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A response to 'The year of the LED lighting fixture'


Gary Heald, Projection Lighting and Roger Sexton, Xicato questions Dr. Geoff Archenold's views on Remote Phospher Technology.

Dear Editor,

Responding to Dr. Geoff Archenhold’s article in mondo*arc issue 47, and particularly to the reference to the AlphaLED module:

• We agree with the disciplined approach of checking specifications of LED luminaires and the adoption of a UK Government strategy similar to the US DOE’s Caliper program. To these ends Projection Lighting’s AlphaLED luminaires have been submitted to the Lighting Association for EST approval.

• We agree with providing lighting specifiers with a complete set of realistic data, for example lumen outputs. Publishing ‘cold’ instead of ‘hot’ data is incorrect since it does not reflect performance at actual operating conditions and as such has caused confusion and possibly even aversion. It is for that reason that Xicato specifies a complete set of product data at 70°C, which is the recommended test point temperature on the module. Projection’s AlphaLED luminaires are designed at an ambient of 30°C to run at these conditions.

• Challenges in perfecting Remote Phosphor based LED modules to ensure CCT and CRI consistency are correctly defined and these are exactly what Xicato (the module manufacturer) is focused on. Xicato uses a combination of technologies to ensure that each product emits uniform and consistent light with the right CCT and CRI values, one of them being a controlled phosphor deposition process: Xicato’s proprietary Cold Phosphor Technology. On top of that, and more importantly, it keeps the phosphor cold. Dr. Archenhold correctly states that phosphor efficiency and lifetime degrade as temperature increases.

Due to the unique integral thermal design of the modules, the heat generated by the phosphor when it converts high energy light to lower energy light is instantly removed from the phosphor itself resulting in a lower operating temperature of the actual phosphor. Phosphor temperatures are significantly below 100°C (as low as 70°C) while regular white LEDs operate the phosphor at an estimated temperature well above 100°C (up to 140°C). The combination of both is what Xicato calls Cold Phosphor Technology and it ensures the best possible colour consistency: within the products, product to product and over time.

Moving to results - thousands of modules have been produced within a colour point specification of almost 2 SDCM, with CRIs over 80 or over 90, depending on the specification of the module. 100% of the Xicato modules are tested for flux, CRI, CCT and DUV using integrating spheres and spectrometers, and cumulative results are available to customers as part of their specification check list. All the test equipment is calibrated against NIST standards and Xicato always verifies test results independently.

Since lifetime is expected to be 50khrs, performance over time needs to be addressed. Xicato uses the most stringent accelerated lifetime testing copied from the automotive industry: WHTOL (Wet High Temperature Operating Life test at 85°C & 85% Relative Humidity; power cycle 1 hr. on / 1 hr. off). The reason this is done is because the combination of both temperature and humidity accelerates degradation. Results have now exceeded 3000hrs and degradation is less than one percent in our worst case. US Energy Star requires less than 8.2% for indoor applications at 6000hrs, which is of course at regular operating conditions.

Finally thermal analyses confirmed by actual testing for all AlphaLED luminaires using a combination of CAD and Thermal Simulation software  optimises integrated thermal design of the AlphaLED luminaire in such a way that module test point temperatures of 70°C are achieved in an ambient of 30°C. This can be directly related to, and is well below, the max LED junction temperature. This results in the junction temperature being lower than those prescribed by LED manufacturers with respect to their L70/B50 figures, ensuring the life time of the LED themselves as thus the fixture.

We believe, stemming from the above technological details, that the AlphaLED luminaire range offers a unique lighting solution, and that Dr Archenhold is too conservative in estimating that it will take one or two years before  “...we will see LED fixtures accelerate into the general lighting market”. Viable LED luminaires are available today.

Regards,

Gary Heald, Projection Lighting
Roger Sexton, Xicato

 

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