Light is necessary for vision; it enables us to sense and perceive our surroundings and in many direct and indirect ways, via eye and skin, affects our physiological and psychological health. The use of light in built environments has comfort, behavioural, economic and environmental consequences. Daylight has many particular benefits including excellent visual performance, permitting good eyesight, effective entrainment of the circadian system as well as a number of acute non-image forming effects and the important role of vitamin D production. Some human responses to daylight seem to be well defined whilst others require more research to be adequately understood. This paper presents an overview of current knowledge on how the characteristics of daylight play a role in fulfilling these and other functions often better than electric lighting as conventionally delivered.

This paper presents a literature review about energy-efficient retrofit of electric lighting and daylighting systems in buildings. The review, which covers around 160 research articles, addresses the following themes: (1) retrofitting electric lighting in buildings, (2) electric lighting energy use and saving potential and (3) lighting retrofit strategies. The retrofit strategies covered in the review are: replacement of lamp, ballast or luminaire; use of task-ambient lighting design; improvement in maintenance; reduction of maintained illuminance levels; improvement in spectral quality of light sources; improvement in occupant behavior; use of control systems; and use of daylighting systems. The review indicates that existing general knowledge about lighting retrofit is currently very limited and that there is a significant lack of information concerning the actual energy performance of lighting systems installed in the existing building stock. The resulting key directions for future research highlights issues for which a better understanding is required for the spread and development of lighting retrofit.

Research indicates that spectral distribution and the direction of the light received at the eye are relevant parameters in studies looking into non-image-forming effects. Nonetheless, lighting conditions are often described with vertical illuminance at the eye and correlated color temperature only, both of which are integral measurements that are not appropriate to give information about the spatial distribution of light and its spectrum. This article describes approaches for spatially and spectrally resolved measurements to properly quantify lighting conditions in research on nonimage-forming effects. The overview of measuring methods indicates that the spectral irradiance, when combined with a luminance image of the lighting setting, is an adequate measure in a large number of research approaches. Nonetheless, lighting conditions in experimental setups with different light sources or various surface reflectances require a higher resolution of detail. Four examples of devices are presented that can be applied in different experimental setups or lighting settings. A decision scheme is included to support the selection of the most suitable measuring equipment. The article concludes with a proposal for analysis and representation of the measurements.

Abstract— High‐intensity light sources illuminating the human eye may create discomfort glare, or at higher intensities even disability glare. In many office lighting conditions, light from overhead luminaires in the ceiling may deliver stray light into human eyes, and as such create discomfort glare, generally referred to as overhead glare. In this paper, overhead glare for a LED luminaire comprising a matrix array of small LED sources using subjective evaluation methodologies and theoretical models, commonly accepted to evaluate glare, were investigated. The perceived overhead glare of the LED luminaire is evaluated at various luminance levels and at different angles (i.e., between 55 and 90°) with respect to the line of sight of the viewer. The results show that a luminaire comprising a matrix of high‐intensity point sources can cause overhead glare and can become glary at lower averaged luminance levels than a luminaire with a uniform light source even at high evaluation angles with respect to the line of sight of the viewer. In addition, the conventional UGR model for predicting discomfort glare needs adaptation for a reliable prediction of perceived overhead glare of complex LED luminaires consisting of a matrix of small‐sized high‐intensity light sources.

This paper analyses the forecast accuracy of current state-of-the-art, data-driven, spectral sky models. The aim is threefold: (i) to determine the forecast accuracy of existing spectral sky models based on a large dataset of spatially, spectrally and temporally resolved measurements, (ii) to investigate the practical implications of spectral forecast accuracies for the assessment of spectrally selective responses (here non-image-forming effects are expressed through melanopic irradiance) and (iii) to study if the use of spectral sky models is more appropriate to predict the non-image-forming effectiveness of daylight than the currently assumed CIE standard illuminant D65. The forecast analysis for CIE Standard Overcast Skies (CIE Sky Type 3) showed that the model published by Chain and colleagues in 1999 performed best, whereas the correlated colour temperature distribution can also be represented with the CIE standard illuminant D65. The analysis showed substantial discrepancies in the forecast for clear skies with low luminance turbidity (CIE Sky Type 12) depending on the correlated colour temperature range. Our findings suggest that for CIE 12 skies, even when simulating with the best performing spectral sky model, forecast inaccuracies affect the estimated non-image-forming effectiveness. Nonetheless, the assumption that the spectral distribution of daylight from a CIE 12 sky corresponds with the CIE standard illuminant D65 underestimates the non-image-forming effectiveness to a greater extent. The results advance the understanding of spectral characteristics of daylight and suggest that considering realistic spectral distributions instead of D65 will lead to a difference in the non-image-forming effectiveness assessment.