Amount of opaque coating on a substrate is determined. An infrared beam is
transmitted into the opaque coating on the substrate. Infrared beams scattered
by the opaque coating are collected and detected at a first wavelength and a second
wavelength. Infrared energy Ic1 and Ic2 of the collected
infrared beams at the first and second wavelengths, respectively, are compared
with predetermined values of infrared energy Io1 and Io2 of
collected infrared beams at the first and second wavelengths, respectively, that
are scattered by a reference substrate without the opaque coating to determine
absorbance values A1 and A2 for the opaque coating at the first and second wavelengths,
respectively. Absorbance values A1 and A2 at the first and second wavelengths are
given by equations A1=-log10(Ic1/Io1) and A2=-log10(Ic2/Io2).
A difference A1-A2 is correlated to an opaque coating amount.
