Lighting Research & Technology

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.

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 = 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 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 ± 0.0054.

The CIE colour rendering index (CRI) has been criticized for its poor correlation with the visual colour rendering of many spiked or narrowband sources, its outdated colour space and chromatic adaptation transform and the use of a small number of non-optimal reflectance samples that have enabled lamp manufacturers to tune the spectrum of a light source to yield, in some cases, inappropriately high general CRI values. The CRI2012 metric proposed in this paper addresses these criticisms by combining the most state of the art colorimetric colour difference model, i.e. CAM02-UCS, with a mathematical reflectance set that exhibits a highly uniform spectral sensitivity. A set of 210 real reflectance samples has also been selected to provide additional information on the expected colour shifts when changing illumination.

High-pressure sodium lamps are currently the main lamps used in public lighting. However, the possibility of using high-power light emitting diode (LEDs) for street lighting is growing continuously due to their greater energy efficiency, robustness, long life and light control. The aim of this paper is to study the power quality of high-power lighting networks based on LED and high-pressure sodium lamps. Both electromagnetic and dimmable electronic ballasts, which can dim the lamp output smoothly and uniformly, have been used connected to high-pressure sodium lamps. High-pressure sodium lamps connected to electronic equipment have been tested with different arc power levels using dimming on a 230 V power supply. The study presented in this paper is completely based on measurements, including harmonic currents in the frequency range up to 150 kHz for all the technologies. The main results show a broadband spectrum in LED lamps which confirms other research in fluorescent lamps powered by high-frequency ballasts. Results also indicate a decrease in the harmonic value with increasing harmonic order, and a decrease in the harmonic value at half load (60%) compared with full load (100%). Although total harmonic distortion of the current is lower with high-pressure sodium lamps connected to electronic rather than electromagnetic ballasts, LED lamps achieved the lowest total harmonic distortion of current.

A novel method of reducing the energy consumption of street lighting using electronic ballasts was simulated on 277 units of 250 W high pressure sodium (HPS) street lights for a month. The design of the electronic ballast is detailed. A control system was utilised to dim the lights at hours of reduced traffic. The energy consumption was compared to that of the conventional system. The electronic ballast operates at a higher frequency and corrects any power distortion in the supply. Together with the light dimming capability, the electronic ballast increased the efficiency of the lighting system resulting in a monthly saving of 10.7 MWh, giving a monthly equivalent carbon dioxide reduction of 6.1 tonnes. In Malaysia, this would translate to a monthly cost saving of 37.7%.

Important efforts are being made to increase electric energy savings in the field of lighting. Of all the different approaches possible, two are used more frequently than others: The use of lighting control systems and the replacement of old light sources with better ones. One direct effect of the employment of new light sources and light dimming controls is a decrease of the power quality of the electrical network. This paper addresses luminous and power quality measurements of different light sources that are suitable for office buildings. The measurements were performed with and without dimming. In this way, the luminous and the power quality characteristics of the light sources, at different light levels, were established.

In this paper, we propose a composite optical system for automotive headlamps. The system is composed of a paraboloid collimating reflector, an off-axis paraboloid reflector, a baffle and an imaging len. By using a LED as the light source, precise light distribution can be achieved. When making ray-tracing simulation for the composite optical system in TracePro, a clear cut-off line can be seen on the measuring screen, as well as a hotspot at the centre. The power consumption for the LED light sources in this system is less than 10 W, resulting in an energy efficiency of more than 58% and a maximum illumination of 26.76 lux at the centre hotspot.