The accuracy of the IES Glare Index System

A new computer programme for calculating initial glare indices has the advantage over the original programme used by the IES of calculating the background luminance as well as the other elements in the glare formula. Discrepancies between the I ES and the new programmes have been found. These appear to be due to an error in the glare formula used in the I ES computer programme and to a misinterpretation of the Jones-Neidhart method for calculating the illumination on walls. Difficulties related to luminous areas and the application of position indices are discussed. The effect of spacing with arrays of luminaires is investigated. The magnitudes of the likely discrepancies between the glare indices calculated by the IES glare index tables and by the BRS glare formula are summarized. As in many respects the IES Glare System is a closed system, lighting installations so far designed using ES Technical Report No. 10 wift, with minor exceptions, be satisfactory.