A technique for optimizing the design of focusing solar collectors was developed through a detailed study of the energy balances for a parabolic-cylindrical reflector with tubular receivers of three diameters. Experimental data for concentration ratios of 10 to 22.5 and surface temperatures of 88 to 353 deg F are presented. Receiver temperature, meteorological variables (including solar radiation), and distribution of reflected radiation in the focal zone of the reflector were measured and correlated so as to permit optimization of concentration ratio.