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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. The term is derived from the Latin word "lumen," meaning light, and the suffix "-escence," meaning "to emit." Luminescence can be classified into several types based on the energy source that excites the electrons to a higher energy level, which then emit photons when they return to their ground state.

The most common types of luminescence include:

1. Photoluminescence: This occurs when a substance absorbs photons (light) and then re-emits photons. It can be further divided into fluorescence and phosphorescence, depending on the time delay between absorption and emission.

2. Chemiluminescence: This type of luminescence occurs when a chemical reaction produces light. It is often used in biological systems, such as in the glow of fireflies.

3. Electroluminescence: This is the emission of light in response to the passage of an electric current or electric field. It is commonly used in flat-panel displays and LEDs.

4. Triboluminescence: This is the emission of light due to mechanical action, such as crushing or rubbing crystals.

5. Radioluminescence: This occurs when a substance is excited by ionizing radiation, such as alpha or beta particles.

Luminescence is widely used in various applications, including lighting, displays, medical imaging, and scientific research. It is also a topic of interest in the study of biological systems, as many organisms use luminescence for communication, camouflage, or attracting mates.