Chromaticity Adrift: Understanding LED Color Stability

23 Pages·2015·2.17 MB·English

by  Michael Royer

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
installation
C
D
• 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
chromaticity diagram.
0.530
• As with MacAdam Ellipses, Δu'v' does
not convey the direction of a shift
' 0.525
v
• 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' approximately circles in
Locus
r = 0.007
0.54
the 1976 (u', v')
0.53 chromaticity diagram
Iso-CCT (±0.02 D )
uv
0.52 • A 1-step ellipse is
approximately equal to
0.51
a Δu'v' of 0.001
' 0.50
v
• ANSI definitions of
0.49
white light allow for
0.48
7-step MacAdam Ellipses fairly large tolerances
For CFLs
0.47 (~14-step difference
ANSI C78.376-2001

from edge to edge)
0.46 SSL Quadrangles
ANSI C78.377-2011
Daylight
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
u'
6
Is color shift a problem for LEDs?
L Prize
LED A19 c. 2009
0.0070
0.0060
0.0050
0.0040

'
v
'
u
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' 0.525 New v' 0.525
4,000 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