scope:

Introduction and scope

Discomfort glare is an aspect of visual comfort that is relevant for determining the quality of a lighting system. It is the type of glare that occurs when people complain about visual discomfort from bright light sources, even when the brightness is not impairing vision. In contrast with so-called disability glare, which is caused by light scattering in the eye, the mechanism for discomfort glare is not well understood. It is usually measured by using subjective evaluation questionnaires. Several empirical formulae based on subjective glare evaluations have been proposed over the past 60 years to predict the degree of discomfort for various types of light sources, electric lighting or daylighting, and for various applications in indoor or outdoor lighting (see e.g. Boyce, 2013).

For indoor lighting, it is generally agreed that discomfort glare produced by an individual source basically depends on four main parameters: the source luminance in the direction of the observer, L_s, the solid angle subtended by the source at the observer's eye , ω, the angular position of the source in the observer's field of view , indicated by the position index, p, and the general field luminance controlling the adaptation level of the observer's eye, or background luminance, L_b. Moreover, it is also acknowledged, that L_s must be greater than 500 cd·m⁻² to 700 cd·m⁻² for discomfort to exist (CIE, 1983). In 1995 the CIE Technical Committee 3-13 developed a practical discomfort glare evaluation system for indoor lighting, including the four above mentioned parameters, which resulted in the Unified Glare Rating, UGR (CIE, 1995). The formula for calculation of a numerical value of UGR, R_UG, is expressed by Equation (1):

It should be noted that this UGR formula is valid only for sources which have a subtense at the eye between 0,0003 sr and 0,1 sr (which corresponds approximately to the range covered by square luminaires with sides of 5 cm up to 1 m at 3 m distance). For smaller sources, the discomfort glare is related to luminous intensity rather than to luminance (CIE, 2002). It should also be noted that the original work on UGR suggested that only luminous areas with a luminance above 500 cd·m⁻² should be counted as glare source area, but there was no described procedure for doing so at the time (CIE, 1995).

With the introduction of light-emitting diodes (LEDs) in general lighting, interest in the topic 'discomfort glare' arose again. The possibility of using LEDs in general lighting allows for many new luminaire types. However, the high luminance contrasts and high local peak luminance in the exit window of some of these luminaires seemed to provoke more discomfort glare than expected based on calculated UGR values. This discrepancy between the calculated UGR and the subjective discomfort glare by these luminaire types gave rise to an increasing number of published studies on this topic.

Many studies show that for the same average luminance, there is a significant dif ference in discomfort glare experience between uniform and non-uniform luminaires, and subsequently imply that UGR, using the average luminance for L_s, is not always a reliable measure for discomfort glare perception of those non-uniform luminaires.

In April 2013, it was proposed during the Division 3 meeting in Paris to start a new Technical Committee on this topic. Given the fundamental nature of discomfort glare, it was decided to form a Joint Technical Committee together with Division 1. The following Terms of Reference for JTC 7 were approved by the BA in October 2014:

1) To review the literature on glare from non-uniform light sources to identify the parameters that influence the discomfort prediction (UGR) and define limits to the applicability of th e UGR formula.

2) To propose a correction to the UGR formula that takes into account the non-uniformity of glare sources.

In this report, a literature review on the effect of various uniformity parameters on perceived glare is provided in Clause 2, together with a literature overview of proposed uniformity corrections to the UGR formula. Clause 3 describes the correction to UGR. This correction is selected on the basis of the following criteria:

1) good agreement with empirical data on perceived glare from non-uniform sources;

2) coverage of the worst-case situation (do not allow a good UGR score to potentially glary sources);

3) minimum deviation from the original definition of UGR;

4) easy implementation for lighting designers and lighting manufacturers (minimum effor t).

Besides non-uniformity of the glare source, there are other issues that may require further improvements or a complete revision of the UGR method in the near future. Clause 4 provides a brief summary of such issues, related to, amongst others, spectral effects and overhead glare.

NOTE Though most studies have been carried out with LED-based light sources, the conclusions may be generalized to any compact bright light source with similar brightness and size. The use of the word "LED" in the following sho uld therefore be interpreted as "a small bright-light source".