1

Michael Royer, PNNL

May 4–7, 2015

Lightfair

Chromaticity Adrift: Understanding LED Color Stability

2

What metrics are used to describe color shift/color stability?

A

B

C D

1-step

2-step

3-step

MacAdam Ellipses

• Experimentally-derived indicators of human color vision tolerances at various chromaticities

• Based on the observations of one highly-trained observer in a very specific scenario/apparatus; results cannot be translated to every installation

• Sometimes referred to as SDCM (standard deviation of color matching)

• Can be reported in multiples (e.g., 3-step ellipse)

• Do not convey the direction of shift/difference

3

0.510

0.515

0.520

0.525

0.530

0.535

0.540

0.250 0.255 0.260 0.265 0.270

v'

u'

Δu'v'

• Δu'v' is the Euclidian distance between two sets of chromaticity coordinates in the CIE 1976 UCS chromaticity diagram.

• As with MacAdam Ellipses, Δu'v' does not convey the direction of a shift

• Δu'v' does not convey whether or not a difference is noticeable

What metrics are used to describe color shift/color stability?

4

0.44

0.45

0.46

0.47

0.48

0.49

0.50

0.51

0.52

0.53

0.54

0.55

0.56

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

v'

u'

Δu'v'

r = 0.007

7-step MacAdam Ellipses For CFLs ANSI C78.376-2001 SSL Quadrangles ANSI C78.377-2011

Iso-CCT (±0.02 Duv)

Daylight Locus

Black Body Locus

• MacAdam ellipses are approximately circles in the 1976 (u', v') chromaticity diagram

• A 1-step ellipse is approximately equal to a Δu'v' of 0.001

• ANSI definitions of white light allow for fairly large tolerances (~14-step difference from edge to edge)

What metrics are used to describe color shift/color stability?

Δu'v' and MacAdam Ellipses

5

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.

6

Is color shift a problem for LEDs?

CALiPER

c. 2008

0.44

0.46

0.48

0.50

0.52

0.54

0.18 0.20 0.22 0.24 0.26 0.28

v'

u'

7

4

ANSI C78.377-2011 chromaticity bins

Black body locus

7

Is color shift a problem for LEDs?

F4

Avg.

C12 A12

Q7

0.0000

0.0010

0.0020

0.0030

0.0040

0.0050

0.0060

0.0070

0 5,000 10,000 15,000 20,000 25,000

Δu

'v'

Hours of Operation

L Prize

LED A19 c. 2009

8

Is color shift a problem for LEDs?

GATEWAY Smithsonian

LED PAR38 c. 2011

Image Courtesy of Scott Rosenfeld, Smithsonian American Art Museum

9

Is color shift a problem for LEDs?

New 4,000

0.510

0.515

0.520

0.525

0.530

0.535

0.540

0.240 0.245 0.250 0.255 0.260 0.265 0.270

v'

u'

New

4,000

0.510

0.515

0.520

0.525

0.530

0.535

0.540

0.240 0.245 0.250 0.255 0.260 0.265 0.270

v'

u'

B1 B2

Bare Lamp Bare Lamp with Secondary Optics Removed

GATEWAY Smithsonian

LED PAR38 c. 2011

7 4

7 4

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?

GATEWAY Smithsonian

LED PAR38 c. 2011

Image Courtesy of Scott Rosenfeld, Smithsonian American Art Museum

12

Is color shift a problem for LEDs?

http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/2013_gateway_color-maintenance.pdf

13

14

Is color shift a problem for LEDs?

0.0000

0.0010

0.0020

0.0030

0.0040

0.0050

0.0060

0.0070

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

Δu

'v'

Hours

LED

CFL

Halogen

CMH

CALiPER Series 20

LED PAR38 c. 2012

15

Is color shift a problem for LEDs?

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

Δu

'v'

Hours

CALiPER Series 20

LED PAR38 c. 2012

Most lamps rated

lifetime = 50,000 hrs

16

Is color shift a problem for LEDs?

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Δu

'v'

Hours

LED

CFL

Halogen

CALiPER Retail 3

LED A19 c. 2013

17

Is color shift a problem for LEDs?

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Δu

'v'

Hours

CALiPER Retail 3

LED A19 c. 2013

Most lamps rated

lifetime = 25,000 hrs

18

Is color shift a problem for LEDs?

Parametric failure criterion (0.007)

0.000

0.005

0.010

0.015

0.020

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Δu

'v'

Hours

CALiPER Retail 3

LED A19 c. 2013

Most lamps rated

lifetime = 25,000 hrs

19

Is color shift a problem for LEDs?

U.S. Border

LED Area Lighting c. 2014

20

Is color shift a problem for LEDs?

U.S. Border

LED Area Lighting c. 2014

0.49

0.50

0.51

0.52

0.21 0.22 0.23 0.24

v'

u'

LM-79 Test Data (Spec)

2,500 Hour In-Situ (Field)

5,000 Hour In-Situ (Field)

7

4

21

Is color shift a problem for LEDs? Smithsonian (Again)

∆u'v' = 0.006

22

What to do?

• LED Packages

• Lamp and Luminaire Materials (Optics)

• Driver

• Application considerations

• HEAT!

23

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