An apparatus and method for optically detecting whether a substance is present at an inspection site on a base material based on light reflected from the inspection site. A light module generates light of a first wavelength and light of a second wavelength. A first optical transmission medium directs the light of the first wavelength and the light of the second wavelength to the inspection site. A photodetector receives light reflected from the inspection site and generates a reflection signal corresponding to the reflected light. A second optical transmission medium directs the reflected light from the inspection site to the photodetector. A control module has an input for receiving the reflection signal. Responsive to the reflection signal, the control module generates a first inspection site absorption value corresponding to absorption of the light of the first wavelength that was directed at the inspection site and a second inspection site absorption value corresponding to absorption of the light of the second wavelength that was directed at the inspection site. The control module generates an output signal indicating whether the substance is present at the inspection site responsive to the first and second inspection site absorption values.