Comparison of the fisheye camera method with goniophotometers for measuring relative angular intensity distributions of light sources

Abstract

Angular intensity distributions of light sources are traditionally measured using goniophotometers. Usually, goniophotometers are expensive to obtain and resource intensive to operate, due to requirements of a dedicated laboratory and the time consuming nature of the measurements. In addition to other applications, such as lighting design, angular intensity distribution data are used for spatial corrections to increase accuracy of luminous flux measurements with integrating spheres. Because goniometric measurements are so laborious, many laboratories omit the spatial correction altogether, increasing their measurement uncertainty by up to a few percent, depending on the integrating sphere and the light source under test. Previously, the fisheye camera method was developed to enable faster and more cost-effective measurements of angular intensity distributions of light sources. Using the fisheye camera method, such angular distribution is obtained in a matter of seconds from a fisheye camera image captured through a port of an integrating sphere, while the sphere is illuminated by the lamp under test. The method was reported in CIE 2017 Midterm Meeting in Jeju, where doubts were presented about the wide applicability of the method and its comparability with traditional goniophotometers. In this study, the performance of the method is confirmed. The fisheye camera method is validated by measuring six LED lamps in eight integrating spheres of various configurations, and by comparing the obtained angular data with that measured using five goniophotometers of different types.