Color rendering performance of smart glazings for building applications

In this paper study of the performance of advanced glazing with respect to color rendering control in buildings is presented. The research is performed by analyzing both absolute color appearance of objects and how color is distorted when daylight is spectrally redistributed by different advanced windows. The analysis takes advantage of a literature survey of the transmittance spectra of the main typologies of dynamic windows which are currently integrated or have the immediate potential to be used in buildings, namely electrochromic, thermochromic, photochromic, gasochromic windows, suspended particle devices and liquid crystal devices. Color rendition is assessed by applying both the traditional CIE CRI methodology and recent color rendering ratings such as the IES TM 30-15 methodology. Results show that the color rendition of the interior lighting can be greatly affected by the application of such advanced devices especially when they are brought in the low transmittance states. In the CIE 1976 color space, in fact, the chromaticity coordinates of the daylight filtered by the windows lie above the blackbody locus in all transmittance states implying that the color appearance of objects is desaturated. The distortion gets worse as the window transmittance is reduced since the distance from the blackbody locus increases. The effect is relevant for the electrochromic device in its lowest transmittance states (<10%) which are characterized by color rendering index values below 80. In all the analysed devices the reduction of the luminous transmittance of the glazing leads also to a reduction of the two color metrics relative to the IES TM 30-15 rating methodology. This reduction reflects the appearance of desaturated colors, the effect being relevant for electrochromic, thermochromic and photochromic windows.