LED

A LED lamp is a type of solid state lighting (SSL) that utilizes light-emitting diodes (LEDs) as a source of illumination rather than electrical filaments or gas.
LED lamps (also called LED bars or Illuminators) are usually clusters of LEDs in a suitable housing. They come in different shapes, including the standard light bulb shape with a large E27 Edison screw and MR16 shape with a bi-pin base. Other models might have a small Edison E14 fitting, GU5.3 (Bipin cap) or GU10 (bayonet socket). This includes low voltage (typically 12 V halogen-like) varieties and replacements for regular AC mains (120-240 V AC) lighting. Currently the latter are less widely available but this is changing rapidly.
A single LED die can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in approximately correct proportions. To achieve this, three approaches are used for generating white light with LEDs: wavelength conversion, color mixing, and most recently Homoepitaxial ZnSe.
Wavelength conversion involves converting some or all of the LED’s output into visible wavelengths. Methods used to accomplish this feat include:
Blue LED & yellow phosphor – Considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light, but produces poor color rendition (i.e., has low CRI).
Blue LED & several phosphors – Similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
Ultraviolet (UV) LED & red, green, & blue phosphors – The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
Blue LED & quantum dots – A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of the LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.
Color mixing involves using multiple colors of LEDs in a lamp to produce white light. Such lamps contain a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency.
Homoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is grown on a ZnSe substrate, which simultaneously produces blue light from the active region and yellow emission from the substrate. The resulting white light has a wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher efficiency white LED.
To be considered SSL, however, a multitude of LEDs must be placed close together in a lamp to add their illuminating effects. This is because an individual LED produces only a small amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively.
Driving LEDs
LEDs have very low dynamic resistance, with the same voltage drop for widely varying currents. Consequently they can not connect direct to most power sources without causing self destruction. A current control ballast is normally used, which is sometimes constant current.

Indicator LEDs
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2mA, 10mA and 20mA are common. Some low current indicators are only rated to 2mA, and should not be driven at higher current.
Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficacy tends to reduce at low currents, but indicators running on 100uA are still practical. The cost of ultrabrights is higher than 2mA indicator LEDs.
LEDs have a low max repeat reverse voltage rating, ranging from apx 2v to 5v, and this can be a problem in some applications. Back to back LEDs are immune to this problem. These are available in single color as well as bicolor types. There are various strategies for reverse voltage handling.
In niche applications such as IR therapy, LEDs are often driven at far above rated current. This causes high failure rate and occasional LED explosions. Thus many parallel strings are used, and a safety screen and ongoing maintenance are required.

Alphanumeric LEDs
These use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. 7 segment and starburst LED arrays are available in both common anode or common cathode forms.

Lighting LEDs on mains
A CR dropper (capacitor & resistor) followed by full wave rectification is the usual ballast with mains driven series-parallel LED clusters.
A single series string would minimise dropper losses, but one LED failure would extinguish the whole string. Parallelled strings increase reliability. In practice usually 3 strings or more are used.
Operation on square wave and modified sine wave (MSW) sources, such as many inverters, causes heavily increased resistor dissipation in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor & rectifier makes a more suitable ballast for such use, and other options are also possible.

Lighting LEDs on low voltage
LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around 2/3 of the supply voltage, and resistor current control for each string.
In resistor-drive devices, LED current is then proportional to power supply (PSU) voltage minus total LED string voltage. Where battery sources are used, the PSU voltage can vary widely, causing large changes in LED current and therefore color and light output. For such applications, a constant current regulator is preferred to resistor control. Low drop-out (LDO) constant current regs also allow the total LED string voltage to be a higher percentage of PSU voltage, resulting in improved efficiency and reduced power use.
Torches run one or more lighting LEDs on a low voltage battery. These usually use a resistor ballast.
In disposable coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type, such as one using a different battery chemistry.
Finally, an LED can be run from a single cell by use of a constant current switched mode inverter. While adding additional expense, this method provides a high level of color and brightness control, and ensures longer LED lifetime.
Comparison to other lighting technologies
Incandescent lamps (light bulbs) create light by running electricity through a thin filament, thereby heating the filament to a very high temperature and producing visible light. The incandescing process, however, is highly inefficient, as over 98% of its energy input is emitted as heat.[citation needed] Incandescent lamps, however, are relatively inexpensive to produce. The typical lifespan of a mains incandescent lamp is around 1,000 hours.[citation needed]
Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. While the heat generated by fluorescent lamps is much less than its incandescent counterpart, energy is still lost in generating the ultraviolet light and converting this light into visible light. In addition, and should the lamp break, exposure to mercury can occur, though the levels involved are not considered hazardous. Linear fluorescent lamps are typically five to six times the cost of incandescent lamps[citation needed], but have life spans around 10,000 and 20,000 hours. Lifetime varies from 1,200 hours to 20,000 hours for compact fluorescent lamps.
Neon lamp (light bulbs) used like night-lamp in children's room. Typically a 230 V (in Europe) is rated 0.5 W of power.
SSL/LEDs LEDs come in multiple colors, which are produced without the need for filters. A white SSL can be comprised of a single high-power LED, multiple white LEDs, or from LEDs of different colors mixed to produce white light. The inherent advantages and disadvantages of SSL are currently the same as those of a LED. Advantages include:
High efficiency - LEDs are now available that reliably offer over 100 lumens from a one-watt device, or much higher outputs at higher drive currents
Small size - provides design flexibility, arranged in rows, rings, clusters, or individual points
High durability - no filament or tube to break
Life span - in properly engineered lamps, LEDs can last 50,000 - 60,000 hours
Full dimmability – unlike fluorescent lamps, LEDs can be dimmed using pulse-width modulation (PWM - turning the light on and off very quickly at varying intervals). This also allows full color mixing in lamps with LEDs of different colors.
Mercury-free - unlike fluorescent and most HID technologies, LEDs contain no hazardous mercury or halogen gases.
Challenges
The current manufacturing process of white LEDs has not matured enough for them to be produced at low enough cost for widespread use. There are multiple manufacturing hurdles that must be overcome. The process used to deposit the active semiconductor layers of the LED must be improved to increase yields and manufacturing throughput. Problems with phosphors, which are needed for their ability to emit a broader wavelength spectrum of light, have also been an issue. In particular, the inability to tune the absorption and emission, and inflexibility of form have been issues in taking advantage of the phosphors spectral capabilities.
More apparent to the end user, however, is the low Color Rendering Index (CRI) of current LEDs. The current generation of LEDs, which employs mostly blue LED chip + yellow phosphor, has a CRI around 70, which is much too low for widespread use in indoor lighting. (CRI is used to measure how accurately a lighting source renders the color of objects. Sunlight and some incandescent lamps have a perfect CRI of 100, while white fluorescent lamps have CRI varying from the 50s to 95.) Better CRI LEDs are more expensive, and more research & development is needed to reduce costs. End user costs are still too high to make it a viable option, for instance, Maplin's website quotes comparable LED spots at £9.99 GBP against the standard Halogen lamp twin pack which comes in at £6.49 GBP(or roughly £3.25 GBP each).
Variations of CCT (color correlated temperature) at different viewing angles present another obstacle against widespread use of white LED. It has been shown, that CCT variations can exceed 500 K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100 K in range from 2000 K to 6000 K, which is the range of CCT variations of daylight.
LEDs also have limited temperature tolerance and falling efficiency as temperature rises. This limits the total LED power that can practically be fitted into lamps that physically replace existing filament & compact fluorescent types. R&D is needed to improve thermal characteristics.

Applications:-

Traffic lights
Automotive lighting
Stage lighting
Bicycle lighting
Electric torches (flashlights)
Domestic lighting
Billboard displays
Floodlighting of buildings
Display lighting in art galleries to achieve a low heating effect on pictures etc.
Train lights (Now common on nearly all modern and most older MU's and Loco's in the UK)