The Prometrix FT-650 Film Thickness Probe uses the CARIS technique (Constant Angle Reflection Spectroscopy) to accurately and quickly measure thin films. This technique uses a white light source to illuminate the sample and measures the reflected light intensity over the range of 400nm - 800nm. The incident light and reflected light combine with resulting constructive/ destructive interference, producing maxima/minima values in the spectrum. The intensity versus wavelength values are then used to calculate film thickness.