Methods of Producing Light
There are a number different means of producing visible Light, The following list is not exhaustive but provides an insight into the 5 main form of artificial light:-
Families of lamps
There are ten principal families of lamps, according to their manner of light emission, which fall into the 5 main categories.
Incandescent lamps are a source of electric light that works by incandescence, (i.e. heat-driven light emissions). An electric current passes through a thin filament, heating it until it produces light. The enclosing glass bulb prevents the oxygen in air from reaching the hot filament, which otherwise would be destroyed rapidly by oxidation. Various gases are used to control premature burnout of the filament and in the case of halogen, prevent blackening of the glass.
- Conventional Incandescent
Light Emitting Diodes (LEDs)
An LED lamp is a type of solid state lighting (SSL) that uses light-emitting diodes (LEDs) as the source of light, rather thanelectrical filaments, plasma (used in arc lamps such as fluorescent lamps) or gas.
LED lamps (also called LED bars or Illuminators) are usually clusters of LEDs in a suitable housing. They come in different shapes such as strips or on a PCB included in a Luminaire. There are also standard retrofit shapes available such as B22/E27 cap to retrofit Incandescent lamps or the MR16 retrofit Halogen lamps with a GU10 or GU5.3 bi-pin base.
- Light Emitting Diode (LED)
- LED Modules
- Organic Light Emitting Diode (OLED)
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through anionized gas. The character of the gas discharge critically depends on the frequency or modulation of the current. Typically, such lamps use an inert gas (argon, neon, krypton and xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury, amalgam, sodium, and/or metal halides. In operation the gas is ionized, and free electrons, accelerated by the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in visible light or ultraviolet radiation. Ultraviolet radiation is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface for some lamp types. The fluorescent lamp is perhaps the best known gas-discharge lamp. Discharge lamps tend to fall into 2 categories:-
Low Pressure Discharge
- Compact Fluorescent (CFL)
- Low Pressure Sodium
High Pressure Discharge
- High Intensity Discharge (HID)
- Mercury Vapour
- Metal Halide
- High Pressure Sodium
- Xenon Arc
In contrast with all other electrical lamps that use electrical connections through the lamp envelope to transfer power to the lamp, in electrodeless lamps the power needed to generate light is transferred from the outside of the lamp envelope by means of (electro)magnetic fields. There are two advantages of eliminating electrodes. The first is extended bulb life, because the electrodes are usually the limiting factor in bulb life. The second benefit is the ability to use light-generating substances that would react with metal electrodes in normal lamps.
Two systems are described below—one based on conventional fluorescent lamp phosphors and a second based on the use of radio waves energizing a bulb filled with sulfur or metal halides.
- Fluorescent Induction
- Radio-generated plasma lamps
1. Fluorescent lamps incorporate a low pressure Mercury discharge but the majority of the light output is from the fluorescence of the phosphors which coat the inside of the glass.
2. High pressure discharge lamps are sometimes known as High Intensity Discharge lamps (HID).
3. The Pressure of the High Pressure Sodium Lamps is less than one atmosphere, but they are so called to distinguish them from Low Pressure Sodium Lamps.
4. Many lamps require separate control gear, but some compact fluorescent and Induction lamps incorporate integral control gear.
Basis of lamp selection (based on bare light sources)
Lighting design guidance may be obtained from the various publications issued by the Society of Light and Lighting. Subject to their recommendations (illumination level, colour rendering, reduction of glare etc.) preference should be given to lamps with an efficacy as high as possible. The chart (Fig.3 page 9) indicates the ranges of efficacies available for common lamps of each family. Nevertheless, the fact that there are so many lamp types indicates that they all have their applications.
Once lamps are installed in luminaires, the overall efficacy would be affected by 3 main factors:
- Control Gear
- Thermal losses
- Luminaire Design (e.g. as measured by Light Output Ratio (LOR))
For example, with an LED installed in a light fixture, the efficacy could reduce to 50% of its original (discrete) LED rating i.e. 50lm/w.
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