Most of the detectors commonly used for radiometric measurements have a more or less poor cosine response. By means of a reflective entrance optics in front of the detection system, we have reduced relative cosine errors below 5&percent; up to zenith angles of 75°. Cosine correction is performed by a roughened polytetrafluoroethylene (PTFE) plate which diffusely reflects parts of the incident radiation into a light guide with the detector at its end. It provides for a neglectible wavelength cosine error dependence and a significantly better radiant efficiency. Originally designed for irradiance measurements of solar global radiation and its photobiological effective irradiance, the entrance optics may also be used for determination of radiant flux from lamps by integrating the lamp irradiance over an enveloping surface as it is done in spectrogoniometric measurements. ©1998 American Institute of Physics.