A special tinted coating is employed to facilitate automatic thickness measurement. The tint transmits the red light, partially attenuates the green light, and absorbs the blue light.
A sample was prepared with an intentional coating thickness gradient to demonstrate the thickness normalizaton process (Fig. 1). A simple relation for thickness is obtained as
where the blue light is used to subtract out the level of light reflected from the surface, and the red light is used to normalize for intensity. Figure 2 shows the thickness map for a coating thickness gradient of 0.15 mm to 1.0 mm.
Fig. 1 RGB Transmittance Through Tinted Coating
Fig. 2 Thickness Measurements