Color Consistency

Willem Sillevis Smitt, XicatoLight and Building, March 2016

"The color of the object illuminated partakes of the color of that which illuminates it" 

- Leonardo Da Vinci

Lighting Aspects of Object Rendition

Saturated Colors• Color Rendering• TM30, IES CIE

Whites or light colors• White point consistency• MacAdam ellipses, SDCM (Standard Deviation of Color

Matching)

For Lightly Colored Surfaces Color Consistency of the Light Sources is Critical

On a white wall, light is just like paint

PART 1: Framework for Describing White Light Consistency

Quantifying White Points and Color

Do you know which colors these are?

(0.4599, 0.4106)(0.4369, 0.4041)(0.4053, 0.3907)(0.3804, 0.3767)

Quantifying White Points and Color

Do you know which color these are?

(0.4599, 0.4106)(0.4369, 0.4041)(0.4053, 0.3907)(0.3804, 0.3767)

CIE 1931, “x,y” coordinates

Or these?

(0.2625, 0.5274)(0.2505, 0.5214)(0.2357, 0.5113)(0.2251, 0.5015)

Quantifying White Points and Color

Do you know which color these are?

(0.4599, 0.4106)(0.4369, 0.4041)(0.4053, 0.3907)(0.3804, 0.3767)

CIE 1931, “x,y” coordinates

Or these?

(0.2625, 0.5274)(0.2505, 0.5214)(0.2357, 0.5113)(0.2251, 0.5015)

CIE 1976, “u’, v’” coordinates

Quantifying White Points and Color

Do you know which color these are?

(0.4599, 0.4106)(0.4369, 0.4041)(0.4053, 0.3907)(0.3804, 0.3767)

CIE 1931, “x,y” coordinates

Or these?

(0.2625, 0.5274)(0.2505, 0.5214)(0.2357, 0.5113)(0.2251, 0.5015)

CIE 1976, “u’, v’” coordinates

How about these?

2,700K Duv 0.0003,000K Duv 0.0003,500K Duv 0.0004,000K Duv 0.000

CCT and CIE 1960 DuvMore intuitive metric for white

Quantifying White Points and Color

CIE 1931, “x,y” coordinates CIE 1976, “u’, v’” coordinates

0K 2,500K 5,000K 7,500K 10,000K 12,500K

-0.006

-0.003

0.000

0.003

0.006

Correlated Color Temperature

Duv

Simple metric specific for white

Quantifying White Points and Color

0K 2,500K 5,000K 7,500K 10,000K 12,500K

-0.006

-0.003

0.000

0.003

0.006

Correlated Color Temperature

Duv

Duv positive: Greenish / Yellowish

Duv negative: Pinkish

Quantifying Color Differences

• MacAdam ellipses• u’v’ CIE 1976 space

More up to date:• SDCM (Standard Deviation of

Color Matching)

McAdam Ellipses Around 3,000K

2800K 2900K 3000K 3100K 3200K

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

2McAdam3McAdam5McAdam

CCT

Duv

0.24 0.245 0.25 0.255 0.260.51

0.515

0.52

0.525

0.53

2McAdam3McAdam5McAdam

V’

U’

CIE 1976, “u’, v’” coordinates CCT and CIE 1960 Duv

Acceptable and Unacceptable Color Differences

2800K 2900K 3000K 3100K 3200K

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

1x2McAdam2McAdam3McAdam5McAdam

CCT

duv

Red Zone:>4 McAdamNot fit for most lighting applications

Yellow Zone:2-4 McAdamNon-critical lighting applications

Green zone:1x2McAdamFor accent and architectural lightingThis corresponds with +/- 0.001Duv and +/- 50K

Energy Star du’v’ 0.006

Conclusions of Part 1:

• CCT – Duv is a more intuitive framework for describing whites and differences in white than

CIE 1931 (x,y) or CIE 1976 (u’,v’)

• 1x2McAdam ellipses is a necessary requirement for non-observable white differences in

accent and architectural lighting

• 1x2McAdam ellipse corresponds with +/- 0.001 Duv and +/- 50K (around 3,000K)

Recommended specification for color consistency:

1x2 McAdam ellipse

Or

+/- 50K CCT AND +/- 0.001Duv

PART 2: Important Steps to Achieve Color Consistency

Step 1: Product Design and Process Control

• Accurate matching of primaries• Keep if consistent over the life of the product

Color Consistency Over the Life of the Product

• Results from external LM-80 testing on Xicato Modules

– At maximum rated current and temperature– 10,000 hours

• Plot shows individual parts at 0h and 10,000h – (0 and 10,000h connected by a line)

• Worst case shift:– CCT +37K– Duv +0.0016

2900K 2950K 3000K 3050K 3100K

-0.004

-0.002

0

0.002

0.004

CCT

Duv

5 Year Field Implementation Evaluation

• Replaced Luminaires after 5 years (2011 – 2016)

• Re-measured the modules and compared to new

• Color shift after 5 years:

– 20-30K

– 0.00056 – 0.00063Duv

St John’s BlackheathFixtures Mike Stoane Lighting

Good Product Design and Process Control

• Provides accurate colors when new

– Within 1x2McAdam

• Keeps colors stable over time

• Enables color warranty

• Enables color stability projections in terms of B0

– B for part of population outside spec

– 0 for no parts out of spec

Recommended specification for color consistency over time:

Must be based on B0LED Module supplier must provide color warranty

Color shift after 50,000h < 3McAdam Ellipse

Step 2: Accurate Testing - Problem

• Problem: standard LED production

testers and procedures not accurate

enough

• Example: LED manufacturer lists +/-

0.007 (x,y)

• Standard tester accuracy alone is far

outside green zone and even into the red-

zone– Not fit for critical (and non-critical)

illumination!

2800 2900 3000 3100 3200

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

LED Manufacturer +/- 0.007 x,y

CCT

duv

Step 2: Accurate Testing – Problem Fixed

• NIST characterizes references sources with

sufficient (great!) accuracy

• With careful testing and procedures, sufficient

testing consistency can be achieved

– CCT: ~ +/- 20k

– Duv: ~ +/- 0.0005

– 10x better capability in Duv and CCT!2800 2900 3000 3100 3200

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

NISTLED Manufacturer +/- 0.007 x,yImproved Tester

CCT

duv

Recommended specification for tester consistency:

CCT < +/- 20CCTDuv < +/- 0.005

Step 3: Improved Calculation of Data (Colorimetry)

• Color calculations done with Color Matching

Functions (CMFs)

• Standard color matching functions can give

inaccurate results

– Two sources have the same test result, but

appear different

• Can be fixed with improved CMFs

– Schanda, Csuti, Harbers - 2011

Can we Make Testing and Calculations Perfect?

• Testing and color quantification done based on “standard observers”

• Many proof-points that we see color similarly, but not the same

Victoria & Albert Museum, LondonDHA DesignsMike Stoane Lighting