Principles of Color Measurement

PrinciplesPrinciples ofof ColorColor MeasurementMeasurement

520520 AnatomyAnatomy ofof thethe CIECIE ChromaticityChromaticity Diagram 540540 SpectralSpectral LocusLocus Colors at maximum saturation. 560560 Colors at maximum saturation. PlanckianPlanckian LocusLocus 580580 ColorColor ofof lightlight emittedemitted byby a black body at a given temperatutemperature.re. 500 UsedUsed toto calculatecalculate CorrelatedCorrelated Color TemperatureTemperature (CCT)(CCT) ofof 500 2500 2500

3000 3000 whitewhite lightlight sourcessources inin KelvinKelvin (K).

40004000 600600 Dominant Wavelength

60006000 Dominant Wavelength

2000

2000 1500 150 IntersectionIntersection pointpoint on the spectral locuslocus 10010000 00 0 620620 byby aa line originating at white and then 490490 %% projectedprojected throughthrough a given color.. 00 5050%% 100100%%

KelvinKelvin ( K( K ) ) PurityPurity (Excitation Purity) TheThe ratioratio of the distance of white to a 480480 stimulusstimulus with respectrespect to the distance ofof whitewhite to the dominant wavelength on thethe ExampleExample spectralspectral locus (expressed(expressed as a perpercentage).centage). 460460 StimulusStimulus

MacAdamMacAdam ellipses ellipses deﬁne areas of color on the CIE chromaticity diagram that are CIECIE ColorColor Matching Functions indistinguishableindistinguishable to to the the human human eye eye (ellipses (ellipses shown shown here here at at ten ten times times their their actual actual size). size). 2.02.0 0.9y y 0.9 0.60.6 v’ v’

0.80.8 0.50.5 1.5 𝒙𝒙풙풙𝒙𝒙풙 0.7 1.5 0.7 𝒚𝒚𝒚𝒚𝒚� 0.6 0.6 0.4 0.4 𝒛𝒛𝒛𝒛𝒛� 0.5 1.0 0.5 1.0 0.3 0.3 0.4 0.4

0.3 0.2 0.5 0.3 0.2 0.5 0.2 0.2 0.1 0.1 0.1 0.1 x u’ 0.0 u’ 0.0 x 400 500 600 700 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 400 500 600 700 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 /nm CIECIE 1931 1931 CIECIE 1976 1976 / Calculating CIE color coo𝜆𝜆rdinates of a stimulus: Color gamut illustrates the RGB limits reproducible by digital displays. Calculating CIE color coordinates of a stimulus: Color gamut illustrates the RGB limits reproducible by digital displays. For a spectral power 0.9 y For a spectral power y 0.6 v’ 0.9 0.6 v’ distribution (SPD), given distribution (SPD), given 0.8 0.8 as , the XYZ tristimulus 0.5 as , the XYZ tristimulus 0.7 0.5 values are computed as: 0.7 values are computed as: 𝑋𝑋푋푋𝑋𝑋𝑋𝑋𝑋𝑋𝑋𝑋𝑋𝑋�𝑋𝑋𝑋𝑋 sRGB DCI-P3 0.6 0.4 Rec.2020 sRGB DCI-P3 𝑃𝑃𝑃𝑃𝑃 0.6 0.4 Rec.2020 𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌𝑌� 0.5 0.5 CIE 1931 CIE 1976 0.3 𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍� 0.4 0.3 CIE 1931 CIE 1976 0.4 sRGB 0.3 0.2 sRGBDCI-P3 0.3 Rec.2020 Cx=X/((X+Y+Z)) DCI-P3 0.2 0.2 Rec.2020 0.2 0.1 0.1 0.1 0.1 x u’ Cx=X/((X+Y+Z)) u′=4X/((X+15Y+3Z)) x u’ Cy=Y/((X+Y+Z)) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 0 CIE0.1 19310.2 0.3 0.4 0.5 0.6 0.7 0.8 0 CIE0.1 19760.2 0.3 0.4 0.5 0.6 CIE 1931 CIE 1976 Cy=Y/((X+Y+Z)) v′=9Y/((X+15Y+3Z))

Radiant Vision Systems engineers imaging colorimeters and photometers, application software, Radiantand goniometric Vision Systems systems engineers to critically imaging evaluate colorimeters light, color and, and photometers, surface quality application. Calibrated software, to and goniometric systems to critically evaluate light, color, and surface quality. Calibrated to simulate human visual perception, Radiant cameras perform scientiﬁc measurement of displays and illuminated components to ensure quality that accurately reﬂects human experience.

www.RadiantVisionSystems.com Copyright ©2019 Radiant Vision Systems LLC. All rights reserved. 2019/03/21 www.RadiantVisionSystems.com Copyright ©2019 Radiant Vision Systems LLC. All rights reserved. 2019/03/21 The Language of Light

Luminous Flux (lumens)

Luminous Intensity (candela)

Luminance Illuminance (candela per (lumens per meter squared) meter squared)

Units of Measurement: Photometry Spectral Power Distribution

Every light source is deﬁned by its unique spectral Luminous Flux Luminous Intensity Illuminance Luminance power distribution (SPD), which is the radiant power (Watts) emitted by the light source at each wavelength in the visible electromagnetic spectrum.

D65 (Sunlight) A B Illuminant A LCD Blue Total output of a light Luminous ﬂux emitted Light incident on a Directional light LED Red source in all directions per unit solid angle surface per unit area emitted or reﬂected HeNe Laser back from a surface 390 430 470 510 550 590 630 670 710 per unit solid angle Wavelength (nm)

Lumen (lm) Candela (cd) Lux (lm/m2) cd/m2 𝜆𝜆 lm = cd * sr cd = lm/sr 1 cd/m2 = 1 nit Human Photopic Response

The human eye is not equally sensitive to wavelengths of light across the visible electromagnetic spectrum. Photometry vs. Photometric Radiometric Sensitivity of the human photopic response is Radiometry Human Visual Perception All Radiation given by the CIE 1931 luminosity function for a standard photopic observer (aligns with the color matching curve), LUMINOUS FLUX Total light RADIANT FLUX peaking at around 555 nanometers. Thus, green lumens (lm) 𝒚𝒚 Watts (W) wavelengths are typically brightest to the eye. output 1 Im = 1 cd * 1 sr

Light from LUMINOUS INTENSITY RADIANT INTENSITY a direction candela (cd) W/sr 1 cd = 1 lm/sr ILLUMINANCE Light incident IRRADIANCE on a surface lux foot-candle (fc) W/m2 2 1 lx = 1 lm/m 1 fc = 1 lm/ft2 Spectral Sensitivity of LUMINANCE the Human Eye RADIANCE Brightness 2 cd/m foot-lambert (fL) W/sr * m2 1 cd/m2 = 1 nit 1 fL = 1/π * cd/ft2 390 400 410 420 430 440 450 460 470 480 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 670 680 690 700 cd = candela sr = steradian lm = lumen W = Watt fc = foot-candle fL = foot-lambert Wavelength (nm)

𝜆𝜆 Radiant Vision Systems engineers imaging colorimeters and photometers, application software, and goniometric systems to critically evaluate light, color, and surface quality. Calibrated to simulate human visual perception, Radiant cameras perform scientiﬁc measurement of displays and illuminated components to ensure quality that accurately reﬂects human experience.