LEDs

LED was discovered accidentally in the early last century (1907). In early development stage, carborundum crystals were used as light-emitting active material. Nick Holonyak Jr. of the General Electric Company has developed the first practical visible-spectrum LED in 1962.

LED Inside: An LED is a semiconductor device that converts electrical energy directly into light. The most important part of an LED is the semiconductor chip located in the centre of the bulb. On its most basic level, the semiconductor comprises two regions. The p-region contains positive electrical charges, while the n-region contains negative electrical charges.

Construction: One way to construct an LED is to deposit three semiconductor layers on a substrate. Between p-type and n-type semiconductor layers, an active region emits light when an electron and hole recombine. Considering the p-n combination to be a diode,

when the diode is forward biased, holes from the p-type material and

electrons from the n-type material are both driven into the active region, and the light is produced by a solidstate process called ‘electroluminescence.’ In this particular design, the layers

of the LED emit light all the way around the layered structure, and

the LED structure is placed in a tiny reflective cup so that the light from the active layer is reflected toward the desired exit direction.

LED emission and color determination: When sufficient voltage is applied to the chip across the leads of the LED, the current starts to flow. Electrons in the ‘n’ region have sufficient energy to move across the junction into the ‘p’ region. When an electron moves sufficiently close to a positive charge in the ‘p’ region, the two charges re-combine. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon. LED emits incoherent LED emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used and can be near ultraviolet, visible or infrared. Usually a combination of the chemical elements like gallium, arsenic and phosphorus is used.

LED Terminology:

AlInGaP: The preferred LED chip technology containing aluminium, indium, gallium and phosphorous to produce red, orange and amber

colors

Bin: The systematic division of distribution of performance parameters (flux, colour or CCT, and Vf) into smaller groups that meet aesthetic

requirements of the assembly

Binning: Subdivision of the manufactured distribution into the bin’s common operating parts (colour, flux and forward voltage)

Candela (Cd): The luminous intensity as defined by the international metric standard (SI). The term, retained from the early days of lighting, defines a standard candle of a fixed size and composition as a basis for evaluating the intensity of other light sources

Chromaticity diagram: A horseshoe shaped line connecting the chromaticities of the spectrum of colors

Hue: The situation when the appearance of different colours is similar; e.g., matching blues and pinks

Lightness: A range of grayness between black and white

Chroma: The degree of departure from gray of the same lightness and increasing color; e.g., red, redder and pure red

Color gamut: The range of colors within the chromaticity diagram included when combining different sources

Color spectrum: All wavelengths perceived by the human sight, usually measured in nanometers (nm)

Color temperature: The effect of heating an object until it glows incandescently. The emitted radiation, and apparent color, changes proportional to the temperature. This can be easily envisioned when considering hot metal in a forge that glows red, then orange and then

white as the temperature increases.

Cool white: Light with a correlated color temperature between 5000ºK and 7500ºK, usually perceived as slightly blue

Correlated color temperature: The phrase used to describe the temperature at which a Planckian black body radiator and an illumination source appear to match, usually specified in Kelvin (K)

Color rendering index (CRI): The calculated rendered color of an object. The higher the CRI (based upon a 0-100 scale), the more

natural the colors appear. Natural outdoor light has a CRI of 100. Common lighting sources have a large range of CRI.

Diffuser: An optical element used to mix light rays to improve uniformity

Driver: Electronics used to power illumination sources

Efficacy (luminous efficacy): The light output of a light source divided by the total electrical power input to that source, expressed in lumens per watt (lm/W)

Epoxy: Organic polymer frequently used for a dome or lens, often prone to optical decay over time, resulting in poor lumen maintenance.

High-power light sources contain no epoxy and deliver superior lumen maintenance.

Flux: The sum of all the lumens (lm) emitted by a source

InGaN LED: The preferred LED semiconductor technology containing indium, gallium, and nitrogen to produce green, blue and white colored LED light sources

Kelvin temperature: Term and symbol (K) used to indicate the comparative color appearance of a light source when compared to a

theoretical blackbody. Yellowish incandescent lamps are 3000K. Fluorescent light sources ranges from 3000K to 7500K and higher.

Lumen (lm): The international (SI) unit of luminous flux or quantity of light. It equals the amount of light that is spread over 929 sq.cm

surface by one candlepower when all parts of the surface are exactly 30 cm from the light source. For example, a dinner candle provides

about 12 lumens. A 60W soft white incandescent lamp provides 840 lumens.

Lumen maintenance: The remaining flux percentage at the rated life of a light source

Lumen maintenance curve: A graph comparing the loss of light output against the time the light source is used

Luminaire: A lighting fixture complete with installed lamps and other accessories

Lux (lx): The SI unit of illuminance or luminous flux incident on a unit area—frequently defined as one lumen per square metre (lm/m2)

Metameric: The term used to describe the visual perception phenomenon where spectrally different sources blend into a third chroma. For example, Sir Isaac Newton discovered that people perceive white when observing mixed blue and yellow light.

Nits: Measurement of display screen brightness. 1 nit = 1 Cd/m2. The more the nits, the brighter the picture.

NTSC color space: The range of colors within the CIE chromaticity diagram included when combining phosphor-based RGB sources in

CRTs such as televisions and computer monitors

Planckian black body locus: The line on the CIE chromaticity diagram that describes the color temperature of an object when heated from approximately 1000K to more than 10,000K

Warm white: Light with a correlated color temperature between 3000K and 3500K, usually perceived as slightly yellow.

White point: The coordinated color temperature (CCT) defined by a line perpendicular to the Planckian black body curve and intersecting

the measured chromaticity

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