A practical method of comparing luminous intensity distributions

Abstract

In the field of goniophotometry, as in many other measurement disciplines, there is an increasing need for laboratories to become ISO-17025 accredited. This involves, amongst other things, characterisation of the goniophotometric measurement for uncertainty analysis and laboratory intercomparisons. Simple comparison of the luminous intensity measurements, however, can lead to large errors where there is a steep luminous intensity gradient when the only difference between the two sets of measurements is a small discrepancy in alignment. Given the difficulty in mounting and aligning some types of luminaires, this makes uncertainty analysis and intercomparisons difficult. Furthermore, the work of several CIE Division 2 Technical Committees (e.g. TC2-62) highlights the need to compare the measurements made on both far-field and near-field goniophotometers. This paper introduces a metric for comparison of two luminous intensity distributions that isolates and removes discrepancies that arise both from misalignment and also the difference in absolute calibration scales or the luminous flux output of the source. Moreover, these discrepancies are reported as part of the metric, so that a complete comparison of the sets of measurements is possible.