Assessing Colour Rendering Properties of Daylight Sources Part II: A New Colour Rendering Index: CRI-CAM02UCS Cheng Li, Ming Ronnier Luo and Changjun Li Department of Colour Science, University of Leeds, Leeds LS2 9JT, U.K. Abstract: In Part I of this paper series, the results of colour fidelity experiment shows that the visual colour difference of a test sample viewed first under the test source and then under the reference lamp was not predicted properly by the CIE colour rendering index (CRI) 1. However, the calculation based on CAM02-UCS 2, an extension of the CIECAM02 colour appearance model 3, includes both CAT02 transform and colour difference equation, gave the best correlation to the visual results. Therefore, the current CIE CRI is modified by calculating colour difference in CAM02-UCS uniform colour space. The new method is named CRI-CAM02UCS and is described in this paper. Key words: CIECRI, CIECAM02, CAM02UCS, CRI-CAM02UCS. INTRODUCTION In Part I of this paper series, three experiments were carried out to investigate the colour rendering property of different daylight sources. The results were uzsed to evaluate the CIE CRI. The colour appearance experiment based on the magnitude estimation method was carried out. It was found the variations of visual results to be very small, and inaccurate to be used to indicate the variations of colour rendering property of different light sources. However, the colour fidelity experiment gave more accurate and consistent results. The colour difference of a test sample viewed first under the test source and then under the reference lamp was judged by observers in terms of colour shift, and the visual results were used to investigate the performance of the CIE colour rendering index. It was found that the CIE CRI, which calculates colour difference based on the CIE 1964 U * V * W * colour space via the von Kries chromatic adaptation transform, could not predict the visual colour difference accurately. Other calculation methods using different colour difference formulae and chromatic adaptation transforms performed better than the CIE CRI method.
2 Moreover, the calculation based on CAM02-UCS, which is an extension of CIECAM02 colour appearance model and includes both CAT02 transform and colour difference equation, gave the best correlation to the visual results. In addition, the similar experiment conducted by Sándor and Schanda 4 also showed the same results as the colour appearance model based colour difference formula gave the best correlation to the visual results. Therefore, the current CIE colour rendering index is modified by calculating colour difference in the uniform colour space based on the CIECAM02 colour appearance model. The new method is named CRI-CAM02UCS and is described in this paper. A NEW COLOUR RENDERING INDEX: CRI-CAM02UCS The new colour rendering index, named CRI-CAM02UCS, follows the fundamentals of the original calculation of the CIE CRI. Figure 1a shows the workflow of CIE CRI, which includes 5 steps. Step 1 is to calculate the spectral power distribution (SPD) of the reference illuminant having the same correlated colour temperature (CCT) as the test illuminant. Step 2: calculate the tristimulus values based on the 8 CIE samples defined in terms of reflectance function under the test and reference illuminants. In Step 3, the tristimulus values of the 8 samples under test illuminant were then transformed to those under the reference illumaint. In Step 4, the 1964 CIEU*V*W* values for each sample were then calculated. The colour difference for each corresponding sample is then calculated and averaged. Finally, Step 5 calculates the CIE CRI. Figure 1b shows the workflow for calculating the new colour rendering index, CRI- CAM02UCS, which calculates the colour difference in the uniform colour space based on CIECAM02 colour appearance model. Thus, the new CRI-CAM02UCS predicts the colour rendering properties of a light source based on the variation in colour appearance of test samples illuminated under the test source and the reference illuminant. Furthermore, it was found in the colour preference experiment that increases in chroma did not yield more pleasant feeling and the saturation factor
3 used in CQS 5 could not be simply used to indicate whether a sample viewed under the test source is more preferred. Thus, the colour difference in the new CRI-CAM02UCS is equally weighted for shifts in lightness, colourfulness and hue of the test samples between the test light source and the reference illuminant. Figure 1b gives the flow chart of calculating the new CIR-CAM02UCS. It can be seen from Figure 1 that, Steps 1 and 2 are the same as CIE CRI. However, Steps 3 and 4 of the CIE CRI is replaced by the extension of CIECAM02, CAM02-UCS. In addition, the final index is different from that of CIE CRI. Test Source SPD 2. Calculate XYZ 1. Calculate SPD of equal CCT 8 CIE samples R% Reference Source SPD 2. Calculate XYZ (XYZ) Test (XYZ) Reference 3. von Kries CAT (XYZ) Trans. 4. CIEU*V*W* 4. CIEU*V*W* (U*V*W*) Trans. ΔE i (U*V*W*) Reference. 5.R a =Σ(100-4.6ΔE i )/8 Figure 1a The workflow for calculating CIE CRI.
4 Test Source SPD 2. Calculate XYZ 1. Calculate SPD of equal CCT 8 CIE samples R% Reference Source SPD 2. Calculate XYZ (XYZ) Test (XYZ) Reference 3. CAM02-UCS 3. CAM02-UCS (J a b ) Trans. ΔE i (J a b ) Reference 4. CRI-CAM02UCS =Σ(100-8.0ΔE i )/8 Figure 1b The workflow for calculating the CRI-CAM02UCS. In CIE CRI, a scaling factor of 4.6 is used to scale colour differences into colour rendering indices, and the average value of the colour rendering indices of the first eight reflective samples is used as the indication of the general colour rendering index (R a ) (see literature survey). The scaling factor of 4.6 is defined so that the R a of a warm white fluorescent lamp has a value of 51. In order to maintain consistency with the current CIE CRI, a different scaling factor is used in calculating the value of the CRI-CAM02UCS, since the colour differences are calculated in a different colour space. The new scaling factor is determined so that the average score of the CRI- CAM02UCS for the CIE standard fluorescent lamps (F1 through F12) is equal to the average score of the CIE CRI (R a =75) for these sources. Thus, this scaling factor maintains consistency of the new colour rendering index scale with the current CIE
5 CRI scale for existing lamps. The following equation gives the calculation of new CRI-CAM02UCS: 8 1 CRI-CAM 02 UCS = (100 8.0 ΔE ) icam 02 UCS 8 i= 1 where Δ E icam 02 UCS is the colour difference for i-th test sample, first illuminated under the test source and then under the reference illuminant, calculated in the CAM02-UCS uniform colour space based on the associated colour appearance model. The new scaling factor of 8.0 is used to keep the CRI-CAM02UCS scale consistence with the original CIE CRI scale. EVALUATING THE CIE CRI AND THE CRI-CAM02UCS WITH DIFFERENT SETS OF SAMPLES In the current CIE CRI, the distances in the CIE U * V * W * space between the points representing the colour of 14 reflective samples when illuminated by the test source and the reference illuminant are calculated, and the CIE special colour rending index, R i, is computed for each sample 1. The average of R i of the first eight reflective samples is used to calculate the CIE general colour rendering index (R a ). However, by using different test samples, the R i values will be different. Therefore, the performances of CRI and CRI-CAM02UCS are tested by using different sets of samples. Five sets of reflectance functions were used: the 8 samples recommended by CIE for calculating the general colour rendering index, the 14 samples recommended by CIE for computing CRI, the 15 samples proposed by NIST for computing the CQS, the set of 24 samples in the GretagMacbeth ColorChecker Chart suggested by CIE for improving the calculation of CRI, and the 684samples representing the gamut boundary of real surface colours 6. They are named: CIE-8, CIE-14, NIST, GMCC and Gamut respectively. The latter sample set represents the colour gamut boundary for all the real surface colours excluding the chromatic fluorescent colours. This set of samples provides the most comprehensive test set amongst all the sets collected.
6 Both the CIE CRI and the CRI-CAM02UCS are calculated by using the 5 sets of samples for the six light sources introduces in Part I of the paper series: GretagMacbeth filtered tungsten lamp, Verivide fluorescent lamp, Philips fluorescent lamp (F20T12/D), Nichia white phosphor LED (NSPL500S), Zumtobel RGB LED (Tempura LED spotlight) and the spectrally tuneable LED cluster which was developed and constructed by the authors 7. In this study, the test and the reference illuminant are assumed to have the same luminance level and background for computing both the CIE CRI and the CRI-CAM02UCS. Note that these light sources were chosen to represent particular types of lamps. Table 1 lists the CRI and CRI- CAM02UCS results based on different datasets. CIE CRI CRI-CAM02UCS Test set CIE-8 CIE-14 NIST GMCC Gamut CIE-8 CIE-14 NIST GMCC Gamut No. of samples 8 14 15 24 684 8 14 15 24 684 GretagMacbeth 96.6 95.9 93.6 96.3 93.7 96.8 96.6 95.5 97.1 95.3 VeriVide 96.9 96.3 95.4 97.3 95.4 97.3 97.1 96.4 97.7 95.8 Philips 74.0 64.0 45.9 67.8 51.4 75.3 72.9 63.1 76.7 67.6 Zumtobel 30.7 13.3-36.5 20.0-28.7 41.8 37.9 14.9 46.3 20.7 Nichia 83.4 81.4 72.4 85.4 72.9 84.4 84.1 75.1 86.3 73.7 LED Cluster 99.3 99.1 98.8 99.2 98.2 99.3 99.2 99.1 99.3 98.6 Table 1 The CIE CRI and the CRI-CAM02UCS values calculated by different datasets. It can be seen in Table 1 that, the values of the CIE CRI and the CRI-CAM02UCS calculated using the NIST set show the largest difference from those calculated using the set of CIE-8 samples. For example, for the Zumtobel source, the values of CRI and CRI-CAM02UCS based on the NIST set are -36.5 and 14.9 respectively, which are much less than those obtained by the CIE-8 set which are 30.7 and 41.8 respectively. Moreover, the values of the CIE CRI and the CRI-CAM02UCS calculated using the NIST set are similar to those calculated using the Gamut set representing the gamut boundary of real surface colours. At the meantime, the values of the CIE CRI and the CRI-CAM02UCS based on the CIE-14 set are also close to those calculated using the GMCC set.
7 Last but not least, no matter which dataset is used, the values of the CIE CRI and the CRI-CAM02UCS give the same rankings for the six light sources. The greatest value of the CIE CRI and the CRI-CAM02UCS is obtained by the tuneable LED cluster, followed by the VeriVide fluorescent lamp and the GretagMacbeth filtered tungsten lamp. These three light sources all have high values for CRI and CRI-CAM02UCS, which are all greater than 90. The Nichia white LED shows slightly less values in CRI and CRI-CAM02UCS, and the Philips fluorescent lamp gives worse colour rendering properties than the Nichia. However, the Zumtobel RGB LED shows the worst colour rendition among the six light sources, and has the lowest CRI and the CRI- CAM02UCS values, no matter which datasets are used. Figure 2 shows the diagram of both the CIE CRI and the CRI-CAM02UCS values calculated by using the five sets of reflectance functions for the six light sources used in Part I of the paper series. It is clear that no matter which dataset is used, the values of CRI and CRI-CAM02UCS give the same rankings for the six light sources, and the Zumtobel has the worst colour rendering properties among the six light sources. Hence, the CIE-8 dataset was adopted to be used in CRI-CAM02UCS because it is currently used by CIE CRI and has the least number amongst the datasets studied. 100 100 80 90 80 Values of CIE CRI 60 40 20 0-20 CIE-8 CIE-14 NIST GMCC Gamut GM VV PH ZTB WLED TLED Values of CRI-CAM02UCS 70 60 50 40 30 20 10 CIE-8 CIE-14 NIST GMCC Gamut 0-40 GM VV PH ZTB WLED TLED Figure 2 The CIE CRI and the CRI-CAM02UCS values by using different sets of reflectance functions (GM, VV, PH, WLED, ZTB and TLED are the abbreviations for GretagMacbeth, VeriVide, Philips, Nichia white LED, Zumtobel and tuneable LED cluster, respectively.)
8 EVALUATING THE CRI AND THE CRI-CAM02UCS BY RED AND MAGENTA SAMPLES Although different sets of test samples were used, the six light sources were given the same rankings by both CRI and CRI-CAM02UCS. However, for the Philips fluorescent lamp and Zumtobel RGB LED lamp, the results calculated by the CRI- CAM02UCS are quite different from those of the CIE CRI. Figure 3 plots the special colour rendering index for the Zumtobel RGB LED lamp calculated by the CRI- CAM02UCS against that of the CIE CRI. Three sets of test samples, CIE-14, NIST, and GMCC, are used here. It can be seen that, for those R i values greater than 60, the CIE CRI and CRI-CAM02UCS give very close predictions. However, for those R i values smaller than 0, the CIE-CAM02UCS gives greatly different results from the CIE CRI; the former values are much greater than those of the latter. For example, the smallest R i achieved by the CIE CRI is -159, however, the corresponding R i calculated by the CRI-CAM02UCS is -10. 100 60 Ri calculated by CIE CRI. 20-180 -140-100 -60-20 -20 20 60 100-60 -100-140 -180 Ri calculated by CRI-CAM02UCS CIE-14 NIST GMCC Figure 3 Special colour rendering index calculated by the CRI-CAM02UCS and the CIE CRI Moreover, the discrepancy of the R i predicted by the CIE CRI and the CRI- CAM02UCS is very large when samples are strong red colours. In Table 2, nine samples are listed, for which the R i values are extremely different between CIE CRI and CRI-CAM02UCS. Table 2 shows the lightness, colourfulness and hue angle predicted by CIECAM02 colour appearance model for these samples under CIE D65, and it clearly can be seen that, these samples are red and magenta colours with high colourfulness values. However, the average R i values predicted by the CIE CRI and
9 the CRI-CAM02UCS are -123 and -9, respectively, which are significantly different. Therefore, the two different indexes give quite different values to saturated red and magenta colours. In the current study, the red and magenta colours with high colourfulness values have already been used in the colour fidelity experiment, and are denoted as C215, MK9, C182, MK32, and C50, respectively. The colour attributes of these samples under CIE D65 are predicted by CIECAM02, and are listed in Table 3. These samples are either red or magenta colours with high colourfulness, and the average values of lightness, colourfulness, and hue angle are 40, 50, 17 respectively. Samples Test sets Lightness (J) Colourfulness (M) Hue angle (h) R i by CIE CRI R i by CRI- CAM02UCS 1 CIE-14 33 72 21-10 -159 2 NIST 61 53 60-33 -130 3 NIST 52 54 41-17 -141 4 NIST 33 65 20-9 -148 5 NIST 31 50 2 6-83 6 GMCC 54 54 57-19 -113 7 GMCC 43 49 16 8-111 8 GMCC 34 63 22-7 -148 9 GMCC 43 50 345 0-77 Mean 43 57 25-9 -123 Table 2 Saturated red and magenta samples whose R i values are predicted quite different by the CIE CRI and the CRI-CAM02UCS Samples Lightness (J) Colourfulness (M) Hue angle (h) C215 25 37 327 MK9 34 44 9 C182 26 60 25 MK32 48 39 32 C50 61 66 51 Mean 38.8 49.2 16.8 Table 3 Colour attributes of red and magenta samples with high colourfulness Moreover, the visual results of the saturated red and magenta samples in the colour fidelity experiment are compared with the predictions by the CIE CRI and the CRI-
10 CAM02UCS. Table 4 shows the CV values between the visual results under each test source and the corresponding colour difference predicted by the CIE CRI and the CRI-CAM02UCS respectively. The results are also similar to those findings in previous paper, for which the colour difference calculated by the CIE CRI method, gives much worse predictions to the visual results, compared to the CRI-CAM02UCS method. The CV values for the four simulators using the CIE CRI method are all much larger than those achieved by the CRI-CAM02UCS method. The average CV value for the CIE CRI method is 35.5, which is approximately 16 units greater than that of using the CRI-CAM02UCS method. The smallest CV value is 7, which is achieved by using the CRI-CAM02UCS to predict the visual results under Sources 3 and 4. Sources 1 2 3 4 mean CIE CRI 59 48 17 18 35.5 CRI-CAM02UCS 34 29 7 7 19.3 Table 4 CV values between visual results and colour differences calculated by the CIE CRI and the CRI-CAM02UCS methods for saturated red and magenta samples In Figure 4, the visual colour differences under 4 test light sources for the saturated red and magenta samples are plotted against those colour differences predicted by the CIE CRI and the CRI-CAM02UCS methods. It can be seen that the CIE CRI method shows larger scatter, and does not agree with the visual colour differences satisfactorily. In contrast, the values of correlation coefficient calculated between the visual results and the predicted colour differences are 0.89 and 0.94 for the CIE CRI method and the CRI-CAM02UCS method, respectively. The colour difference calculation based on the CRI-CAM02UCS method shows better correlation to the visual results than the CIE CRI method.
11 18 15 r=0.89 18 15 r=0.94 Visual colour differenc 12 9 6 c Visual colour differen 12 9 6 3 Test Simulator 1 Test Simulator 2 Test Simulator 3 Test Simulator 4 0 0 3 6 9 12 15 18 CIE CRI method predictions 3 Test Simulator 1 Test Simulator 2 Test Simulator 3 Test Simulator 4 0 0 3 6 9 12 15 18 CRI-CAM02UCS method predictions Figure 4 Visual colour differences under 4 test simulators for the saturated red and magenta samples plotted against colour differences predicted by the CIE CRI and the CRI-CAM02UCS methods Therefore, although the CIE CRI and CRI-CAM02UCS give the same rankings for the six light sources, they differ greatly for the strong red colours. The visual results for those saturated red and magenta colours obtained in the colour fidelity experiment also shows that, the CRI-CAM02UCS fits much better to the visual results than that of the CIE CRI. Thus, not only the new CRI-CAM02UCS is much simpler than the CIE CRI, but also correlates better to the visual results. COLOUR RENDERING INDEX AND COLOUR GAMUT The size of colour gamut represents the volume of colour space that can be realised under any particular light source. The physical measure for the size of the colour gamut is its volume, which can be used to quantifying the quality of colour rendering. The 684 reflectance functions representing the gamut boundary of real surface colours are used to calculate the colour gamut, and the volume of the 3-D colour gamut can be considered as the sum of volumes of tetrahedrons 6. Moreover, when the gamut boundary of the test source is compared with that of the reference illuminant, a volume shared by both the test source and the reference illuminant can be defined. The shared colour gamut will have a smaller, but similar to that given under the reference illuminant, hence the shared volume can be computed as a sum of shared volumes of tetrahedrons.
12 The volumes of the colour gamut of real surface colour are calculated for the CIE illuminant D65 and the six light sources described in the colour appearance experiment. The volumes shared by the CIE illuminant D65 and each test light source are also calculated. All the values are normalised against the corresponding values under CIE D65. Table 5 shows both the colour volumes and shared volumes for the light sources. It can be seen in Table 5 that, the Zumtobel source has the largest colour volume among the six light sources, while the Nichia white LED has the smallest. Also, the LED cluster has the largest shared volume with the reference CIE illuminant D65, while the Nichia source again has the smallest shared volume. CIE CRI CRI-CAM02UCS Colour Volume Shared Volume D65 100.0 100.0 100.0 100.0 GretagMacbeth 96.6 96.8 99.9 98.5 VeriVide 96.9 97.3 101.0 99.1 Philips 74.0 75.3 91.2 87.1 Zumtobel 30.7 41.8 118.5 91.7 Nichia 83.4 84.4 88.6 85.4 LED Cluster 99.3 99.3 100.7 99.8 Table 5 Colour volumes and shared volumes for the six light sources used in the colour appearance experiment. The values calculated based on the colour gamut volume or shared volume give different rankings to the six light sources, compared to the values obtained by the CIE CRI and the CRI-CAM02UCS. The most significant difference is for the Zumtobel source. It shows the lowest values in both the CIE CRI and the CRI-CAM02UCS, but has the largest volume in the colour gamut of the real surface colours. Besides, the Zumtobel source shared more volume with the CIE D65 than the Nichia and Philips sources. However, the visual results showed that the Zumtobel source had the largest variation in colour appearance compared with that of the reference source. Therefore, the volume or the shared volume of the colour gamut of a light source is not suitable to indicate the colour rendering properties in terms of colour appearance and colour fidelity. In contrast, it can be used to demonstrate the volume of colour space that can be realised under this particular light source.
13 40 30 20 10 b 0-40 -30-20 -10 0 10 20 30 40-10 -20-30 -40 a Zumtobel Reference Figure 5 The colour gamut of the eight reflective samples under the Zumtobel source and the reference illuminant An example is given in Figure 5, for which the colour gamut of the CIE recommended eight reflective samples under both the Zumtobel source and the reference illuminant are plotted. It is clear that the colour gamut under the Zumtobel has a larger area than that under the reference illuminant. However, most of the samples under Zumtobel source shift in the red-green direction, and show a significant variation in colour appearance compared with that of the reference illuminant. CONCLUSION A new colour rendering index, CRI-CAM02UCS, which calculates colour difference in a uniform colour space based on the CIECAM02 colour appearance model, is developed. The calculation method based on the CRI-CAM02UCS method shows better agreement to the visual colour difference than the original CIE CRI. Furthermore, different sets of reflectance functions were also investigated, and the results show that, no matter which set of reflectance functions is used, the values of the CIE CRI and the CRI-CAM02UCS give the same rankings for the six light sources. Nevertheless, the Zumtobel has the worst colour rendering properties among the six light sources. However, the CIE CRI and CRI-CAM02UCS gives extremely different values for the strong red colours. Nevertheless, the visual results of the saturated red and magenta samples used in the colour fidelity experiment have shown that the performance of the CRI-CAM02UCS is better than that of the CIE CRI.
14 Furthermore, the volumes of colour gamut for the real surface colours under different light sources are also calculated. These results show that, for the colour rendering properties in terms of colour fidelity or colour appearance, the new CRI-CAM02UCS should be used to indicate the performance of the light source. However, the calculation based on the colour gamut volume of real surfaces can be used to indicate the volume of colour space based on the gamut set that can be realised under this particular light source. REFERENCES 1. CIE 13.3:1995, Method of Measuring and Specifying Colour Rendering Properties of Light Sources, 1995. 2. M. R. Luo, G. Cui and C. J. Li, Uniform Colour Space Based on CIECAM02 Colour Appearance Model, Color Research & Application, Volume 31, Number 4, Aug 2006. 3. C. J. Li, M. R. Luo, R. W. G. Hunt, N. Moroney, M. D. Fairchild and T. Newman, "The performance of CIECAM02", IS&T/SID 11 th Color Imaging Conference, Scottsdale, Arizona, USA: 28-32, 2002. 4. N. Sándor and J. Schanda, "Visual colour rendering based on colour difference evaluations", Lighting Research and Technology 38: 225-239, 2006 5. W. Davis and Y. Ohno, "Toward an improved color rendering metric", SPIE Proceeding of Fifth International Conference on Solid State Lighting, San Diego, California, USA, 2005. 6. C. J. Li, M. R. Luo, M. R. Pointer, X. Li, C. Li and W. Ji, "A new method for quantifying colour rendering", Proceeding of 26 th Session of the CIE, CIE 178:2007, Beijing, 2007. 7. C. Li, C. J. Li and M. R. Luo, Quality of LED Based Daylight Simulators, CGIV2006, Leeds, UK, 2006.