Phototoxicity is defined as a toxic response is elicited by topically or systemically administered photoreactive chemicals after the exposure of the body to environmental light. Several classes of photoreactive chemicals could cause phototoxic reactions when activated by light at otherwise non-toxic doses. Phototoxicity can be categorized as photoirritation, photoallergy, and photogenotoxicity. The purpose of this test is to measure the phototoxicity of a chemical. The main event in any phototoxic reaction is the absorption of photons of a wavelength that induces the excitation of the chromophore. Excitation energy is often transferred to oxygen molecules, followed by the generation of ROS (reactive Oxygen Species). Measurements are performed using a spectrometer. The determination of the ROS generation from irradiated chemicals with simulated sunlight is indicative of phototoxic potential.