Predicting lamp spectrum effects at mesopic levels. Part 1: Spatial brightness

This paper reports on experimental work carried out to test metrics for predicting spatial brightness at mesopic levels under lamps of different spectral power distribution. The side-by-side matching technique was used following an extensive review of experimental design. Five different types of lamps were presented in all 10 possible pairs, these being selected to compare brightness predictions based on established characteristics of lamp spectrum such as colour rendering index, correlated colour temperature and the scotopic/photopic ratio. The results were also used to test proposed systems for predicting brightness and visual performance at mesopic levels. Of the lamp characteristics examined the scotopic/photopic ratio exhibited the highest correlation with the test results. The new CIE recommended system for visual performance based mesopic photometry was found to give an acceptable prediction of the brightness results.