New GELcore phosphors improve color-rendering index

For white LED lamps that use phosphor downconversion of blue or UV light, the development of new phosphors is a key requirement to enable improvements in color temperature and color rendering, and to open up new markets for LEDs.

Aug 31st, 2004
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Several speakers at the recent Fourth International Conference on Solid-State Lighting from GE Global Research and GELcore described efforts to develop improved phosphors, notably GELcore's phosphor technology manager Emil Radkov, who unveiled a range of new phosphor blends that he described as having "unprecedented" performance.

4-phosphor blend
Radkov reported one specific white LED with an output of 25 lm at 23 lm/W, which had a color-rendering index (CRI) of 97 and color temperature of 3000 K. The figure shows the spectrum and CRI values of this LED. The color coordinates were x = 0.441 and y = 0.410, and the contribution of LED light to the spectrum was less than 2%.

Compared with deluxe linear fluorescent lamps, which have a broad spectrum, the combination of a blue LED with a YAG phosphor has distinct deficiencies in the violet and blue-green regions. Both these types of lamp are also deficient in the red region, which makes them poor at rendering deep red colors.

Compared to a traditional blue LED with YAG phosphor, the combination of blue, cyan and orange phosphors pumped with a near-UV chip represents a major improvement in lumen output at color temperatures below 4000K, while further CRI enhancement is achieved by adding a fourth phosphor emitting in the deep red region.

The highest CRI value for commercial white LEDs is ~90, which falls short of the value of 100 defined for incandescent lamps. Radkov described a range of white LED blends with four-color phosphors, which had CRI values exceeding 95 over the entire range of color temperatures from 2500 to 8000 K.

In some cases the CRI values (particularly the R9 component, which refers to the rendering of deep red colors) can approach the theoretical maximum value of 100. Although there is a trade-off between luminous efficacy and color rendering, improvements continue to be made to both parameters through optimization of phosphor blends.

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This is an extract from a longer article originally published in the October 2004 issue of Compound Semiconductor magazine.

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