Design Approaches for High-CRI LEDs

Ninety-CRI LEDs have been about 20% less efficient than 80-CRI, limiting adoption. Here are two approaches to closing that gap.

White LEDs are typically fabricated by combining blue LEDs with yellow, green, and red phosphors. Historically, cool white LEDs were the first to reach the market, for applications in which high CRI was not a requirement. These LEDs were made with only yellow phosphors and had a bluish-white tinge, in the CCT range of 5,000 to 6,500 K. As illumination applications became more prevalent, LEDs with a warmer white appearance in the CCT range of 2,700 to 4,000 K became available. These warm white LEDs used a combination of yellow and red phosphors to achieve the desired CCT and minimum 80 CRI but with an efficiency penalty when compared to the cool white LEDs.

Figure 1. Lumen loss of current red phosphors in warm white LEDs

Figure 1 shows the spectrum for a typical 2,700 K, 80-CRI white LED (green line) and the human photopic response (blue dashed line). Although the addition of conventional red phosphors enables warmer CCTs and higher CRI than yellow phosphors alone, there is significant lumen loss since much of the red phosphor emission occurs outside the human eye's sensitivity. To make a 90-CRI LED, the typical approach is to use an even redder phosphor to reach the higher CRI, but with even less emission in the eye's sensitive region. As a result, a 90-CRI LED at 100 lm/W is about 20% less efficient than an equivalent 80-CRI LED.

One approach to improve the efficiency of the 90-CRI LED is to focus on phosphor improvements such as the introduction of a narrow (< 50nm FWHM) red phosphor. Figure 1 also shows the modeled spectra of a 2,700 K, 90-CRI white LED (red line) that includes a narrow (30 nm FWHM) red phosphor. The narrower red phosphor increases the lumen efficiency by substantially reducing the emission outside the eye's sensitivity range. By optimizing the material quality, the wavelength peak, and the width of the red phosphor's emission, up to 20% gains in LED efficiency may be achieved.¹ This approach is expected to benefit all L1 LEDs including single emitters such as LUXEON TX, LUXEON 3535L, or LUXEON COB.

Another approach is the to use direct red LEDs in combination with white LEDs to create a hybrid LED module with high efficiency and high CRI. A direct red LED has a significantly narrower emission than a red phosphor, leading to a similar reduction to the lumen loss shown in Figure 1. In 2013, as part of a DOE SSL Product Development project, a hybrid LED module was demonstrated by Philips Lumileds with 712 lm, 140 lm/W, CRI = 91, and R9 = 75 operating at 85°C.² The direct red wavelength (610 to 615 nm) was selected for optimal efficiency and R9 impact, and an off-white LED was used to provide the green part of the spectrum. The combination of red and white LEDs does result in more complicated system designs, but it can provide the added benefit of color tunability or integrated color control.

Approaches for efficient 90-CRI LEDs

High-CRI technology	Examples	Typical specs at 85°C	Features
High-CRI phosphor	Luxeon TX, MP 3535L, COB	95–100 lm/W, CRI = 95, R9 = 80	Simple configuration Narrow red phosphor improves efficiency Large efficiency penalty w/ conventional red phosphors
Direct red and off-white	Hybrid LED for DOE SSL Product Development project	140 lm/W, CRI = 91, R9 = 76	High efficiency w/ high CRI Color tunable High LED and system cost

The table summarizes these two technical approaches towards enabling high-efficiency and high-CRI LED solutions for solid-state lighting. As these technologies mature, we expect to see increasing adoption of 90-CRI solutions as the color quality benefit begins to exceed the efficiency tradeoff.

References

1. O. Shchekin, J. Bhardwaj, "LED technologies for Breakthrough in performance and cost," China SSL 2012


2. G. Huang, W. Soer, G. Vu, F. Jin, "High Power Hybrid Warm White LEDs for Illumination," poster presented at DOE SSL R&D Workshop, Tampa, FL (2014)