The perceived color is a very important property for architectural windows, including “smart” ones [9]. Quantitative assessments can be performed in several different ways; here we employ the CIE Colorimetric System [20,21]. The purpose of colorimetric analysis is to give color specifications for observers with normal vision in terms of tristimulus values or chromaticity coordinates. The ideal observer’s color matching functions are denoted x, y, and z, and represent red, green, and blue primaries, respectively. One can describe any color as an additive mixture of these. The y curve is chosen so that it coincides with the luminous efficiency of the light — adapted eye. The CIE 1964 tristimulus values corresponding to a certain color stimulus Ф(А) are obtained from

j" x (A W (A )dA *aE = j 7 (A)S (A )dA

and analogously for Vcie and Zcie. The color stimulus of present interest is

V(A) = S(A)T(A) (4)

where S(A) is the relative spectral irradiance function. Chromaticity coordinates, denoted x, y, and z, are then obtained from

X =——————— ^CIE——————— ^ (5)

XCIE + VCIE + ZCIE

and correspondingly for y and z. These formulas lead to the chromaticity diagram shown in the lower parts of the various panels of Figs. 3 and 4. Any color can be represented as a point within the shown boundary. The chromaticity coordinates for a colorless (achromatic) material are x = y = z = 0.333.

The chromaticity coordinates were calculated for four different standard illuminants representing a chosen set of light sources. Illuminant D65 signifies the average north sky daylight at 6500 K, illuminant A represents a tungsten halogen incandescent light

source at 2856 K (typical home or store accent lighting), illuminant F11 pertains to a commercial rare-earth-phosphor narrow-band fluorescent light source at 4000 K (used in Europe and the Pacific Rim for typical office or store lighting), and illuminant F2 represents a commercial wide-band-fluorescent cool white light source at 4150 K (typical office or store lighting in the U. S.A.).

Figure 3. CIE chromaticity diagrams representing the color of optimized electrochromic nickel-oxide-based films in fully bleached and colored states under daylight illumination (CIE D65; panel a), incandescent illumination (CIE A; panel b), narrow band fluorescent illumination (CIE F11; panel c), and cool white fluorescent illumination (CIE F2; panel d). Coordinates signifying the illuminant as well as colorlessness (i. e., the achromatic point) are plotted as a reference. The upper part of each panel is a magnification of the central region of the lower part. The designation NiXO (with X being Mg, Al, Si, V, Zr, Nb, Ag, or Ta) indicates that X is present in the oxide but does not specify the amount.

Figure 4. CIE chromaticity diagrams representing the color of optimized electrochromic iridium-oxide-based films in fully bleached and colored states under daylight illumination (CIE D65; panel a), incandescent illumination (CIE A; panel b), narrow band fluorescent illumination (CIE F11; panel c), and cool white fluorescent illumination (CIE F2; panel d). Coordinates signifying the illuminant as well as colorlessness (i. e., the achromatic point) are plotted as a reference. The upper region of each panel is a magnification of the central part of the lower part. The designation IrXO (with X being Mg, Al, Zr, or Ta) indicates that X is present in the oxide but does not specify the amount.

Figure 3 shows chromaticity coordinates for the optimized nickel-oxide-based thin films reported on in Fig. 2(a) under CIE standard illuminants D65 (panel a), A (panel b), F11 (panel c), and F2 (panel d), respectively. In panel (a), the data pertaining to the bleached state lie very close to the point representing colorlessness for illuminant D65. The data for the colored state are rather scattered, with the chromaticity depending on the specific additive. Also the trajectories in color space between the bleached and colored states are different for each of the additives. Another characteristic of the bleached state is that the dominant wavelengths are short so that the eye has less sensitivity. The chromaticity coordinates for the incandescent light, reported on in panel (b), indicate a light yellow color in the bleached state and a yellow-green color in the dark state. The data points for the bleached state appear less dispersed than the corresponding points for D65. For fluorescent lights, represented in panels (c) and (d) for narrow-band fluorescent and cool white fluorescent light sources, respectively, the data are similar and dispersed as regards the dark states. The films show a green-yellow color in the bleached state.

Figure 4 shows analogous chromaticity coordinates for the iridium-oxide-based films. From panel (a) it can be seen that the films, in both colored and bleached state, have chromaticity coordinates close to the achromatic point, and they are therefore almost colorless. Their dominant wavelength changes significantly from greenish blue to green, but the chromaticity remains close to the achromatic point. With the incandescent illuminant, for which panel (b) is appropriate, the color coordinates indicate that the films appear yellowish-orange to orange, and the color is more evident than for the daylight illuminant. In fluorescent light, panels (c) and (d) show that the films appear close to yellow. On the whole, it can be stated that the chromaticity coordinates for the iridium-based oxide films do not change to the same extent as the coordinates for the nickel-based oxide films. It can also be noticed that the iridium-based film that changes the most is the one containing Mg, while the film with the least change in color is comprised of pure iridium oxide.

Figure 5 shows Vqie, which corresponds to the luminous transmittance. The results in panel (a) are entirely consistent with those in Fig. 2(a). Films of nickel oxide mixed with Mg and Al show as much as 85 % luminous transmittance, whereas films containing Si and Zr yield up to ~83 %. For additives of Nb or Ta, the transmittance can be ~80 %. The case of the vanadium admixture is different, though, and shows a luminous transmittance not exceeding 75 %, i. e., lying ~4 % below the value for pure nickel oxide. Generally speaking, the luminous transmittance shows a rather weak dependence on the specific illuminant.

Panel (b) in Fig. 5 shows luminous transmittance of iridium oxide films mixed with Al, Mg, Ta, or Zr. Excepting the Zr-containing film, the additives lead to an improved luminous transmittance in the bleached state. Al and Mg rises the luminous transmittance to ~83 to 84 % from the initial 79 % of pure iridium oxide, while the Ta — containing film has a luminous transmittance of ~82 %. Zr, on the other hand, lowers the luminous transmittance by ~4 % due to a higher reflectance; thus such a film unsuited for applications requiring high transmittance.

Conclusions

We have shown that electrochromic nickel-oxide-based films can display enhanced short-wavelength transmittance in the bleached state when Mg, Al, Si, Zr, Nb, or Ta was added. However, admixtures of transition metals such as V and Ag did not improve the optical properties of the films. The colored state was not strongly affected by any of these additions. Larger admixtures are likely to change color, though, as one may infer from an analogy with mixed tungsten-based oxides [1,22]. Iridium-oxide-based films show improved transmittance in their bleached state when mixed with Al, Mg, or Ta, but not when mixed with Zr.

Acknowledgements

This work was supported by the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden. E. Avendano gratefully acknowledges a scholarship from the University of Costa Rica to complete his Ph. D. work at Uppsala University.