Editorial: Daylight and illuminance measurement

This issue focuses on daylight-related research and a camera-based method to measure illuminance. Any form of illumination for occupied spaces – natural or artificial – needs to be considered in terms of both its quantity and its quality. A useful distinction, often made, is to consider quantity in terms of lux received on the task plane and quality in terms of the spectral power distribution of the light. While incomplete, it provided a useful starting point. Both the quantity and quality of illumination received at the eye are now a consideration for the assessment and/or prediction of the non-visual effects of light, for example, for circadian entrainment. The paper ‘Evaluation of sky spectra and sky models in daylighting simulations’ by Inanici et al. delves deeply into the various ways that the spectral component of illumination from sky light can be predicted. This timely study reveals much about the inner workings of these simulations and provides a much needed inter-model comparison. The prevailing qualitative character of illumination can also be inferred from measures of correlated colour temperature (CCT). While CCT has long been used to categorise sources of artificial light, the paper by Villalba et al. describes how to quantify the changes in CCT when direct solar radiation passes through various commonly used window shading materials. Titled ‘The impact of woven shade fabrics on correlated colour temperature and illuminance with daylighting’, the article presents empirical findings for a variety of shading materials. For a basement room where it is not possible have a window to the outside, the potential may exist to ‘collect’ daylight on the roof of the building and ‘transport’ it to the windowless space. One approach, which today has an established market presence, uses highly reflective light-pipes, typically 500 mm or greater in diameter. A promising rival approach uses instead a much thinner conduit made from a fibre-optic bundle, offering the potential for installation in buildings where space constraints may prohibit the use of the much larger light-pipes. Necessarily, the daylight collected on the roof must be concentrated for transport through the fibre-optic. The paper ‘Analysis, evaluation and integration of modular natural illumination system using a rectangular Fresnel lens for high performance’ by Garg et al. describes recent advances which could form the basis of a practical light-transport system based on fibre-optic bundles. Lastly, the paper ‘A joint validation study on camera-aided illuminance measurement’ by Mahlab et al. provides further evidence to support the use of camera-based methods to measure illuminance. As demonstrated in the article, using digital cameras to capture highdynamic range images of scenes has greatly expanded the potential for quantitative lighting analysis in realistic/complex scenes. This edition also offers a timely opportunity to remind potential contributors that the call for papers for the Special Issue on ‘Daylight in Buildings and the New Standards’ is now open. Please see the LR&T website for details.