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.
This paper studies the colorimetric characteristics of daylight within the framework of the CIE Standard General Skies. The correlated colour temperatures (CCTs) were studied based on long-term measurements. The colorimetric analysis employed in this study comprises: (i) the CCT occurrence; (ii) the CCT ranges; (iii) the CCT variation depending on the solar altitude; in function of season, time of day, and orientation, all corresponding to the fifteen CIE Standard Skies. The last part of the analysis illustrates the practical application of the results in the urban structure, represented in spectral daylight potential diagrams. The findings show significant colorimetric discrepancies both between and within the sky types and support the key argument against a fixed CCT for all sky types. The paper helps to understand the spatio-temporal variability of the CCT and indicates what possible effect that variation might have on the colorimetric daylight characteristics on façades, and therefore on the potential of a building regarding the ipRGC influenced responses to light.
Based on 622 spectral measurements, a reconstruction approach for spectral power distributions of daylight was proposed in 1964. This procedure allows to derive the spectral power distribution of daylight illuminants from their chromaticity coordinates or correlated colour temperature. It suggests that inexpensive sensors can be used to measure daylight and a representation of colour characteristics of daylight can be done with just one single value, without large errors. The reconstruction approach was adopted by the CIE, nonetheless, emphasising that geographical and seasonal variations might exist. Up until today, these variations are investigated in single studies. This paper includes a review of relevant parameters that affect the reconstruction procedure and proposes a template for spectral measurements, to be used in a large worldwide measurement campaign, to modify or complement the existing CIE approach for reconstitution of daylight's spectral power distribution.
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