Lighting LEDs
> committees of CIE, the international lighting body, Independent lighting designer Kevan Shaw of
are working on methods for the measurement of LED Kevan Shaw Lighting Design agrees: ‘It depends why
performance. you are specifying LEDs. If you are trying to achieve
There is little doubt that white LEDs are an extremely the ultimate in energy efficiency then you are likely to
important source and, as with most new technologies, be disappointed. There are too many people prepared
those technical and quality issues will be largely ironed to accept the marketing hype that LEDs are the “ultra
out and the price will come down with increased efficient light source”. There are an increasing number
introduction. But is it wise to specify them now? Does of applications where LEDs are the right choice –
it make more sense to stick with the tried and trusted emergency lighting, coloured effect lighting, signage,
sources for the time being? to name a few. It is likely that other applications will
‘The use of LEDs for white light is rapidly growing, become appropriate territory for LEDs in time.’
but it needs to be the right application, and they have But actual efficiency is not an issue, argues Peter
to offer advantages compared to other technology,’ Barton, managing director of LED designer and
says LEDs consultant Gordon Routledge. ‘For general manufacturer Forge Europa. It’s more a matter of
lighting, T5 fluorescent and CMH are still tough cost effectiveness in comparison with other sources,’
competition to beat on cost of ownership.’ he says. ‘They are two to three times more efficient >
Key questions to ask about LEDs
What is the hot and cold junction or board temperature
of the LED?
The temperature will have a direct bearing on the life of
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the LED. The cooler the board/junction temperature the
The use of
longer life the product will probably have. The supplier
should advise what the temperature is when tested in
LEDs for white
the lab (cold) and when tested live (hot). It is usual for
light needs to
laboratories to test LEDs at an ambient temperature of
be the right
25 deg C.
application, and
What forward current are the LEDs being driven at?
offer advantages
Life expectancy is usually given in terms of the
over other
percentage of the flux compared to a new LED. L80 – the
life to 80 per cent flux – is typical. Long quoted life may
technologies
be simply the life to a lower flux output (for example,
– Gordon
L50). A standard test method for the measurement
Routledge
of lumen maintenance of LEDs is IESNA LM80 and
is commonly used by the major LED manufacturers. What is the total circuit/system wattage?
Fixture manufacturers should be able to substantiate It is important to understand the real electrical load.
lifetime claims through determining the LED junction
temperature at the highest design ambient temperature What colour temperatures are the LEDs working at?
that the fixture is suitable for. The operating current for The most efficient LEDs are cooler in appearance (5000–
the LED relating to its operating temperature will provide 6000K) but may not be perceived as warm enough in
information on the light output and projected life. the UK and other northern European counties.
What test conditions is the photometry based on? Do they maintain a stable colour over the entire rated
The output of LEDs is dependent on temperature lifetime? How do you know?
and therefore it is important to know the conditions Colour stability is a common problem in lower quality
on which the output is based. Unrealistically low test LED fixtures. It can be a result of poor LED selection,
temperatures in the laboratory will yield a higher output poor thermal management, or both. LEDs should be
than is achievable in practice and the fixture may appropriate for the application (lighting-class LEDs,
therefore not deliver what is expected. not 5mm lamps designed for toys and novelties, for
example), and the luminaire should have a good thermal
Average life expectancy of installed driver? management capacity for the worst-case expected
The driver life should be rated and tested in a similar operating environment. Colour shift is measured
manner to HID ballasts. The life of a driver may have an through Macadam Ellipses and the fewer steps the LED
impact on the life of the luminaire. In other words it may is rated at (for example, two Macadam Ellipses), the less
be the part with the shortest life. As a general rule the variation is likely through life.
cooler the driver and its components are running, the Source: Understanding LEDs, published by the Institution
longer and more trouble free it is likely to be. of Lighting Engineers, www.ile.org.uk
46 CIBSE Journal February 2010 www.cibsejournal.com
CIBSEfeb10 pp44-45,46,48_Leds.indd 46 28/1/10 16:39:31
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