Solid State Lighting: Using Light Emitting Diodes, Energy Efficiency & Renewable Energy, US Department of Energy, 2009
from http://www1.eere.energy.gov/buildings/ssl/using_leds.html




TERMINOLOGY

Breakdown voltage: It is a parameter of a diode that defines the largest reverse voltage that can be applied without causing an exponential increase in the current in the diode. As long as the current is limited, exceeding the breakdown voltage of a diode does no harm to the diode.

Candela: The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelenghts, also known as the luminous efficiency function). A common candle emits light with a luminous intensity of roughly one candela. The candela is sometimes still called by the old name candle, such as in foot-candle and the modern definition of candlepower.

Color rendering index: The color rendering index (CRI) sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source.

Color temperature: Is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, and other fields. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black- body radiator. The temperature (usually measured in kelvins (K)) at which the heated black-body radiator matches the color of the light source is that source's color temperature; for a black body source, it is directly related to Planck's law and Wien's displacement law.

Constant current: Is a term most often used in electronics to describe a system that can vary the voltage across an electronic circuit to maintain a constant electric current. LEDs usually take their drive from a constant dc-current source to maintain constant luminescence. An important usageof constant current power supplies are LEDs that have very dynamic electrical resistance and optimally must be operated within a short range of currents.

Correlated color temperature: The correlated color temperature (Tcp) is the temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under.

Current limiting resistor: LEDs are current-driven devices, not voltage driven. Although drive current and light output are directly related, exceeding the maximum current rating will produce excessive heat within the LED chip due to excessive power dissipation. The result will be reduced light output and reduced operating life. LEDs that are designed to operate at a specific voltage contain a built-in current-limiting resistor. Additional circuitry may include a protection diode for AC operation or full-bridge rectifier for bipolar operation. The operating current for a particular voltage is designed to maintain LED reliability over its operating life.

Electrical polarity of LEDs: LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.

Heat dissipation: When many LEDs are mounted into a small area, heat generation must be taken into consideration. If there is a possibility that the ambient temperature may exceed 60 degrees centigrade, some kind of forced cooling system will be needed. The ambient operating temperature must be taken into consideration when a product/ system is being designed. There are certain limits to maximum current, at certain temperatures which must be kept in mind.

Illuminance: In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception, measured in lux (lx) or lumens per square metre (cd.sr.m-2).

LED current overdriven: Driving a LED above a specific point will shorten the useful life. You may also get a maximum rating for either current or forward voltage. Exceeding those ratings will dramatically shorten the useful life, generally ending it suddenly in the process. But driving an LED “hot” will make it burn more brightly for a shorter time. The heat dissipated by the junction has to be conducted through the leads, which aren’t very big and aren’t designed to heatsink the package. When the junction runs hot, the light output will also degrade much more rapidly. LED won’t light up at all without being overdriven somewhat.

Lifetime and failure: Solid state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Typical lifetimes quoted are 25,000 to 100,000 hours but heat and current settings can extend or shorten this time significantly. The most common symptom of LED failure is the gradual lowering of light output and loss of efficiency.

Lighting efficiency: Artificial light sources are usually evaluated in terms luminous efficacy of a source, also sometimes called overall luminous efficacy. This is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. It is also sometimes referred to as the wall-plug luminous efficacy or simply wall-plug efficacy. The overall luminous efficacy is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the “luminosity function”). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called overall luminous efficiency, wall-plug luminous efficiency, or simply the lighting efficiency.

Luminance: Is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2).

Luminous efficacy: Is a figure of merit for light sources. It is the ratio of luminous flux (in lumens) to power (usually measured in watts). As most commonly used, it is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source provides visible light from a given amount of electricity. This is also referred to as luminous efficacy of a source.

Luminous flux: The SI unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. In other systems of units, luminous flux may have units of power.

Luminous intensity: In photometry, luminous intensity is a measure of the wavelength- weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

Reverse breakdown voltage: The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse. Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. The cause of damage is overcurrent resulting from the diode breakdown, not the voltage itself. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may be protected by placing a diode (or another LED) in inverse parallel.

Surface-mount technology (SMT): Is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices or SMDs. In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.

Thermal runaway: When a junction gets warmer, the current through it at a given voltage will increase. The increased current in turn heats the junction further, and the problem gets worse. Eventually, if nothing limits the current, the junction will fail due to the heat. Because of thermal runaway, it’s important to use some current limiting circuit even with a regulated voltage source.



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