Light solar energy distribution

Fig. 40 Energy distribution of solar radiation (according to CIE, No. 20) and filtered xenon arc light (Xenotest 1200) [102].

Figure 5 in particular illustrates the shift of the spectral peak to longer (red) wavelengths associated with red skies at sunset and sunrise. Figure 7 schematically shows the relationship of absolute temperatures, and color as perceived by the human eye for various outdoor natural conditions, and artificial indoor sources. Our eyes adapted to take advantage of the energy peak in solar spectral distribution between 400 nm and 700 nm, and evolved under the spectral distribution of our sun. Therefore, the match (or mismatch) of artificial source spectral distributions with the solar spectral distribution is important for both natural and artificial lighting applications. 

It is well known that the spectral distribution and irradiance of the solar radiation at the Earth s surface depend on the location and is subjected to seasonal and diurnal variations. Therefore, a reference spectrum is needed as a basis for comparison with the spectral energy distribution of artificial light sources. Data from CIE No. 15 1971 (colorimetry official recommendations of the International Commission on Illumination) that recommend a standard illuminant D65 with a scheduled color temperature of approximately 6500 K have been used as a basis over the years. 

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