Luminaire Glare Metrics
While there have been many different models created to categorize glare as a metric, none do a good job in making it universal. Therefore, none of these models are useful as a comparative metric between luminaire specifications. The most popular metric is the unified glare rating (UGR), which is used mainly for interiors.3 For outdoor applications, glare concepts like the "threshold increment IT" and "glare control mark G" have been developed primarily for road lighting for motorized traffic. Reference4 formulates the GR rating as a function of the vailing luminance produced by both the luminaires and the environment. As with other methods, there is still difficulty in establishing parameters to specify the luminaire, due to interdependency with the environment and the observer's position and viewing direction with respect to the glare source.
The most commonly used method to measure glare for outdoor lighting is the G rating, a system on the BUG rating scale (based on IES TM-155). In this metric, there is a scale for the glare rating, which is based on an absolute value in lumens depending on the zonal lumens of the distribution. For luminaire comparison, you could use this metric to extract the environmental factors that are independent of the luminaire. Note however, this metric is incomplete and not always ideal - for few reasons. First, the G rating value is based on luminous flux, and not true luminaire luminance. Second, it does not consider the other factors affecting glare directly, like luminaire uniformity and size of luminance opening.
While lighting technology has evolved to solve many issues as it pertains to glare, the standards and metrics to help designers specify luminaires have not. Existing standards have many drawbacks, so it is challenging to specify a luminaire without resorting to expensive and time-consuming mock-ups. More research will need to happen in this area, so that luminaire manufacturers can adopt a glare metric that is consistent across the industry.