Marjukka Puolakka

User preferences for LED lighting in terms of light spectrum

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.

Subjective preferences and colour quality metrics of LED light sources

To understand peoples 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 = 80, high feeling of contrast index (FCI) keeping Ra = 80, high CQS relative gamut area scale (Qg) keeping Ra = 80, low FCI keeping Ra = 80, low Qp keeping Ra = 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 peoples 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 ± 0.0054.

Alerting effects of daytime light exposure - a proposed link between light exposure and brain mechanisms

The effects of light on alertness have been shown several times and the proposed cause has been suppressed melatonin levels. The relation of melatonin and alertness applies at night but not by day when there is hardly any melatonin. Still, light can be used to improve daytime alertness, but how? This paper describes the brain mechanisms involved in light-induced daytime alertness and proposes a novel model of two parallel mechanisms. In addition to the well-established circadian pathway, it is suggested that light can use the amygdala in the limbic system to send signals to the cerebral cortex. The participation of the amygdala in light-induced alertness means that light is provoking and modulating emotions that induce alerting responses. The model is assembled from known relations but has not yet been verified as a functional system. The paper proposes methods to test the model.

Road Lighting Quality, Energy Efficiency, and Mesopic Design – LED Street Lighting Case Study

Abstract The topics of this paper are mesopic dimensioning, glare properties and energy efficiency of LED street lighting. Road lighting luminance measurements and threshold increment (TI) measurements are made for four different LED street lighting installations. The effect of mesopic dimensioning on energy consumption and life cycle costs of the LED installations are examined. Through the calculations and measurements, the energy efficiency and glare properties of the LED installations are analyzed. The measurement results indicate that the measured and calculated threshold increment values differ from each other due to varying calculation parameters in real installations. It also shows that energy can be saved when mesopic dimensioning is applied to street lighting design.

User acceptance studies for LED office lighting: Preference, naturalness and colourfulness

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) + 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.

User acceptance studies for LED office lighting: Lamp spectrum, spatial brightness and illuminance

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.

Life cycle assessment of a fluorescent lamp luminaire used in industry - a case study

Environmental impacts of light sources and control gears have been analysed in several life cycle assessments (LCAs). In contrast, this paper analyses the environmental impacts of an entire fluorescent lamp/luminaire combination. The environmental impacts of a fluorescent lamp luminaire with two T5 lamps and an electronic ballast are analysed in a variety of potential environmental impacts. The results show that the energy consumption during use dominates the total environmental impacts by 93% on the average, while manufacturing accounts for 7% and end-of-life less than 1%. The importance of energy consumption is dependent of the used energy source. Compared to a previous LCA of fluorescent lamp luminaire, the division of the results into life cycle stages is similar but there are differences in the numerical results.

Thermal Management of Outdoor LED Lighting Systems and Streetlights—Variation of Ambient Cooling Conditions

Abstract Optimized cooling design of outdoor LED luminaires, and other outdoor electronics, is not possible without better knowledge of ambient cooling conditions and their variation. Temperatures and wind speeds have a significant effect on heat transfer between the enclosure surfaces and their surroundings. In the case of LEDs, where optical characteristics, reliability, and lifetime depend strongly on temperature, information on not only the worst case, but also on average operating conditions is needed. This study presents outdoor night time temperature and wind speed data for 50 locations worldwide. Dew point temperatures, precipitation and snowfall data are also discussed. The average night time temperature and wind speed are found to be 12.5 degrees Celsius and 3.1 meters per second, respectively. Variation of local average and maximum night time temperatures is found to be significant from a thermal management point of view. Wind speed is, on average, above zero over 90% of the time, suggesting, its effect on convective cooling of outdoor LED luminaires should not be neglected. The data indicates that the design parameters for optimized cooling of outdoor LED luminaires vary significantly between installation locations. In an example simulation presented, over 40% reductions in weight and cooling area are achieved using a location specific approach. The data presented provides information on cooling parameter variation, as well as on the average and worst case night time ambient conditions experienced by outdoor LED luminaires. The information should be taken into consideration in luminaire cooling design, as well as in research on thermal management, reliability, and optical performance of outdoor LED lighting systems.

User-acceptance studies for simplified light-emitting diode spectra

In previous user-acceptance studies conducted at Aalto University, it was found that the preferred light-emitting diode spectral power distributions (SPDs) were not characterised by a high-CIE colour-rendering index but by a high-colour quality index (CQS) colour preference scale (Qp) and a high-CQS gamut area scale (Qg). In these studies, the SPDs were realised with a 12-channel LED spectra simulator. It is, however, foreseen that LED light sources consisting of 12 different types of LEDs will not be commercially exploitable due to complexity. The objective of this work was to investigate the possibility of generating simplified LED SPDs having CQS Qp and CQS Qg values similar to those of the preferred complex LED SPDs found in the previous user-acceptance studies. User-acceptance studies were carried out in lighting booths to investigate people’s preferences for the lit environment under both the complex and simplified LED spectra. The results suggest that the preferred complex LED SPDs can be optimized both for efficiency and cost without sacrificing the colour quality of the light.

Investigation of pavement light reflection characteristics

ABSTRACT Current road lighting standards and recommendations are based on luminance levels and luminance distributions on the road surface. The luminance of the road surface depends on the amount of light falling on it and the reflection properties of the road surface. Using light aggregate for road surface pavements, significant energy savings in road lighting could be achieved due to significantly higher reflectance properties of the surface. In this study several experimental pavement samples with light surface characteristics were manufactured and their reflection properties were measured. The results indicate that lighter pavement materials result in higher average road surface luminance values and lower average lighting energy usage costs per kilometer.