Light emitting diodes (LEDs) have been used in electronic systems for many years. There is nothing new about the use of LEDs as panel display lights or even in optical fibre communications.
But the use of higher power LEDs as general purpose lighting sources, which have the potential to replace filament bulbs and even fluorescent tubes, has now become one of the most interesting component markets in the industry.
Firms have been designing LED lighting systems into buildings and public arenas for around five years. But advances in technology, which have steadily boosted power efficiencies and improved design flexibility, have promoted more widespread use of LEDs as creative lighting in place of traditional filament bulbs and fluorescent tubes.
“If it lights up you can use LEDs, if it is colour then definitely use LEDs,” says Colin Beale, technical sales manager for opto semiconductors at Osram.
The use of LED lighting inevitably started with higher cost applications, such as prestige buildings and rock concert stages, where the colour design features of LEDs were the main attraction. “LEDs give designers new ways of using lighting,” says Gordon Routledge, managing director of Dialight Lumidrives, the Yorkshire-based LED lighting system design firm.
“The market is looking at alternative ways of lighting. This is because it sees lighting as being more important in peoples’ lives,” says Graham Cathrall, UK sales manager at distributor EBV Elektronik, which offers a range of LED lighting products.
“That is the Holy Grail for manufacturers, that white LEDs will enter the traditional consumer lighting markets,” says Allan Morris from PixelRange.
“We need to be careful here,” points out Routledge. “Where’s the value-proposition? That’s the issue with white light LEDs. Lighting in an undersea vehicle, for example, there is a value-proposition, it is reliability and extra cost is not an issue.”
“But it is hard to say that the general replacement of white lighting with LEDs will never happen,” adds Morris.
Routledge also believes the domestic LED lighting market will happen but it is likely to be driven by the manufacturing of lower cost LEDs in China. “The Chinese took the cost out of compact fluorescent lights and they will do the same in the white LED lighting market. I won’t say the domestic market won’t happen, but for us it is a long way down the list,” adds Routledge.
Colin Beale from European manufacturer Osram has a warning. “Not all LED technologies are the same and it is not as easy to make as it may first appear. We are already seeing lower standard products from the Far East and they will fail,” says Beale.
So if the emergence of a domestic lighting market for LEDs is likely to be the domain of lower cost products manufactured in China, it is the high value architectural lighting markets where the interesting technology and product developments are already making a commercial impact.
“There are two markets for LED lighting, the professional systems market and the high volume consumer market,” says Routledge.
One example of a professional systems market, says Beale, is in retail store lighting where LED lighting systems can be used to subtly control the mood lighting. “We are changing the construction of the LEDs to address the quality of the white light they produce. So with LEDs you can change the tone and colour of the white lighting throughout the day to attract customers,” says Beale.
But this is still a specialist application where cost and power consumption is not the issue. “Yes, it can be done today with fluorescent tubes and would be easier with LEDs, but cost and power can still be restrictive,” says Beale.
So how efficient are LEDs in comparison to fluorescent tubes?
Efficiency is typically measured as the total quantity of light emitted by a device against the power consumed and it is usually quoted as a figure in lumen per Watt (lm/W). The most efficient light sources are fluorescent tubes which can be as high as 100lm/W.
In comparison white LEDs in production today have an efficiency of around 40-60lm/W. “So today white LEDs will use more power than fluorescent lights to produce the same light,” says Beale.
But manufacturers are improving the efficiencies of LEDs all the time. Today white LEDs can match the efficiencies of halogen lamps and they are starting to rival compact fluorescent lights (CFLs).
LEDs also have the advantage of being able to produce a light which is less diffuse than with fluorescent tubes.“Within two to three years LEDs will start to rival efficiencies of linear fluorescents,” says Beale.
“But cost per lumen is another issue. Here the LED will never match that of the fluorescent,” adds Beale.
Although this starts to look more attractive if you are interested in the total lifetime cost of the product. LEDs are inherently more reliable and longer lasting than fluorescent tubes. This is one reason why LEDs are commercially attractive for use in a growing number of professional lighting applications.
LED manufacturers can quote lifetimes of over 50,000 hours, but when heat management of the devices is taken into account in the design this lifetime figure can be doubled, which corresponds to more than 11 years of continuous use.
One development is tuning the wavelength of the LED so that the colour of the light can be modified. Another feature of this is that the wavelength can be adjusted to maintain efficiency of the LED over time and across different temperature ranges. “Temperature control is particularly important and there is a lot of work going on right now in driver circuit design,” says Routledge.
With the wide range of LED lighting products available on the market designers need to be careful in selecting the right device for their application - they need to read data sheets carefully. In practice LEDs start to degrade as soon as they are turned on.
This should be quantified on the data sheet, but it is worth remembering that the large print figures on the front of the data sheet may not give the true efficiency figures of the operating device. “So the message for designers is to read the small print on the data sheet,” says Beale.