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© 2010 Rensselaer Polytechnic Institute. All rights reserved.11
Introduction to ColorimetryIntroduction to Colorimetry
Jean Paul FreyssinierJean Paul Freyssinier
Lighting Research Center, Rensselaer Polytechnic InstituteLighting Research Center, Rensselaer Polytechnic InstituteTroy, New York, U.S.A.Troy, New York, U.S.A.
sponsored by
www.lrc.rpi.edu/programs/solidstate/assist
IES NY
Issues in Color Seminar
February 26, 2011
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
AcknowledgmentsAcknowledgments
NYC IES organizers of NYC IES organizers of Issues in Color SeminarIssues in Color Seminar, , especiallyespecially›› Jason Livingston, Wendy Jason Livingston, Wendy LuedtkeLuedtke, Dan Rogers, and , Dan Rogers, and
Meg SmithMeg Smith
LRC faculty, staff, and studentsLRC faculty, staff, and students
Sponsors of ASSIST ProgramSponsors of ASSIST Program
22
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.33
Radiometry
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.44
Radiometry
Detection and measurement of electromagnetic energy› Purely physical – no consideration of how it stimulates
the eye
Unit of measurement: watt› The watt is a unit of power› Power is the rate of energy; energy per time
• 1 watt = 1 joule/second
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.55
Radiometry: Geometry and units
The geometry of how radiant energy is produced, emitted, propagating, defines the units of measurement
Description Quantity Unit
Energy per time Power W
Incident on a surface Irradiance W/m2
Leaving a surface Exitance W/m2
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.66
Sources of radiance
High Medium Low
Sun
1.6x109 1.2x107
to 3.9x107
3.0x104 1.4x104
Approximate luminance, cd/m2
1.5x106
to 1.0x109
30
Electroluminescent
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.77
Daylight 5700 K
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
Incandescent
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
Spectrum: Radiation as a function of wavelength
The electromagnetic spectrum can be divided into smaller and smaller bands, or expressed as a continuous function of wavelength (or frequency)
Units: W/nm
curveunder area0
dPPtotal
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
350 450 550 650 750
Wavelength(nm)
Rel
ativ
e En
ergy
88
0
1
390 440 490 540 590 640 690 740Wavelength (nm)
Spectra of typical light sources
Incandescent Fluorescent High pressure sodium
0.0
0.2
0.4
0.60.8
1.0
1.2
350 450 550 650 750
Wavelength(nm)
Rel
ativ
e En
ergy
0
1
2
3
4
5
6
350 450 550 650 750
Wavelength(nm)
Rel
ativ
e en
ergy
Light emitting diodes
© 2010 Rensselaer Polytechnic Institute. All rights reserved.99
PhotometryPhotometry
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.10101010
What is photometry?
A simple, mathematically precise system of measuring and specifying light agreed to by an international community involved with its commerce and specification
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.11111111
Why is photometry important?Why is photometry important?
Promotes international tradePromotes international trade Provides a quantitative language for Provides a quantitative language for
communicating between stakeholderscommunicating between stakeholders
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1212
Light
IESNA Definition: Radiant energy capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 780 nanometers. CIE defines it over 360 to 830 nm.
Official (CIE) definition: radiant energy weighted by the photopic luminous efficiency function, V().
Based on flicker photometry.
300 350 400 450 500 550 600 650 700 750 8000
0.2
0.4
0.6
0.8
1
Wavelength, nm
Lum
inou
s ef
ficie
ncy
V() - Photopic
300 350 400 450 500 550 600 650 700 750 8000
0.2
0.4
0.6
0.8
1
Wavelength, nm
Lum
inou
s ef
ficie
ncy
V() - PhotopicV() - Scotopic
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1313
What does flicker photometry mean?
Related to response of photoreceptors in central fovea› L and M cones› 2L + 1M V()
Cone Fundamentals and V( )
0
0.2
0.4
0.6
0.8
1
400 500 600 700
wavelength (nm)
rela
tive
valu
e
photopicL coneM coneS cone
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1414
Light: Calculation of luminous flux
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1515
Light: Calculation of luminous flux
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
1616
Light: Calculation of luminous flux
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
Fluorescent lamp, 4100 K (F32T8/841)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
360 410 460 510 560 610 660 710 760
Wavelength (nm)
Relativ
e po
wer
1717
Light: Calculation of luminous flux
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1818
Light: Calculation of luminous flux
Flux Luminousfunction efficiency luminous Photopic
WPower Wlm683
nm830
nm380
VP
k
dVPk
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.1919
Photometry
Description Quantity UnitLight Luminous flux Lumen
Amount incident per surface area
Illuminance Lumen/m2
Amount leaving per surface area
(Luminous) Exitance Lumen/m2
In a particular direction (range of directions)
(Luminous) Intensity Lumen/srcd
In a direction, the amount emitted per surface area
Luminance Lumen/(m2 sr)cd/m2, nit
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2020
Photocell and photopic response
415 0.0022 420 0.393194 3.85E+02 4.0420 0.004 430 0.407356 3.90E+02 4.3425 0.0073 440 0.419748 3.95E+02 4.7430 0.0116 450 0.434304 4.00E+02 5.0435 0.0168 460 0.446499 4.05E+02 5.3440 0.023 470 0.463415 4.10E+02 5.5445 0.0298 480 0.476397 4.15E+02 5.7450 0.038 490 0.487215 4.20E+02 5.9455 0.048 500 0.500787 4.25E+02 6.1460 0.06 510 0.512982 4.30E+02 6.3465 0.0739 520 0.52439 4.35E+02 6.5470 0.091 530 0.536389 4.40E+02 6.7475 0.1126 540 0.548387 4.45E+02 7.0480 0.139 550 0.560386 4.50E+02 7.2485 0.1693 560 0.571597 4.55E+02 7.4490 0.208 570 0.583202 4.60E+02 7.6495 0.2586 580 0.594611 4.65E+02 7.8500 0 323 590 0 606412 4 70E+02 8 1
CIE Photopic Luminous Efficiency Function and Silicon Photocell Spectral Response
0.0
0.2
0.4
0.6
0.8
1.0
350 450 550 650 750 850 950 1050Wavelength (nm)
CIE PhotopicSilicon
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2121
Errors applying V()
Filters work well for broadband, “white” light sources, but not for narrowband sources
400 450 500 550 600 650 7000
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Rel
ativ
e re
spon
se
Illuminance MeterCIE Photopic
440 450 460 470 480 490 5000
0.05
0.1
0.15
0.2
0.25
0.3
Wavelength (nm)
Rel
ativ
e re
spon
se
Illuminance MeterCIE Photopic
Blue LED
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ColorimetryColorimetry
© 2010 Rensselaer Polytechnic Institute. All rights reserved.2323
What is color?What is color?
Perception Perception –– opponent color theoryopponent color theory›› Red vs. greenRed vs. green›› Blue vs. yellowBlue vs. yellow›› HueHue›› Saturation (chroma)Saturation (chroma)›› Lightness (brightness)Lightness (brightness)
Color matching Color matching –– trichromatictrichromatic color theorycolor theory›› Any light can be perfectly matched with a combination of Any light can be perfectly matched with a combination of
just 3 standard lightsjust 3 standard lights Specification of the light stimulusSpecification of the light stimulus
›› Color matching functionsColor matching functions›› Equivalent to photometryEquivalent to photometry
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2424
Human color perception
Trichromatic vision› 3 cone photoreceptors› Overlapping spectral
sensitivity› A lot of not completely
understood neural processing both at the retina and within the visual cortex of the brain
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2525
Opponent color encoding
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Metamers
Lights of the same color appearance can be made up of different spectral power distributions as seen in the diagram at the right.
Sources with the same color appearance, but different spectral power distributions will render colors differently.
Broad spectral power distributions are more likely to produce better color rendering
These three spectra can produce the same color perception
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2727
Metamers
400 450 500 550 600 650 7000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1x 10
-4
Wavelength, nm
Spec
tral P
ower
, W/n
m
Yellow-filtered incandescentYellow LEDYellow LED Yellow-filtered white source
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.2828
CIE colorimetric system
Based on color matching – not color perception Principle of univariance
› Once a photon is absorbed by a photoreceptor all wavelength information is lost
› Photoreceptor response is determined by the number of photons absorbed
› Color information is contained in the relative strength of the signals from each type of photoreceptor
Matching done under very particular and controlled conditions› 2° observer and 10° observer› Bipartite field
Reference field
Matching field
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
CIE 1931 System
The basic CIE (Commission Internationale de L’Eclairage)system was developed in 1931.› Cartesian graph of chromaticity coordinates (x,y)
• Chromaticity coordinates describe the color of the source or thelight reflected from a surface under given lighting conditions.
• Set of 3 chromaticity coordinates, (x,y,z) represent the proportional amounts of 3 established primary colors that must be added together to form the test color.
• The coordinate z can be calculated if x and y are known.
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Introduction to tristimulus values
Tristimulus values, R,G,B, or X,Y,Z show the absolute amounts of the three primaries required to make a match being specified
Tristimulus values are psychophysical quantities› Based on functions derived from averaged data of multiple
observers› Do not correspond to perceptual color› Y describes luminance
CIE system is for specifying difference or equivalence of light stimuli
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.3131
Color matching functions
XYZ System employing imaginary primaries
400 450 500 550 600 650 7000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength, nm
Tris
timul
us V
alue
x()y()z()
ZYXYy
ZYXXx
dzPZ
dyPY
dxPX
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
X tristimulus value calculation
400 450 500 550 600 650 700 7500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)
spectral power spectral power distributiondistribution3500 K T83500 K T8
x color x color matching matching functionfunction
weighted weighted spectral spectral powerpower
X tristimulus X tristimulus value (area)value (area)
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Y tristimulus value calculation
400 450 500 550 600 650 700 7500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)
spectral power spectral power distributiondistribution3500 K T83500 K T8
y color y color matching matching functionfunction
weighted weighted spectral spectral powerpower
Y tristimulus Y tristimulus value (area)value (area)
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Z tristimulus value calculation
400 450 500 550 600 650 700 7500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)
spectral power spectral power distributiondistribution3500 K T83500 K T8
z color z color matching matching functionfunction
weighted weighted spectral spectral powerpower
Z tristimulus Z tristimulus value (area)value (area)
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
X, Y, and Z
400 450 500 550 600 650 700 7500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)
X = 20.88X = 20.88 y = 19.85y = 19.85 z = 9.99z = 9.99
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.3636
Spectrum Locus
Purple Boundary
Blackbody Locus
CIE 1931 chromaticity space
Fluorescent lamp, 3500 K
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.3737
Properties of the CIE chromaticity diagram
Gamut of possible colors using these three LEDs
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.
Limitations of 1931 CIE Chromaticity Diagram
There is no luminance level. Sources may have identical chromaticity coordinates, but
SPD will be different and colors can be rendered differently with these sources. (Metameric)
Color space is not represented in a uniform fashion. The visual difference between two points separated by a particular distance on the diagram varies with the position of the colors.
Other color systems have been developed which represent more uniform color space.
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.3939
Perception of chromaticity differences
The distance between the end points of each line segment are perceptually the same.
Diagram is not perceptually uniform.
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Discriminating differences in chromaticity
MacAdam’s ellipses of chromatic discrimination, plotted 10 times their actual size in the CIE chromaticity diagram.
Lamps with chromaticities within a 3-step ellipse should appear to be the same color by most observers.
ANSI specifies 4-step ellipses for fluorescent lamp chromaticities.
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4141
CIE 1976 Uniform Chromaticity Space (UCS)CIE 1976 Uniform Chromaticity Space (UCS)
The CIE 1976 UCS diagram is perceptually uniformThe CIE 1976 UCS diagram is perceptually uniform›› uu’’ = 4= 4X X / (/ (XX + 15+ 15YY + 3+ 3ZZ) = 4) = 4xx / (/ (--22xx + 12+ 12yy + 3)+ 3)›› vv’’ = 9= 9Y Y / (/ (XX + 15+ 15YY + 3+ 3ZZ) = 9) = 9yy / (/ (--22xx + 12+ 12yy + 3)+ 3)
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4242
Brightness of saturated colors
Saturated colors,
especially deep reds
and blues, appear
brighter than
photometric
measurements imply
Contours of enhanced brightness factors
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4343
Luminance is linear
+ =
L(g) + L(r) = L(y)1.5 + 1 = 2.5
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4444
Brightness is nonlinear!
B(g) + B(r) B(y)
+ =
In fact...
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4545
Brightness is nonlinear!
B(g) or B(r) > B(y) !!!
+ =
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4646
Other ways of specifying color
Many other color spaces have been developed and used for various tasks.› L*, u, v
• Based on CIE 1976 UCS diagram• Basis of CRI calculation (currently uses 1964 version)• Hue, lightness, chroma and saturation
› L*, a, b• Based on CIE 1976 UCS diagram• Hue, lightness, chroma
Generally not used for lighting industry No system is perfect
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
CCT is CCT is ›› an indication of the color appearance of the light an indication of the color appearance of the light
emitted by a sourceemitted by a source›› applicable to nominally white light sourcesapplicable to nominally white light sources›› derived from the chromaticity of a reference derived from the chromaticity of a reference
(blackbody radiator)(blackbody radiator)
Correlated Color Temperature (CCT)Correlated Color Temperature (CCT)
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Correlated Color Temperature (CCT)Correlated Color Temperature (CCT)
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.4949
Correlated color temperature (CCT)
Isotemperature lines: Lines perpendicular to the CIE 1960UCS defining constant CCT
0 0.2 0.4 0.6 0.8
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
CIE 1931 Chromaticity Diagram with Planktain Locus
xy
0.1 0.2 0.3 0.4 0.5 0.6
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45CIE 1960 Chromaticity Diagram with Planktain Locus
u
v
2000 K3000 K
4000 K∞ K
8000 K
5000 K6000 K
7000 K
2000 K
8000 K
©© 2010 Rensselaer Polytechnic Institute. All rights reserved.2010 Rensselaer Polytechnic Institute. All rights reserved.5050
Graphically determining CCT
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Limitations of CCTLimitations of CCT
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ANSI Tolerance Zones for SSL Lamps
ANSI Tolerance Zones for CCT of Linear ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State SourcesFluorescent and Solid State Sources
ANSI Tolerance Zones for Linear Fluorescent Lamps
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
ANSI Tolerance Zones for CCT of Linear ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State SourcesFluorescent and Solid State Sources
ANSI Tolerance Zones for Linear Fluorescent Lamps
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
CCT of Compact Fluorescent LampsCCT of Compact Fluorescent Lamps
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
0.42
0.47
0.52
0.57
0.15 0.20 0.25 0.30
u'
v'2700 K3000 K
3500 K
4000 K
5000 K
6500 K
Blackbody Locus
CCT of Linear Fluorescent LampsCCT of Linear Fluorescent Lamps
© 2010 Rensselaer Polytechnic Institute. All rights reserved.
Thank you.Thank you.
5656