Abstract

The color of a material, such as solution of a dye, can change by changing parameters like pH, temperature, illumination direction, and illumination type. Dichromatism—a color change due to the difference in thickness of the material—has long been known as a property of only a few materials. Here we show that dichromatism is a common property of many substances and materials, and we introduce a method for its quantification. We defined dichromaticity index (DI) as the difference in hue angle (Δhab) between the color of the sample at the dilution, where the chroma is maximal, and the color of four times more diluted (or thinner) and four times more concentrated (or thicker) sample. The two hue angle differences are called dichromaticity index toward lighter (DIL) and dichromaticity index toward darker (DID), respectively. High dichromaticity was found for materials that were previously known as dichromatic (pumpkin oil, bromophenol).

© 2009 Optical Society of America

1. INTRODUCTION

The three perceptual attributes of color—lightness, hue, and saturation—are generally presented as being independent. The color difference is a result of disparities in one or more of these attributes, but it is often difficult to identify which attributes are involved [1]. Moreover, variations in lightness can be caused by changes in hue or saturation (Helmholtz–Kohlrausch effect), and variations in hue can be caused by changes in lightness (Bezold–Bruecke effect) or in colorimetric purity (Aubert–Abney effect) [2].

Materials can change their hue with temperature [3], crystal structure [4], direction of illumination [5, 6], and layer thickness [7]. A phenomenon where some transparent substances change hue due to the change in layer thickness is called dichromatism [8, 9]. Its physoicochemical and physiological background was revealed only recently [7]. It was shown that it is caused by two minima in the absorption spectrum: one wide shallow minimum and one narrow deep minimum. The phenomenon of dichromatism is present in a broad range of substances, but in most it is barely detectable or not detectable by the human observer. The importance of dichromatism was acknowledged in the use of ophthalmic filters [10], but other applications are expected in all areas where a transparent colored material of changeable thickness or changeable concentration is used, e.g., acrylic glass products or beverages that are diluted (syrup, liqueur).

In textiles, cosmetics, furniture, and many other industries color is important. Unless special color effects are created, it is usually preferred that the color of a product be constant and independent of observation conditions. For this purpose, the phenomenon known as the alexandrite effect is commonly investigated. In this effect, the hue of the object appears different in daylight illumination and artificial illumination (e.g., an incandescent lamp). One of the extreme examples of the effect is a gemstone alexandrite, which appears green in daylight and turns red in incandescent light. The absorption spectrum of alexandrite is characterized by two absorption minima in the blue-green and red regions and by an absorption maximum in the yellow region [9, 11]. A smaller but significant change in hue is observed in many other colored materials. For this reason, different types of illumination are available in most stores with fashion clothes, furniture, and ceramics [12]. In gemology, gemstones are divided into four categories according to the extent of the alexandrite effect [13]. The hue angle change (Δh°) is used as a measure of this effect in other areas such as dentistry [14] and industrial design [15]. Computer programs are available for prediction of this phenomenon from the spectra, and the metamerism index is defined for quantification of the color changes that are due to illumination in different materials [16].

In contrast to the alexandrite effect, to our knowledge no attempt has yet been made to quantify dichromatism. Here we present a procedure and a computer algorithm that calculates the level of dichromatism from the absorption spectra of the substance. We propose a measure named dichromaticity index (DI), which, as far as we know, is the first measure for the quantification of dichromatism.

2. EXPERIMENTAL PROCEDURE

The spectra of pumpkin oil, bromophenol blue, and resazurine were measured on a sapphire spectrophotometer (Tecan, Austria). The spectra of other substances were obtained from published data (OMLC 2007; Optical Spectra; The Oregon Medical Laser Center at Providence St. Vincent Medical Center and the biomedical optics program at Oregon Health and Sciences University. http://omlc.ogi.edu/spectra/). The source code for the Dichromaticity Index Calculator software is available in Appendix A.

3. RESULTS

3A. Algorithm for Dichromaticity Measurement

The computer algorithm “Dichromaticity Index Calculator” was developed to calculate the two measures of dichromaticity: the dichromaticity index toward a lighter (DIL) and the dichromaticity index toward a darker (DID) substance. Lighter or darker color may be an outcome of diluting or concentrating transparent solutions of dyes or increasing/decreasing the layer thickness. When the resulting color is lighter or darker, this is usually accompanied by a change of the hue of the color, due to the phenomenon called dichromatism. The two indexes (DIL and DID) are newly defined quantitative measures of color inconstancy resulting from the dichromatic properties of substances. The indexes are calculated on the basis of their absorption spectra.

The Dichromaticity Index Calculator computer software tool was written as a script in Matlab (MathWorks, Natick, Massacusetts, USA). An absorption spectrum is first loaded into the software algorithm (Fig. 1 ). The software interpolates the spectral samples to match a wavelength range 380770nm and then calculates the transmittance spectrum of the originally entered absorption spectra. The absorption spectrum is then multiplied by the tenth root of 2 (1.07177), and the transmittance spectrum is calculated again. The process is repeated 200 times (the last spectrum corresponds to a sample that is 220-fold thicker than the sample used for measuring the original spectrum). The original absorption spectrum is then repeatedly divided by the tenth root of 2, and a series of another 100 transmittance spectra are calculated (the last spectrum corresponds to a sample that is 1024-fold (210) thinner than the sample used for measuring the original spectrum). The obtained combined array of transmittance spectra represents the spectra of a substance by increasing/decreasing the thickness of the layer (or increasing/decreasing concentration), where the spectrum number n+10 represents the spectra at double thickness compared with the nth spectra.

The CIE (Commission Internationale de l’Eclairage) chromaticity coordinates X, Y, and Z are then calculated from each spectrum by use of the corresponding CIE color matching functions [17, 18, 19]. The coordinates are transformed into a perceptually uniform CIELAB color space (also known as L*, a*, and b* color space). L* in the color space represents the luminance of the color (or value) on a numerical scale from 0 (black) to 100 (white). The color coordinates a* and b* represent a position between red (+a*) and green (a*), and between yellow (+b*) and blue (b*).

The next step in the computer algorithm is to calculate the chroma and the hue angle (hab) for each of the dilution steps. The dilution step with the maximal chroma is selected as the reference. From this reference, the hue angle difference (Δhab) is calculated toward a four times more diluted (or thinner) dilution step and toward a four times more concentrated (or thicker) dilution step of the sample. The two hue angle differences are called dichromaticity index toward lighter (DIL) and dichromaticity index toward darker (DID), respectively, where lighter and darker may be due to dilution/concentration or due to thinner/thicker sample. The units of DIL and DID are angular degrees.

3B. Graphical Output of the Dichromaticity Measurement

The computer program also draws the graphical output, which is composed of four panels (Figs. 2, 3 ). In both figures, the top-left panel shows the absorption spectrum of the substance. The top-right panel shows the CIE diagram of the x and y chromaticity coordinates. The inside curve marks the varying thickness of the layer or the dilution of the colored substance. Monochromatic colors are represented at the spectral locus and the purple line of the CIE chromaticity diagram, where the saturation is maximal, and increasingly less saturated colors are represented toward the central white color (the dot at the origin of the curve: CIE illuminant D65, which equates to average daylight). Color category regions are marked with the corresponding color on the perimeter of the diagram, and the hue centers are marked by large empty circles [20]. In all panels, the color positions of every twofold thickness of the layer are marked by small empty circles. In all panels, green squares mark the dilution/thickness with maximum chroma and the plus signs mark the dilution/thickness with 50% transmission. The bottom-left panel shows the CIELAB color space diagram of increasing thickness/concentration of the substance. Straight lines are vectors showing hue (angle) and chroma (length) of the color at maximal chroma (toward the squares) and the colors of fourfold less/more diluted or thick substance (DIL and DID). The bottom-right panel shows chroma at each dilution step.

Using the Dichromaticity Index Calculator software we calculated indexes DIL and DID for a set of 32 different spectra (Table 1 , Fig. 4 ). Our results show that the spectra used for calculation result in negative or positive index values, where negative index is attributed to the clockwise hue angle change, and vice versa. Furthermore, we show that spectral properties of many substances result in DIL that is different from DID.

4. DISCUSSION

So far, we believe that no attempt has been made to quantify dichromatism, which is a well-known property of the color of transparent objects or solutions. We have developed a procedure and a computer algorithm that calculates the level of dichromatism from the absorption spectra of the substance.

The software calculates the transmittance spectrum for each tenth-root-of-2 dilution step. The CIE chromaticity coordinates are calculated and transformed into a perceptually uniform CIELAB color space. The chroma and the hue angle are calculated for each dilution step. The dilution step with the maximal chroma serves as the reference for the hue angle difference (Δhab) toward a four times more diluted/concentrated dilution step. The two hue angle differences are called the dichromaticity index toward lighter (DIL) and the dichromaticity index toward darker (DID), respectively.

Several color-difference equations ΔE are used to describe whether the changes in the overall shade between two samples were perceivable to the human observer [21]. Significant improvements have been achieved by advanced CIELAB-based color-difference formulas, of which the CIEDE2000 is the latest [22, 23]. The CIELAB formula likely distorts hue differences around a hue circle and may not be assumed as the exact predictor of hue differences [24]. CIEDE2000 proposes a redefinition of the a* coordinate, leading to new chroma and hue differences [25]. In the CIEDE2000 the T function adjusts hue differences depending on the hue angle, but it might not be applicable to all levels of chroma and lightness [23]. Moreover, CIELAB may in part conflate hue and chroma differences, and similarly, due to the Helmholtz–Kohlrausch effect, it may also in part conflate chromatic and lightness differences [23]. For simplicity and clearness of our procedure, we did not introduce any of the hue difference correction functions.

Calculation of dichromaticity indices for a set of selected substances shows that the three substances where a dichromatic color phenomenon was previously described (pumpkin seed oil, bromophenol blue, and resazurine) have dichromaticity indexes exceeding 40°, whereas some other substances where the dichromatic properties were not yet described have smaller but still observable dichromaticity (Table 1, Fig. 4). Chlorophyll a diluted in ether has even higher dichromaticity indexes than previously described dichromatic substances. Interestingly, indexes DIL and DID are often not similar; for example, pumpkin seed oil has DIL8.97° and DID44.12°. This means that the hue angle difference is distinct when a substance is diluted or thickened. The negative values indicate clockwise hue angle changes, while positive values indicate counterclockwise hue angle changes.

To test the robustness of the system, we calculated DIL and DID from three pumpkin oil absorption spectra measured at three different concentrations. At highest concentration, the peak at 400nm was greater than 3, which was close to the overflow of the spectrophotometer, and the peak height at 560nm was 0.8; at the intermediate concentration, the peak at 400nm was 3 and the peak at 560nm was 0.4; at the lowest concentration the heights of the two peaks were 1.5 and 0.2, respectively. The DIL calculated from the three spectra were 8.97, 11.31, and 12.50, and the DIL calculated from the three spectra were 44.12, 45.21, and 45.44, respectively. This shows that the system is not very sensitive to inaccuracy in the entered spectra, but for accuracy it is recommended to use appropriately measured absorption spectra.

For further validation we measured the spectra of presumably nondichromatic “normally” colored blue, green, yellow, and red substances and calculated their DIL and DID. A 5mg tip of four water-soluble color pencils (color codes 451, 563, 407, and 421, Faber-Castell, Germany) were dissolved in 1ml of water and centrifuged at 130rpm for 10min, and absorption spectra were measured. DIL and DID were in all cases below 20 except for DID of yellow color, which was 20.63, and DIL of red color, which was 23.18.

5. CONCLUSIONS

We conclude that dichromatism is a common property of many substances and materials, and we have introduced a method for its quantification. We defined dichromaticity index (DI) as the difference in hue angle (Δhab) between the colors of the sample at the dilution step with maximal chroma and the four times more diluted (or thinner) and the four times more concentrated (or thicker) sample. High dichromaticity was found for materials that were previously known as dichromatic. Moreover, many other substances also have significant dichromaticity (acridine orange, vitamin B12), which should be considered when they are used as dyes.

APPENDIX A: Matlab Code for Computing Dichromaticity

The input absorption spectrum S is multiplied 200 times by the increasing thickness factor Th, and each time the CIE chromaticity coordinates X, Y, and Z (Xcie, Ycie, and Zcie, respectively) are calculated from each spectrum by use of the corresponding CIE color matching functions from the array “cie,” where columns represent the wavelength; X, Y, and Z color matching functions; and standard illuminant data for D65, respectively. K is the normalizing constant.

for n=1:200
Th=(11024)*2̂(0.1*n);
K=100sum(cie(:,3).*cie(:,5));
Xcie=sum(10.̂(S.*Th).*cie(:,2).*cie(:,5))*K;
Ycie=sum(10.̂(S.*Th).*cie(:,3).*cie(:,5))*K;
Zcie=sum(10.̂(S.*Th).*cie(:,4).*cie(:,5))*K;
The chromaticity coordinates are transformed into a CIELAB color space, whereL” is luminance, “a” and “bare two color coordinates, “chroma” is chroma, and “hab” is hue angle:
if Ycie100>0.008856
L=116*(Ycie100)̂(13)16;
else
L=903.3*(Ycie100);
end
if Xcie94.82>0.008856
fx=(Xcie94.82)̂(13);
else
fx=7.787*(Xcie94.82)+16116;
end
if Ycie100>0.008856
fy=(Ycie100)̂(13);
else
fy=7.787*(Ycie100)+16116;
end
if Zcie107.38>0.008856
fz=(Zcie107.38)̂(13);
else
fz=7.787*(Zcie107.38)+16116;
end
a=500*(fxfy);
b=200*(fyfz);
chroma=(â2+b̂2)̂(12);
hab=atan2(b,a);
hab=hab+2*pi*(hab<0);
The results for each dilution are stored in a matrix “res”:
res(n,1)=chroma;
res(n,2)=hab;
end
The dilution step with the maximal chroma is selected as the reference. From this reference, the hue angle difference (dhab) is calculated toward a four times more diluted (“step” is 20) and toward a four times more concentrated sample. The two hue angle differences are called dichromaticity index toward lighter (DIL) and dichromaticity index toward darker (DID), respectively:
step=20;
dhab=res(find(res(:,1)==max(res(:,1))),2)
res((find(res(:,1)==max(res(:,1))))step,2);
dhab=dhab2*pi*(dhab>pi);
dhab=dhab+2*pi*(dhab<pi);
DIL=dhabpi*180;
dhab=res(find(res(:,1)==max(res(:,1)))+step,2)
res((find(res(:,1)==max(res(:,1)))),2);
dhab=dhab2*pi*(dhab>pi);
dhab=dhab+2*pi*(dhab<pi);
DID=dhabpi*180;
Chroma at four times more diluted and concentrated solution is calculated (chromaL and chromaD, respectively); maximal chroma (ChromaMAX) and the hue angle of the maximal chroma (AngleAtChromaMAX) are calculated:
chromaL=res(find(res(:,1)==max(res(:,1)))step,1);
chromaD=res(find(res(:,1)==max(res(:,1)))+step,1);
ChromaMAX=max(res(:,1));
AngleAtChromaMAX=res(find(res(:,1)==max(res(:,1))),2)pi*180;

Tables Icon

Table 1. Dichromaticity (DIL and DID) and Related Parameters of Selected Substances, Calculated from Their VIS Absorption Spectra by the Computer Algorithm Dichromaticity Index Calculatora

 

Fig. 1 Flowchart of the Dichromaticity Index Calculator software algorithm.

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Fig. 2 Graphical output of the Dichromaticity Index Calculator computer algorithm for a typical dichromatic substance, pumpkin seed oil. Top left: absorption spectrum of pumpkin oil. Top right: CIE diagram of the x and y chromaticity coordinates. The inside curve marks the varying thickness of the layer or the dilution of the pumpkin seed oil. The central dot at the line origin represents white color. Color category regions with hue centers are marked by large empty circles (clockwise from lower-left: blue, green, yellow, orange, red). In the top-right, bottom-left, and bottom-right panels, the color positions of every twofold thickness of the layer are marked by small empty circles. Green squares mark the dilution/thickness with the maximum chroma, and plus signs mark the dilution/thickness with 50% transmission. Note that the latter marks are overlaid in all panels of the figure. The bottom-left panel shows the CIELAB color space diagram of increasing thickness/concentration of the pumpkin oil. Straight lines are vectors showing hue (angle) and chroma (length) of the color at maximal chroma (toward the square mark) and the colors of the fourfold less/more diluted or thick pumpkin oil (DIL and DID). Note that DID is 44.1     deg, and DIL corresponds to 8.97  deg but is not visible on the image, since it is hidden behind the curve. The bottom-right panel shows chroma at each dilution step.

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Fig. 3 Graphical output of the Dichromaticity Index Calculator computer algorithm for a nondichromatic substance, oxygenized hemoglobin. Top left: absorption spectrum of oxygenized hemoglobin. Top right: CIE diagram of the x and y chromaticity coordinates, as in Fig. 2. Bottom left: CIE a*b* diagram of increasing thickness/concentration of oxygenized hemoglobin. Straight lines are vectors showing hue (angle) and chroma (length) of the color at maximal chroma (toward the square mark) and the colors of fourfold less/more diluted or thick oxygenized hemoglobin (DIL and DID). Note that DID is 5.63deg and DIL corresponds to 1.17deg. The bottom-right panel shows chroma at each dilution step.

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Fig. 4 Bar chart of dichromaticity values DIL and DID for a selection of colored solutions. Note low dichromaticity values for hemoglobin and high DID value for pumpkin oil. Bromophenol blue has different dichromaticity values, which are dependent on pH.

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1. M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356–367 (2000). [CrossRef]  

2. M. J. Luque and P. Capilla, “Adaptación cromática y apariencia del color,” in Fundamentos de Colorimetría, P. Capilla, J. M. Artigas, and J. Pujol, eds. (Universidad de Valencia, 2002), pp. 55–70.

3. Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48–51 (2008). [CrossRef]  

4. N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43–47 (2008). [CrossRef]  

5. F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691–692 (2000). [CrossRef]   [PubMed]  

6. D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33–45 (1990). [CrossRef]  

7. S. Kreft and M. Kreft, “Physicochemical and physiological basis of dichromatic colour,” Naturwiss. 94, 935–939 (2007). [CrossRef]   [PubMed]  

8. I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405–421 (1941).

9. W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560–1566 (1975). [CrossRef]  

10. S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666–673 (1984). [PubMed]  

11. K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147–164 (1980).

12. W. A. Thornton, “Method and device for efficiently generating white light with good rendition of illuminated objects ,” Patent number: US 4176294 (December 3, 1976).

13. Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186–191 (1994). [CrossRef]  

14. S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374–379 (2007). [CrossRef]  

15. D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274–284 (2004). [CrossRef]  

16. Hunterlab, Metamerism, Application Notes , Vol. 6, No. 13 (Hunterlab, 2008). http://www.hunterlab.com/appnotes/an11_95.pdf.

17. CIE 015:2004, Colorimetry, 3rd ed. (Commission Inter nationale de L’Eclairage, Vienna, 2004).

18. L. Dunwoody, “Methodological consideration in color research,” Percept. Mot. Skills 72, 1125–1126 (1991). [CrossRef]   [PubMed]  

19. R. M. Boynton, “History and current status of a physiologically based system of photometry and colorimetry,” J. Opt. Soc. Am. A 13, 1609–1621 (1996). [CrossRef]  

20. A. Petzold and L. T. Sharpe, “Hue memory and discrimination in young children,” Vision Res. 38, 3759–3772 (1998). [CrossRef]  

21. R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760–1764 (1989) . [CrossRef]   [PubMed]  

22. CIE,Technical report: “Improvement to industrial colour-difference evaluation ,” CIE Publication 142, (CIE Central Bureau, Vienna, 2001).

23. R. G. Kuehni, “Ciede2000, milestone or final answer?” Color Res. Appl. 27, 126–127 (2002). [CrossRef]  

24. R. G. Kuehni, “Towards an improved uniform color space,” Color Res. Appl. 24, 253–265 (1999). [CrossRef]  

25. M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340–350 (2001). [CrossRef]  

References

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  1. M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
    [CrossRef]
  2. M. J. Luque and P. Capilla, “Adaptación cromática y apariencia del color,” in Fundamentos de Colorimetría, P.Capilla, J.M.Artigas, and J.Pujol, eds. (Universidad de Valencia, 2002), pp. 55-70.
  3. Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
    [CrossRef]
  4. N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
    [CrossRef]
  5. F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
    [CrossRef] [PubMed]
  6. D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33-45 (1990).
    [CrossRef]
  7. S. Kreft and M. Kreft, “Physicochemical and physiological basis of dichromatic colour,” Naturwiss. 94, 935-939 (2007).
    [CrossRef] [PubMed]
  8. I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405-421 (1941).
  9. W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
    [CrossRef]
  10. S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
    [PubMed]
  11. K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147-164 (1980).
  12. W. A. Thornton, “Method and device for efficiently generating white light with good rendition of illuminated objects ,” Patent number: US 4176294 (December 3, 1976).
  13. Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
    [CrossRef]
  14. S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
    [CrossRef]
  15. D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274-284 (2004).
    [CrossRef]
  16. Hunterlab, Metamerism, Application Notes, Vol. 6, No. 13 (Hunterlab, 2008). http://www.hunterlab.com/appnotes/an11_95.pdf.
  17. CIE 015:2004, Colorimetry, 3rd ed. (Commission Internationale de L'Eclairage, Vienna, 2004).
  18. L. Dunwoody, “Methodological consideration in color research,” Percept. Mot. Skills 72, 1125-1126 (1991).
    [CrossRef] [PubMed]
  19. R. M. Boynton, “History and current status of a physiologically based system of photometry and colorimetry,” J. Opt. Soc. Am. A 13, 1609-1621 (1996).
    [CrossRef]
  20. A. Petzold and L. T. Sharpe, “Hue memory and discrimination in young children,” Vision Res. 38, 3759-3772 (1998).
    [CrossRef]
  21. R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
    [CrossRef] [PubMed]
  22. CIE,Technical report: “Improvement to industrial colour-difference evaluation ,” CIE Publication 142, (CIE Central Bureau, Vienna, 2001).
  23. R. G. Kuehni, “Ciede2000, milestone or final answer?” Color Res. Appl. 27, 126-127 (2002).
    [CrossRef]
  24. R. G. Kuehni, “Towards an improved uniform color space,” Color Res. Appl. 24, 253-265 (1999).
    [CrossRef]
  25. M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
    [CrossRef]

2008 (2)

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

2007 (2)

S. Kreft and M. Kreft, “Physicochemical and physiological basis of dichromatic colour,” Naturwiss. 94, 935-939 (2007).
[CrossRef] [PubMed]

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

2004 (1)

D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274-284 (2004).
[CrossRef]

2002 (1)

R. G. Kuehni, “Ciede2000, milestone or final answer?” Color Res. Appl. 27, 126-127 (2002).
[CrossRef]

2001 (1)

M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
[CrossRef]

2000 (2)

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

1999 (1)

R. G. Kuehni, “Towards an improved uniform color space,” Color Res. Appl. 24, 253-265 (1999).
[CrossRef]

1998 (1)

A. Petzold and L. T. Sharpe, “Hue memory and discrimination in young children,” Vision Res. 38, 3759-3772 (1998).
[CrossRef]

1996 (1)

1994 (1)

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

1991 (1)

L. Dunwoody, “Methodological consideration in color research,” Percept. Mot. Skills 72, 1125-1126 (1991).
[CrossRef] [PubMed]

1990 (1)

D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33-45 (1990).
[CrossRef]

1989 (1)

R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
[CrossRef] [PubMed]

1984 (1)

S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
[PubMed]

1980 (1)

K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147-164 (1980).

1975 (1)

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

1941 (1)

I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405-421 (1941).

Adams, A. J.

S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
[PubMed]

Bank, H.

K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147-164 (1980).

Barber, D. J.

D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33-45 (1990).
[CrossRef]

Bednarik, J.

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

Boynton, R. M.

Calogero, S.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Capilla, P.

M. J. Luque and P. Capilla, “Adaptación cromática y apariencia del color,” in Fundamentos de Colorimetría, P.Capilla, J.M.Artigas, and J.Pujol, eds. (Universidad de Valencia, 2002), pp. 55-70.

Cui, G.

M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
[CrossRef]

Dunwoody, L.

L. Dunwoody, “Methodological consideration in color research,” Percept. Mot. Skills 72, 1125-1126 (1991).
[CrossRef] [PubMed]

Fink, W.

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

Freestone, I. C.

D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33-45 (1990).
[CrossRef]

Fritsch, E.

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

Fukui, K.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Gubelin, E.

K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147-164 (1980).

Ha, N. Y.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Haslbeck, S.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Hemphill, S.

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

Hewlett, E. R.

R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
[CrossRef] [PubMed]

Hita, E.

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

Howell, D. H.

I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405-421 (1941).

Ishikawa, K.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Jeong, S. M.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Kennard, I. G.

I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405-421 (1941).

Kim, J.

R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
[CrossRef] [PubMed]

Kim, S.

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Kishida, H.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Kitagawa, H.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Kreft, M.

S. Kreft and M. Kreft, “Physicochemical and physiological basis of dichromatic colour,” Naturwiss. 94, 935-939 (2007).
[CrossRef] [PubMed]

Kreft, S.

S. Kreft and M. Kreft, “Physicochemical and physiological basis of dichromatic colour,” Naturwiss. 94, 935-939 (2007).
[CrossRef] [PubMed]

Kuehni, R. G.

R. G. Kuehni, “Ciede2000, milestone or final answer?” Color Res. Appl. 27, 126-127 (2002).
[CrossRef]

R. G. Kuehni, “Towards an improved uniform color space,” Color Res. Appl. 24, 253-265 (1999).
[CrossRef]

Lee, Y.

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Lim, B.

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Little, A. C.

S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
[PubMed]

Liu, Y.

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

Luo, M. R.

M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
[CrossRef]

Luque, M. J.

M. J. Luque and P. Capilla, “Adaptación cromática y apariencia del color,” in Fundamentos de Colorimetría, P.Capilla, J.M.Artigas, and J.Pujol, eds. (Universidad de Valencia, 2002), pp. 55-70.

Martinek, K.-P.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Melgosa, M.

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

Morita, Y.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Naito, A.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Nakasuji, K.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Nakazawa, S.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Nishimura, S.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Ohashi, Y.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Ohtsuka, Y.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Okamoto, H.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Oulton, D. P.

D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274-284 (2004).
[CrossRef]

Pankhurst, Q. A.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Petzold, A.

A. Petzold and L. T. Sharpe, “Hue memory and discrimination in young children,” Vision Res. 38, 3759-3772 (1998).
[CrossRef]

Res, M. A.

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

Rhee, S.

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Richer, S. P.

S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
[PubMed]

Rigg, B.

M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
[CrossRef]

Rivas, M. J.

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

Sasaki, K.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Sato, K.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Schmetzer, K.

K. Schmetzer, H. Bank, and E. Gubelin, “The alexandrite effect in minerals: chrysoberyl, garnet, corundum, fluorite,” Neues Jahrb. Mineral., Abh. 138, 147-164 (1980).

Schneider, W.

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

Seghi, R. R.

R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
[CrossRef] [PubMed]

Sekine, A.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Sharpe, L. T.

A. Petzold and L. T. Sharpe, “Hue memory and discrimination in young children,” Vision Res. 38, 3759-3772 (1998).
[CrossRef]

Shigley, J.

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

Shiomi, D.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Shiro, M.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Stievano, L.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Suzaki, G.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Suzuki, S.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Takanishi, Y.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Takezoe, H.

N. Y. Ha, Y. Ohtsuka, S. M. Jeong, S. Nishimura, G. Suzaki, Y. Takanishi, K. Ishikawa, and H. Takezoe, “Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals,” Nature Mater. 7, 43-47 (2008).
[CrossRef]

Takui, T.

Y. Morita, S. Suzuki, K. Fukui, S. Nakazawa, H. Kitagawa, H. Kishida, H. Okamoto, A. Naito, A. Sekine, Y. Ohashi, M. Shiro, K. Sasaki, D. Shiomi, K. Sato, T. Takui, and K. Nakasuji, “Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers,” Nature Mater. 7, 48-51 (2008).
[CrossRef]

Thornton, W. A.

W. A. Thornton, “Method and device for efficiently generating white light with good rendition of illuminated objects ,” Patent number: US 4176294 (December 3, 1976).

Vienot, F.

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

Wagner, F. E.

F. E. Wagner, S. Haslbeck, L. Stievano, S. Calogero, Q. A. Pankhurst, and K.-P. Martinek, “Before striking gold in gold-ruby glass,” Nature 407, 691-692 (2000).
[CrossRef] [PubMed]

Yang, H.

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Young, T.

D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274-284 (2004).
[CrossRef]

Am. J. Optom. Physiol. Opt. (1)

S. P. Richer, A. C. Little, and A. J. Adams, “Effect of ophthalmic filter thickness on predicted monocular dichromatic luminance and chromaticity discrimination,” Am. J. Optom. Physiol. Opt. 61, 666-673 (1984).
[PubMed]

Am. Mineral. (1)

I. G. Kennard and D. H. Howell, “Types of coloring in minerals,” Am. Mineral. 26, 405-421 (1941).

Archaeometry (1)

D. J. Barber and I. C. Freestone, “An investigation of the origin of the color of the lycurgus cup by analytical transmission electron-microscopy,” Archaeometry 32, 33-45 (1990).
[CrossRef]

Color Res. Appl. (5)

M. Melgosa, M. J. Rivas, E. Hita, and F. Vienot, “Are we able to distinguish color attributes?” Color Res. Appl. 25, 356-367 (2000).
[CrossRef]

Y. Liu, J. Shigley, E. Fritsch, and S. Hemphill, “The alexandrite effect in gemstones,” Color Res. Appl. 19, 186-191 (1994).
[CrossRef]

R. G. Kuehni, “Ciede2000, milestone or final answer?” Color Res. Appl. 27, 126-127 (2002).
[CrossRef]

R. G. Kuehni, “Towards an improved uniform color space,” Color Res. Appl. 24, 253-265 (1999).
[CrossRef]

M. R. Luo, G. Cui, and B. Rigg, “The development of the CIE 2000 colour-difference formula: CIEDE2000,” Color Res. Appl. 26, 340-350 (2001).
[CrossRef]

Dent. Mater. (1)

S. Kim, Y. Lee, B. Lim, S. Rhee, and H. Yang, “Metameric effect between dental porcelain and porcelain repairing resin composite,” Dent. Mater. 23, 374-379 (2007).
[CrossRef]

Int. J. Clothing Sci. Technol (1)

D. P. Oulton and T. Young, “Colour specification at the design to production interface,” Int. J. Clothing Sci. Technol 16, 274-284 (2004).
[CrossRef]

J. Dent. Res. (1)

R. R. Seghi, E. R. Hewlett, and J. Kim, “Visual and instrumental colorimetric assessments of small color differences on translucent dental porcelain,” J. Dent. Res. 68, 1760-1764 (1989) .
[CrossRef] [PubMed]

J. Opt. Soc. Am. A (1)

J. Phys. D: Appl. Phys. (1)

W. Fink, M. A. Res, J. Bednarik, and W. Schneider, “Visual dichroism in glasses,” J. Phys. D: Appl. Phys. 8, 1560-1566 (1975).
[CrossRef]

Nature (1)

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Figures (4)

Fig. 1
Fig. 1

Flowchart of the Dichromaticity Index Calculator software algorithm.

Fig. 2
Fig. 2

Graphical output of the Dichromaticity Index Calculator computer algorithm for a typical dichromatic substance, pumpkin seed oil. Top left: absorption spectrum of pumpkin oil. Top right: CIE diagram of the x and y chromaticity coordinates. The inside curve marks the varying thickness of the layer or the dilution of the pumpkin seed oil. The central dot at the line origin represents white color. Color category regions with hue centers are marked by large empty circles (clockwise from lower-left: blue, green, yellow, orange, red). In the top-right, bottom-left, and bottom-right panels, the color positions of every twofold thickness of the layer are marked by small empty circles. Green squares mark the dilution/thickness with the maximum chroma, and plus signs mark the dilution/thickness with 50% transmission. Note that the latter marks are overlaid in all panels of the figure. The bottom-left panel shows the CIELAB color space diagram of increasing thickness/concentration of the pumpkin oil. Straight lines are vectors showing hue (angle) and chroma (length) of the color at maximal chroma (toward the square mark) and the colors of the fourfold less/more diluted or thick pumpkin oil ( DI L and DI D ). Note that DI D is 44.1       deg , and DI L corresponds to 8.97   deg but is not visible on the image, since it is hidden behind the curve. The bottom-right panel shows chroma at each dilution step.

Fig. 3
Fig. 3

Graphical output of the Dichromaticity Index Calculator computer algorithm for a nondichromatic substance, oxygenized hemoglobin. Top left: absorption spectrum of oxygenized hemoglobin. Top right: CIE diagram of the x and y chromaticity coordinates, as in Fig. 2. Bottom left: CIE a * b * diagram of increasing thickness/concentration of oxygenized hemoglobin. Straight lines are vectors showing hue (angle) and chroma (length) of the color at maximal chroma (toward the square mark) and the colors of fourfold less/more diluted or thick oxygenized hemoglobin ( DI L and DI D ). Note that DI D is 5.63 deg and DI L corresponds to 1.17 deg . The bottom-right panel shows chroma at each dilution step.

Fig. 4
Fig. 4

Bar chart of dichromaticity values DI L and DI D for a selection of colored solutions. Note low dichromaticity values for hemoglobin and high DI D value for pumpkin oil. Bromophenol blue has different dichromaticity values, which are dependent on pH.

Tables (1)

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Table 1 Dichromaticity ( DI L and DI D ) and Related Parameters of Selected Substances, Calculated from Their VIS Absorption Spectra by the Computer Algorithm Dichromaticity Index Calculator a

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