Light pollution is an environmental issue of rapidly increasing concern. Among the various types of light pollution, a major issue in residential areas is discomfort glare due to light trespass. Discomfort glare makes seeing and sleeping in a room difficult. Typical discomfort glare evaluation methods include the Daylight Glare Index and the Unified Glare Rating. However, no evaluation method exists for discomfort glare from artificial lighting entering through a window at night, i.e. light trespass. Therefore, this study investigated the existing methods for evaluating discomfort glare and subjective evaluation methods related to the characteristics of light trespass. We also examined the possible application of lighting simulation software programs for light trespass evaluation. We then performed a subjective evaluation of discomfort glare due to light trespass, compared the experimental values to our calculated values, and finally suggested a modified formula for evaluating light trespass.