Lighting Research & Technology

Colour rendering for picture gallery lighting means colour fidelity, showing the colours of the pictures as seen by the painter in the light he used in creating the pictures. As up to the beginning of the 20th century illuminance high enough for good colour vision was possible only in daylight, daylight would be the optimum illuminant. For art preservation and energy saving reasons this is not feasible. Museums often use light of 3500 K correlated colour temperature (CCT). A method is described that takes chromatic adaptation into consideration to determine the spectral power distribution producing least colour distortion of object colours while changing from a higher adaptation luminance at 6500 K to 3500 K illumination at a lower adaptation luminance. The method can be used for any CCT and adaptation luminance values.

An earlier paper has described a new method to optimise the spectral power distribution of solid state lighting systems used in museums. This paper will discuss the question of the selection of the test samples used for the spectral power distribution optimisation for the illumination of renaissance paintings. For fine tuning the spectrum the help of museum curators was needed, who remember the appearance of the pictures under traditional light sources and who were able to advise lighting engineers to tune the spectrum to obtain a pleasing appearance. The paper will show the results obtained by testing the optimum spectrum in the museum. Part 1 of the present publication dealt with a new method to optimise the spectral power distribution of Solid State Lighting systems used in museums. Part 2 will discuss the question of test samples for the illumination of renaissance paintings and will show results obtained by testing the optimum spectrum in the museum.

This paper describes design of daylight collectors using multiple Fresnel lenses, each facing the sun for a particular period of time in a day. The influence of solar movement on flat Fresnel lens was studied. The influence of groove density of the lens was also determined by which the suitable Fresnel lens was selected. Passive and profiled north-facing and south-facing collectors with multi-lens system have been designed based on the solar altitude and azimuth angle of a geographical location. Performance of the north- and south-facing collector at different times of a day with an acrylic dome indicated better efficiency and uniformity of lighting throughout the day time for the proposed profiled Fresnel collectors.

After dark, road lighting should enhance the visual component of pedestrians’ interpersonal judgements such as evaluating the intent of others. Investigation of lighting effects requires better understanding of the nature of this task as expressed by the typical distance at which the judgement is made (and hence visual size) and the duration of observation, which in past studies have been arbitrary. Better understanding will help with interpretation of the significance of lighting characteristics such as illuminance and light spectrum. Conclusions of comfort distance in past studies are not consistent and hence this article presents new data determined using eye-tracking. We propose that further work on interpersonal judgements should examine the effects of lighting at a distance of 15 m with an observation duration of 500 ms.

In daylight science the availability of sunlight and skylight at different times has been studied because of the desire to use daylight in both exterior and interior spaces. Exterior skylight illuminances under overcast skies were adopted as a standard for window design in the past. Current ISO/CIE sky types are standardised as relative luminance patterns normalised to the zenith. In this paper, the zenith luminance in candela/square metre and the resulting diffuse illuminance in lux are determined for all sky types. Furthermore, the proportions of sunlight and skylight under different levels of turbidity are found and documented. Efforts to harmonise electric lighting and daylighting standards need to analyse the data on available daylight in physical units in order to ensure energy savings that respect human requirements as well as providing information suitable for computer-aided design.

An experiment has been carried out to investigate the effect of lighting on the perception of atmosphere in a living room, using three types of light sources: halogen, fluorescent and LED lamps. In a psychophysical experiment, 29 native Chinese observers assessed eight lighting conditions having different luminances and correlated colour temperatures. For each condition, 71 scales were employed using the categorical judgment method. Factor analysis identified two underlying dimensions: liveliness and cosiness. This agrees with those found by Vogels who used Dutch observers to assess atmosphere perception. Both observer groups also agreed that an increase of luminance would make the room more lively. However, there were also some disagreements such as a higher CCT source would make the room more lively for Chinese observers but less lively for Dutch observers.

An experiment was carried out to investigate spatial brightness at photopic levels under lighting of different spectral power distributions. One aim was to replicate the experiment reported in 1990 by Berman et al. demonstrating that light with a higher scotopic / photopic (S/P) ratio would be perceived as brighter. In addition, a third SPD was included to investigate gamut area and two additional procedures were employed to provide concurrent validity of the findings. It was concluded that while lighting of higher S/P ratio was brighter, the S/P ratio alone was insufficient to predict spatial brightness. A metric for the chromatic contribution is also needed, this being provided by gamut area in the current work.

The use of LEDs is increasing rapidly in both the commercial and residential markets. Nowadays, the main way to measure the luminous intensity distribution of a LED is by using a goniophotometer. This takes a long time which means its test efficiency is low. In this paper, we will propose a new way to measure the luminous intensity distribution of LEDs based on the Luneburg lens, which is a kind of dielectric lens with continuously changed index of refraction, with high efficiency and low cost. We also develop a complete test system and give the result of a simulation of the test system.

There are vast numbers of light-emitting diode (LED) luminaires to choose from, but not all LED luminaires perform reliably. More than half of the input electrical power to a LED luminaire is wasted in the form of heat. Heat management is the one of the most critical issues faced by LED luminaire designers. The long term reliability of a LED luminaire is mainly dependent on the junction temperature of the LEDs. Hence, accurate junction temperature information during luminaire operation is critical for monitoring and assessing the health of the luminaire. In practice, it is extremely difficult to measure the junction temperature of LEDs in modern day luminaires. With the optical system and heat sink surrounding the tiny LED, measurement of junction temperature with direct methods like infrared cameras and thermocouples becomes more complex. This paper explores the possibility of monitoring and measuring the junction temperature of LEDs in a luminaire by making use of the strong correlation between the forward voltage drop at the LED junction and the temperature of that junction. Results of thermal investigations of a LED downlight are presented. The results suggest that the inherent forward voltage/ junction temperature dependency of LEDs can be used to measure and monitor LED junction temperature in a luminaire under operational conditions.