Exposure to visible light (400-700 nm wavelengths) is an unnatural stress factor to preimplantation embryos cultured in vitro. This study investigated the spectral composition and intensity of light during IVF procedures, and calculated radiation doses reaching the embryo during handling and manipulation. The study shows that normal IVF procedure may result in stressing radiation doses, unless filters are applied. This is at present not sufficiently recognised. No Danish IVF clinics use filters to protect embryos against visible light. 95% of the radiation was from microscopes. Ambient light, in contrast, was not a significant contributor to light stress and the use of dark laboratories is not justified.