Luminescence is the property of a material to glow with its own light, but there are many types of luminescence with their own mechanisms of action. The word comes from the root lumen, meaning light. Often the light given off comes from absorbed light prior, but other times the light can come from the inner structure of the material.
Luminescence, the general emission of light, not along with heat as in the case of fire and incandescence, but from a change in electron states within the material. The material structure on a very small scale leads to its ability to absorb light, and then release it. You’ll find that the terms for describing different “types” of luminescence often overlap and which term is used in a given context simply depends on the field it’s studied in.
Electroluminescence is a broad category of luminescent phenomena, in which changes in chemical bonds, from electric current or chemical reactions, for example, release light without heat. And, there are also several distinct types of mechanoluminescence, which result from physical stress or deformations.
Photoluminescence comprises the broadest context, in which absorbed light is later released, within which the phenomena of fluorescence and bioluminescence lie.
While fluorescence was first noted in 1565, it was not named until 1862 by Sir George Stokes. He used the term to describe the glow of fluorspar and uranium glass. This is the most antiquated notice of luminescent phenomena I could find, but the broader was noted another time in the 14th century by an Italian cobbler and alchemist.
Vincenzo Casciarolo & The Bologna Stone
Vincenzo Casciarolo (1571-1624) was a shoemaker with a hobby for alchemy. He would go mining and looking for rare rocks. In the hills of Bologna, around 1602, he found a strange rock that he did some experiments on. He used a method called calcination which endowed the stone with glowing properties after sun exposure. He called it the “sun sponge.” Today some still call it sun stone or “lapis solaris.”
After being rumored and observed by some throughout the area, it took a little bit of work to find the exact circumstances to make certain stones glow. That is, it required copper impurities in the case of baryte (BaSO4), and calcination heat treatment – controlling for oxygen exposure. The calcination process turned the barium sulphate (BaSO4) into barium sulfide (BaSO). The presence of copper and the oxygen deprived heat treatment contribute to the chemical reaction needed.
This was the first example of a persistent luminescent material. The actual term luminescence was coined first by Eilhard Wiedemann in 1888.
1. Photoluminescence – Classic types of luminescence
A molecule exhibits photoluminescence when it can absorb and release a photon in the visible spectrum. If it weren’t in the visible spectrum, we wouldn’t be able to perceive it as light. Fluorescence is a type of photoluminescence, and so is phosphorescence, when occurring in the visible range of light (400-1200 nm about.)
2. Fluorescence
These types of luminescence are usually at the top of mind as they are everywhere in daily life and applications. Fluorescence specifically absorbs and releases on very short time scales with a red-shift between the absorbed and emitted light. This happens instantly to our eyes. The absorbed light excites an outer electron to an excited, unstable state, which it then falls back down in emitting the photon. In between absorption and emission, non-radiative transitions sap away from the photon’s energy, so it emits at a redder (lower energy) color. Here is the whole step by step for just how fluorescence works.
Examples of fluorescence in nature include scorpions, jellyfish (which have both fluorescent and phosphorescent bioluminescent capabilities), and many natural stones like fluorite, fluorescence’s namesake. Commercial products have solidly applied fluorescence, mainly to increase visibility, such as with street signs, fabric and paper lighteners, and blacklight reactive products. Since fluorescence red-shifts its absorbed light, the blacklight, which is bluer than what we normally see, gets spat out as a bright color in the visible spectrum.
Fluorescence can be a form of electroluminescence as well when the same electron state changes happen due to a chemical reaction. The photon is still absorbed and emitted, but it originates from within the substance during the chemical reaction. The categories of luminescence are a bit overlapping at times, but fluorescence is definitely one of the most primary forms!
3. Phosphorescence
Phosphorescence, similar to fluorescence, is a kind of photoluminescence in which photons that have been absorbed prior emit. But while fluorescence emits these photons in less than a second, phosphorescence has a delayed emission. The electrons can drop back down in energy states and release their light many hours after it has been absorbed.
Despite the name, phosphorus isn’t phosphorescent, but rather chemiluminescent in the oxidized state (below). Unlike fluorescence, the electron, excited to a higher energy state from photon absorption, gets “stuck.” In quantum terminology, the electron is excited into triplet state, where there is a higher multiplicity of spin, so it doesn’t decay as quickly. Phosphorescence also has a redshift upon emission due to nonradiative transitions, just like fluorescence.
A commonly known example of phosphorescent materials is anything that glows in the dark, like stick-on ceiling stars, or glow-in-the-dark paints. These product often use zinc sulfide. If you have experience with similar “glow-in-the-dark” products, you’ll know that the amount they glow depends on what they’ve been able to absorb. If you keep those stars in a drawer in the day they will barely glow at night. Take them in the sun and they will glow much brighter that night.
Phosphorescence in the sea
Many sea creatures have luminescent molecules in their skins. Some have fluorescent properties, but most are phosphorescent and release the light absorbed in day during the night for visibility.
Bioluminescence
Bioluminescence is any luminescence occurring in a biological organism. The ocean has the most organisms that are bioluminescent. Some bioluminescent insects, like fireflies, are more properly designated as chemiluminescent, like glow sticks, their luminescence results from a chemical reaction within them.
A well-known example of bioluminescence is the jellyfish aequorea victoria, which produces light in the dark ocean by way of a chemical called Luciferin. This chemical is not unique to the jellyfish, as it sometimes gets it from feeding on algae that make it too. Luciferin on its own must be catalyzed by an enzyme Luciferase to bring the molecule into its unstable excited state. As with fluorescence, the molecule drops down in energy which releases the light. These processes evolved for hunting, communication, mating, and defense, among other uses.
Bioluminescent Algae
Since the bioluminescent algae is so strikingly beautiful, many marine tourist locations offer night guided tours to appreciate it. Sometimes the effect is only pronounced at certain times of year.
4. Chemiluminescence
These types of luminescence come from chemical reactions. Chemiluminescence results from a chemical change that drops the electron energy states, resulting in light. In photoluminescence the light is absorbed prior to release. Though chemiluminescence can happen over short or long time frames depending on the energy level the excited electron occupies, the photon comes from the energy released in bonds during the reaction. Excited molecules naturally decay into their ground state after absorbing the photon from a chemical reaction.
For commercial and industrial purposes, researchers have identified many chemical reactions capable of chemiluminescence, such as luminol + hydrogen peroxide into 3-aminophthalate (C8H7N3O2 + H2O2 –> C8H7NO4 + photon). Luminol reacts with hydrogen peroxide to give off blue light. When catalyzed with copper or iron, a large amount of light comes off of the reaction. Elemental phosphorus, as it oxidizes in air, also gives these light-producing chemical reactions.
Luminol is used in a lot of forensics applications, like finding old bloodstains and in DNA sequencing.
Fireflies and glow sticks are phosphorescent reactions.
Candoluminescence
The main appearance of candoluminescence is with the Welsbach gas lamps of the 1880s that used thorium dioxide. A flame brought to the gas gave off more light than expected from typical blackbody radiation (as in incandescence.) The studied mechanism turned out to by a type of chemiluminescence due to the properties of thorium dioxide. Other rare-earth metals and ceramics have this effect as well.
Lyoluminescence
Another rarely-spoken of chemiluminescence is lyoluminescence, resulting from a dissolved solid in liquid. This type could also be considered a kind of radioluminescent, because the materials have at one point been inundated with gamma-radiation (higher energy than x-rays.) Lyoluminescence is mainly studied in the area of food science, where some spices and processed goods are found to exhibit it, but not much else is known yet.
Electrochemiluminescence
Another niche chemiluminescence is electrochemiluminescence (a very long word), resulting from an electrochemical reaction. In an exergonic chemical reaction, the free energy of the products is greater than that of the reactants. Thus light is emitted as the molecules settle into their lower energy states. Some chemical reactions are catalyzed with electric currents, and these can show electrochemiluminescence. Scientists have fine-tuned certain instances of electrochemiluminescent reactions for applications in analyzing immunoassays.
5. Crystalloluminescence
When crystals form, their molecules bind together in a set structure, the precipitation of the crystal. The forces that bring these building blocks together is based on the bonding energies and attractions amongst the molecules. Crystalloluminescence can occur during crystallization or any other change of state that happens energetically “downhill” – with no input of additional energy into the system.
Potassium iodide emits at 380-480 nm and potassium chloride 240-380 nm. Potassium sulfate was the first studied example of crystalloluminescence as it crystallizes from its aqueous for rather rapidly. While also occurring in sodium chloride (table salt), crystalloluminescence is also very niche, and hasn’t found many applications yet.
6. Mechanoluminescence – Physically effected types of luminescence
Luminescence can also occur from purely physical, mechanical perturbations. The category of mechanoluminescence is easily broken up into friction (triboluminescence), cracks (fractoluminescence), and strain (piezoluminescence).
Triboluminescence
When some materials are cracked, crushed, or rubbed, the chemical bonds are broken and light is released from the energy of the former bonds. The common science experiment showing triboluminescence is the crushing of certain candies – such as chewing wintergreen lifesavers in a dark room. You can read about the piezoelectric properties of rock-type sugar candies in this list of examples of piezoelectricity. Also the adhesive layer on scotch tape produces triboluminescence when peeled.
Francis Bacon first noted triboluminescent properties of sugar in 1620. Robert Boyle further documented triboluminescence in the 1660s, and in 1675 Jean-Felix Picard observed it in a mercury filled barometer, with light flashes coming from the interface of glass and mercury.
Triboluminescence works in a few possible ways. One way is by the physical kinetic energy given to the electrons, raising them to the unstable energy level only to drop back down and release light. Another is the friction causing molecules to polarize, and separate into positive and negatively charged “clouds”, which release energy in the form of light when they snap back like a rubber band into their depolarized rest state. In either case, the static charge separates and then is reunited to its stable state, releasing the energy absorbed. If you see a common theme yet, it should be that unstable = absorbed energy and stable = released energy.
Triboluminesence also happens in our bodies, though rarely directly observed. Blood, ligaments, and mucus membranes all have the necessary material structure for producing triboluminescence. During sex, blood circulation, chewing, and exercise, the separations of charge occur repeatedly.
Fractoluminescence
When certain crystal structures are broken by fractures, light is released in the process of fractoluminescence. In many cases fractoluminescence is indistinguishable from triboluminescence, and the choice of term is dependent on context.
Piezoluminescence
We have covered a lot about piezoelectricity already, and piezoluminescence is indeed related. Piezoluminescence doesn’t require the breaking, scratching, or fracturing of the material, just a pressure or stress on the crystal structure. It’s simply the deformation, the shifting of the crystal planes temporarily, causing the energy release for the light, and not the actual breaking of bonds, or any kind of friction. However, piezoelectricity and triboluminescence often come along together in many materials, like sugar.
Walking on very dry sand at night can display triboluminescence and piezoluminescence too. Sand is made up of silicon dioxide, quartz. Especially in tiny grains, quartz will glow under friction or pressure.
As a piezoelectric crystal is stressed, like potassium bromide for example, the electrons change configuration to match the temporarily shifted planes. Similar to the “holes” (areas of positive charge) and electron concentration in transistors, the flow of electrons after the planes relax to their equilibrium state, gives off the light. Again, think of returning to a stable configuration as releasing energy as light.
Sonoluminescence
Sonoluminescence is super unique amongst all forms of luminescence. When acoustic pressure waves (sounds) travel through a liquid, bubbles of air pockets can form and as they implode on the surface, create light.
sonoluminescence was discovered incidentally in 1934 at University of Cologne in Germany. Since then, little more has been discovered. Peter Jarmon proposed in 1960 that microshocks could play a role, at the interface of the liquid and gas. Some also think a form of bremsstrahlung, which is a phenomenon of collision-induced radiation well-known in nuclear physics, could play a role. Proton tunneling has been proposed too. Some are still very interested in the idea that sonoluminescence could be harnessed to make a thermonuclear reactor, but little work has played out on that front.
7. Radioluminescence
Radioluminescence is caused by ionizing radiation shot at a material. Luminous paints used to be used on watch dials and other instrument displays so that they would glow in the dark. These could be tritium excited paints or a mixture of a radioactive substance like radium with a phosphor. Even with phosphorescent coatings, the stored energy would give out too quickly for those purposes. They stopped using them because they were radioactive, of course.
Now, radioluminescence is seen around nuclear plants and hazardous waste treatment centers as a byproduct. Former applications like the watch displays and gun sights have phased out. They are limited to those researchers specifically studying radioactivity.
8. Thermoluminescence
Some materials can absorb heat energy and then re-emit it as heat. Quartz and calcite when heated release light as they cool down. There is a distinct different in thermoluminescence and incandescent light, as incandescence releases heat as well.
Geologists and archeologists take particular interest in thermoluminescence because it can happen over thousands of years, like in beach sands (quartz). Defects in the crystal structure trap the electrons in their “unstable” states for long times. Cosmic rays of the past give radiation to crystals that are able to receive, so scientists can use thermoluminescent properties of geologic material to read the clock on processes and formations.
The moon and meteorites may also have thermoluminescent properties, such as discussed in this and prior studies.
9. Electroluminescence
While above we’ve defined chemiluminescence, there is some overlap in chemi- and electro-luminescence in many examples. When electricity passed through a system results in luminescent, it’s called electroluminescence. LEDs work like this, exploiting the photoelectric effect through two junctions of transistors.
There is intrinsic electroluminescence and charge injection. Intrinsically the material must have a large conduction band where electrons are excited into. In the conduction band collisions with luminescent atoms ionize the electrons.
Charge injection electroluminescence is more like a electrode supplying a crystal with an electric current. LEDs need the PN junction.
Fluorescent lighting work by electroluminescence and fluorescence combined. A discharge passes through a gas, usually mercury, inside the tube. Strong emissions come from the electricity’s interaction with the gaseous molecule’s electrons. This produces ultraviolet light, then absorbed by the phosphor coating that re-emits it as the broadband visible light. The coating of the phosphor tunes the output color from warm to cool toned.
Cathodiluminescence
When electrons are sent into a luminescent material, the resulting light is cathodiluminescence. Like the photoelectric effect inherent in simple transistors, cathode ray tubes like in old TV sets set a good example of cathodiluminescence
10. Cryoluminescence- Rare luminescence alert!
Of the types of luminescence, this is maybe even more mysterious that sonoluminescence. Cryoluminescence refers to fluorescent properties induced in normally non-fluorescent materials through the act of cooling. This is a bit shocking to many since usually we must add energy into a system to get the luminescence effect. Chemists do sometimes use the term “fluorescence thermochromism” to refer to a luminescence upon cooling.
Wulfenite, pictured below, exhibits cryoluminescence, and most other examples we know about today are engineered nanomaterials.
The first finding of cryoluminescence dates to as early as geologic studies from 1954, and 1968, in a work characterizing Zinc Sulfide. Since then, there has been some efforts to probe for novel effects, with this 2019 study finding anti-stokes shifted luminescence due to a form of triboluminescence from inherent defects in crystal structure. Anti-stokes behavior means the emitted light is blue-shifted, or gains energy in the absorption and excitation phases. This is found in low-energy systems, such as supercooled noble gases, ethanol, and in nitrous oxide. Noble gases tend to have anomalous ordering as they are supercooled and begin to condense into fluid or solid phases, especially at the triple point, where their thermodynamic properties are not unlike that of black holes.
Iridescence – outside the Types of Luminescence
I wanted to include iridescence to differentiate it as something different than the luminescences. A common thread we see to qualify as luminescence is that the molecules release light as an electron drop down in energy levels after excitation. Iridescence is equally awe-inspiring but has actually a quite different mechanism.
Iridescence is a more direct form of light interacting with molecules structurally. In iridescence, spaced shingles or leaves of matter, which have a thinness on the order of nanometers, cause the light to interact with itself. In the case of iridescence, the electron energy is not involved, and there is no “release” of light, but rather it is all the incoming light shifting phase and combining in reflections.
Structural color
The most well studied example of structural color resulting in iridescence is the Morpho butterfly. Other examples abound in abalone shells, mostly in keratin rich mollusks and insects.
So many types of Luminescence, but really it’s all in the name
I hope this round up of every type of luminescence I could find was comprehensive. From the commonly used and experienced fluorescence to the obscure and mysterious sonoluminescence.
But remember, usually light is emitted along with heat. In all these types of luminescence, the molecules are snapping back to stable electron configuration, which simply incidentally emits light. No heat is produced because electrons don’t particularly have frictional effects. So you take this life message along, keep stable in adversity and emit light while keeping your cool! 0_0
