A method for measuring the thickness of coatings on metal substrates comprising: illuminating a sample comprising a substrate and a coating with light waves of varying wavelengths from a light source; receiving the light waves reflected by a top surface of the coating on the sample at a light collector; receiving the light waves reflected by a bottom surface of the coating on the sample at the light collector; diffracting the light waves into a plurality of component wavelengths with a grating; detecting the light intensities of the plurality of component wavelengths at a detector array; wherein the method further comprises: generating a resultant reflectance spectral curve by mathematically superimposing the detected light intensities from the top and bottom surface for each of the plurality of component wavelengths, the resultant reflectance spectral curve have a periodic wave pattern; calculating the thickness of the coating from a frequency of the periodic wave pattern of the resultant reflectance spectral curve of the component wavelengths; and detecting changes in the distance between the sample and the light source and varying the intensity of the light waves in real time based on the distance between the sample and the light source; wherein the sample is continuously moving; and wherein calculating the thickness of the coating occurs in real-time. illuminating a sample comprising a substrate and a coating with light waves of varying wavelengths from a light source; receiving the light waves reflected by a top surface of the coating on the sample at a light collector; receiving the light waves reflected by a bottom surface of the coating on the sample at the light collector; diffracting the light waves into a plurality of component wavelengths with a grating; detecting the light intensities of the plurality of component wavelengths at a detector array; wherein the method further comprises: generating a resultant reflectance spectral curve by mathematically superimposing the detected light intensities from the top and bottom surface for each of the plurality of component wavelengths, the resultant reflectance spectral curve have a periodic wave pattern; calculating the thickness of the coating from a frequency of the periodic wave pattern of the resultant reflectance spectral curve of the component wavelengths; and detecting changes in the distance between the sample and the light source and varying the intensity of the light waves in real time based on the distance between the sample and the light source; wherein the sample is continuously moving; and wherein calculating the thickness of the coating occurs in real-time. generating a resultant reflectance spectral curve by mathematically superimposing the detected light intensities from the top and bottom surface for each of the plurality of component wavelengths, the resultant reflectance spectral curve have a periodic wave pattern; calculating the thickness of the coating from a frequency of the periodic wave pattern of the resultant reflectance spectral curve of the component wavelengths; and detecting changes in the distance between the sample and the light source and varying the intensity of the light waves in real time based on the distance between the sample and the light source; wherein the sample is continuously moving; and wherein calculating the thickness of the coating occurs in real-time.