We developed a method of measuring the spectral radiance of all sky elements using a general purpose digital camera and a circular fisheye lens. The accuracy of the fisheye camera is adequate for practical daylight design. The instrument is portable for field measurements. The digital camera with the fisheye lens is colorimetrically calibrated to obtain the calibration functions which transfer the video signal values RGB on a digital color image into the absolute values of CIE 1931 tristimulus values XYZ pixel by pixel. The calibrated fisheye digital camera measures the all sky distribution of the XYZ tristimulus values. The XYZ tristimulus values are transferred into the all sky spectral radiance distribution in two ways: (1) by using the CIE daylight illuminant and (2) by using the eigenvectors obtained by principal component analysis of measured spectral radiances of the sky elements by a spectroradiometer. The accuracy of the all sky spectral radiance distribution is validated by comparison with the spectral radiance measured by the spectroradiometer.
In the previous paper 7) , 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). In the previous paper 7) , 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). Keywords :
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