Prismatic structures in a thermal solar collector are used as overheating protection. Such structures reflect incoming light efficiently back whenever less thermal power is extracted from the solar collector. Maximum thermal power is generated when the prismatic structure is surrounded by a switching fluid with an index of refraction comparable to that of the prismatic structure. Thermal heat can be harvested via extra fluid channels in the solar absorber or directly via the switching fluid near the prisms. The light reducing effect of prismatic structures is demonstrated for a typical day and a season cycle of the Earth around the Sun. The switchability and the light reducing effect are also demonstrated in a prototype solar collector. (C) 2009 Elsevier Ltd. All rights reserved.