A fixed visible input and tunable visible input are directed to a location on a surface to be interrogated. The fixed visible input and the tunable visible inputs are aligned so that their surface locations of optical illumination overlap on the interrogated location. An output wavelength discriminator receives the reflected difference-frequency generated on the interrogated location. The output wavelength discriminator is substantially non-transmissive at frequencies higher than the difference-frequency, but substantially transmissive at the difference-frequency of the fixed visible input and the tunable visible input. Signal collection optics receives an output of the output wavelength discriminator and directs the propagation of the output thereof so that a collected optical light signal is formed after propagation through the signal collection optics. The collected optical light signal is converted to an electronic signal. Thus, the intensity of the difference-frequency wavelength is monitored for providing surface-sensitive infrared spectroscopic characterizations on the surface to be interrogated.
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