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Gareth - can you tell us a bit about your background?
I have gained over ten years of experience in optical physics. Between 2001 and 2005 I undertook studies that lead to a PhD in quantum physics at Southampton University. In 2006, I was appointed Laboratory Manager at an optical testing laboratory, dealing with a wide range of optics applications, including laser machining, environmental testing, optical communications, high speed imaging and optical spectroscopy. I played a leading part in refocusing the laboratory towards the emerging solid state lighting photometric testing market, utilising my knowledge of optics and device physics to build a state of the art, temperature-controlled photometric testing facility. I also developed a curriculum of training courses, inviting trainers to run courses on photometry, and LED luminaire design and thermal management. I am a member of the Institute of Lighting Professionals and a founder member of the Solid State Lighting Metrology Working Group.
Why "Photometric" and "Optical" testing?
The company name was chosen to reflect the breadth of services we offer. We serve both the lighting as well as the wider optical and photonics industries. Beyond the usual photometric testing of lighting products, we are also equipped to measure the photobiological safety of LEDs and lighting and the effective intensity of flashing light sources and visual alert devices. We are also able to measure the optical properties of materials, for example spectral reflectance and transmittance as well as BRDF/BTDF and scatter. We can also provide field normalisation and luminance calibration of camera systems and measure the brightness or reflectance of road traffic signals and signs
Why do many regard LED specifications as works of fiction?
LEDs are mass produced. There is a significant variation in the brightness and colour of the LEDs as they come off the production line. Manufacturers test and group LEDs by brightness and colour (a process called "binning"). However, in general, LEDs are tested under very idealised conditions - a short flash of current is passed and the LED does not have chance to achieve a steady state temperature. When LEDs are clustered into a luminaire or other product, the LED temperature rises above the nominal 25°C and the light output falls. At the 70-80°C operating temperature typical of many light fittings, the LED flux (lumen output) has dropped by 30% compared to the initial level (some people refer to this as "hot lumens" versus "cold lumens"). At the same time, the correlated colour temperature (CCT) of the LEDs rises and the colour rendering (CRI) also changes. Unlike traditional lamps, you have to test the assembled luminaire with LEDs fitted as a finished product - simply estimating luminaire performance based on the output of the component LEDs will be very misleading.
How does Photometric Testing help its clients save time and money?
We provide clients with a choice between standard measurement services or laboratory access. A standard service might involve a goniophotometric test of a luminaire and the generation of a photometric data file in .ldt (EULUMDAT) format. On the other hand, a client may wish to book the laboratory for a day to experiment with different luminaire configurations (for example, testing the output with different LEDs, lenses, reflectors or diffusers). This way, a client not only has access to our equipment but also our expertise. The laboratory service is aimed at those who are developing the next generation of ultra efficient lighting and who value a development partner who not only knows how to measure the product correctly, but who can also advise on design optimisation. This way, our clients can avoid common design mistakes and get their luminaires to market more quickly and with reduced development costs.
Your laboratory is state-of-the-art - why is that important?
For solid state lighting (SSL) to gain market acceptance, it is vital that luminaire light output specifications are accurate. Traditional photometric equipment is known to give large errors when used to measure LEDs and SSL. For example, our integrating sphere is coated with a highly diffuse, 98% reflectance integrating sphere paint. The combination of a spherical surface, high reflectance coating and matte finish gives a near-perfect uniform luminance field within the sphere. That way, the photodetector sees a representative proportion of the light coming from the fitting under test. In comparison, integrating "cubes" (or other geodesic shapes) suffer from hot spots or dim patches - in my experience, such integrators tend to under-record the flux from a source. Not only that, we use a spectroradiometer rather than a filter photometer to measure the light in the sphere. Spectroradiometers do not suffer from the large colour correction errors of simple photometers and are recommended in recent lighting standards, for example IESNA LM79-08 and CIE 127:2007. We also employ auxiliary correction on our sphere to allow for the effects of sample self absorption. Our goniophotometer system is a near-field device. Near-field (or imaging) goniophotometers record the 2D luminance from a light source for each angle of azimuth and inclination. Software then extrapolates the near-field luminance to a far-field luminance intensity distribution. The main advantage of this technique over traditional direct far-field measurements is that we don't have to worry about near-field errors with LED arrays or non-diffused light sources that are measured at an insufficient distance with a far-field goniophotometer.
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