LED's - Are they ready to enter the mainstream lighting industry?
The LED is a form of Solid State Lighting (SSL). LEDs are almost universally thought of as being a very recent technology but it might surprise Voltimum users to know that dim yellow light was first produced from a silicon carbide (SiC) semiconductor using electricity in 1907 by Henry J Round. Because of many constraints, this discovery was not really acted upon for several decades until the first commercial LEDs were produced in 1968. However, the first bright LEDs did not appear until the 1980s, and development has accelerated since then to include white light LEDs (WLEDs).
LEDs are essentially microchips but are not made from silicone. Instead, they are made from crystals derived from combinations of inorganic substances (typically gallium indium nitride (GalnN), and gallium phosphide (GaP). They are 'cooked' as wafers in a reactor under high pressures and temperatures. The design of the LED is such that these substances are arranged in ways (layers) that allow light to be emitted when an electric current is applied. This lighting effect is termed 'electro luminescence'.
An LED is a particular type of semiconductor diode, comprising a chip of semiconducting material impregnated with 'impurities' (the materials) to create a structure called a pn junction. A DC electric current of the correct polarity passing through the junction creates charge carriers, so that when an electron meets a 'hole', it falls into a lower energy level. In this way, energy is released in the form of a photon, so light is emitted as electro luminescence.
The wavelength of the emitted light varies according the materials, and there are now many of these being used to obtain the necessary brightness and colour. It is in the chemistry of the material combinations that much progress is currently being made. Different bright, pure colours can be produced depending upon the precise mix of materials. Deeply saturated red, green, blue, ultraviolet, orange and yellow colours are all attainable, as is white with different colour temperatures (eg: warm white and pure white). Colours can be near ultraviolet (UV), visible or infrared.
Despite the colours available, LEDs actually produce narrow spectrum monochromatic light. This means that these devices are around 90% efficient ('efficacy' is the correct measure of efficiency in lighting terms - in lumens/W), many times more efficient than conventional incandescent lamps (a mere 9% or so). This is a crucial benefit bearing in mind the increasing risks associated with man-made global warming. However, it is important to remember that LED lighting may not always be the most energy efficient available; it depends on the specific application, and on the LED colour. For example, the 90% efficiency is only achieved by low output red LEDs. LEDs for general lighting are typically around 40% efficient at best.
The monochromatic light output of LEDs also means that special techniques must be used to produce white light from the coloured output. This is achieved in the following three ways: Firstly, by adding a downshifting yellow phosphor to blue LEDs. This mixes the light to eventually produce white light. Secondly, by combining red, green and blue LEDs so that relative intensity colour mixing produces blended white light. Lastly, it is achieved by introducing phosphors to ultraviolet LEDs to produce visible white light.
Gallium nitride, with various doping materials added, and / or a phosphor coating, is one way of producing WLEDs, but some recent WLEDs have used no phosphors. Instead, they make use of homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate. This allows emission of blue light from its active region and yellow light from the substrate at the same time.
It should be noted that LED efficacy can vary greatly. For example, low light output LEDs (such as for instruments and back lit displays) have a higher efficacy than bright ones. Efficacy also depends on the colour, as already stated.
White light from LEDs has already been possible for many years but early on, the resulting light was of poor quality and not very bright. The 'holy grail' of LED lighting technology is to produce a WLED that will eventually be bright enough and cheap enough to replace many conventional lamps.
LEDs are extremely compact. This means that to produce enough light for general lighting applications, they have to be made in clusters - usually called 'arrays'. Although LEDs are highly efficient, some heat is still produced, and in arrays, this heat will need to be conducted away. In fact, high output LEDs can produce a lot of heat, and for these, thermal management can be problematic in keeping the LEDs sufficiently cool to stay healthy.
Also, to achieve the best lighting performance from LEDs, dedicated fittings are necessary. This, in the long term, should be a significant benefit in itself, because there are major and exciting possibilities available to lighting designers and architects, as well as for end-users.
Semiconductors based upon special organic materials can also be used to make OLEDs. These hold great promise but are less highly developed, though some in the lighting industry think that OLEDs may be even more exciting longer term than 'conventional' LED lighting.