Measurement of the All Sky Spectral Radiance Distribution Using a Digital Camera With a Fisheye Lens

Abstract

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.