Ideally measurements of spectral ultraviolet irradiances require a perfect adaption of the entrance optics to the cosine of the incidence angle. Other requirements of the entrance optics are: No ageing, high throughput at all wavelengths, weatherproofness, and no fluorescence of their material. In practice, however, available entrance optics differ by more than 10% from the ideal cosine response for incident angles greater than 60 degree(s). Without a correction this introduces a great uncertainty in the absolute measurement of irradiances, especially when the sun is low. A measurement of the angular dependence of the entrance optics and a knowledge of the ratio of the direct to the diffuse component of the global irradiance can be used to correct this cosine error. The correction is dependent on wavelength and sun elevation. For our cosine diffuser the corrections vary between 3% and 18%. The accuracy of the corrections is limited by the accuracy of the measurement of the angular dependence of the cosine diffuser, by the knowledge of the ratio of the direct to the diffuse radiation and by the knowledge of the angular dependence of the radiance of the diffuse component. We assume that our method reduces the overall cosine uncertainty from about +/- 10% to about +/- 3%.