Luminescence

Luminescence is the emission of light by a substance that has not been heated to incandescence. The light is produced when the material is excited by energy from sources other than heat, such as photons, chemical reactions, electricity, or mechanical action. The emitted light arises from electronic transitions within atoms or molecules and can occur at ambient temperatures.

There are several broad classes of luminescence according to the excitation mechanism. Photoluminescence covers light emission triggered by absorption of photons and includes fluorescence, which occurs promptly after excitation, and phosphorescence, which can persist after the exciting source is removed. Chemiluminescence describes light emitted from a chemical reaction. Bioluminescence is chemiluminescence generated by living organisms and commonly involves enzymes such as luciferase. Electroluminescence results from electrical excitation, as in light-emitting diodes and related devices. Triboluminescence arises from mechanical action such as scratching or rubbing. Sonoluminescence refers to light emitted from collapsing gas bubbles in a liquid under ultrasound.

In practical terms, luminescent outputs are characterized by their quantum yield, emission spectrum, and lifetime. Fluorescence is typically fast, while phosphorescence can last from microseconds to hours. Applications span lighting, displays, bioimaging, sensors, and security features. Common materials include fluorescent dyes and phosphors, semiconductors in LEDs, and biological systems such as fireflies and certain jellyfish.