Knowledge: Forms of Luminescence

Fluorescence is one of many forms of luminescence, the emission of light not resulting from heat. This distinguishes luminescence from incandescence, which is light emitted by a substance as a result of heating. All luminescence requires some input of energy to cause the light emission, and the varieties are distinguished by where that energy comes from. Note that there are several different schemes for classification of the types of luminescence and the format presented below is not absolute.

  • Photoluminescence
    • Energy source – Absorption of electromagnetic radiation (photons)
    • Includes:
      • Fluorescence – rapid (nanoseconds) emission of photons as electrons jump from excited state to ground level
      • Phosphorescence – delayed (milliseconds to hours) emission of photons that have been trapped in a ‘forbidden’ state
  • Chemiluminescence
    • Energy source – chemical reaction
    • Includes:
      • Bioluminescence – biochemical reactions in living organisms (like fireflies)
      • Electrochemiluminescence – resulting from electrochemical reactions
  • Crystalloluminescence
    • Energy source – crystallization, the process in which solid crystals precipitate from a solution, a melt or more rarely deposited directly from a gas
  • Electroluminescence
    • Energy source – electric current passing through a substance
    • Includes:
      • Cathodiluminescence – resulting from electrons striking a luminescent material
  • Mechanoluminescence
    • Energy source – mechanical action on a solid
    • Includes:
      • Triboluminescence – generated when bonds in a material are broken when that material is scratched, crushed, or rubbed
      • Fractoluminescence – generated when bonds in certain crystals are broken by fractures
      • Piezoluminescence – produced by the action of pressure on certain solids
      • Sonoluminescence – generated by imploding bubbles in a liquid when excited by sound
  • Radioluminescence
    • Energy source – bombardment by ionizing radiation
  • Thermoluminescence
    • Energy source – the re-emission of previously absorbed energy when a substance is heated

Some examples:

– ‘Traditional’ filament-type light bulbs work by incandescence. That’s why they get so hot.

– Fluorescent lights, including both the long tubes in many buildings and the spiral compact fluorescents that replace the traditional bulbs, work by a combination of electroluminescence and fluorescence. An electric discharge passes through mercury gas vapor inside the tube, producing strong emissions at very specific wavelengths, with the main energy in the ultraviolet. The inside of the glass is coated with a material called a phosphor that absorbs the ultraviolet and re-emits it as broadband visible light by – you guessed it – fluorescence. By changing the composition of the phosphor you get ‘warm white’, ‘cool white’, etc.

– Light-emitting diodes (LEDs) work by electroluminescene, with an current passing through a semiconductor material.

– There are many bioluminescent organisms that produce their own light for mating, prey avoidance, counterillumination, communication, and other purposes. Fireflies are a well-known example on land. Bioluminescence is found very widely in the ocean, both near the surface in plankton, and in many deep sea critters. Check out the Bioluminescence Web Page at UC Santa Barbara.

– Glow-in-the-dark materials, like the plastic stars in kids’ rooms, work by phosphorescence, absorbing energy while they are exposed to light and releasing it over time.

– You rarely see it now, but radioluminescence used to be commonly used to make things like instrument displays and clock or wristwatch hands glow steadily in the dark. You can’t use phosphorescence for that since the stored energy would fade out too quickly. The items that you wanted to glow were painted with a mixture of radium, a radioactive substance, and a phosphor. The particles emitted by the radium would strike the phosphor, producing a continuous glow. We know more about the harmful effects of radiation now, so these have largely disappeared.

You can find more information on all of these forms of luminescence on Wikipedia or with general web searches.