Last updated: 15 February 2015
Richard A . Steenblick and Dar-Veig Ho invented this design while Steenblick was a student at the Georgia Institute of Technology, circa 1980. He started The Suntwist Company, which published and sold instructions and a large paper pattern to allow construction of the novel fresnel-type reflector, from a single flat sheet of reflective material. It was intended to be both effective and affordable, though ultimately the design has not found a large following. He and his associate were granted a US patent regarding the Suntwist design, and many other variations of spiral concentrators, with both positive and negative focal lengths. The Suntwist Company ceased operations after a time, and Steenblick went on to pursue a distinguished career as a professor at GA Tech, in the field of optics.
After cutting the spiral from a 4-foot (1.2m) square sheet, pairs of holes were drilled at four points along each turn of the spiral. The holes were plotted in a special pattern, so that if the spiral were "wound up" somewhat like a torsion spring, the holes could be aligned, and wire passed through them to tie the spiral to a supporting X-frame. The winding set up spring tension in the turns of the spiral with the result that the continuous strip tilted inward toward the center, to varying degrees. The resulting geometry constituted a fresnel-type reflector, of depth less than 2" (5cm), yet approximating the effect of a paraboloid shape.
For low cost, The Suntwist Company recommended using fiberboard substrate and a glued-on aluminum foil reflective surface. While these materials satisfied the performance requirements of the concentrator, the practical issue of material durability was amplified in significance by the design. In particular the anchor holes, necessarily located quite near the inner edges of the spiral's turns, were at risk of breaking through to the edge under stress from the wire, during construction and with handling/use. This would result in de-alignment of the spiral and degradation of the focus. Construction using sheet metal could overcome the issue, but increased cost and tooling needed for construction.
In the example pictured below, the sheet metal is stainless steel. While stainless steel offers corrosion resistance, reflectivity is only ~ 65%. Aluminum offers significantly better reflectivity, but is structurally weaker.