In the United States, in most of the states, glare from the portable light towers in the work zones is evaluated subjectively. Subjective evaluation of glare usually has the inherent bias of the person performing the evaluation. Efforts to determine an objective glare metric that could be measured in situ at work zones have largely been unsuccessful. The need for such a metric is important as traffic volumes increase and more construction activities occur at night. In an attempt to determine an objective measure of perceived glare, this study characterized the vertical illuminance of three commercially available portable light towers in various orientations, correlated the vertical illuminance to perceived glare, and modelled the relationship between vertical illuminance at the driver eye level and perceived glare in a realistic work zone setup on Virginia Smart Road. The results from the characterization showed that vertical illuminance increases rapidly between a distance of 80 and 20 m to the light tower. This critical range was consistent across all the light tower types in each orientation. Results also indicated that vertical illuminance is correlated with perceived glare and could be accurately modelled with a generalized logistic function. At a mean vertical illuminance level of 17 lux in the critical range, the perceived glare transitions from low to high. These results could help in developing objective glare metrics for work zones.