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Luminescence is a general term for light emission by a substance that is not at high temperature. It is a form of cold body radiation. There are two main types of luminescence: fluorescence and phosphorescence. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. The emitted light has a longer wavelength and is typically of lower energy than the absorbed light. This process is usually fast, occurring on the order of nanoseconds. Phosphorescence, on the other hand, is a type of photoluminescence related to fluorescence. In phosphorescence, the emission of light continues after the excitation light is removed, which can last from microseconds to hours. This is because the excited state from which the light is emitted has a longer lifetime. Luminescence is used in various applications, including lighting, medical imaging, and chemical analysis. It is also a key concept in the study of photochemistry and photophysics.