How to Develop White Light Recipes with LED Phosphors

White LEDs are engineered using a combination of blue LEDs and various red, yellow, and green phosphors to achieve specific color coordinates and color quality ratings. To achieve high-quality white light, white LEDs use multiple phosphors to convert blue-LED “pump” light into a broader spectrum that closely resembles the reference curve for the targeted color temperature. There is a large set of possible phosphors that can be blended together to achieve a desired white SPD. Examples of individual phosphor SPDs are shown in Figure 1. The phosphor engineer has a large palate of color components to work with.

 The general method of developing a white LED is as follows.

1. Find a phosphor mixture that “hits” the desired color point. This might have one blue LED and a number of phosphor components.
2. Analyze the TM-30-18 data for this result and determine what color components need to be adjusted to better achieve the CRI specification.
3. Adjust the phosphor mixture and iterate to an acceptable result.

 Phosphors can be characterized by their peak wavelength and a width parameter, usually full width half max (FWHM). They can also be characterized by the phosphor’s color point expressed in CIE 1931 x and y coordinates. Figure 2 shows this representation on the CIE 1931 diagram.

 The color point of a mixture of light sources is subject to metamerism. There are an infinite number of SPD combinations that can achieve a specified color point if the light sources have variable spectra, but if a precise phosphor blend is specified and the SPDs are fixed, there is only one solution that can achieve a desired system color point. Normal production variations result in a distribution of color points and manufacturers use binning based on color perception to group parts into specified ranges.

Figure 1. Phosphor emission SPDs.[1]

The equations below show the method to calculate the color point of a mixture using the CIE x, y coordinates and the lumen output of each source. This example is for a three-component system where the subscripted L is lumens, and x, y are the CIE color coordinates of each source. This equation can be expanded for any number of sources.

Figure 2. CIE 1931 diagram illustrating the variety of phosphor color points that are available to achieve a specified white LED color point. Increasing the lumen contribution of a component pulls the color point towards the color point of that component. This information is combined with the data in a TM-30-18 report to develop a phosphor recipe meeting color temperature and color quality goals.

White LED phosphor recipe development involves blending a phosphor mixture to achieve the desired color point (as shown in Figure 2) and the desired CRI specification.

 If a phosphor engineer wanted to improve the 70 CRI part shown in Figure 3, it is immediately apparent that adding red is needed and the “cyan gap” needs to be closed by adding shorter wavelength green phosphors. However, after adding red and green, all other values shift. Phosphor tuning is an iterative process of making small changes and evaluating each step using color quality data. The visualizations (Figure 1, Figure 2, and Figure 3) are all used during this process.

Figure 3. Comparison of the TM-30 measurements for 70 and 95 CRI, 5000K COBs. The reference curve for higher CCT components uses an idealized solar spectrum which in this case is called D50.

For more information about KSF Phosphors and Luminus’ LUX Series COB LEDs with KSF phosphor technology, read the white paper:  LUX Series COBs: Achieving High CRI Lighting with High Efficacy Using Luminus LUX Technology.

REFERENCES:

[1]       He, G., and Yan, H., "Optimal spectra of the phosphor-coated white LEDs with excellent color rendering property and high luminous efficacy of radiation," Optics Express 19(3):2519-2529, 2011. DOI: 10.1364/OE.19.002519

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