Abstract: An ideal or near ideal radiation concentrator for a distant source of radiation or a radiation distributor in which a non concave linear radiation absorber or source is arranged between facing reflecting side walls to receive radiation reflected from the side walls or to emit radiation. One side wall is made up of at least a portion the curvature of which is in accordance with equation (2) of the specification and the other side wall including at least a portion the curvature of which is in accordance with equation (3) of the specification. Where a circular or other convex section absorber or source is employed it is preferred that each side wall commence at a point on the absorber or source, the curvature of the side wall adjacent the absorber or source being the involute of the cross-section of the absorber or source.