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138 TECHNOLOGY / LEDS


At the recent IALD Enlighten Americas conference in Montreal, Tad Trylski raised a few eyebrows... and laughs... with his presentation challenging the confusing messages coming from manufacturers regarding LED performance.


LIES, DAMN LIES... AND PHOTOMETRICS


Sales professionals call it ‘spec-dumping’ – a litany of numbers, technical attributes, reeled off in the hope of convincing a pro- spective client that a product is measurably superior to others. Generally, it’s consid- ered poor sales technique. Nevertheless, we’re seeing a conspicuous resurgence of the spec-dump in our industry, seemingly connected to the march of new technology and its new parameters. We have taken a step backwards in how we quantify and specify lighting as we re-learn the technol- ogy. The trouble is that we, lighting designers, are complicit, if not largely responsible for this state of affairs. We ask the questions we think we’re supposed to ask. Sales peo- ple assume that we know what we’re talking about so they investigate the answers; the ones that look good become the headlines. So the vicious circle of collective deception ensues. We begin to compete using mea- sures which we have become conditioned to care about. We have developed a common-law under- standing of photometrics that owes as much to folklore as to rigorous science. Miscon- ceptions, legends and urban myths abound. The measures themselves are all familiar: lumen output, colour temperature, colour rendering, efficacy, beam angle, lamp life and, most fashionable of all, MacAdam ellipses - a 70-year-old concept but unheard of outside academic circles until LEDs came along in the last few years. Now it’s a met- ric of quality, yet still poorly understood. It’s not the only one, though. Measures which were once clear and well defined - designed to measure the performance of lamps and reflectors - have now become misleading indicators of performance. We are learning that two LEDs of the same colour temperature won’t necessarily be the same colour; that two fittings with the same lumen output and LOR may appear very different in brightness and that two luminaires with the same quoted beam angle could have widely differing light distribution.


The fact is, you can’t define a colour with a number. Plotting a CCT of 3000K, for ex- ample, on a colour chart, gives a line - not a point - which covers many colours; anything from light pink to pale green. Colour ren- dering, expressed as CRI, gives no indication of the ability to discriminate colours (as demonstrated in a recent academic study). Similarly, quoted beam angles tell us only one thing - how far from the peak of the beam is it half as bright. Nothing more. In fact, one of the major lamp manufacturers developed and published their own mea- sure, VBA, in an attempt to provide a better idea of the visible beam angle one would perceive. Measurement of light output, in particular, has inconsistencies when applied to LEDs - be they discrete devices or complete ‘light engines’; relative or absolute photometry in which a number of testing standards has been applied. Yet this is perhaps the fierc- est specification battleground. Manufactur- ers will invest significant effort to outper- form each other. The results can be mildly ridiculous: one major luminaire manufactur- er published design data for its downlights which included products boasting light output ratios up to 111%. When considered as a ratio or efficiency measure, such figures appear to be in the fantasy realm of the perpetual motion machine - something for nothing. It takes a very close analysis - usually in the differ- ence in laboratory conditions between the testing of the LED and the testing of the luminaire - to discover the source of the magical extra lumens. The ambiguities in measurement and in- terpretation give rise to another time-hon- oured, sales and marketing technique - that of ‘specmanship’. A major lamp manufacturer boasts a 25- year lifetime for its MR16 LED replacement - printed on the box with an asterisk to tell the customer this is based on the device burning four hours per day. The specifica- tion sheet for the same product calculates the annual cost saving... based on twelve


hours per day. It’s not uncommon, also, for manufacturers to create new metrics. A major luminaire manufacturer has gone beyond lumens per watt to give us comparisons in lux per watt. Why not? Their mathematics are entirely correct... if applied to a single spotlight, but don’t stack up for a room full of downlights. None of these companies lacks technical ex- pertise; all are respected for their R&D and generally their integrity is not questioned. Nevertheless, without stricter standardi- sation, the numbers can be used to tell a story. Politicians quote statistics, lighting people quote photometrics. The shortcoming of many of the measures in use today is that in attempting to quantify light, they fail to quantify what we see. What does CRI 85 look like? It is amusing how obsessed we have become by the numbers when, in fact, the base unit of all light measurement, the Candela, is not entirely accurate and, indeed, the only one of the seven SI base units derived from subjective human response (from experi- ments conducted in the 1930s). Taking numerical obsession and specman- ship to an extreme, we are occasionally con- fronted with specification sheets that might be described as the ‘edited highlights’ from a family of products - highest light output, warmest colour, longest lamp life... Anyone who ever played Top Trumps in the school playground will remember that there was always one card to have - the best one in the pack - but never one that was unbeatable. You could never have a car that was the best for speed, acceleration, fuel efficiency and price. As a child, I knew which car was the best from the cards in front of me and the one I wanted to have. As an adult, I make a more careful, reasoned assessment. When it comes to specifying LEDs, is it possible that we are still in the Top-Trumps phase of our lives? Tad Trylski is an independent lighting designer - tad@trylski.co.uk.


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