A system and method for measuring coating thickness upon a substrate is disclosed. A near infrared light is directed upon the coating and reflected near infrated light is collected to determine the coating thickness. A diffuser is placed between the coating and the reflected near infrared light collector to improve the accuracy of the measurement, especially for coating thickness of less than about 2 mils and for coatings with shiny surfaces. The diffuser is formed of a low density polytetrafluoroethylene fluoropolymer film.