A set of general graphical relationships has been developed for establishing the receiver-reflector area ratio which will provide the maximum useful heat delivery from a focusing collector. The method, applied to paraboloids and parabolic cylinders, gives the optimum area ratio in terms of incident radiation intensity, optical properties (including precision of reflector), and thermal loss rate. Use of the method for design and for evaluation of existing reflectors is illustrated; its use in economic optimization will be dependent on availability of adequate cost data.