Abstract: This application relates to solar receivers and solar energy concentrators, and windows for use therein. The solar receiver comprises a housing (2) and a window (3) disposed in or on a wall (4) of the housing (2), wherein the window (3) comprises vitreous silica having an OH content of 30 ppm or more by weight. In a prior art solar receiver having a high purity vitreous silica window, the window generally requires cooling if the receiver is operated at temperatures above 800° C., which limits the receiver's efficiency. The solar receiver of this application addresses the problem of the limited efficiency of the prior art receivers by using a window comprising vitreous silica having a relatively high OH content of 30 ppm or more by weight.