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
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 bins 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 areafrequently 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