The main source of naturally occurring light is the one we are most familiar with - the Sun. However this is only available for an average of 12 hours a day. To provide light at other times, as well as deep within the caves they inhabited, early man had to look to other sources.
There are a wide range of naturally occurring light sources in nature other than sunlight and daylight. These include lightning, the occasional Aurora Borealis display, bioluminescence and pyroluminescence. Early man turned to the only one of these sources that was readily controllable, pyroluminescence or the light produced during combustion. Up until the mid-nineteenth century this was the only significant source of man-made night light available, produced in a variety of ways ranging from campfires and oil lanterns to the much brighter and more recent gas lamps.
With the development of electricity came the development of electric lighting. One of the first attempts to produce electric light was by Warren de la Rue in 1820, however it wasn't until 1880 that the first commercial installation of an electrically powered lighting system occurred on the steamship Columbia, with land-based businesses finally taking to Edison's new invention in 1881. Since then there has been significant scientific development in the field of artificial light. Whilst by far the majority are based on electricity, there has also been developments in chemical, thermal and radioactivity based systems.
Artificial Light Sources
As a generalisation, there are only two types of artificial light source: incandescence and luminescence. In both types, the fundamental cause of the radiant emission is the same, the excitation of valence electrons and the energy released when they return to their original states. It is the way that this radiant energy is produced as visible light that differentiates the two.
As discussed in electromagnetic radiation, any physical object with a temperature above 0K emits some level of electromagnetic radiation due to the oscillation of atoms and molecules within its structure. The higher the temperature, the higher the frequency of this radiation - to a point where the object begins to glow dull red (around 873K), then yellow, and eventually gets to a point we term "white hot" when it is so hot (> 2500K) it emits light over all visible frequencies. This continuous spectrum of radiation results from the irregular excitation of very large numbers of free electrons at different states within the solid, only some of which will be at the highest temperature.
When electrical current is passed through a conductor, there is always a certain level of resistance to its flow (as per Ohm's law). Good conductors have a very low resistance whilst very poor conductors (what we may call an insulator) have a very high resistance. This resistance depends on the type of material as well as the cross-sectional area through which the flow is occurring The effect of this electrical resistance is to heat up the material. If a large amount of current passes through a small cross-section of resistive material, it can get very hot indeed - to the point where it is white hot and emits visible light.
This is the principle behind the electrical filament lamp, commonly known as the incandescent bulb. A material with the optimum resistance (usually Tungsten) is used so as not to impede the flow too much but still heat up enough without requiring too much energy.
In addition to resistance-induced, there a three other forms of incandescence.
Basically the light produced within a flame. This occurs mainly due to the incandescence of hot oxidised carbon or other solids but may also result from the recombination of ions into molecules. The amount of light produced depends on the material being combusted, some burning very brightly whilst others produce very little flame. One important point about pyroluminescence is that hot gasses may incandesce at very specific frequency bands, mainly due to a sparser molecular structure and hence a lesser influence from neighbouring molecules. Thus it is possible to observe a blue coloured flame whereas it would be impossible to get a solid object to glow blue solely by heating it up.
Most commonly produced within a gas mantle, this occurs when materials such as zinc oxide, cerium or thorium are excited by incandescent radiation. This induces fluorescence within the material (described later), causing them to produce shorter wavelength emissions (higher frequency light) than would be expected from the temperature of the materials and gasses within the process.
When a large electrical potential difference occurs between two conductors spaced very slightly apart, a spark may occur as electrons spontaneously flow from one to the other. This normally occurs very quickly, producing a bright flash of light and then ceases when the potential difference becomes more even due to the transfer of charge. However, if the potential difference is sustained between the two, a continuous discharge can be produced that emits light due to the incandescence of the ends of each electrode as well as the luminescence (described later) of vaporised electrode material between them.
Luminescence occurs when single valence electrons change state. This occurs mainly in gasses or materials in which neighbouring atoms exert little influence. It can be induced when a flowing free electron collides with such an atom, thus imparting some of its energy and disturbing the 'orbit' of the valance electron. After a short delay, the valence electron returns to its original state, in the process releasing the additional energy as a photon of electromagnetic radiation. This released radiation need not be at a visible frequency.
Within a relatively homogenous gas the emitted radiation is likely to be mostly of the same frequency (as electrons all have the same mass travelling at the speed of light and the atoms of the gas have the same number of electrons in the same 'orbit') - leading to the characteristic line spectra of luminescent sources.
This raw process is termed gaseous discharge, however there are other forms of luminescence.
This process is based on the same principles as gaseous discharge, however it occurs in crystalline inorganic compounds instead of gasses. The main difference is that the disturbance of the valence electron in one atom disturbs its neighbouring atoms as well, meaning that some of the imparted energy is lost as heat. When the electron returns to its normal state, the amount of energy it releases is less, so the frequency of emitted radiation is slightly lower. This can be useful in some materials such as phosphor which, when excited by ultraviolet radiation, produces visible light.
This is basically the same as fluorescence but occurs in materials in which a meta-stable state exists for valence electrons just above their normal 'orbit'. Thus, when disturbed, the valence electron can be bumped into this new higher-energy state and remain trapped there for anything from a few milliseconds to a few days. They will eventually decay to their original state, emitting photons when they do. The effect is a sustained 'glow' which can last much longer than the excitation.
Light Amplification by Stimulated Emission of Radiation (LASER) produces intense radiation ideally all at the same frequency and more importantly, fully in phase. In terms of wave theory, in phase means that the maximum and minimum amplitudes of the oscillation line up whereas in particle theory it means that the emission occurs in regular pulses at the same frequency as the radiation itself. In a laser this is induced in materials that are excited by radiation at exactly the same frequency as they emit.
If contained within specially reflective containers, 'standing waves' can be built up within the material, where the initially random emission by atoms reflect around within the material until a pulsing effect is produced. As this standing wave oscillates at the same frequency as the emitted radiation, and is produced within a material that is excited by radiation at this same frequency, a feedback loop occurs in which the passing wave-front excites atoms which then decay at the same frequency, further building up the strength of the wave-front This build-up occurs to a point where the energy is sufficient to pass through one end of the container (usually the one designed to be slightly less reflective) and results in a flow of perfectly coherent, high energy light. Solid, gas and semiconductor lasers simply induce this pulsing effect using different mechanisms.
Lower - Energy Luminescent Sources
There are a wide range of other luminescent effects which are much weaker than those described above. These include:
The effect produced in an Light Emitting Diode (LED) where low voltage direct current is applied to a dielectric-phosphur mixture deposited on a flat surface, usually a doped crystal.
The effect that occurs in a cathode-ray tube (computer monitor) when a substance (usually phoshor) is struck by electrons flowing from a cathode.
Light that appears at the anode or cathode when a solution is electrolysed.
An effect observed when a solution crystallises. It is thought to be due to the rapid formation of molecules from free ions.
This refers to light given off during chemical reactions occurring at room temperature.
Small amounts of light given off by a material when it is heated slightly. Marble apatite, quartz and diamond are examples of thermoluminescent materials.
Light produced by rubbing, shaking or crushing crystals. This is thought to result from a small electrical discharge when a crystallised molecular chain fractures.
The light observed when sound waves pass through some fluids. This is thought to occur when the pressure fluctuations in minute bubbles of gas cause their surface areas to change. When they expand they develop a capacity which is released when they contract.
Light emitted from a material when exposed to alpha, beta, gamma or X rays.
These different sources of light, and the artificial lighting systems derived from them, all have specific spectral characteristics and can vary quite significantly in the amount of input energy required to produce the same quantity and quality of light. It is important, therefore, that the lighting designer has a reasonably understanding of this before specifying any system.
- Lighting - the Electronic Textbook
- Sources of Light - Compton's Encyclopedia Online
- A History of Quantum Mechanics