Influence of wavelength red‐shift on color‐rendering index and luminous efficacy of LEDs

Abstract

The peak wavelength of light-emitting-diodes (LEDs) shifts towards the longer wavelengths when the temperature of the LED chip increases. The color-rendering index (CRI) and luminous efficacy may change with the peak wavelength of the LED, and it is not expected to do so when the LEDs are used in fields which require a high CRI and luminous efficacy. A mathematical model of the LED spectrum was used to analyze the influence of the wavelength shift on the CRI and luminous efficacy of three-and four-chip-packaged LEDs, respectively. The results show that the CRI decreases with the peak wavelengths shifting towards longer wavelength for both three-and four-chip LEDs; especially, the CRI of the four-chip LEDs decreases more for a green-color wavelength shift than for other colors. The luminous efficacy increases with a green-color wavelength shift and decreases with a red-color wavelength shift. FIGURE 5 -The normalized CRI as a function of shifted wavelength for the four-chip LED.FIGURE 6 -The luminous efficacy as a function of shifted wavelength for the four-chip LED.FIGURE 4 -Spectral power distribution of the four-chip LED. Journal of the SID 18/12, 2010 1031 13 S. Chhajed et al., "Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes," J. Appl. Phys. 97, 054506 (2005). 14 Y. Ohno, "Color rendering and luminous efficacy of white LED spectra," Proc. SPIE 5530, 88-98 (2004). papers on display devices and systems. She holds three U.S. patents and five Chinese patents. She is a senior SID member. Her recent research interests include optics and optoelectronics, especial display technologies. Jin-Ren Yan is a graduate student at the School of Electronics and Information Engineering, Sichuan University. His major is optical engineering. His recent research interest is information-display technologies. Lei Li is working toward his Ph.D. degree at the School of Electronic and Information Engineering, Sichuan University. He has published four papers. His major is optical engineering. His recent research interest is information-display technologies and optical design. Da-Hai Li is a professor of optics at the School of Electronics and Information Engineering, Sichuan University. He received his M.S. degree from the University of Electronic Science and Technology of China in 1996 and Ph.D. degree from Sichuan University in 2002, respectively. He has published more than 40 papers. He is a member of the Society for Information. His recent research interests include optics and optoelectronics, especially display technologies and optical measurements.Jian-De Zhang is a graduate student at the School of Electronics and Information Engineering, Sichuan University. His major is optical engineering. His recent research interest is informationdisplay technologies.