Subjective preference of light colour and LED lighting

The main objective of this work is to investigate the subjective preferences for lighting environments under different LED spectral power distributions (SPDs) and to analyse the different existing colour quality descriptors in order to recommend the best descriptor. An additional aim of the work is to find out the correlated colour temperature (CCT) and illuminance levels that users prefer for LED lighting. The experiments were conducted in lighting booths and in office rooms, where the subjective preferences for different LED light spectra were studied. In the lighting booth experiments, seven different LED SPDs were studied at CCTs of 2700 K, 4000 K and 6500 K at 500 lux. The study showed that the observers preferred the LED SPDs which increased the object chroma and colourfulness values (calculated in CIECAM02-UCS). Also, the preferred LED SPDs had higher values of reference-based metrics (such as colour quality scale (CQS) colour preference scale) and higher values of area-based metrics (such as CQS colour gamut scale or gamut area index (GAI)). The observers preferred the light sources at CCT of 4000 K and 6500 K over the CCT of 2700 K. The work was continued by simulation work and user acceptance studies to find out the simplified LED SPDs that the observers would prefer. The simulation results suggested that it is possible to generate simplified LED SPDs that have CQS Qp and CQS Qg values similar to those of the preferred complex SPDs that were generated by 9 to 11 different types of LEDs. The user acceptance studies conducted in the lighting booths also showed that the simplified LED SPDs using three different types of LEDs were preferred over complex LED SPD. Later, similar simplified LED SPDs were also studied in office rooms, and it was found that the observers preferred simplified LED SPDs over fluorescent lamp. The three different LED SPDs at each CCT of 4000 K and 6500 K were studied in the office room experiments. The observers preferred most the LED SPDs with high CQS Qp and CQS Qg or GAI values and least the LED SPDs that had the lowest CQS Qp and CQS Qg values. Also, the light sources (having high CQS Qp and CQS Qg) with negative Duv values were more preferred over light sources with positive Duv values maintaining the Duv values within the limit of ±0.0054. Moreover, for the office lighting, the observers preferred CCT of 4000 K over CCT of 6500 K at 500 lux. It was also found that the observers preferred the illuminance level of 500 lux over 300 lux. The results of the experiments conducted in the lighting booths to test the performance of different fidelity metrics showed that the CIE CRI, CRI2012 and CQS provide similar predictions for LED light sources that do not enhance the object chroma. It was found that the best prediction of colour fidelity was provided by CQS for LED light sources that enhance object chroma

Colour Rendering Index and colour rendering of LEDs

The purpose of this study was to understand CIE (Commission Internationale del Eclairage) Colour Rendering Index (CRI) and its deficiencies. Another aim was to find out limitation of CIE CRI for LEDs. Finally, current works on colour rendering of LEDs was examined in the study. CIE (Commission Internationale del Eclairage) Colour Rendering Index (CRI) is the only internationally recognized colour rendering metric. This metric expresses the colour rendering properties of light sources based on colour shift of test objects when illuminated by reference illuminant and test source. The CIE method to obtain CRI is called CIE test colour (sample) method. Despite of prominence, CIE test colour method has numerous deficiencies. The problems of CIE test colour method became more serious when applied to white light emitting diode based sources. Various studies have indicated that even LEDs with low values of CIE CRI can produce visually appealing, vivid, and natural light. CIE technical committee TC1-62 concluded that the CIE CRI is generally not applicable to predict the colour rendering rank order of white LEDs light sources. The colour quality of light sources is not just a colour difference. Colour discrimination, colour harmony, colour preference, colour acceptability, visual clarity, and brightness are some known dimensions of light source colour quality. For different application different light sources are suitable and relevant dimensions of light source colour quality should be used to select appropriate light sources as per the application. The universal colour rendering metric should be able to define all the dimensions of light source colour quality

Meaningful design in a multicultural community. A case study on multi-functional urban parks.

Urban planning is responsible for the arrangement of environments that we are living in as well as for the design of urban parks that allow us to escape from everyday stressors. However, we no longer live in culturally homogenous societies, and people of different backgrounds seem to have different perspectives on urban park aesthetics as well as the range of facilities and features that parks should provide. This study focuses on preferences and perspectives that people of different cultural backgrounds have of urban parks. This research was based on a single-case study of a multi-functional park – Waterloo Park, located in Kitchener-Waterloo (Ontario, Canada), and was focused on investigating urban park preferences of seven ethnic groups: Caucasian Canadians, East and North Asians, South Asians, Middle- Eastern, Arabic, African/Caribbean and African/Zimbabwean or Kenyan. The feedback obtained from face-to-face interviews with Waterloo-Park users have been analyzed in order to establish how do people of different cultural backgrounds conceptualize urban parks and what their breadth of needs are when utilizing park space. Demographic information, such as ethnic association, was obtained from the participants on a self-descriptive basis. Findings from this study indicates that there are apparent differences in expectations and needs that culturally diverse users have regarding urban parks, and provides substantial evidence that culture plays an influential role in perception and evaluation of urban parks. Recommendations for professional practice advocate shifting Canadian design practices towards a true comprehensive and multifunctional park design and incorporating the various motives and needs of a culturally diverse Canadian society

Photometric measurements of lighting quality: an overview

This paper aims to provide an overview of various aspects used to measure the overall lighting quality, instead of solely focusing on a single aspect. Lighting quality is a concept that allows excellent vision while providing high comfort. However, it cannot be measured directly; although, it can be indicated by measuring different aspects of lighting quality individually. By reviewing thirty eligible studies, based on forward and backward citation, eleven lighting quality aspects were determined that influence the overall lighting quality. Preferably, control algorithms for high quality lighting implement all these aspects; however, current control algorithms generally have a limited focus on energy or one or two specific aspects. Consequently, these control algorithms do not necessarily provide high quality lighting. This paper reviews measurement methodologies for the variable aspects of lighting quality, namely quantity, distribution, glare, spectral power distribution, daylight, directionality, and dynamics. We distinguish ad hoc and continuous measurements for each aspect. Ad hoc measurements are single measurements that provide a high accuracy “snapshot”; continuous measurements have a lower accuracy but provide a good overview over time that is relevant as input for lighting control algorithms optimizing the lighting quality. This overview shows that luminance distribution measurement devices are highly suitable for measuring lighting quality for both ad hoc and continuous measurements. Except the spectral power distribution, all variable aspects of lighting quality can be measured using such a device although the aspects directionality and dynamics still require more suitable luminance based indicators. Hence, it can be concluded that a luminance distribution measurement device is very suitable device to provide relevant input for lighting quality control algorithms

Effect of Window Glazing on Colour Quality of Transmitted Daylight

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, Ra,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

Feasibility of ceiling-based luminance distribution measurements

There is a high relevancy in the luminance distribution related to the perceived visual comfort. Moreover, the required technology is maturing such that it is feasible to integrate such devices in lighting control systems, which is expected to improve the overall lighting quality in office environments, or to conduct long-term field studies. Preferably, the luminance distribution measurement corresponds to the visual field of the user. However, for long-term measurements this is not feasible as this causes interference. Therefore, this study aimed to find a suitable ceiling-based position for luminance distribution measurements. In a first phase, the most suitable ceiling-based measurement position was identified for four luminance based metrics: Desktop Luminance, Monitor Luminance, B40 Luminance, and Retinal Illuminance. The results showed that a ceiling-based position above the aisle with a 20° angle relative to the ceiling was the most suitable position because its field of view has large similarities with the field and angle of view of the user. In a second phase, the performance of this most suitable position found in phase 1 was assessed under real office conditions, and compared with the visual field of the user. The Desktop and Monitor Luminance achieved an acceptable accuracy with very basic commissioning. The Retinal illuminance was measured with a reasonable accuracy when an elaborate calibration procedure was applied. For the B40 Luminance, in all scenarios, inaccuracies above 20% were found. This study shows that ceiling-based measurements are feasible, except for the B40 luminance, however, one should account for the introduced uncertainty

Recommendations for long-term luminance distribution measurements: the spatial resolution

Currently, luminance distribution measurement devices are increasingly used because there is high relevancy in the luminance distribution to the perceived visual comfort, also technology is maturing. It is now feasible to conduct long-term measurements and integrate these devices into lighting control systems. This, however, can result in new issues such as privacy controversies and high computational costs, induced by high spatial resolutions. Therefore, this study aims to propose a spatial resolution that is able to measure the luminance accurately while minimizing privacy sensitivity and computational costs. This is done based on luminance distribution measurements in office environments. The accuracy of lower resolution luminance distributions is tested for the mean and maximum luminance and the illuminance. Additionally, the ability to recognize faces is measured as an indicator for privacy-sensitive content. Finally, the processing time is measured as an indicator for the computational costs. The results show that for mean luminance or illuminance measurements the spatial resolution can be reduced significantly to 440 x 330 and 720 x 540 pixels, respectively. This spatial resolution does not compromise the accuracy while minimizing the ability of automated facial recognition and reducing the computational costs significantly. However, for maximum luminance measurements, a high resolution of 3000 x 2250 pixels is deemed appropriate, although this does allow automated facial recognition and results in high computation costs. A toolbox has been developed to assist others in choosing a relevant spatial resolution for their luminance camera during long term luminance measurements in typical office environments

Spectral tuning of luminance cameras: a theoretical model and validation measurements

Presently, luminance distribution measurement devices, using High Dynamic Range technology, are increasingly used as they provide a lot of relevant data related to the lit environment at once. However, the accuracy of these devices can be a concern. It is expected that the accuracy would be improved by incorporating the effect of the camera spectral responsivity and the spectral power distribution of the illuminant under which the measurements are conducted. This study introduces two optimization criteria incorporating these aspects to improve the spectral match and the performance of luminance distribution measurement devices. Both criteria are tested in a theoretical model and in practical measurements using two cameras and three illuminants: LED, halogen and fluorescent. Both methodologies support the hypothesis that the conventional method to determine the luminance introduces spectral mismatches that can be limited by optimizing relative to the spectral responsivity of the camera. Additionally, substantial evidence was found, by both the theoretical model and the validation measurements, that the spectral power distribution of the illuminant also has an effect on the performance

Interaction of daylight and electric light on subjective light appraisals in office environments

Background To improve human daytime functioning and provide healthy lighting regimes in office environments, better insights are required in how to provide light conditions that cater optimally to both non-image forming processes and visual performance and comfort. Controlled lab studies are suggesting that we should offer more light during daytime to users for healthy circadian entrainment and higher state alertness, as brighter light in the evening/night may impede healthy circadian entrainment. However, providing pleasant and bright light conditions has proved to be quite challenging. The challenge is even bigger when we consider that most office workers work in environments where there is also a contribution of daylight, resulting in light patterns that change substantially over the day and over seasons. Objective A field study was conducted in a real office environment, with the objective to explore the relationship between subjective light appraisals and objective light conditions. To allow for meaningful variations in both daylight and electric light, longitudinal measurements were performed across the day, across seasons, and under both standard and bright electric lighting. Method The field study was performed twice, over three weeks in summer and three in winter, in a Dutch office environment. The study was approved by the local ethical committee. In summer 12 office workers (1 female, mean age 45, SD = 10.06, range 25 to 55) participated; in winter 11 (1 female, mean age 43, SD = 10.59, range 25 to 55). Each of the three weeks had a unique electric lighting regime. In the first week, participants received extra bright light (average electrical desktop luminance 110 cd/m2) in the morning, standard lighting (average electrical desktop luminance 15 cd/m2) in the afternoon, in the second week only standard lighting, and in the third week they received extra bright light in the afternoon. Ecological momentary assessment was employed, combining frequent experience sampling (ES, self-report) with continuous measurements using ambulant and office-bound sensors. The ES questionnaires probed subjective light appraisals, alertness, well-being and thermal comfort, 10 times a day. Light loggers provided estimates of personal light exposure. Additionally, the lit environment was continuously measured using three image-based luminance distribution measurement devices, referred to as Bee-Eyes, attached to the ceiling with a sampling interval of ten minutes. Based on these luminance measurements relevant light performance indicators such as the desktop luminance, luminance ratio between electric light and daylight, and luminance contrasts were extracted. Results The measurements resulted in a rich dataset consisting of both subjective and objective data captured in a real office environment with seasonal and daily variations in daylight under varying levels of electric light. Statistical analyses will relate objective measures such as desktop luminance, luminance ratio between electric light and daylight, and luminance contrast in the periphery to subjective appraisals of pleasantness, brightness, and color. Discussion This field study can provide relevant insights for designing light interventions applicable to other office situations. The combination of both objective and subjective measures allows for a deeper understanding of the relationship between actual light exposure and the potential interaction between NIF and IF effects

Lighting for road tunnels

Drivers' visual performance is closely related to traffic safety in a real driving environment. In order to improve the traffic safety of road tunnel lighting, the effect of LED lighting on human visual performance was investigated using reaction time as a parameter. The experiment was performed with a scale model that can simulate a road tunnel lighting environment. Reaction times were measured under different values of luminance, correlated colour temperature (CCT), eccentricity, and contrast. The results show that visual performance can be improved by increasing the CCT of the light sources. The improvement of visual performance is greater in peripheral vision than that in foveal vision. The shortest reaction times were measured at a luminance level of 10 cd/m(2) and at a CCT of 5000 K. An appropriate luminance value with high CCT is recommended for tunnel lighting in interior and transition zones.Peer reviewe