the re-absorption probability R in the framework of fluorescent concentrators was first analysed by weber and lambe on the example of infinite strip geometry. by symmetry, such geometry also applies to a rectangular collector where a solar cell is attached to one edge and the other three edges are covered by perfect mirrors. in this case, noting that for fluorescent collectors with a reasonably high gain, it is sufficient to take the probability p(r) = 1/V ,where V is the volume of the collector. the volume integral the reduces to an integral over a single coordinate which can be evaluated analytically to give