How White LEDs Differ in Color Rendering. A Study by Steve Bush

Aug 15, 2011 Electronics Weekly has recently  published an excellent article by Steve Bush bringing light to all the complicated nuances of white light classifications and color rendering differences that you can observe using different white LEDs.

The author takes time to go back to basics of color rendering explaining the color classification system developed by Albert Munsell, followed by the definition and Color-Rendering Index (CRI), developed in the first half of 20th century, and by the explanation of LED color temperature. He consults with the three leading LED experts to explain how the white lED diodes are classified and what applications hay can be best at.

Author also notes that, power efficiency - which is the main reason LEDs are being introduced for lighting - is good in all LEDs, but falls with reducing colour temperature: from 100-120 lm/W for the best cool whites to 70-90 for the best warm whites. These figures will rise as the technology improves.

Cree has a high CRI brand for light engines and end-products called TrueWhite.
"TrueWhite specifically refers to a technology. It uses a greenish-white from BSY [blue + saturated yellow] LEDs, mixed with red from red-emitting LEDs," said Cree's Scheidt. "It is a very efficient way to create warm white colours and gives a very high CRI - over 90 - with broad spectrum as well as high red content." Cree also produces traditional lighting LEDs using extended-red phosphors to get warm whites. "Some are available giving 80, 85 or 90 CRI minimum," he said.

Lumileds has a twist on this traditional approach with its Lumiramic technology, using a pre-moulded sliver of yellow fluorescent ceramic instead of fluorescent paint. This allows the colour temperature to be precisely controlled during manufacture despite natural LED die variations, as each die gets a custom-selected ceramic sliver on the production line.

Lumiramic has allowed the firm to produce its 'Luxeon A' range which simplify things for customers because there are only tiny LED to LED variations for each colour temperature band.
"We know how the independent parameters of phosphor and die behave over temperature," said Lumileds' Hechfellner. "Luxeon A is product line with CRI options from 80-90."

In conclusion Steve Bush suggests that here are better systems than CRI, but none of them are dramatically better than CRI in most circumstances. And that any system which produces a single number is going to be flawed. He emphasizes that it is necessary to avoid over-specifying warm colour temperature or CRI because you will pay for it in power. He recommends to pick the colour temperature that your customer desires, work out the CRI that your customer needs, and then check for special spectral requirements.  

We highly recommend you to read the original article at:

Images from Wikimedia Commons

Author:  Alice Jones

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