Luminescence is defined as which of the following?

• A. Emission of visible light after exposure ends.
• B. Emission of visible light during exposure by UV or X-ray radiation.
• C. Absorption of light without emission.
• D. Emission of visible light by a material when it is exposed to a radiation source.

Answer: (D)

Luminescence is the emission of visible light by a material that has been energized by a radiation source. This includes light emitted while the material is exposed to excitation (fluorescence) and light that continues after exposure ends (phosphorescence). The essential idea is that light is produced by the material as a result of absorbed energy, not from heat. In gemology, this property helps identify stones because different substances glow in characteristic colors under UV or other radiation. The other descriptions capture only a subset of luminescence or describe absorption without emission, so they don’t fit the general definition.