Theoretically speaking all the terms mean the same and describe emission of light by substances not involving high temperatures. The generic term luminiscence can be used to describe all such light emission processes. Luminiscence means relaxation of excited molecule to ground state accompanied by liberation of light. On the other hand light emission from a material on account of application of heat is termed as incandescence.
The objective of this article is to clarify the differences between these apparently similar sounding terms.
Fluorescence is a process in which light is emitted by a substance at the longer wavelength subsequent to absorption at a shorter wavelength. The difference in the two wavelengths is referred to as Stoke’s shift. A fluorochrome is a substance which is capable of emission of light at longer wavelength after absorption at a shorter wavelength.
On absorption of light at a particular wavelength electrons shift to higher energy states which are inherently unstable and there is a tendency to return to the ground state. The return transition is very fast and takes place within \(10^-^9 to 10^-^6 sec\).
Phosphorescence is similar to fluorescence but re- emission is slower (\(10^-^4 to 10^2\)) or even slower (minutes to even hours). The transition to a low energy ground state is slowed down by transition of electrons from excited singlet state to an intermediate triplet state and then eventual return to the ground state. Re- emission of light occurs at a lower energy (longer wavelength) in comparison to fluorescence due to extra energy loss in the intermediate state transition. Examples of phosphorescence are coatings on hands of a table clock which glow in the dark or even idols and toys which glow when lights are switched off
Bioluminescence refers to light emanating from living organisms and has mystified mankind the most. It gives the glow to fireflies, makes the waves glow at night and gives colourful glow to deep sea creatures. Bioluminescence has one unique feature which differentiates it from other categories of luminiscence. It is observed in pitch dark of night or under the darkness of the deep sea as it does not require irradiation of the organism by a light source. The biological organism derives its energy through from the food that it consumes. Certain proteins, such as luciferin, store the energy and result in excited electrons. Enzymes called luciferase de- activate the excited molecules resulting in emission of light.
Chemiluminiscence is a process which results in light emission due to a chemical reaction. The reaction is seldom initiated by light irradiation. A reaction product has an excited intermediate which emits light on stabilization to the ground state. A well known example is oxidation of luminol in alkaline media to aminophthalate ion which is produced as an excited state and emits blue light on relaxation to the ground state.
Luminiscence behaviour of a substance is depressed by several factors which will be taken up in a subsequent article.