Today, measurement methods like deflectometry allow accurate measurements of specular surfaces. The measurement methods are often more precise than human vision. If the aim is to inspect surfaces for defects that would disturb humans, so called aesthetic defects, it is important to understand the connection between the measurement method and human vision. This problem is addressed in this report. In contrast to matte surfaces, there are different influencing factors for the perception of specular surfaces. We are proposing a model which introduces a lower bound for the visibility of defects on specular surfaces. This means that defects smaller than this bound cannot be identified by an average human observer.