In defining the best design, one criterion should certainly be: the best design maximizes the temperature of the absorber , while maintaining the source at (a heat reservoir, like the sun). Introducing probabilities where is the probability of radiation from wall 1 reaching wall 3, and so on (Fig 5). The condition is . In case is not 0 (as in a cavity receiver), the generalization is which still maximizes . These conditions are intuitively self-evident. Here we sketch a proof: If , then some energy, say is exchanged with a body which is, in general at a lower temperature that the source 1. Therefore at equilibrium, will reach a temperature intermediate between and . Stated the other way, if reaches at equilibrium, then all the energy exchange must be between and since the other objects will in general be at a lower temperature.Additionally, for an efficient concentrating system () the concentration value is . In the case of photovoltaic systems, the condition states that light starting from the solar cell would invariably reach the source. The application of this concept should lead to the development of more thermodynamically efficient thermal designs as well as higher-efficiency solar cells.