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Lighting terms

Terminology of illumination.

A basic overview

There are a number of technical terms associated with illumination such as: incandescence, candoluminescence, ( bio/ chemi/ electro/ photo) luminescence, cathodoluminescence, fluorescence, phosphorescence , (emission, absorption, spectrum, warm, cold, chromatic) electromagnetic radiation.  In this page I hope to elaborate on these terms to give a short but clear description.  I have also included a youtube video where I hope to help cement the description.  The video I've chosen is a "best fit" short video which I feel illustrates the term or shows the actual process in action.

The headings below are the a quick way to get to the description within this page.

Incandescence really refers to the generation of visible light by the glow of heat.  Usually a lot of very focused heat. The heat can be generated by a number of means such as directly through the burning of a fuel, an example being a candle flame or flare, conversely, burning indirectly such as in a Tilley lamp (see candoluminescence) or electric filament light bulb. Unlike other forms of light generation, such as the various forms of luminescence which tend to have specific frequencies equal to the specific quantised energy released, there is a consistent distribution of energy over the frequency spectrum. If this output was plotted as a graph with frequency on the x-axis and energy on the y-axis it would  form a Poisson distribution. The head of the Poisson would be dependent on the temperature of the material.

  • A perfect neutral material is often referred to as a "black-body".  ie a material that contributes nothing in its own right and is entirely dependent on an external source to induce energy. This induced or given energy is absorbed in its entirety (no reflections) and is then re-radiated perfectly with respect to its surroundings.  This idealised radiation is called, "black-body radiation."

In perfect back-body radiation, it is possible to reliably predict the light output energy and frequency by knowing the "temperature" of the black-body and Visa verse. This is not true with regard to candoluminescence.

The filament light bulb usually comes in two flavors, the less effective but extremely cheap incandescent bulb and the slightly more effective but comparatively more expensive halogen. Technically both are incandescent but the halogen version is commonly referenced by the additional added "halogen" gas. This gas is to aid in the tungsten filament's longevity and capsule clarity. The small amount of halogen gas (originally chlorine but commonly bromine) allows the tungsten filament to run at a much higher temperature and remain functional for longer.
Luminescence is defined by the ability to generate light without producing heat.  It is rare that absolutely no heat will be generated but it is considerably reduced, thus can be considered different to incandescent lighting. The current accepted reason for this phenomenon is that, at an atomic level, electrons are excited to a different shell quantisation level before decomposing to a lower shell level. For any atom the quantisation levels are discrete (ie fixed) they can only radiate energy equal to that particular quantisation jump, which results in a specific radiated frequency. The larger the difference the higher the radiated frequency. Where frequency is in the range of visible light, we see it as a specific colour or where there are multiple quantisation jumps we may see certain hues but the perceived colour does not vary.

All atoms and molecules can have electrons excited to different quantisation levels but the resulting radiation due to its decay may not be visible to the human eye. This property is termed as, "atomic spectral absorption and emission," and the scholarly field is called Spectrum analysis or Spectroscopy. Luminous materials are special in that they readily re-radiate light we can see.

There are many ways that a luminous material may get their initial quantised excitation and the terms such as, (cando/bio/ chemi/electro/photo/cathodo/mech) luminescence, really describe the mechanism of that initial excitation. There are two main branches of luminescence: fluorescence and phosphorescence. and these describe the rate of quantisation decomposition or the time they remain visible after the initial excitation event.
Candoluminescence has, in the past, been a disputed term. It is defined by the ability to generate light by excitation of a material through heating. Candoluminescence is essentially a subset of incandescence. Materials that candoluminesc also appear to defy the normal rules regarding black-body radiation and appear to radiate energy much higher up the electromagnetic spectrum than would be normally associated for a particular type of energy input. Similar to incandescents, it produces a Poisson type distribution of light output but appears to be shifted higher as though it was much hotter than the energy source would be capable of producing.

Examples of candoluminescence are: a limelight, or through the mantel of a Tilley lamp, also known as a pressure lantern. In the case of the Tilley lamp the heat energy is very effectively utilised to produce light from a thin and fragile structure called a mantel. The materials that comprise the mantel are excited by the combustion of fuel and so will candoluminesce producing a much brighter light than can be solely attributed to temperature alone. Known materials which candoluminesce are: Calcium oxide (CaO), Thorium oxide (ThO2), Zirconium oxide (ZrO2) and Magnesium oxide (MgO). The candoluminescent properties of the previously mentioned oxides can be further enhanced by mixing with a tiny quantity of more exotic rare earth compounds that act as a sort of catalyst.

  • These materials with catalytic properties most commonly used are: Yttrium oxide (Y2O3) and Cerium Oxide (CeO2) but other rare earth oxides carry this property such as: Terbium, Erbium or its elements, Lanthanum, Neodymium, Samarium, and Praseodymium. It has been speculated that they work by a temporary interaction with oxygen, enhancing the combustion process, thus releasing the energy more effectively to the candoluminescing material, increasing its brightness.

  • The same catalytic property was also observed with metalic compounds of platinum and iridium which enhanced the brightness of candoluminescent materials.

Phosphorescence is defined as the production of light over a longer period after external excitation, by either incidental radiation or a chemical reaction. Essentially a persistence in light output that is greater than 10ms. The most common place that you would see phosphors, that phosphoresce, would be in a Cathode Ray Tube (CRT) such in old oscilloscopes. An oscilloscope requires the persistence in the trace display to visibly denote the electron ray path. The radiation which excites the phosphor is effectively beta radiation (see cathodoluminescence). Beta radiation is energised electrons generated from an electron source (usually hot bulb) and accelerated within an electric field. There are further subsets of phosphorescence such as cathodoluminescence.

Cathodoluminescence is defined as light generated by the collision of high energy electrons (beta radiation) into a phosphor such is in a CRT. Examples are old TVs and old oscilloscopes.
Chemiluminescence (chemoluminescence) is defined by the generation of light by the mechanism of chemical reaction. An example being the light sticks often seen in festivals and night clubs. It is generally, but not always, a subset of phosphorescence.
Bioluminescence is defined by the generation of light by a living organism such as a firefly or glow worm. The organisms manufacture organic substances which, when combined, allow them to luminesce at will. Some organisms (notably the scorpion) will glow when exposed to Ultra Violet (UV) light but this fluorescence is caused by photoluminescence and should not be confused with biolumiescence. Essentially bioluminescence is a subset of chemiluminescence. The organism is manufacturing the chemicals which, when combined, bioluminesce.
Fluorescence is similar to phosphorescence but is defined as instantaneous light generated by the absorption of incidental radiation such as electromagnetic radiation (photons) or radiation (alpha, beta or gamma rays). It is different property from a phosphorescent material as the light output is brief for any single excitation event.  The most common examples being the compact fluorescent light (CFL) bulb or the plasma display on a TV. Plasma screens utilise phosphors and they are excited by a plasma gas in a very similar mechanism to a fluorescent tube. In the two examples high energy UV light generated by the excitation of gaseous mercury is converted by a phosphor coatings to red, green and blue light. In a fluorescent tube the phosphor coating can be combined to make white or even specialist colour balanced lights.
Photoluminescence is defined as the generation of light by the absorption of photons. Essentially a conversion of light frequencies, usually from higher to lower or shorter wave, to longer wavelengths. An example is blue to green light in some "white light" Light Emitting Diode (LED). Confusingly, the phosphorous coating of a fluorescent light or LED are photoluminescing and are not phosphorescing even though the coatings are called phosphors. Also to add insult to injury the phosphors in a fluorescent tube, plasma display or LED don't even contain phosphorous, the element the name is derived.
Electroluminescence is defined by light through excitation of electricity or current. Electroluminescent materials are often manufactured as a translucent capacitor with a phosphor layer between the plates. The electric field excites the phosphor causing light. An example being the Organic Light Emitting Diode (OLED).
There are some other fringe types of luminescence such as Mechanoluminescence (light from mechanical action) and its various sub categories:
Sonoluminescence (light from sound,specifically sonar),
 (light by mechanical pressure),
Fractoluminescence (light from the process of fracturing a material),
Triboluminescence (light by rubbing or scratching a material).

Listed technical terms:  (from the above article in Alphabetical order)

Further reading and references for this article: take a look at the resource page:

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