LEDs and Getting Color Right

Megan Carroll, LC, IESNA

Xicato Director of Sales, Northeast

Color science: describing Color and Color rendering

Color science applied: getting Color right with LEDs

Checklist when specifying LED luminaires

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Chromaticity spaces

Color points

Consistency of Color points

Color rendering

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Light

Object

Observer

Why is Color?-Dr. Mark Fairchild

“Our world is color coded so that all living

creatures know what or whom to attract, what to

eat, when to be afraid and how to behave”

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Objects can modify incident light in various ways

Reflection

Transmission

Emission

Object properties to

consider

Reflection

Transmission

Absorption

Scattering

Surface Texture

Fluorescence

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Observer Metamerism

Biological factors

Genetics

Age

Cognitive factors

Color AppearanceObserver 2 no match

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Color

Discrimination

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Cognitive Factors Color is an interpretation of the stimulus we see, the biological response

• Different Color appearance: ‘simultaneous contrast’• There are many other appearance phenomena, eg Chromatic Adaptation, Helmholtz-

Kohlrausch Effect and Hunt Effect

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Power as a function of wavelength

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• If Colored lights are

mixed, the result will

always be brighter than the

individual component

Colors

• If the primary Colors, red,

green and blue are mixed

in the correct intensities,

the ultimate result will be

white

• Yellow, magenta and cyan

are called the secondary

Colors

Based upon theory of additive Color mixing its possible to map every

Color within the gamut defined by the primary Colors

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Under a set of fixed viewing conditions, Colors can be matched,

quantified and compared but limitations apply

CMFs

Light spectrum

CIE XYZ

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After Judd and Wyszecki, 1975

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1931 x-y

• 2D via mathematical

transform of CIE XYZ

to xy (x+y+z=1)

• Chroma (from a

chromaticity point),

hue, lightness

• Color mixtures

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Black body An ideal object that absorbs all electro-

magnetic radiation falling on it

Its SPD, and hence color, depends only on it’s temperature

Black body locus (BBL) It is the path that the color of an

incandescent black body would take in a particular chromaticity space

CCT The temperature in [K] that a “black

body” must reach in order to produce the same color as the given source

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+ve Duv = Color point above BBL-ve Duv = Color point below BBL0 Duv = Color point on BBL (only CCT sufficient)

The distance from a given Color point

to that of a blackbody radiator of the same

CCT in the 1960 uv chromaticy space is

called Duv.

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MacAdam Ellipse

Region on the

chromaticity diagram

which contains all colors

which are

indistinguishable to the

“average human eye”

SDCM

1-step : 68.26%

2-step : 95.44%

3 – step : 99.44%

• MacAdam ellipses

in u’v’ CIE 1976

space

• SDCM: Standard

Deviation of Color

Matching

Δu’v’ = √(u’0–u’1)² + (v’0-v’1)²

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• Color rendering is the ability of a light

source to provide information on the

Color of illuminated objects

• To provide information on an object a

light source must either:

1. Display all Colors naturally

2. Make Colors easily distinguishable

3. Display selected vividity

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Index measures change in chromaticity of selected color

patches between test source and an ideal color source

for naturalness (Planckian radiator or reconstituted

daylight illuminant)

Test color patches. Calculation in 1960 uv color space

R1 – R8 used to calculate CRI or Ra

• Averaged – doesn’t tell you about specific Colors

• It is a measure of naturalness based on datums from stored mental

connections. Sometimes controlled vividity needed.

For the above GAI is a more appropriate metric

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– The area of the polygon

formed by the same set of

Colors in a given Color

space – here 1976 u’ v’

– Correlates well with increase

in chroma (saturation/

vividness)

• Gamut Area Index

– GAI = GASx100

GABB

• GAI & CRI• Position and shape of gamut area denotes where saturation

occurs

• Should be used with CRI: low CRI and high GAI will give overly

saturated and unnatural appearance

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• Halogen

• LED module Artist

• LED module Standard

• CFL

• Compact Metal Halide

• LED module Vibrant

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CRI Ra=99

R9=100

Gamut Area CRI: 99.1

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CRI Ra=77

R9=-3

GAI: 87.5

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CRI Ra=88

R9=0

GAI:113.2

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CRI Ra=98

R9=93

GAI: 100.7

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CRI Ra=83

R9=18

GAI: 96.4

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CRI Ra=84

R9=38

GAI:110.6

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CRI Ra = 96

R9= 98

GAI: 126.3

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Check at what temperature module performance is quoted

Check initial and maintained tolerances on Color point

Check Color rendering properties across all 15 CIE test Colors and

GAI

• LEDs shift in Color with current and temperature

• Not all products behave in the same way

A, B & C product examples, operated from low to max

operating current & temperature

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LM-80-08 requires 6000h CIE 1976 data

White LED Supplier A CCT 3000K, Tj~112C,

test time 6000hr 1000mA

Remote Phosphor Module Supplier B

CCT 3000K, Tj~117C, test time 4000hr

1000mA, 2000lm

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2200K 2700K 3000K 4000K

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LRC (2004)

2-step MacAdam ellipse: ”…when these

fixtures are used to illuminate an achromatic

(white) scene. Accent lighting a white wall

and lighting a white cove are some

examples.”

4-step MacAdam ellipse: “…when these

fixtures are used to illuminate a visually

complex, multicolored scene. Lighting a

display case and accent lighting multicolored

objects or paintings are some examples.”

Recommended Research for 2- Step Spaces

Color consistency maintained:

1 x 2-step initial, < 3-Step maintained.

Acceptable?

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•Phosphor on

chip LEDs

•Halogen

•CMH

•CFL

•SDW

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Color Consistency: Getting it Wrong

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Ra R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15

LED 81 80 85 89 81 78 80 86 66 16 64 79 58 81 93 75

LED 98 98 99 98 98 98 97 98 98 96 99 98 88 98 98 98

A

A

B

B

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Ra CRI Examples

95-100 Excellent Galleries, graphics arts industry, fashion shops, boutiques,

hotels, homes

90-94 Very Good High end shops

80-89 Good Offices, most shops, transport hubs, leisure

70-79 Fair Warehouses, amenity, residential outdoor

<70 Poor Motorways, garages

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A

B

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LED A LED B

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• ≥110 GAIBB and look at area shape/position in Color space

• ≥80 CRI

• Sometimes a need for vivid blues, whites, and reds, without excessive enhancement

• Balance (higher CCT has a higher GAI)

Vividness

•Applications: Retail: fashion, toys,

cars. Galleries: some contemporary

art

•Application need: bold and distinct

target colors, crisp whites

Naturalness

•Applications: retail: fresh foods, general

department stores

•Application need: flowing, natural,

balanced colors. Harmony is better than

divergence

Vivid / Natural

Continuum

Mix in an installation, e.g. for general and accent

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LED source LM-80 report for the same drive current and case

temperature (or higher) used in the luminaire. Some reports may be at a relatively low case temperature (TC) such as 55˚C, while in the

luminaire it may be operating at 85˚C.

Color consistency (initial and maintained) specification, supporting

reliability data, and warranty. Make sure the Color consistency specification is suitable for the application.

Color rendering properties across all 15 CIE test Colors, including

saturated ones. For some applications check gamut area or GAI. Having CRI alone (Ra) will only communicate the ability to render 8 CIE pastel Colors).

Working production sample of the module in the luminaire that

corresponds to the LM-79/LM-80 report. This ultimately is the route to qualitative and quantitative performance.

LEDs and Getting Color Right

Megan Carroll, LC, IESNA

Xicato Director of Sales, Northeast