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Luminous Efficacy

The luminous efficacy of a light source is the ratio between the emitted luminous flux and the amount of the absorbed energy to transmit it. It is expressed in lumen/watt (lm/W). The lumen, the measure unit of the luminous flux, is equal to the luminous flux detected in a solid angle of one steradian and emitted towards all directions from a source with a luminous intensity of 1 candle. (the steradian –sr symbol- is the SI unit of solid angle and it is defined as " the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r ²". Since the area of the sphere is 4πr², as a consequence the solid angle subtended by the whole sphere is equal to 4π sr).

Moreover, the luminous flux is defined on the basis of the subjective perception of the medium human eye and it corresponds to a particular curve inside the spectrum of the visible light. A light bulb emits radiation even outside the visible spectrum, (usually in the infrared and in the ultraviolet wavelength), which does not contribute to the brightness perception. A lamp has a higher luminous efficacy as much as it is able to emit a spectrum suitable for the human vision.

At the beginning of the nineties, when LED began to be produced their luminous efficacy was comparable to the one of the light bulbs but, above all, the luminous power that they were able to emit was a few milliwatt. For this reason they were used only as indicators and warning lights in small systems of light indication.

The technology has made big steps from then on the emitted power - today high power LED are available (HPLED -High Power LED) - with powers of 1, 3 and 5 Watt, but above all they have efficacies ten times more than the incandescent light bulbs.

As you can see on the above graph, HPLED - with luminous efficacy between 80 and 100 lm/W -are in the market by now and the laboratories have already produced components -between 150 and 160 lm/w - that will be in the market within one or two years. If we consider a 60 W standard light bulb, with an efficacy of 15lm/W, we get a luminous source of 60 • 15 = 900 lumens. If we suppose using 3 Watt HPLED, with a luminous efficacy of 100 lm / W, we can get the same luminous capacity using 3 of these LEDs ( 3 • 3 • 100 = 900 lm), but using only a total power of 9 watt instead of 60 W of a standard light bulb. An 85% cost saving !

Read more here: http://dev.emcelettronica.com/luminous-efficacy

Renewable & Alternative Energy Resources: http://www.alternativeenergybase.com

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