Glossary - LED basics

The angle between the two directions for which the intensity (candlepower) is 50% of the maximum intensity as measured in a plane through the nominal beam centre line.

The angle between the two directions in the plane in which the candlepower is equal to a stated percent of the maximum candlepower in the beam.

During manufacture, LEDs are broadly grouped into colour categories such as red, red-orange, cyan, cool white and warm white, which encompass a wide colour and temperature range. For example, Cyan LEDs can range from 490…520 nm. Light output from a 520 nm rating will look very ‘greenish’.  Many applications require greater precision, so every LED is tested to determine its exact colour, lumens output and forward voltage and are then assigned a 'bin' number - 'binning'. Users can request a specific bin number.

Unit measurement of luminous intensity. One candela is one lumen per steradian.

A curve, often on polar coordinates, illustrating the variation of luminous intensity of a lamp or luminaire in a plane through the light centre.

Luminous intensity of a particular light source in a specific direction expressed in candela. (see also Luminus intensity).

The aspect of colour that includes consideraton of its dominant wavelength and purity.

Conventional LEDs are made from a variety of inorganic semiconductor materials. The following table shows the available colors with wavelength range, voltage drop and material.

Refers to a lamp with a special phosphor or coating to give it a colour rendering profile like natural daylight.

Colour rendering is not a physical quantity, but a qualitative characteristic of light. The more spectral colours that are contained in light, the better the colour rendering is, since an increase in the spectral colour proportion makes the illuminated objects appear more natural. The colour rendering quality of a lamp is given by the colour rendering index (Ra). Objects appear most natural when the colour rendering index Ra = 100.

The visible light spectrum, ranging between 380 (violet) and 770 (red) nanometers in the electromagnetic spectrum (see also Visible spectrum).

The actual colour of a light source, also referred to as Correlated Colour Temperature (CCT), measured in degrees Kelvin (K) . It is also defined as appearance of white light in terms of warmth or coolness. The colour temperature of light source is that temperature that a black body must have, such that the colour of the emitted light (its chromaticity) mathches that of the real light source. Black body is defined as theoretical body used in physics which fully absorbs electromagnetic radiation of any wavelength striking.

See Light Density.

The dominant wavelength and complementary wavelength are ways of describing non-spectral (polychromatic) light mixtures in terms of the spectral (monochromatic) light.On the CIE color coordinate space, a straight line drawn between the point for a given color and the point for the color of the illuminant can be extrapolated out so that it intersects the perimeter of the space in two points. The point of intersection nearer to the color in question reveals the dominant wavelength of the color as the wavelength of the pure spectral color at that intersection point. The point of intersection on the opposite side of the color space gives the complementary wavelength, which when added to the color in question in the right proportion will yield the color of the illuminant (since the illuminant point necessarily sits between these points on a straight line in CIE space, according to the definition just given).

See Luminous (Light) Flux.

Broad spectrum light source capable of producing colours throughout the entire range of the visible spectrum; simulating actual sunlight. High CRIs and Kelvin temperatures ranging between 5500 and 5900.

Illuminance is purely a physical measurement value. It is the ratio between the luminous flux and the area to be illuminated, irrespective of the reflectance of the surface. An illuminance of 1 lux occurs when a luminous flux of 1 lm falls evenly on an area of 1 m². Illuminance reduces with the square of the distance between the light source and the surface

Radiant energy in the wavelength bands by the (CIE) recommendation:
IR-A: 700 nm–1400 nm (0.7 µm – 1.4 µm, 215 THz - 430 THz)
IR-B: 1400 nm–3000 nm (1.4 µm – 3 µm, 100 THz - 215 THz)
IR-C: 3000 nm–1 mm (3 µm – 1000 µm, 300 GHz - 100 THz)
Energy in this range is not visible to the naked eye, but can be sensed by the skin. Applications include heat lamps, photography, medical therapy and industry.

The light emitted from a source. Intensity varies given the direction at which one views the source. Intensity does not vary with distance. See also Luminus Intensity.

Scientific unit of temperature. Colour temperature is measured on the Kelvin scale.

See Correlated Colour Temperature (CCT).

A measure of electrical power equal to 1000 watts.

The standard measurement of electrical energy consumption. 1000 watts of electricity used in one hour. Also the typical billing unit used by electrical utilities.

A factor that represents the reduction of lumen output over time. The factor is commonly used as a multiplier to the initial lumen rating in illuminance calculations, which compensates for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1.

Light colour is the colour of the light emitted by a lamp. In the CIE normed valence system, it is calculated from the spectral composition and represented by X-Y co-ordinates in a continuous two dimensional diagram. The light colour is characterised by the colour temperature, given in Kelvins.

Light density is a measure of the Impression of the brightness which the eye has of a luminous or illuminated surface. Light density is measured in candelas per unit area (cd/m2 or cd/cm2)

See Luminus Intensity.

Used to calculate or project the performance of a lighting system after a given period of time under certain conditions; includes environmental conditions, such as temperature, voltage, dust and/or dirt and lamp depreciation.

This is a measure of the efficiency of a lamp and indicates how many lumens per Watt a lamp generates. The higher the lumens to Watt ratio, the more energy is converted to light by the lamp.

Unit of measurement of the light flux.

The gradual decline in light output from a light source over time. Due to filament deterioration and bulb darkening.

A measurement of how a lamp maintains its light output over time.

Unit of measurement of the light output.

Luminance indicates the luminous intensity of a light source or an illuminated surface, referred to its observed area. For humans, light is not visible until radiation enters the eye. Luminance is the only variable that can be perceived by humans.

Luminous (also called Light) flux measured in lumens (lm), describes the total light output of a light source.

Luminous intensity is the luminous flux of a light source in a particular direction and not dependent on the size of the recipient. It can be indicated by a vector. It is measured in candelas (cd) and its spatial distribution characterises the light emission of lights and reflector lamps. The shape and symetry of this light intensity distribution determines whether the lights or reflector lamps have low or broad, symetric or asymetric beams. The light intensity distribution is shown in a light intensity distribution curve (LDC) in which the light intensity of a lamp, represented in polar co-ordinates to accommodate the various directions of emission, are connected up to form a curve.

A unit of illuminance equal to 1 lumen per square metre.

The average light output of a lamp over its rated life.

A unit of length equal to one billionth of a metre. The nanometre (nm) is commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light, in particular, ranges from 400 to 1000 nm.

The power rating of lamps, as published by lamp manufacturers.

An inorganic chemical compound processed into a powder and depositied on the inner glass surface of fluorescent tubes and some mercury and metal-halide lamps. Phosphors are designed to absorb short wavelength ultraviolet radiation and to transform and emit it as visible light.

Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye.[1] It is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function (a.k.a. visual sensitivity function) that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function—and others—may also be applied in the same way.

A measurement indicating how efficiently a lighting system is using the power it is drawing. The ratio between power used (watts) and power being drawn from the line (volts-amperes). Expressed as a percent or range from 0 to 1.0. The higher the percent the more efficiently the power is being used. High power factors are ratings of 0.9 (90%) or higher. Incandescent lamps are close to 1.0, magnetic ballasts can be as low as 0.5.

Depending on the optics (lens) used, LEDs emit light at differing radiation angles.

Most electro-optical sensors respond to optical radiations of wavelengths typically comprised between 10-7 and 10-5 m (or photons of energies between 10-18 to 10–20 J), , by converting them into electrical signals, by means of its detector. At each instant, the detector output is proportional to the radiation instantaneous rate, or flux ?, that is incident upon its sensitive area. Consequently, the flux arriving at the detector is the primary parameter to be optimized by the designer of the sensor.

Mixtures of colours which are created with the aid of lamps in the primary colours  red, green and blue. These are particulary suitable for decorative lighting with changing colours. To achive a better white light, an additional white source of light is often used. This is then known as a RGBW colour mix. LEDs utilise the RGB mixing principle to create white light.

See SMT LEDs.

Refers to LED Surface mount technology or Surface mount LED package that has solder pads on the body of the LED package. SMT technology brings some advantages enabling LEDs to be placed with high speed automated pick-and-place equipment, using  reflow soldering process and overall increasing production rates and decreasing production costs

Every wavelength in the visible electromagnetic band has a specific spectral colour. A prism makes the spectral colours of light visible - for example a rainbow is created when raindrops act as prisms.

In optical communications applications, the usual method of specifying spectral width is the full width at half maximum. This is the same convention used in bandwidth, defined as the frequency range where power drops by less than half (at most ?3 dB).

Thru-hole technology refers to an LED package using a lead frame to provide electrical connections. Typical thru-hole LEDs are three and five millimeters in diameter and have a narrow viewing angle and low power-handling abilities due to the lack of a dedicated thermal path. Thru-hole LEDs are installed into holes drilled in the printed circuit board (PCB) and are then placed in a wave solder oven to make connections with electrical connections on the bottom side of the circuit board.

A measure of the distortion of an electrical wave form. Excessive THD may cause adverse effects to the electrical system.

Underwriters Laboratory. Commonly referred to as 'UL'. An independent organisation whose responsibilities include rigorous testing of electrical products. When products pass these tests, they can be labelled and advertised as 'UL Listed'. UL tests for product safety only.

Radiant energy in the wavelength range of about 100 to 380 nanometres (nm). This light is invisible to the naked eye and is also known as black light. See also Visible Spectrum.

Radiant energy in the wavelength range of about 380 to 770 nanometres (nm). The light that can be seen by the naked eye and produces what we also call the 'colour spectrum'.

A unit of electrical power. One watt is equal to one ampere of current flowing through one ohm of resistance.

The amount of electricity consumed by a bulb.

Distance between two successive points of a periodic wave; the wavelength of light are expressed in nanometres.

There are several methods used to produce white light with LEDs and more on the R&D horizon. Most white LEDs are built from a blue LED die covered with special phosphor. In operation, much of the blue light is converted to yellow, and the mixture of blue and yellow appears as white to the eye. Manufacturers provide a range of available colour temperatures by making changes in the phosphor composition, warm-white, neutralwhite and cold-white devices can be created in this way.