In a conventional Compound Parabolic Concentrator (CPC), the reflector formation criterion is that light tangential to the edge of the closest tube (the tube nearly touching the cusp) is reflected back in the same direction by the CPC reflector, so that light which is not tangential strikes the closest tube more directly or via the reflector on a second bounce. Light cannot be reflected from one reflector to another and still be collected.

In Fig 1, two dotted tangent lines are shown. Light passing along these lines will be reflected straight back along the incident direction. Light more normal to the collector aperture will be collected. Between the left and the centre tube, the light ray is collected after a single bounce. Some light will be lost through the gaps between the absorber surface and the cover tube but about 98% is aimed at the tubes before reflection and absorption losses are accounted for.

In a CPC the curvature usually used is that of a mathematical involute, and the curve is truncated at the height approximately as shown in Fig. 1. The actual height of the reflector rim between the tubes depends upon the tube spacing. A full involute is not used; it would finish level with the top of the inner absorber tubes and would allow about 12.5 standard evacuated tubes having a 34.2 mm absorber tube diameter in a 1350mm wide panel. In this case the spacing between the tube centrelines is equal to the absorber tube circumference (107.4 mm), so that peak optical concentration is about 1. Such a "full” CPC would use much more reflector area than a practical design and the shape could not be as easily fabricated, so a 14 tube panel with a lower reflector is probably a more acceptable compromise these days, especially as tubes are relatively low cost.