It is well-known that CIE Illuminant C has been used as typical colorimetric characteristics of daylight. However, we have experience shows that ordinary daylight around us is not always constant but usually changes with seasons, weather, directions and so on. Unfortunately these data are scarce in Japan.
Therefore, the authors have been trying to measure spectral distributions and chromaticities of north daylight in Nagaoka, Japan. A monochrometer and an amplifier with photomultiplier tube were both trially manufactured for this research.
The results are as follows:
1) The spectral distribution shows very wide variation with measuring place, weather, time and so on.
And, the comformity index of spectral distribution of daylight from north sky is about 360 for Planckian radiator in relation to the equal color temperature.
2) The chromaticity of daylight is distributed between+40 MPCD and-20 MPCD, in relation to the locus of Planckian radiator.
And, points of the average chromaticity with each color temperature are arranged along the formulated curve;
y=2.224
x-2.102
x2-0.160, and this curve falls slightly on the green side of the Planckian locus.
3) Some studies on the same subject have been recently presented in America, Canada and England. Comparison of the authers' results with theirs shows that there is very similar tendency between them.
This research will be continued to collect a statistical data on daylight in Japan.
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