Introduction to Colorimetry

Transcription

1 IES NY Issues in Color Seminar February 26, 2011 Introduction to Colorimetry Jean Paul Freyssinier Lighting Research Center, Rensselaer Polytechnic Institute Troy, New York, U.S.A. sponsored by 1

2 Acknowledgments NYC IES organizers of Issues in Color Seminar, especially Jason Livingston, Wendy Luedtke,, Dan Rogers, and Meg Smith LRC faculty, staff, and students Sponsors of ASSIST Program 2

3 Radiometry 3

4 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 4

5 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/m 2 Leaving a surface Exitance W/m 2 5

6 Sources of radiance Sun Electroluminescent Approximate luminance, cd/m 2 1.6x x x10 7 to to 1.0x x x x High Medium Low 6

7 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 P total 0 P d area under curve Relative power Daylight 5700 K Wavelength (nm) Relative power Incandescent Wavelength (nm) 7

8 Spectra of typical light sources Incandescent Fluorescent High pressure sodium Light emitting diodes Relative Energy Wavelength(nm) Relative Energy Wavelength(nm) Relative energy Wavelength(nm) Wavelength (nm) 8

9 Photometry 9

10 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 10

11 Why is photometry important? Promotes international trade Provides a quantitative language for communicating between stakeholders 11

12 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( ). 1 Based on flicker photometry. 0.8 V( ) - Photopic V ( ) - Scotopic Luminous efficiency Wavelength, nm

13 What does flicker photometry mean? Related to response of photoreceptors in central fovea L and M cones 2L + 1M V( ) Cone Fundamentals and V( ) relative value photopic L cone M cone S cone wavelength (nm) 13

14 Light: Calculation of luminous flux 1.0 Fluorescent lamp, 4100 K (F32T8/841) Relative power Wavelength (nm) 14

15 Light: Calculation of luminous flux 1.0 Fluorescent lamp, 4100 K (F32T8/841) Relative power Wavelength (nm) 15

16 Light: Calculation of luminous flux 1.0 Fluorescent lamp, 4100 K (F32T8/841) Relative power Wavelength (nm) 16

17 Light: Calculation of luminous flux 1.0 Fluorescent lamp, 4100 K (F32T8/841) Relative power Wavelength (nm) 17

18 Light: Calculation of luminous flux 830nm 380nm lm W k 683 P Power V k P W V Photopic luminous efficiency function Luminous Flux d 18

19 Photometry Description Quantity Unit Light Luminous flux Lumen Amount incident per surface area Amount leaving per surface area Illuminance Lumen/m 2 (Luminous) Exitance Lumen/m 2 In a particular direction (range of directions) In a direction, the amount emitted per surface area (Luminous) Intensity Luminance Lumen/sr cd Lumen/(m 2 sr) cd/m 2, nit 19

20 Photocell and photopic response CIE Photopic Luminous Efficiency Function and Silicon 3.85E Photocell 420 Spectral Response E E E E E E E E E E CIE Photopic 4.40E Silicon E E E E Wavelength (nm) E E

21 Errors applying V( ) Filters work well for broadband, white light sources, but not for narrowband sources 0.3 Illuminance Meter CIE Photopic Relative response Illuminance Meter CIE Photopic Relative response Blue LED Wavelength (nm) Wavelength (nm) 21

22 Colorimetry 22

23 What is color? Perception opponent color theory Red vs. green Blue vs. yellow Hue Saturation (chroma) Lightness (brightness) Color matching trichromatic color theory Any light can be perfectly matched with a combination of just 3 standard lights Specification of the light stimulus Color matching functions Equivalent to photometry 23

24 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 24

25 Opponent color encoding 25

26 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 26

27 Metamers 1 x Yellow-filtered LED incandescent Yellow-filtered LED white source Spectral Power, W/nm Wavelength, nm 27

28 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 28

29 CIE 1931 System The basic CIE (Commission Internationale de L Eclairage) system was developed in Cartesian graph of chromaticity coordinates (x,y) Chromaticity coordinates describe the color of the source or the light 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.

30 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

31 Color matching functions XYZ System employing imaginary primaries Tristimulus Value x( ) y( ) z( ) X Y Z x y P P P x y z X X Y Z Y X Y Z d d d Wavelength, nm 31

32 X tristimulus value calculation 1.8 spectral power distribution 3500 K T x color matching function weighted spectral power X tristimulus value (area) Wavelength (nm)

33 Y tristimulus value calculation 1.8 spectral power distribution 3500 K T y color matching function weighted spectral power Y tristimulus value (area) Wavelength (nm)

34 Z tristimulus value calculation spectral power distribution 3500 K T z color matching function weighted spectral power Z tristimulus value (area) Wavelength (nm)

35 X, Y, and Z Wavelength (nm) Wavelength (nm) Wavelength (nm) X = y = z = 9.99

36 CIE 1931 chromaticity space Spectrum Locus Blackbody Locus Fluorescent lamp, 3500 K Purple Boundary 36

37 Properties of the CIE chromaticity diagram Gamut of possible colors using these three LEDs 37

38 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.

39 Perception of chromaticity differences The distance between the end points of each line segment are perceptually the same. Diagram is not perceptually uniform. 39

40 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. 40

41 CIE 1976 Uniform Chromaticity Space (UCS) The CIE 1976 UCS diagram is perceptually uniform u = 4 4X / ( (X + 15Y + 3Z) 3 ) = 4 4x / ( (-2x + 12y + 3) v = 9 9Y / ( (X + 15Y + 3Z) 3 ) = 9 9y / ( (-2x + 12y + 3) 41

42 Brightness of saturated colors Saturated colors, especially deep reds and blues, appear brighter than photometric measurements imply Contours of enhanced brightness factors 42

43 Luminance is linear + = L(g) + L(r) = L(y) =

44 Brightness is nonlinear! + = B(g) + B(r) B(y) In fact... 44

45 Brightness is nonlinear! + = B(g) or B(r) > B(y)!!! 45

46 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 46

47 Correlated Color Temperature (CCT) CCT is an indication of the color appearance of the light emitted by a source applicable to nominally white light sources derived from the chromaticity of a reference (blackbody radiator)

48 Correlated Color Temperature (CCT)

49 Correlated color temperature (CCT) v CIE 1960 Chromaticity Diagram with Planktain Locus K 3000 K 4000 K K 5000 K 6000 K 7000 K 8000 K u y CIE 1931 Chromaticity Diagram with Planktain Locus K K x Isotemperature lines: Lines perpendicular to the CIE 1960 UCS defining constant CCT 49

50 Graphically determining CCT 50

51 Limitations of CCT

52 ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State Sources ANSI Tolerance Zones for Linear Fluorescent Lamps ANSI Tolerance Zones for SSL Lamps

53 ANSI Tolerance Zones for CCT of Linear Fluorescent and Solid State Sources ANSI Tolerance Zones for Linear Fluorescent Lamps

54 CCT of Compact Fluorescent Lamps

55 CCT of Linear Fluorescent Lamps v' K 3500 K 4000 K 2700 K K 6500 K 0.47 Blackbody Locus u'

56 Thank you. 56