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GENERALINTEREST More Light! Practical high-power LEDs By Professor M. Oßmann In recent years the LED has seen many improvements so that we see them fitted in more and more situations where filament lamps were traditionally used. They have many advantages and their excellent reliability is often an important consideration. `power' of a light source is expressed in lumens. The lumen (lm) is a photometric unit for the luminous flux from a light source. The term photometric indicates that it takes into account the spectral response of the human eye and so gives an indication of how bright the source will be perceived. Figure 1 shows the (daylight adapted) relative spectral luminous efficiency of the light wavelength to stimulate the eye (also known as the V-lambda curve). The eye is most sensitive to light with a wavelength of 555 nm (giving a relative efficiency of 1 on the curve). A monochromatic light source emitting at this frequency with a radiant flux of 1 W is equivalent to 683 Lumen. If we could keep the same output power but change the frequency of the light to 650 nm (red) the lumen calculation now becomes: 683 × 0.107 = 73 Lumen because at this frequency the relative efficiency has fallen to 0.107. Infrared or ultraviolet light sources emit their energy at a frequency outside the eye response curve so the lumen value will be zero. The luminous flux is a measure of all the light emitted from the source irrespective of direction. Lamps are also classified in terms of their luminous performance or efficacy. This parameter indicates how LED technology has come on leaps and bounds in the last few years and recently some of the higher output devices have become available commercially. Component supplier Conrad Electronics (www.int.conradcom.de) kindly donated samples for use in producing this article. Electronic engineers are not necessarily lighting specialists so this article sets out to fill in some of the technical background and also explore a couple of simple circuits to drive a 1-watt high power LED. Lumens or watts ? In the electrical world we are familiar with the watt as a measurement of power. Transmitters or IR lasers for example, all have their outputs defined in watts. The equivalent 38 Elektor Electronics 9/2003 GENERALINTEREST much light is produced for how much electrical energy consumed and is expressed in Lumens per watt (lm/W). This calculation always takes into account the power consumed by the complete lighting unit including electronic ballasts etc that are necessary for some types of lighting. Table 1 shows a comparison of some common lamps. It is interesting to see the spread of figures for different types of lamp technology. The lowpressure sodium lamp holds the record for the greatest efficacy and its unflattering yellow light is a Candela Take a look at any LED data sheet and you will soon come across the expression Candela (cd) rather than luminous flux when the light output of the device is quoted. The can

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