COLOR RENDERING INDEX For White Light Sources
Color Rendering Index A measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source. Color Rendering Index R a The color rendering index describes how naturally a light source displays a set of different test colors. Reference and test light sources are calculated at the same CCT, measuring shift in chromaticity. Lamps that render color samples similarly will have a high CRI. The lower the CRI, the higher the luminous efficacy (lm/w) of an LED. 8 1 2 3 4 5 6 7
Reference Illuminants (Light Sources) CRI measures the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of the same Correlated Color Temperature. Black Body Radiator: Color Temperatures in the range of 2300K < 5000K Daylight Spectrum: Color Temperatures in the range of 5000K 25000K
Color Rendering Index Color Rendering Index R a Additional six special test color samples 8 1 2 3 4 5 6 7 14 13 9 10 11 12
Color Rendering Index (CRI) Example CRI 50 8500K CRI 70 5000K CRI 95 3500K
Color Quality Rating of Color Rendering Index Definition of Color Rendering Classes for Color Quality
Color Quality Industry Ratings Energy Star SSL Indoor Luminaires: CCT: 2700, 3000, 3500, 4000/4100K and 5000K for commercial CRI: Ra 80 SSL Indoor Lamps: CCT: 2700, 3000, 3500, 4000/4100K and 5000K CRI: Ra 80, R9>0 Designlights Consortium (DLC) Based on lighting application Application Minimum Light Output Minimum Luminaire Efficacy Allowable CCTs (ANSI C78.377-2008) Minimum CRI Outdoor Pole/Arm-Mounted Area and Roadway Luminaires 1,000 lm 60 lm/w <5700K 50 Linear Panels (2x2 Troffers) 3,000 lm 60 lm/w <5000K 80 Four-foot Linear Replacement Lamps 2,200 lm 96 lm/w <5000K 80, R9>0
Limitations of CRI Small number of color samples, which are unsaturated. Test color samples 9-12 are for saturated colors, but they do not contribute to the computation of general CRI. CRI does not reasonably characterize highly structured, narrowband spectral emissions like those from SSL sources (red, green, and blue components) which may increase or decrease saturation of certain colors. One number cannot tell you everything. Cannot fully characterize the multidimensional experience of color such as color appearance, color fidelity, chromatic discrimination, vividness and observer preferences. Mock-ups remain the recommended method of assessing lamp color rendering properties, particularly in color critical applications.
Limitations of CRI Example Same Chromaticity Same CCT Different CRI Different illumination lumens = 31.2 CCT = 3223 x = 0.4217 y = 0.3978 D(uv) = -0.0002 CRI=94 lumens = 217 CCT = 3231 x = 0.4217 y = 0.3987 D(uv) = 0.0002 CRI=1
New Metrics Color Quality Scale (CQS) (CIE TC1-69) Developed at the National Institute of Standards and Technology (NIST) to define and better qualify LED lighting. CQS involves several facets of color quality, including: color rendering, chromatic discrimination, and observer preferences. Main differences from CRI: Takes into account subjective color saturation perception - Gives weight to an increase of saturation addressing perception Measures color fidelity with regard to color saturation preferences Moderate increase of saturation is often correlated with desired color perception. Identifies color fidelity but is only one number!
New Metrics CRI / Gaumut Area Index (GAI) Developed by ASSIST program to give subjective assessments of vividness, naturalness and acceptability. CRI and CRI-like metrics only evaluate a single aspect: fidelity. Good color rendering by a light source requires an optimum amount of color saturation. Gamut area index (GAI) is a good indicator of saturation. This two-metric recommendation using CRI and GAI together can ensure good color rendering. Results are representative of observers of different geographical origins and for sources of warm and cool CCTs, different spectral makeup, and light level. www.lrc.rpi.edu/programs/solidstate/assist/recommends/lightcolor.asp
LED Spectrum is Critical for Interior Design Color Temperature (CCT) is not a perfect measure of lighting quality Color rendering index is another imperfect lighting quality metric, with its emotional impact dependent on the interaction of the illuminated surfaces Lighting spectrum, material textures and optical property interactions influence interior design results Managing the LED color spectrum can create varied emotions from sterile / clean to soft / warm
LED Phosphors
LED Phosphors Entire white CCT-range (2700-6500K) LCW.CC CRI95 2700-4000K LCW.EC CRI82 2700-5000K LCW.PC CRI70 4000-5000K LUW CRI70 5500-6500K
Traditional Light Sources Fluorescent CCT: 3000K CRI: 85 High Pressure Sodium CCT: 1800-2200K CRI: ~20 Tungsten Halogen 3000K CRI: ~95-100
New Techniques For Achieving High CRI
Phosphor-based High CRI Typical CRI of 95 R9 > 90 R13 >90 2700-4000K Indoor Lighting Retail Lighting
Phosphor-based High CRI Warm white (3000K) LED with a typical CRI R a = 95 and very good R 9 and R 13 values.
Efficiency on luminaire level (lm/w) The Efficiency and CRI Dilemma With standard white LEDs it is not possible to realize LED luminaires and lamps with both high efficiencies and high CRI. With the Brilliant Mix concept, high CRI and high efficiencies can be realized! Color Rendering and Luminaire Efficiency* Brilliant Mix Solution *Luminaire efficiency considers thermal, optical and electrical losses
Hybrid Approach - High CRI LEDs Brilliant Mix Concept Mixing of EQ White and amber LEDs enables warm white light sources. Combination of EQ White and Amber LEDs results in efficient light with a high CRI
Hybrid Approach - High CRI LEDs Brilliant Mix Example Combination of Amber (617nm) and EQ White
Hybrid Approach - High CRI LEDs Spectral Power Distribution
Hybrid Approach - High CRI LEDs Brilliant Mix concept R values (2700K) CCT: 2700K