Don’t be afraid, the doctor will explain all…
Many factors limit the efficacy of solid-state lighting
the conditions in which the source will be used. The LED package can be tested in a number of ways. The first is the quick screening test performed on the LED, in which a current pulse is applied to the LED and the light output is measured. This is the ‘cold’ lumen test and results in the highest lumen value that can be achieved. The second way is to apply constant DC current to the LED with the case at a temperature that represents the temperature that may be reached inside a lamp or luminaire. This is the ‘hot’ lumen value and is much closer to the value that the LED package will emit in real world conditions. Specifiers should always make sure that ‘hot’ lumens are used as the starting point for any discussion of luminous efficacy. Of course most LED lamp or luminaire products have lenses or reflectors on top of the LED. These optics introduce losses and so the number of lumens emitted from the LED will not be the same as the number emitted from the lamp or luminaire. The driver circuitry will also introduce losses and further lower the efficacy. So, manufacturers can make a number of different claims, but if specifiers ask the right questions, they should get the right answers. A group of lighting bodies in the UK has developed guidelines for specification of LED products – bit.ly/ledspecs – and I’ll discuss the questions specifiers should ask about product efficacy in more detail in a future column.
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