The Guth position index P is a key geometric parameter used in glare-evaluation models to quantify the positional contribution of a light source to visual discomfort. Its practical relevance spans lighting design, photometric assessment and the calibration of discomfort glare metrics. Despite its longstanding use in lighting practice, the technical literature reveals significant inconsistencies in how the index is defined, parameterized and computed. These discrepancies concern the interpretation of the operational angles, the choice of reference geometry and the compatibility between institutional models. The purpose of this paper is to document and systematize the technical literature concerning the terminology used in defining the P /Guth position index, with emphasis on longstanding inconsistencies among models belonging to the same technical family. The motivation arises from persistent disagreements regarding the definition and geometric placement of the operational angles. The literature reveals a field shaped by partially compatible models, competing angular conventions and institution-specific computational practices rather than theoretical convergence. The contribution of this paper lies in exposing the methodological, notational and descriptive fragmentation affecting the equation and practical use of the Guth position index.
AA Ianos (Tue,) studied this question.