Sunlight and typical electric lamps have several orders of magnitude more visible than UV-B radiation (280-320 nm). This makes measuring UV-B radiation challenging because trace amounts of visible radiation can be internally reflected to the UV-B detectors. This contamination of the UV detectors by visible radiation is called stray light. The spectroradiometer has baffles to minimize internal reflection but it still has about 0.1% stray light, which can significantly affect measurement of UV-B radiation in sunlight. In applications without significant visible radiation, such as germicidal lamps or black lights, the stray light is not a factor. UV-A (320-400 nm) radiation is a relatively large fraction of sunlight (8.5%) so UV-A measurements are minimally affected by stray light. However, UV-B radiation is only about 0.3% of the visible radiation on a clear day near noon. UV-B measurements in sunlight, therefore, are affected up to 33% by stray light, so the following procedure should be used to improve the accuracy of UV-B measurements in sunlight or electric lamps.