Evaluation of new color metrics: guidelines for developing narrow-band red phosphors for WLEDs

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are rapidly becoming more popular in the lighting industry due to their energy savings, long lifetimes and environmentally friendly characteristics. The color rendering index (CRI, Ra) and the visual energy efficiency (luminous efficacy of radiation, LER) are the critical criteria to be considered when developing novel red phosphors for use in warm white pc-WLEDs to replace incandescent and fluorescent lamps. In this regard, narrow-band red-emitting materials have been intensively developed in terms of the CRI and LER in an effort to complement the red deficiency of the widely commercialized Y3Al5O12: Ce3+ (YAG) phosphor-based pc-WLEDs. However, CRIs are limited in their inability to guarantee good saturated colors of illuminated objects under a warm white color. Instead of using CRI values as criteria, a two-measure system encompassing the color fidelity score (R-f) and the color gamut score (R-g) was developed and adopted as Illuminating Engineering Society of North America (IES) technical memorandum TM-30-2015 for correct evaluations of the color rendition and to guide the optimization of LED light sources. In this review, we summarize the recent findings on novel narrow-band red phosphors, the improved color and visual energy properties of these phosphors, and their ability to improve the optical properties of corresponding warm white pc-WLED lightings. To solve the complex problem of overestimation of high-CRI values, we discuss the ways in which the narrow-band red phosphors affect the new color metrics (R-f, R-g, and color icon) and the LERs of tri-color pc-WLEDs while varying the narrow scale and the blue-shifted peak position of red phosphors. These new color metrics and LER criteria provide guidelines with which many material and chemistry researchers can develop new red phosphors by optimizing the crystal structure, crystal rigidity, local symmetry, the number of available sites for activators, the selection of the host and activator, and other factors.