LEDs (light emitting diodes) have been used in electronics for a good three decades. In fact they are everywhere now. As well as using up low levels of energy, LEDs cope consummately with shocks and vibrations – and they last a long time. On the downside however, until a few years ago blue LEDs did not even exist and luminous power was relatively weak. Also, LEDs did not yield much light making it difficult – or even unthinkable – to use them in large areas. Until recently.
Relatively powerful LEDs – offering up to 130 lm/chip – have been on the market for around five years, but now there are a variety of monochrome LEDs (red, green, blue etc) as well as white-light LEDs yielding high levels of luminosity. Put enough of these types of LEDs together and you can now illuminate practically anything. White-light LEDs in particular are an attractive option for illuminating objects. Apart from the electrical and optical benefits of white light (light yield, in lumens per watt of power), this is due to two key factors. The first is luminous color. For this we use a reference point, taken as the light spectrum of a black light source at a given temperature. Accordingly, the luminous color of white LEDs falls into three main areas:
The second factor affecting light quality is the Color Rendering Index, or CRI. This shows the color rendered by the object being illuminated. The reference point for this is sunlight with an index of 100 CRI. Artificial light sources can not exceed 100 CRI. The CRI of today’s LEDs can be as high as 93, the equivalent to the color rendered by a top-of-the-range fluorescent tube or halogen lamps (which use much more energy).
From an economical point of view, electrical and optical effectiveness is the main motivation when using LEDs, apart from the initial outlay and light quality. Halogen lamps achieve luminous flux levels of around 20lm/W whereas modern LEDs range from 50-80lm/W although fluorescent tubes can go as high as 100lm/W. Under laboratory conditions, however, LEDs do hit more than
100lm/W, making them the most efficient technology we have to create light.
The only problem is, the cost of the types of LEDs needed to provide sufficient illumination under modern conditions exceeds the purchasing cost of fluorescent systems. Nevertheless, depending on the application, there are times when it really does make sense to use LEDs rather than conventional lighting systems based on fluorescent tubes – despite the initial outlay. There are a number of reasons for this, including:
Staff at the Heilbronn-based Steinbeis Transfer Center for Applied Electronics have been working on the development of customized LEDs for many years, as well as related power electronics. The graph on the left shows a lighting unit developed by the transfer center using LED technology instead of conventional T5/35W fluorescent tubes. When you plot the illumination provided by the LED unit on a 3D graph, the illumination that would be provided by a fluorescent tube is shown in blue. As the graph shows, LED illumination is no longer second-best compared to fluorescent tubes, especially in terms of effectiveness and yield.
The illumination unit itself – as well as the performance electronics needed to work the unit – is highly compact and thanks to the effectiveness of both, units do not overheat. Once special lenses are added to the LED illumination unit, lighting can be optimized on a minimum amount of power. In fact, with the right lens the amount of light lost can be minimized making it possible to design the LED unit any way you want. It is even possible to achieve the levels of performance previously only offered by systems equipped with fluorescent tubes – and sometimes even outperform them.
Of course there is still plenty of work to do in developing high efficiency LEDs, but progress is rapid. In the years to come we should expect to witness a rich variety of highly interesting and innovative LED lighting solutions – all of which would have been scarcely possible (if at all) using previous technology.
Prof. Dipl.-Ing. Manfred Dorsch
Dipl.-Ing. (FH) Thorsten Gatzka
Steinbeis Transfer Center Applied Electronics (Heilbronn)
stz58@stw.de