The present disclosure is generally directed to methods and systems for measuring the thickness of coatings or thin films on various substrates. For example, one disclosed method includes the steps of providing and directing light waves of varying wavelengths toward a moving substrate comprising a coating, linearly polarizing the light waves, converting the linearly polarized light waves to circularly polarized light waves, analyzing elliptically polarized light waves reflected by the moving substrate, capturing analyzed light waves, generating light wave data based on the captured light waves, and determining a thickness of the coating based on the light wave data.