A study was carried out in light booths to investigate user preference for different light spectra. Twenty-one LED spectra and three fluorescent lamps, all at three different correlated colour temperatures were used for the study. Sixty observers evaluated the lit scenes under different spectra for naturalness of selected objects, colourfulness of a Macbeth Colour Checker Chart, and the visual conditions of the lighting booths. The observers preferred the spectral power distributions (SPD) under which the chroma and colourfulness values of the object colours were higher. The CIE Colour Rendering Index was not a good indicator of the observers' preference for the LED SPDs. It seemed that light sources with a higher CQS Gamut Area Scale and CQS Colour Preference Scale values were preferred by the observers as far as LED spectra were concerned. However, the CQS Gamut Area Scale calculated with a new version of the software (version 9.2) and the Gamut Area Index were suitable as indicators of observers' preference for both the LEDs and the fluorescent lamp spectra. Because chroma and colourfulness values of object colours influence the observers' preference, they should be considered as important factors for the selection of a colour preference index for light sources.

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Two mock-up office rooms, one with a spectrally tuneable LED lighting system and the other with a fluorescent lamp (FL) lighting system were built for user acceptance studies for office lighting. One room was illuminated with six LED luminaires and the other with six FL luminaires. Each LED luminaire has 480 LEDs (20 different LED types and 24 of each type). Each FL luminaire has eight fluorescent lamps, four with a correlated colour temperature of 4000 K and four of 6500 K. User acceptance studies in the office environments were conducted with 40 observers, each observer having 16 sessions (six LED spectral power distributions (SPDs) þ two FL SPDs and two illuminances (500 lux and 300 lux)). The observers performed office-related tasks while being totally immersed in one of these lighting conditions. The objective of this study was to verify previous results obtained in small-scale booth experiments and to further study peoples' preferences for LED office lighting. The small-scale experiments showed that LEDs with a higher value of reference-based colour metrics (such as colour quality scale (CQS) colour preference scale) and with a higher value of area-based or volume-based metrics (such as CQS gamut area scale or gamut area index) were preferred by the observers. The office room study of the present work validated the findings of the earlier experiments but now in an office lighting environment. Furthermore, the observers preferred 4000 K to 6500 K at a light level of 500 lux and the light level of 500 lux over 300 lux. It was also found that the simplest LED SPD with three peaks has similar colour quality characteristics to complex LED SPDs with several peaks.

To understand people's judgement of the naturalness of object colours and preference for the lit environment, lighting booth experiments were conducted. Seven different LED spectral power distributions, all at correlated colour temperatures of 2700 K, 4000 K and 6500 K, were simulated for high CIE colour rendering index (Ra), high colour quality scale (CQS) colour preference scale (Qp) keeping Ra ¼ 80, high feeling of contrast index (FCI) keeping Ra ¼ 80, high CQS relative gamut area scale (Qg) keeping Ra ¼ 80, low FCI keeping Ra ¼ 80, low Qp keeping Ra ¼ 80, and for a mimic of a fluorescent lamp (FL) in terms of the closest possible value of the Ra, FCI and Qp to the reference FL. Other metrics such as the gamut area index (GAI), memory CRI, and nCRI were also considered in the analysis. Sixty observers evaluated 24 lighting situations. This paper deals with a comparison evaluation, where LED lighting was compared to FL lighting. It was found that Ra does not explain the subjective naturalness of objects and the subjective preference of the lit environment for the different light sources. The results also indicate that people's judgement of naturalness and overall preference were explained better when a reference-based metric (such as Qp) and an area or volume-based metric (such as Qg or GAI) were both considered while maintaining the light source chromaticity difference (Duv) value within the limit AE 0.0054.

In order to determine user acceptance for light-emitting diode (LED) office lighting, a full-scale experiment was conducted in two mock-up office rooms with different light spectra at two different illuminances. Six LED spectra and two fluorescent lamp spectra were used for the study. There were four spectra (three LED spectra and one fluorescent lamp spectrum) at a correlated colour temperature (CCT) of 4000 K and four spectra (three LED spectra and one fluorescent lamp spectrum) at a CCT of 6500 K. Forty observers evaluated the lighting environments under different spectra and rated the lighting environments sitting at a working desk and at a meeting table. The observers preferred the task illuminance of 500 lux to 300 lux and the CCT of 4000 K to 6500 K. The observers preferred the spectral power distributions (SPDs) under which they found the lighting environment to look brighter and more spacious. The observers' preferences showed that spatial brightness was affected by illuminances and SPDs. The lighting with a SPD having higher values of a reference-based metric (CQS Colour Preference Scale) and an area-based metric (CQS Gamut Area Scale or Gamut Area Index) was preferred most at a CCT of 4000 K. It was also found that the lighting with the fluorescent lamp was least preferred among the other SPDs at the CCT of 4000 K. The observers preferred both the simple and complex SPDs over fluorescent lamps at 4000 K in the office environment. It would be a good option to develop simple LED SPDs while maintaining the criteria of good lighting for an office environment.

In this study, the colour quality of the daylight transmitted through different window glazing types is evaluated. The analysis considered four different types of window glazing: laminated, monolithic, coated and applied film glazing ranging in luminous transmittance from around 0.97 to <0.1. The spectral transmittance data of different window glazing types are taken from the International Glazing Data Base (IGDB), which is maintained by Lawrence Berkeley National Laboratories (LBNL). The study showed that the CIE CRI does not always seem to be the suitable method to predict the colour quality of daylight in building for particular situations. However, in the context of this study, the prediction of colour rendering properties of window glazing by other metrics such as Colour Quality Scale (version 9), Memory CRI, R a,D65 (adjusted CRI metric with D65 as the reference illuminant) performed better. For most of the daylit situations inside the building, the chromaticity difference criterion was not met. Judging the colour quality of such situations requires different method.

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To investigate the performance of three different colour fidelity metrics, Commission Internationale de l'Eclairage (CIE) colour-rendering index (CRI), colour quality scale (CQS) and CRI2012, lighting booth experiments were conducted. Three different light-emitting diode spectral power distribution, all at correlated colour temperatures of 2800 K, 4000 K and 6500 K, were simulated for Ra490, Ra % 80 and Ra560. It was found that CIE CRI, CQS and CRI2012 were highly correlated with visual colour difference. It was also found that colour quality scale provides better prediction for the light-emitting diode light source that highly increases object chroma. However, more colour fidelity experiments using lightemitting diode light sources that highly increase object chroma are required before recommending any final colour fidelity metric.