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14 September 2005 Light recycling in solid state devices
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The idea of light recycling is rather simple. Assume that part of the light emitted by a light source is returned to the light source itself. If the light source does not completely absorb this light then the part which is not absorbed, is still available for further use. The hidden virtue of light recycling is that the recycled light is superposed in the same phase space (etendue) as the original radiation. Thus the average radiance of the source is increased albeit at the price of a reduction of total radiant power. This seems to violate the Second Law of Thermodynamics because the temperature of the radiation is related to the spectral radiance. Increasing the radiance amounts to reducing the entropy. However, radiating into free space is an irreversible process in which entropy is created. Light recycling reduces the entropy carried by the radiation by reducing the entropy production rate in the emission process itself. We show that the maximum radiance which can be achieved by light recycling is marked by the equilibrium radiance. The equilibrium luminescent spectrum diverges as photon energies approach the splitting of the quasi Fermi levels. The familiar spectrum of LEDs clearly does not diverge because the absorptivity/emissivity approaches zero in this regime. These features render light recycling particularly attractive. We report on preliminary laboratory measurements which show encouraging results.
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Ling Fu, Ralf Leutz, and Harald Ries "Light recycling in solid state devices", Proc. SPIE 5941, Fifth International Conference on Solid State Lighting, 59411L (14 September 2005);


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