Chromaticity Adrift: Understanding LED Color Stability
Michael Royer, PNNL Lightfair
May 4–7, 2015
1 What metrics are used to describe color shift/color stability?
MacAdam Ellipses • Experimentally-derived indicators of human color vision tolerances at various chromaticities 1-step • Based on the observations of one 2-step highly-trained observer in a very 3-step specific scenario/apparatus; results A cannot be translated to every C D installation • Sometimes referred to as SDCM (standard deviation of color B matching) • Can be reported in multiples (e.g., 3- step ellipse) • Do not convey the direction of shift/difference
2 What metrics are used to describe color shift/color stability?
0.540 Δu'v' • Δu'v' is the Euclidian distance 0.535 between two sets of chromaticity coordinates in the CIE 1976 UCS 0.530 chromaticity diagram. • As with MacAdam Ellipses, Δu'v' does
not convey the direction of a shift
v' 0.525 • Δu'v' does not convey whether or not a difference is noticeable 0.520
0.515
0.510 0.250 0.255 0.260 0.265 0.270 u'
3 What metrics are used to describe color shift/color stability?
Δu'v' and MacAdam Ellipses
0.56 • MacAdam ellipses are 0.55 Black Body Δu'v' Locus r = 0.007 approximately circles in 0.54 the 1976 (u', v') 0.53 Iso-CCT (±0.02 Duv) chromaticity diagram 0.52 • A 1-step ellipse is 0.51 approximately equal to
a Δu'v' of 0.001 v' 0.50 • 0.49 ANSI definitions of white light allow for 0.48 7-step MacAdam Ellipses fairly large tolerances For CFLs 0.47 ANSI C78.376-2001 (~14-step difference
0.46 SSL Quadrangles from edge to edge) Daylight ANSI C78.377-2011 0.45 Locus
0.44 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 u'
4 When does color shift become noticeable?
The million dollar question… It depends. Viewer Field of View Surface Characteristics Proximity Time
In MacAdam’s experimental setup, a just noticeable difference was determined to be three times the standard deviation of color matching (or a 3-step ellipse) for a given observer. However, a 1-step ellipse is often called a MacAdam unit of color difference.
5 Is color shift a problem for LEDs? CALiPER c. 2008 0.54
4 7 0.52
0.50
' v
0.48
0.46
ANSI C78.377-2011 chromaticity bins Black body locus 0.44 0.18 0.20 0.22 0.24 0.26 0.28
6 u' Is color shift a problem for LEDs? L Prize LED A19 c. 2009
0.0070
0.0060
0.0050
0.0040
u'v' Δ 0.0030 F4
0.0020
Avg. 0.0010 Q7 C12 A12 0.0000 0 5,000 10,000 15,000 20,000 25,000 Hours of Operation
7 Is color shift a problem for LEDs? GATEWAY Smithsonian LED PAR38 c. 2011
Image Courtesy of Scott Rosenfeld, Smithsonian American Art Museum 8 Is color shift a problem for LEDs? GATEWAY Smithsonian LED PAR38 c. 2011
B1 0.540 B2 0.540
0.535 0.535 4,000
0.530 0.530
' ' v v New 0.525 4,000 0.525 New 4 4 0.520 7 0.520 7
0.515 0.515
0.510 0.510 0.240 0.245 0.250 0.255 0.260 0.265 0.270 0.240 0.245 0.250 0.255 0.260 0.265 0.270 u' u'
Bare Lamp Bare Lamp with Secondary Optics Removed
9 Is color shift a problem for LEDs? GATEWAY Smithsonian LED PAR38 c. 2011
Image Courtesy of Scott Rosenfeld, Smithsonian American Art Museum 10 Is color shift a problem for LEDs? GATEWAY Smithsonian LED PAR38 c. 2011
Image Courtesy of Scott Rosenfeld, Smithsonian American Art Museum 11 Is color shift a problem for LEDs?
http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/2013_gateway_color-maintenance.pdf
12 13 Is color shift a problem for LEDs? CALiPER Series 20 LED PAR38 c. 2012
0.0070
0.0060
0.0050
0.0040 u'v' Δ 0.0030
0.0020 LED CFL 0.0010 Halogen CMH 0.0000 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 Hours
14 Is color shift a problem for LEDs? CALiPER Series 20 LED PAR38 c. 2012
0.007
0.006
0.005
0.004 u'v' Δ 0.003
0.002
0.001
0.000 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 Hours Most lamps rated lifetime = 50,000 hrs
15 Is color shift a problem for LEDs? CALiPER Retail 3 LED A19 c. 2013
0.007 LED 0.006 CFL Halogen 0.005
0.004 u'v' Δ 0.003
0.002
0.001
0.000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Hours
16 Is color shift a problem for LEDs? CALiPER Retail 3 LED A19 c. 2013
0.007
0.006
0.005
0.004 u'v' Δ 0.003
0.002
0.001
0.000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Hours Most lamps rated lifetime = 25,000 hrs
17 Is color shift a problem for LEDs? CALiPER Retail 3 LED A19 c. 2013
0.020
0.015
u'v' 0.010 Δ Parametric failure criterion (0.007)
0.005
0.000 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Hours Most lamps rated lifetime = 25,000 hrs
18 Is color shift a problem for LEDs? U.S. Border LED Area Lighting c. 2014
19 Is color shift a problem for LEDs? U.S. Border 0.52 LED Area Lighting c. 2014
LM-79 Test Data (Spec) 0.51 2,500 Hour In-Situ (Field)
5,000 Hour In-Situ (Field)
' v
4 0.50 7
0.49 0.21 0.22 0.23 0.24 u'
20 Is color shift a problem∆u'v' = 0.006for LEDs? Smithsonian (Again)
21 What to do?
• LED Packages • Lamp and Luminaire Materials (Optics) • Driver • Application considerations
• HEAT!
22 What to do?
• Determine if color shift is a concern, and to what degree. – Will early failures be a financial concern? – Is mitigation possible? • Seek data (LM-80, LM-84 are only current relevant standards) • Look for products with a warranty (there are few) – Understand what monitoring/measurement is necessary
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