Light emitting diodes LED, together with increased use of daylighting indoors, can provide energy efficient and healthy lighting systems. The Swedish "oracle" Alf Henriksson claimed that the arrival of artificial or electric lighting was the technical advance that had the greatest effect on people's lives during the previous century. Before that, the possible replacements for sunlight and daylight were far too limited and costly to make possible a demonstrable change in the diurnal rhythm, wakefulness and sleep behaviour. Most work was carried on during the light hours of the day, and when there was no longer enough light, the bed was the only alternative for most people. Especially during the latter half of the 20th century, access to electric light made possible a diurnal activity that could be largely carried on without regard to the position of the sun in the sky.
Beautiful urban environment. Stadshus Bridge in Stockholm with aesthetic lighting by light emitting diodes. Lighting design by Klas Möller.
Increased security. Pedestrian bridge across the E18 motorway at Hägernäs, with Pipeline LED.
When Edison managed to create light by heating a filament, development underwent a paradigm shift which enabled living conditions to change by altering daily conditions. Gas discharge lamps and fluorescent coating then prepared the way for another paradigm shift over the period 1940-2000. Development of the LED light source is another stage in the paradigm shift in the field of lighting. This enables energy use for lighting to be reduced, at the same time as the quality of lighting is enhanced and the quantity of generated light is greatly increased.
Low voltage and colours
Development in the field of LED is expected to bring about further improvements in the near future with regard to light quality and luminous efficiency. Light emitting diodes which provide different well accentuated colours can already very effectively replace other light sources such as neon tubes, fluorescent tubes and incandescent bulbs. Today there is only one firm that produces LED for direct connection to mains voltage. Otherwise it is already possible to regulate and control the quantity and colour of light in a way that is completely different from all discharge lamps.
Evolution of heat
At present, however, a large quantity of the generated photons is absorbed by the conducting material, and this gives rise to serious heating problems. In order to maintain the luminous characteristics and the promised long service life, it is important that certain low temperature levels should be observed, which requires thermal sinks at even relatively moderate powers. Thermal sinks do not absorb any energy. Improved properties will provide higher luminous efficiency and less photon absorption, and thus lower direct heat generation. In the long run this can mean that the heating problem will be almost completely eliminated.
Human perception of lighting by LED depends to a great extent on the type of LED used. Colour temperature – blue, yellow or red hues, colour rendition, good (up to CRI = 100) or poor (CRI below 70). A yellow or red hue with good colour rendition is perceived favourably. Comprehensive research is in progress at present concerning human perception of light from light emitting diodes.
Changeable hues and quality
The semiconductor material from which light emitting diodes are made is produced in large wafers. These are divided into small pieces, each of which gives rise to a light emitting diode. From each wafer, tens of thousands of LED of varying quality and hue can be made. Further development must be capable of producing several diodes of high quality and the same hue from the same wafer. At present, "white light" is produced from fluorescent coating (in the same way as in conventional fluorescent tubes), but the fluorescent radiation consists of photons with wavelengths around 450 nm (1 nm = 10-9 m) which at the same time produces a certain quantity of blue light from the diode. Hue and colour rendering index can vary quite a lot between diodes even when they come from the same source.
Absorption problems
Photons are generated in the semiconductor material and pass through this to come out into the surrounding medium. During this passage a large quantity of photons is absorbed (capture loss); at present, capture loss amounts to more than fifty per cent. Losses also occur in the process that generates the photons. These are however lower than the capture losses. Further development must result in a considerable reduction in both these losses.
Flexible colour and strength. The facade of a museum that can be made to change colour with LED lighting. Photographer: Zumtobel
Ingenious lens optics are needed
If development of the energy saving technology is to be completed, there is a great need for the development of ingenious methods for the design of innovative reflector and lens optics for LED. Development of the light source as such has come much further than light fittings, optics and other fittings technology, something that considerably hampers total development and thus the possible energy savings. Development of optics for road and street light fittings has progressed furthest.
Development potential
Development of LED technology is very promising: Luminous efficiency in excess of 200 lm/W (light is measured in lumen; the best fluorescent tube has a luminous efficiency of about 100 lm/W), colour rendering index CRI of over 90 ( a light source with the best possible colour rendering has CRI = 100). Development is proceeding towards higher quality, lower costs and larger volumes. At present, light emitting diodes of good colour properties and a luminous efficiency of over 100 lm/W are available in the market. There are also LED of high power, several watts.
The diodes are being used over an increasing area for lighting purposes. Luminous efficiency, service costs and the total cost are already sufficiently advantageous in comparison with compact fluorescent lamps in professional applications. In the home environment, owing to their excellent facilities for stepless regulation, LED may in five years become the light source which will, instead of fluorescent lamps, replace incandescent bulbs. Incandescent bulbs will soon disappear completely because of the prohibition by the EU. It is expected that within thirty to forty years light emitting diodes will have outcompeted all other light sources, and will provide the same amount of light as that produced at present, with only ten per cent of the present energy use. Globally, this should mean that the present energy use of about 3000 TWh for lighting purposes will decrease to about 300 TWh. This implies an unprecedented saving and reduced environmental loading, even if the use of lighting increases all over the world. At present, the global need for lighting increases by 2-5 per cent annually. As far as Sweden is concerned, this would represent a decrease from 14 to 1.4 TWh.
The function of light emitting diodes
The development of a completely new type of light source the LED (light emitting diode) which, with regard to performance and service conditions, is completely different from fluorescent tubes and other similar light sources based on gas discharge, may result in the energy use in some lighting installations being reduced by up to eighty to ninety per cent. It is already possible to reduce the use of electrical energy for highly coloured signs by ninety to ninety-five per cent compared with those where the light source is the traditional low pressure fluorescent tube. The way LED generate light is that the stream of electrons flows in a solid body in a higher energy band and then emits this energy in the form of photons (light) of a certain wavelength. When the electrons pass a small gap in the conductor material, energy is emitted down into a lower energy band. The stream of photons generated has a certain wavelength range and the eye perceives the photon stream as a coloured light within a narrow coloured region. Different bands of coloured light can be generated in this way, and if a sign is to have a certain colour, the result can be achieved at a very high efficiency. For example, broad-band "white" light can be produced by making short wave blue radiation "pump" on levels in a phosphor which then fluoresces broad-band white light.
Author
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Nils Svendenius
is Professor of illumination technology at Jönköping University College