LED Explained

Light-emitting diode

A light-emitting diode (LED) is a semiconductor light source.  LEDs are used as indicator lamps in many devices and are increasingly used for other lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

When a light-emitting diode is forward-biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.  LEDs are often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching. LEDs powerful enough for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output.

Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology. Infrared LEDs are also used in the remote control units of many commercial products including televisions, DVD players, and other domestic appliances.

Semiconductors

Some insulators, however, can be forced to allow the flow of electrons by applying energy in the form of heat or light. The added energy excites the electrons in the valence band enough for them to jump over the band gap and into the conduction band. These types of materials, called semiconductors, make up most electronic components. One type semiconductor is the diode.

Diodes

Electricity tends to flow in all directions at once. However, electronic circuits require that electrons flow in a certain direction for devices to work. Diodes make this possible by allowing the current to move only in a single direction. A diode acts like a one-way lane; on one direction, the diode serves as a conductor; on the other direction it acts as an insulator and stops the flow of electrons.

What Makes an LED Light Work?

You have probably seen some kind of LED display or an LED light source recently. You may even be looking at one now if you’re reading this on a particular type of computer monitor. LED, which stands for “light emitting diode,” has become a ubiquitous component behind everything from lamps to digital watches, traffic lights, auto headlights, text displays and in a plethora of electronic devices including cell phones and TVs, just to name a few. But behind this seemingly simple device, there lies a complex yet fascinating technology involving the fields of electronics and even advanced physics.

Energy Bands

In order to learn exactly how LEDs work, it is important to first understand the fundamentals. Electricity consists of electrons that can be either particles or waves. Now think of any solid material and imagine it possesses two energy bands: a valence band and a conduction band. For a solid to conduct electricity, electrons must move from the valence band to the conduction band. The difference between a metal that conducts electricity and an insulator, which does not, is that the insulator has a boundary called a “band gap” between the valence and conduction bands. This band gap makes it more difficult for electrons to move to the conduction band, so an insulator generally does not conduct electricity.

Where LED’s Come In

The only thing separating a “normal” diode and the light-emitting variety is the material it consists of. A standard diode has silicon-based semiconductors, while an LED utilizes a combination of elements, among them gallium, indium, aluminum, arsenic, phosphorus and nitrogen. In every diode, every time you apply current in the “proper” direction, electrons move from the valence band to the conduction band. This leaves empty spaces in the valence band, and after a time, electrons will shift back to fill these empty spaces. This shifting process releases energy. Silicon doesn’t lend itself to emitting light; as a result, silicon diodes only give off heat. But with materials such the ones used in LEDs, this shifting of electrons causes the LED to give off light.

Comparison to other lighting technologies

Incandescent lamps (light bulbs) generate light by passing electric current through a resistive filament, thereby heating the filament to a very high temperature so that it glows and emits visible light. A broad range of visible frequencies are naturally produced, yielding a “warm” yellow or white color quality. Incandescent light is highly inefficient, as about 98% of the energy input is emitted as heat. A 100 W light bulb emits about 1,700 lumens, about 17 lumens/W. Incandescent lamps are relatively inexpensive to make. The typical lifespan of an AC incandescent lamp is around 1,000 hours. They work well with dimmers. Most older light fixtures are designed for the size and shape of these traditional bulbs.

Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn emits 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 a fluorescent lamp is much less than its incandescent counterpart, energy is still lost in generating the ultraviolet light and converting this light into visible light. If the lamp breaks, exposure to mercury can occur. Linear fluorescent lamps are typically five to six times the cost of equivalent incandescent lamps 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.  Most fluorescent lamps are not compatible with dimmers. Those with “iron” ballasts flicker at 100 or 120 Hz, and are less efficient. The latest T8-sized triphosphate fluorescent lamps made by Osram, Philips, Crompton and others have a life expectancy greater than 50,000 hours, if coupled with a warm-start electronic ballast. The life expectancy depends on the number of on/off cycles, and is lower if the light is cycled often. The efficiency of these new lamps approaches 100 lumens/W. The efficiency of fluorescent tubes with modern electronic ballasts and compact fluorescents commonly ranges from 50 to 67 lumens/W. Most compact fluorescents rated at 13 W or more with integral electronic ballasts achieve about 60 lumens/W. For comparison, general household LED bulbs available in 2011 emit 64 lumens/W.

LED Lights

An LED lamp (LED light bulb) is a solid-state lamp that uses light-emitting diodes (LEDs) as the source of light. The LEDs involved may be conventional semiconductor light-emitting diodes, organic LEDs (OLED), or polymer light-emitting diodes (PLED) devices. However, PLED technologies are not commercially available. Diode technology improves steadily.

LED lamps can be made interchangeable with other types of lamps. Assemblies of high power light-emitting diodes can be used to replace incandescent or fluorescent lamps. Some LED lamps are made with identical bases so that they are directly interchangeable with incandescent bulbs. Since the luminous efficacy (amount of visible light produced per unit of electrical power input) varies widely between LED and incandescent lamps, lamps are usefully marked with their lumen output to allow comparison with other types of lamps. LED lamps are sometimes marked to show the watt rating of an incandescent lamp with approximately the same lumen output, for consumer reference in purchasing a lamp that will provide a similar level of illumination.

One high power LED chip used in LED lights can emit up to 7,500 lumens for an electrical power consumption of 100 watts. LEDs do not emit light in all directions, and their directional characteristics affect the design of lamps. The efficiency of conversion from electric power to light is generally higher than with incandescent lamps. Since the light output of many types of light-emitting diodes is small compared to incandescent and compact fluorescent lamps, in most applications multiple diodes are assembled.

LED lamps offer long service life and high energy efficiency, but initial costs are higher than those of fluorescent and incandescent lamps. Life cycle of LED lamps is multiple compared to incandescent lamps, however, degradation of LED chips reduces luminous flux over life cycle as with conventional lamps.

Diodes use direct current (DC) electrical power. To use them from standard AC power they are operated with internal or external rectifier circuits that provide a regulated current output at low voltage. LEDs are degraded or damaged by operating at high temperatures, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.

What is Colour Temperature?

Colour temperature is a measure of how warm or cool the light given off by a lamp appears. ‘Warm’ colours appear tinged with yellow and generally feel soft and cosy. Cool colours are tinged with blue and appear whiter, making them a more ‘honest’ and unforgiving light more suitable for working environments than relaxing.

Colour temperature is measured in Kelvin (K). The lower the colour temperature value in Kelvin, the warmer the colour so obviously higher values refer to cooler colours. Confused? Imagine a naked flame, as it’s temperature increases, it’s colour becomes whiter (cooler). A white flame is always a higher temperature than an orange flame.

 

Colour Temperature

Classification

Application

2700K Very Warm White Recommended colour for average household use, similar to standard indoor incandescent bulbs that most people are used to.
3000K Warm White Average colour of halogen lamps. Slightly whiter than incandescent. Often used in white rooms for a clean and modern feel.
3500K Amber White Average coour of fluorescent tubes and CFL lamps.
4000K Cool White More honest and cold light. Ideal for use in areas where clear illumination is paramount, e.g kitchens and offices.
5400K Daylight The lower end of the ‘daylight’ colour temperatures. Representative of natural light on a bright, sunny day. Very cold and generally not used for household applications unless specifically desired.
6000K Cool Daylight Upper end of the consumer daylight colour temperatures. Very white light, approx. colour of daylight simulating fluorescent tubes and CFL lamps.
6500K Specialist Daylight Slightly harsh blue/white light used in specialist applications such as photography and architechural drawing.

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