Evacuated receivers used with parabolic trough solar collectors can be characterized in terms of their heat loss and optical efficiency. The optical efficiency is the ratio of the energy collected to the incident solar radiation when operating at ambient temperature. If one restricts attention to the active portion of the absorber receiving incident sunlight, this is equal to the product of the transmittance of the glass cover (τ) and the absorptance of the absorber surface (α). This paper describes a new outdoor transient test method for measuring the optical efficiency. An aluminum tube is inserted into the center of the stainless steel absorber tube, and the annulus between the two tubes is filled with a measured mass of aluminum shot. The receiver is precooled and then exposed on an outdoor test rig to solar radiation. The slopes of the temperature versus time curves for the aluminum filler and the steel tube are taken during a period of steady solar radiation and near the point at which the average glass temperature is close to the average absorber temperature (i.e., when there is minimal heat loss from the absorber tube to ambient). The slopes are then used to determine the optical efficiency. This method has the advantage of using the actual solar spectrum and has an uncertainty of about ±3%, which can be improved upon if a reference receiver is used for comparison. When this method was applied to the active section of the receiver tube, measurements of an actual receiver tube yielded a τα that, to within experimental uncertainty, is consistent with the manufacturer's values of τ and α.