert-Beer law for the decay of light intensity,
as a beam propagates into the medium, with the kinetic equation, one obtains a
generalization of the Wegscheider equation from photobleaching theory. It is
shown that this equation can be solved exactly, but implicitly, and can be
reduced to a first order ordinary differential equation in a single reduced
variable.
