A method is provided for determining an amount of a contaminant on a surface. Base values of infrared energy reflected from the surface without the contaminant at first and second wavelengths is determined. The values of infrared energy reflected from the surface with the contaminant at the first and second wavelengths are also determined. The values of infrared energy absorbed by the contaminant on the surface are determined at the first and second wavelengths. A difference of the absorbance at the first and second wavelengths is determined. The difference is correlated to an amount of the contaminant on the surface.