Geometrical errors and their uncertainty in goniophotometers: an application to road surface photometry

The measurement of luminance spatial distribution of road surfaces provides a full description of the light-material interaction. This quantity is related to road lighting system design and its measurement is performed by goniophotometers. Uncertainty analysis of goniophotometers is usually performed on laboratory instruments with very low beam divergence. Road surface luminance uncertainty budget needs a detailed approach because samples are large and due to the size constraints of on-site measurements, portable instruments tend to have larger beam divergence. In addition, due to the nature of samples, the uncertainty depends on the sample under test. The paper investigates the geometrical errors implicit in luminance measurements made by goniophotometry in the road surface analysis. It presents a theoretical model of a goniophotometer for spatial luminance distribution and provides an evaluation of errors dealing with sensors and source optics characteristics necessary to assess the uncertainty budget of the measurement results.