Abstract
The author considers the design of an axially-symmetric, two-stage concentrator for photovoltaic cells. The primary stage is a Fresnel lens whose first surface is flat, the secondary a single-surface spherical lens in the form of a domed pillar glued to the cell. The author describes a method by which the dimensions and positions of the lenses may be chosen to maximize the concentration for a given acceptance angle. Use of an analytical expression for the caustic formed inside the secondary avoids the need for extensive ray tracing. The narrowest part of the caustic yields the maximum concentration and optimum cell position. Ray tracing confirms the results in selected cases and is used to examine the uniformity of cell illumination.