主成分分析を用いた天空要素の分光放射輝度の推定法

摘要

In the previous paper, we have developed a method of measuring the spectral radiance of all sky elements using a general purpose digital camera and a circular fisheye lens. In this method, the fisheye camera measured the XYZ tristimulus values of each sky element The XYZ tristimulus values were transferred into the spectral radiance in two ways: (1) by using the CIE daylight illuminant and (2) by using the eigenvectors obtained by principal component analysis of spectral radiance of sky elements measured by a spectroradiometer. The method (1) was mostly accurate, but was sometimes unable to estimate spectral radiance of blue skies because the CIE daylight illuminant was defined with the correlated color temperature being within the range of approximately 4,000 to 25,000Kelvin. The correlated color temperature of the method (2) was unlimited, but the accuracy was lower than the method (1). In this study, to improve the method (2), we obtained eigenvectors by principal component analysis of normalized spectral radiance of sky elements. The XYZ tristimulus values were transferred into the xy chromaticity coordinates, and the relative spectral power distribution was computed. Then it was transferred into the spectral radiance by tristimulus values Y [cd/m~2]. We found the improved method (2) was able to estimate spectral radiances more accurately than the original method (2).%JISの照明基準に演色性が追加されたことゃサーカディアンリズムが特定の波長に影響されることが指摘されていることなどから、室内の光環境の設計の際に分光分布を考慮した照明設計を行う必要性が高まっている。そのためには室内の分光相互反射計算が必要であり、その入力値として昼光の分光分布のデータが必要である。昼光の分光分布としては相関色温度毎の相対分光分布として標準化されたCIE昼光があるが、分光相互反射計算には絶対値である分光放射輝度の全天空分布（全天空分光放射輝度分布）のデータが必要である。