Process modeling languages such as BPMN or EPC provide a set of graphical constructs defining their visual notations. The visual notation is one comparison criteria in favor of a process modeling language. Usually, the first choice for the evaluation of visual notations are the principles of the Physics of Notation (PoN) theory. Their vague operationalization, however, gives room for contradictory recommendations how to improve visual notations. Therefore, the intention of this paper is to identify recent empirical studies to visual notations of process modeling languages, which might contribute to a better understanding of PoN principles. A comprehensive literature survey has been conducted showing a confirmation of the PoN principles and identifying refinements for their operationalization. We applied our findings to an evaluation of the visual notation of BPMN from 2012 and showed advancements. Our findings define the current guidelines for evaluating and improving visual notations of process modeling languages.