Approach to Analyse Seasonal and Geographical Variations in Daylight Illuminants

Abstract

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.

Evaluation of Visual and Nonvisual Levels of Daylight from Spectral Power Distributions Considering Orientation and Seasonality

“…Peyvandi et al [26] calculated the 25% and 75% interquartile values of the CCT to find the range containing 50% of the CCTs for each condition, obtaining a range for clear sky without aerosol particles of 5889-6294 K, a range for clear sky with aerosol particles of 5712-6325 K, and a range for overcast sky of 6218-7757 K. The overcast sky and atmosphere with aerosol particles represented extreme conditions where the limits of the CCT were observed. Daylight measured in Granada (Spain), with the most observed CCTs of 5555-5715 K [24], and in Boulder, Colorado (United States), with obtained CCTs of 5500-6400 K, showed that lower CCTs represented atmospheres with higher aerosol particle levels [23,24]. Similar results were found in Rochester, New York (United States), where daylight with CCT as low as 3530 K in a hazy sky was reported [27].…”

“…Conversely, in the opposite locations (north and west), the CCTs are altitude-dependent; and the higher the CCT is, the cooler the results that are found. The measured CCTs show variations as a function of the season, orientation and altitude in correspondence with CIE D illuminances, even with clear sky conditions [22,23]. When the value of the sky is considered, daylight approaches 6500 K. The average value of the data taken in this study is 6122 K, similar to the dominant CCT of the study conducted in the city of Granada, Spain (5700 K).…”

“…Conversely, in the opposite locations (north and west), the CCTs are altitude-dependent; and the higher the CCT is, the cooler the results that are found. The measured CCTs show variations as a function of the season, orientation and altitude in correspondence with CIE D illuminances, even with clear sky conditions [22,23].…”

Colorimetry to assess the visual impact of dust deposition on mosaics at sheltered archaeological sites

“…The X, Y, Z values obtained from the LumiCam ® were adjusted to the levels of illumination at the time of the measurement through the white reference from the X-Rite ColorChecker ® panel. In addition, the reference illuminant (D100 CIE) has spectral characteristics very similar to the indirect natural light [29].…”