UNEP-lites.asia Laboratory Training Workshop

Transcription

1 UNEP-lites.asia Laboratory Training Workshop Beijing, China April 2015

2 UNEP GELC Lamp Performance Testing Training Workshop April 22-24, 2015, Beijing Fundamentals of Colorimetry and Practical Color Measurements Yoshi Ohno CIE VP-Technical Elect, President-Elect NIST Fellow, Sensor Science Division National Institute of Standards and Technology Gaithersburg, Maryland 2

3 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 3

4 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 4

5 Three cone sensitivities 5

6 Color Matching Experiments Wright (1929) l R =650 nm l G =530 nm l B =460 nm Guild (1931) Broadband Primaries (lamp+filters) 2 6

7 Results Wright (1929) Monochromatic primaries (460, 530, 650 nm) 10 observers 2 field of view Guild (1931) red, green, blue filters plus lamp 7 observers 2 field of view B G R B G R 7

8 CIE 1931 XYZ Color Matching Functions (CIE 1931 Standard Colorimetric Observer) f(l) x 牋 ( )? y 牋 ( )? z 牋 ( )? Tristimulus Values X = k Y = k Z = k ò f(l)x(l)dl l ò f(l) y(l)dl l ò f(l)z (l)dl l Wavelengt h ( nm ) 2 field of view observer (applicable to 1 to 4 field of view) 8

9 CIE Color Matching Functions (CIE 1964 Supplementary Standard Colorimetric Observer) Tristimulus Values X 10 = k f(l)x 10 l Y 10 = k Z 10 = k ò ò ò f(l)y l f(l)z l (l)dl 10 (l)dl 10 (l)dl Applicable to field of view greater than 4. Used for some applications in object color, but not used for light source specification. 9

10 Chromaticity coordinates Tristimulus Values Chromaticity Coordinates X Y Z x = y = X X +Y + Z Y X +Y + Z Y is a measure of visual intensity of light stimulus. x, y, Y fully describes a light stimulus. 10

11 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 11

12 CIE 1931 (x, y) Chromaticity Diagram Spectrum locus Color mixing Purple line Chromaticity of a mixture of two lights lies along the line between the two chromaticity points of the lights. 12

13 MacAdam Ellipses Just noticeable color differences. (magnified by 10 times) If the color space were uniform, these ellipses would be circles of the same size. 13

14 CIE 1960 (u, v) Chromaticity Diagram In 1960, the CIE developed Uniform Chromaticity Scale diagram, based on the work of MacAdam. (Now obsolete) u = v = 4X X +15Y + 3Z 6Y X +15Y + 3Z u = v = 4x (-2x +12y + 3) 6y (-2x +12y + 3) 14

15 CIE 1976 (u, v ) Chromaticity Diagram In 1976, CIE adopted an amended Uniform Chromaticity Scale which gave better agreement with experimental data: u' = u ; v' =1.5v u = v = 4X X +15Y + 3Z 9Y X +15Y + 3Z u = v = 4x (-2x +12y + 3) 9y (-2x +12y + 3) 15

16 CIE (u,v ) for chromaticity difference specification CIE 1931 (x, y) Diagram CIE 1976 (u, v ) Diagram 7 step MacAdam Ellipses R= step MacAdam Ellipses» 7-step MacAdam ellipses a circle with radius on (u v ) diagram. Do not use MacAdam ellipses. x-step MacAdam ellises radius 0.00x» 16

17 CIE TN 001 (2014) Primary author Y Ohno Published July

18 CIE TN 001 (2014) The u'v' circle is specified with a centre point and radius r on the (u',v') diagram, and expressed by, Recommended to replace n-step MacAdam ellipses Available free at: 18

19 Conversions (x,y) (u,v ) u = v = 4x (-2x +12y + 3) 9y (-2x +12y + 3) (u,v ) (x, y) 9 u x = (6 u -16 v +12) 2 v y = (3 u -8 v + 6) (x,y) (u,v) u = v = x = y = 4x (-2x +12y + 3) 6y (-2x +12y + 3) (u,v) (x, y) 9u (6u - 24v +12) 3v (3u -12v + 6) 19

20 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 20

21 Relative Spectral Power Distribution Color Temperature Temperature [K] of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus Planckian radiation 1000 K 2000 K 3000 K 5000 K K K Wav elength (nm) 21

22 Correlated Color Temperature (CCT) Temperature [K] of a Planckian radiator whose chromaticity is closest to that of a given stimulus on the CIE (u,2/3 v ) coordinate. (CIE 15:2004) CIE (u,2/3 v ) is the CIE 1960 (u, v) diagram, which is now obsolete. 22

23 Chromaticity expression for lighting CCT (Correlated Color Temperature) Duv (Shift from Planckian locus) Duv (u, v ) = (0.245, 0.528)? 23

24 Duv Duv scale on (u, v ) diagram Defined in ANSI C Closest distance from the Planckian locus on the (u', 2/3 v') diagram, with + sign for above and - sign for below the Planckian locus. 24

25 CCT- Duv chart 3000 K 4000 K 5000 K 6500 K 2700 K 3500 K 4500 K 5700 K 7-step MacAdam ellipses (CCT in log scale) 25

26 Direct approach (1) to calculate CCT and Duv T m T m+1 Tx CCT u v distance d Triangular solution Tx l (1) Create a table of CCT vs distance d i to BB locus on (u,v) coodinate. (2) Find the closest point in the table. (3) Solve the triangle for the neighboring 2 points x = d 2 m d m+1 2l + l 2 ( ) x T x = T m-1 + T m+1 - T m-1 ( ) 1/ 2 2 D uv = [±sign] d m-1 - x 2 l Use Planck s equation and color matching functions at 1 nm interval. 26

27 Direct approach (2) to calculate CCT and Duv Parabolic solution T m+1 T m Tx (1) Create a table of CCT vs distance d i to BB locus on (u,v) coodinate. (2) Find the closest point in the table. (3) Parabolic fit for the neighboring 3 points. d(t) = at 2 +bt +C D uv = [±sign] ( at 2 x + bt x + C) 27

28 CCT Error in Triangular Solution 28

29 Conversion from (CCT, Duv) back to (x, y) Input: CCT T (K) Duv D uv (u, v) 1) Calculate (u 0, v 0 ) of the Planckian radiator at T (K). 2) Calculate (u 1, u 1 ) of the Planckian radiator at T+DT (K). DT=0.01 (K) 3) Calculate (u 0, v 0 ) du = u 1 - u 0 dv = v 1 - v 0 u = u 0 + D uv sinq = u 0 + D uv dv / du 2 + dv 2 v = v 0 + D uv cosq = u 0 + D uv du / du 2 + dv 2 u = u v =1.5v x = 9 u /(6 u -16 v + 12) y = 2 v /(3 u - 8 v + 6) (u 1, v 1 ) (Included in Revision draft of C78.377) 29

30 Simple calculation of Duv from (x, y) or (u,v ) Duv is normally calculated in the process of calculating CCT. Below is a simple approximation formula, without calculation of CCT. 1) Convert (x, y) or (u, v ) to (u, v) u = 4x/(-2x +12 y + 3) v = 6y/(-2x +12y + 3) 2) Duv is obtained by or u = u v = 2 v / 3 L FP = (u ) 2 + (v ) 2 æ a = arccos u ö ç è ø L FP L BB = k 6 a 6 + k 5 a 5 + k 4 a 4 + k 3 a 3 + k 2 a 2 + k 1 a + k 0 D uv =L FP - L BB (Included in C ) k k k k k k k

31 Simple calculation from (x,y) or (u,v ) to Duv Accuracy of this method within in the range from 2600 K to K and Duv ± within in the range from 2160 K to K and Duv ± (Included in C ) 31

32 LEUKOS 10:1, 47-55, 2014 (DOI: / ) 32

33 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 33

34 ANSI and IEC (for Fluorescent Lamps) ANSI C IEC for Fluorescent Lamps CIE 1931 (x, y) Diagram 34

35 ANSI C for Solid State Lighting Products First published in Used in Energy Star and many regulations worldwide 2011 revision (current) 2015 revision (in ballot) Duv ±

36 4-step version in C (Informative Annex) Annex B. 4-step quadrangles Annex C. 4-step u v circles 36

37 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 37

38 Chromaticity below Planckian Locus Current standard Anecdotes say Lights below Planckian locus look better. An example in neodymium lamp 38

39 NIST Spectrally Tunable Lighting Facility 39

40 40

41 2013 Vision Experiment at NIST on Preferred and Acceptable level of Duv Experiments made at 4 CCTs, at 6 Duv points at each CCT, at total 23 points. Total 50 spectra used. 18 subjects participated. Subjects viewed fruits on the table, his/her skin tone and the whole room. Selected lights that looked more natural. 41

42 2013 Vision Experiment at NIST on Preferred and Acceptable level of Duv Results Proposal made to ANSI C to allow such products. Duv Another experiment planned for summer 2015 at NIST. Y. Ohno and M. Fein, Vision Experiment on Acceptable and Preferred White Light Chromaticity for Lighting, CIE x029:2014, pp (2014) 42

43 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 43

44 Relative reflectance Relative reflection Relative power Object Color Measurement Light source S( ) Eye or detector Wavelength (nm) Reflectance factor R(λ) Reflected light S(λ) R(λ) Wavelength (nm) sample Wavelength (nm) 44

45 Tristimulus Values for Object Colors Trisimulus values X = k Y = k Z = k ò ò ò S(l)R(l) l S(l)R(l) l x(l)dl y(l)dl S(l)R(l) z(l)dl l R( ): Spectral reflectance factor of object surface S( ): Spectral distribution of illumination (standard illuminant). k =100 ò S(l) y(l)dl l Y= 100 (%) for a perfect diffuser. Y gives luminance factor of the surface in % (for the given illumination). 45

46 Relative power Relative power CIE Standard Illuminants To Calculate Object Color, one of standard illuminants is used. Standard Illuminant A: Representative of tungsten-filament lighting with a color temperature of 2856 K. Standard Illuminant D65: Representative of average daylight with a CCT of ~6500 K. Other Daylight Illuminants D50, D55, D75 Wavelength (nm) <Now obsolete> Illuminant B: direct sun light with a CCT of ~4900 K Illuminant C: average daylight with a CCT of ~6800 K ( realized by a tungsten source with a prescribed liquid filter.) Wavelength (nm) *Formulae to calculate values of Illuminants A and D are available in CIE 15:

47 Light Color vs. Object Color Chromaticity diagrams such as (x,y), (u,v ) are two-dimensional and are only for light color. These are not for object color. No black, grey, or brown Object color needs another axis: black white Object color needs a 3-dimensional color space. 47

48 Lightness Object Color Space Three attributes of object color are hue, chroma (saturation), and lightness, and are expressed in a three dimensional space. Chroma white To allow accurate specification of object colors and color differences, CIE recommended CIELAB and CIELUV in black 3D color space Hue 48

49 CIE 1976 (L * a * b * ) color space (CIELAB color space) X, Y, Z X n, Y n, Z n : for object surface : white reference (perfect diffuser) L*=100 L * =116 (Y /Y n ) 1/3-16 a * = 500 é ë(x / X n ) 1/3 - (Y /Y n ) 1/3 ù û b * = 200 é ë(y /Y n ) 1/3 - (Z / Z n ) 1/3 ù û (when X / X n, Y /Y n, Z / Z n > ) See CIE 15:2004 for more details. L*=0 49

50 Example: (a *,b * ) plots of 1200 Munsell color samples Ref. D65 50

51 Color difference formulae CIELAB space CIEDE2000 (Improved formula) * DE 00 See CIE 142:2001 is occasionally used for displays as they simulate object colors. is used for uncertainties of object color measurements. 51

52 Comparison of object color spaces Plot of 15 saturated Munsell samples (used in CQS). CIE L * a * b * (CIELAB) or CIE L * u * v * (CIELUV) are the current CIE recommendations. Illum. A W*U*V* (used in CRI, obsolete) CIELAB CIELUV D65 52

53 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 53

54 Investigating problems of the CRI Color Rendering Index (CRI) CIE 13.3 Test source Reference source Same CCT [K] CIE Dxx Planckian (CCT<5000 K) Standard Daylight (CCT > 5000 K) #1 #2 #3 #4 #5 #6 #7 #8 R a #9 #10 #11 #12 #13 #14 R 9 54

55 Problems of CRI 1. CRI (Ra) badly penalizes visually preferred lights CCT 4929 K, Duv CRI Ra = Good CRI (Ra) score does not guarantee good color rendering CCT 5020 K, Duv CRI Ra = 82, R9 =

56 Color Quality Scale (CQS) Proposed by NIST to improve CRI on these problems Improvement of CRI, produces one number score that correlates well with perceived naturalness for real objects. 15 saturated test color samples Update the old formulae in CRI CIELAB color space CMCCAT2000 chromatic adaptation 0 to 100 scale RMS averaging of color differences Saturation factor (address the Hunt effect) Standards work for new metric is still on-going (in CIE, IES) CQS is used as a tool for color quality design. W. Davis and Y. Ohno, Color Quality Scale, Optical Engineering 49 (3), March

57 CQS 9.0 EXCEL sheet (Color Rendering Simulation) Used by many companies as a design tool for color quality 57

58 Looking at Luminous Efficacy of Radiation B-Y + broad Red B-Y + narrow Red RGBA (simulation) LER= 310 lm/w LER= 375 lm/w LER= 382 lm/w ~20 to 25 % increase Narrowband theoretically more efficient 58

59 Outline 1. Background of CIE 1931 colorimetry system 2. Chromaticity diagrams and MacAdam ellipses 3. CCT and Duv 4. ANSI chromaticity specification for SSL 5. Preferred white light chromaticity 6. Object color specification, CIELAB space 7. CRI and color quality 8. Practical color measurements of LED products 59

60 Color Quantities of Light Sources All light sources: Chromaticity coordinates (x,y), (u, v ) White light sources: Correlated color temperature T c (K) Duv D uv Color Rendering Index (CRI) R a Narrow-band sources (LEDs): Dominant wavelength d (nm) 60

61 Dominant Wavelength Wavelength of the monochromatic stimulus that, when additively mixed in suitable proportion with a specified achromatic stimulus, yields a color match with the color stimulus considered. Achromatic stimulus is usually equal energy spectrum: (x,y)=0.3333, ) If this is the colored stimulus and this is the achromatic stimulus, then a line connecting the two and going to the spectral locus ends at the dominant wavelength 61

62 Peak wavelength vs. Dominant wavelength Dominant wavelength (618 nm) Peak wavelength (628 nnm) 62

63 Peak vs. Dominant Wavelength Examples of real LEDs Peak WL (nm) Difference (peak-dom) (nm) Dom. WL (nm)

64 Conversion from Photometric to Radiometric Quantities [cd] [lm] [W/sr] [W] Conversions can be made by knowing the relative spectral power distribution S ) of the light source: X v = K = K ò m l S(l)V (l)dl [lm/w] X e S(l)dl ò l X v : photometric quantity (e.g., I v [cd]) X e : radiometric quantity (e.g., I e [W/sr]) K m = 683 [lm/w] This ratio K is called luminous efficacy of radiation (LER). 64

65 Measurement of Spatially-averaged color A sphere-spectroradiometer directly measures the spatially averaged color quantities. < Sphere-spectroradiometer system > 65

66 Gonio-spectroradiometer This method may be used when a spherespectroradiometer system is not available, or the test sample is too large for such a system. 1. A goniometer equipped with a spectroradiometer (gonio-spectroradiometer) 2. A goniometer equipped with a colorimeter (gonio-colorimeter). This must be calibrated against a spectroradiometer for each SSL product measured. LM-79 66

67 Calculation of spatially averaged color Example for chromaticity x LM-79 67

68 Sources of uncertainty in a spectrometer Uncertainty of reference standards (spectral irradiance, total spectral radiant flux) Input optics geometry Wavelength scale error Bandpass and scanning interval Random noise Stray light Detector non-linearity Detector zero drift Ref: Colorimetry Understanding the CIE System, edited by J. Schanda, John Wiley and Sons, pp (2008). Chapter 5 Spectral Color Measurement (Y. Ohno) Appendix 2 Uncertainties in Spectral Color Measurement (G. Gardner) 68

69 Bandpass error Cool White FL Measured (5 nm BP) Error in u v Wavelength (nm) 1.2 LED model Measured (10 nm BP) Wavelength (nm) Bandwidth of 5 nm (FWHM) or less is acceptable for colorimetry of most light sources. Error is proportional to the square of bandwidth increase. 69

70 Bandpass error correction Stearns and Stearns method (S-S method) (also, ASTM E308) Corrected value: S 0 S 0 = 1 98 M M M M M 2 Applies only to a triangular bandpass Bandwidth and scanning interval must be matched. 70

71 Ohno-Gardner method for bandpass correction Applicable to any bandpass functions, non-triangular, asymmetric, not matched with scanning interval. I 0 = s(l, l 0 ) dl ò I 1 = ò s(l,l 0 )l dl ò I 2 = s(l, l 0 ) l 2 Corrected value S 0 is obtained from the neighboring five points. dl æ Calculated numerically ö ç è for any bandpass functionø S 0 = b -2 M -2 + b -1 M -1 + b 0 M 0 + b 1 M 1 + b 2 M 2 with b -2 = a 2-1 X, b = - a -1-1 X, b = a 0 0 X, b = - a 1 1 X, b = a X, and X = a 0 2-2a -1 a 1. Y. Ohno, A Flexible Bandpass Correction Method for Spectrometers (AIC 2005) J. Gardner, Bandwidth correction for LED chromaticity, Color Res. Appl. 31(5)

72 How to determine the bandpass of an array spectrometer (1) Measure emission line. (3) Fit to a model (2) Wavelength-reversed data. (4) Normalize it b(m, l 0 ) = b rel(m, l 0 ) b rel (m, l 0 ) ò m Y. Ohno, Measurement of Bandpass for Array Spectrometers, Proc., CIE 27 th Session, July

73 Relative power Stray Light Error 1.0E E E E E E E-05 Diode-array NI ST/ 5nm BP Example x=0.7035( x= ) y=0.2951( y= ) x= y= RED # E Wav elength (nm) Correction methods available Ref: Y. Zong et al, Appl. Opt (2006) 73

74 THANK YOU for your attention. Contact: 74