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Eyes On... Eyes On…

Photometric Testing

A1 Lighting talks to Dr Gareth John, Technical Director at Photometric Testing about the testing of lighting products.

A1: Photometric Testing has established itself as a trusted provider of precision measurements for the lighting industry. How has the team made such a name for itself? Dr Gareth John: Our laboratory has been operational since September 2011, but our team has worked in the light measurement field since 1989. I previously spent 6 years serving as laboratory manager at another lighting test laboratory so I have practical experience in how to test both traditional and next-generation solid state lighting (LED) fittings. I am told that I have an eye for detail and I pride myself on understanding our clients’ requirements and delivering an expert, timely and cost effective service. Our board of directors includes Pro-Lite’s Robert Yeo, who has 23 years experience in photometry and is able to bring his expertise to bear in defining best practice for our quality management systems. Robert also delivers our regular photometry training programmes, which our clients value for helping them to understand LED specifications and to learn how to measure and express the output of luminaires and other LED-based products.

A1: How has the testing of lighting products changed over time?

GJ: The basic principles remain the same. Clients still require so-called “photometric


data” for their luminaires. This is a goniophotometric measurement of luminous intensity over all angles, with the data compiled into one of two standard file formats, either .ies or .ldt (EULUMDAT). This data is used to calculate the number and spacing of fittings required in a particular scheme to achieve the desired illuminance. The main difference today is the shift to testing LED lighting – this requires a great deal of care to ensure that measurements are made once the fitting has reached operating temperature. LED output goes down with temperature, so we avoid making tests straight from cold. The other difference is the need for “absolute” rather than “relative” photometry – LEDs are generally integral to the fitting and you cannot reliably scale the luminaire’s output for different LEDs, meaning that you have to test the fitting as a whole.

A1: Your facilities are state of the art, how important is this to the testing process? GJ: Very. No client will be happy if our test results under-state the performance of their fitting, which would be the case if we hadn’t invested in a state-of-the-art facility and the very latest photometric test equipment. Our dark room is painted with low reflectance paint, equipped with baffles for stray light control and air conditioned to

a constant 25°C. Our integrating sphere (which we use for measuring the total luminous flux of lamps, LEDs and fittings) is spherical and painted with a 98% diffuse coating – this ensures that the highly directional readings that you get with non- spherical integrators with lower reflectance paints are avoided. We perform auxiliary correction to ensure that we allow for the error from self-absorption of the luminaire in the sphere. All tests that we perform are based upon spectroradiometric measurements – avoiding the errors associated with cheap lux meters.

A1: Testing LEDs can occasionally arrive at misleading results. What does Photometric Testing do to ensure that LED specifications are accurate?

GJ: The biggest source of frustration for our clients is not understanding that the rated flux and colour temperature of the component LEDs does not reflect in the overall lumen output of the luminaire. Component LEDs are tested and binned in production at a junction temperature of 25°C, whereas in a typical fitting the operating temperature may be as much as 80°C. With LEDs, as temperature rises lumens go down – CCT also shifts. Estimating luminaire lumens based on the rated flux of the LEDs does not take into

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