Reshaping divergence with lenslet arrays

Two lenslet arrays, separated by one mutual focal length, can be used to reshape the divergence of a light beam.

Optics near the central focus of a solar plant operates at high energy flux, for example, at the beam-down optics of a glass-making solar furnace the flux will be on the order of 100 suns. This creates both the need to operate at high optical efficiency, and the opportunity to employ more expensive optical solutions than are possible on surfaces of larger area.

Each point on the beam-down optics of a solar furnace will see the circular opening of the furnace (because it is distorted by perspective) as approximately an ellipse. The beam-down optics needs to redirect approximately horizontal radiation coming from the field of telescopic heliostats downward toward the furnace opening. It also needs to reshape the divergence of the radiation to fit inside the approximately elliptical angular extent of the furnace opening.

A simple way to reshape divergence is with two lenslet arrays of the same focal length, separated by one focal length. In effect, each array acts as a field lens for the other. As the figure above illustrates, the lenslets in the two arrays do not have to have the same shape (though they must have the same area) in order to be in perfect correspondence with the lenslets in the other array. For example, in the figure above, circularly divergent light entering through the elliptical lenslets, would exit the round lenslets with elliptical divergence.