by Franco Musiari [ Assipe ]
Efficacy, power and costs of high brightness LEDs have got levels that make them an attractive solution in many lighting system applications.

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 – see Fig 1 –

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 !

In the comparison table of the different technologies, an analysis more accurate of the different lighting systems has been done, taking into account the whole system that leads to the final luminous yield.
• Source efficacy (lm/W) – it takes into account the real yield of the technology according to the parameters and the opening definitions.
• Electrical source efficacy (%) – it defines possible losses to adapt the standard electrical source (220 V AC) to the considered technology. For example, for the incandescent standard lamps, the connection to the powerline is made without any adjustment. This does not happen to the other technologies such as the fluorescent lamps. They require a ballast that has an energy efficacy of 60-70%. An inverter, for compact fluorescent lamps, has an yield of 80-90% while it is estimated that a LED AC/DC converter can get levels higher than 80%.
• Radiant object efficacy (%) – Generally the lamps have a system that allow them to direct the light beam towards the wanted direction. Even this radiant object has its own performance, which is estimated in the 30-50% range for luminous systems that use light bulbs irradiating towards almost all directions. On the other hand a 95% performance is assumed for LEDs, since the generated beam is much oriented starting from the emission point.
• Total Efficiency (lm/W) – multiplying all the coefficients previously mentioned, medium -medium/high values have been considered obviously, we get the total efficacy of the considered luminous system and we get the lumen quantity that can be achieved for each watt provided to the system.
In the total chain, you can see as the incandescent lamps (with a yield of 7lm/W) are the most inefficient one compared to 38lm/W, which is the value obtained by HPLED.
You can evaluate the consumption that every solution generates in order to obtain a luminous source equal to 800 lumens: 114 watts for old light bulbs, 10.5 watts for LEDs - an energy saving of 90%!

We are not considering another important parameter: the life, to be precise the efficacy time of the considered source, that is reported in the last raw of the comparison table. An assumption of 25.000 working hours for LEDs is realistic, if the generated heat is properly managed . If we suppose using the system for 6 hours per day, they can work more than 11 years.