Patent ID: 6876441

Claim:
A method for determining phase-shift between first and second electromagnetic radiation signals, the first electromagnetic radiation signal being modulated by a first waveform signal having a first frequency, the first electromagnetic radiation signal being transmitted at a surface which results in at least a portion of the first electromagnetic radiation signal being reflected as a second electromagnetic radiation signal comprising the steps of: multiplying the first waveform signal by a second waveform signal modulated at a second frequency, the second frequency being different from the first frequency, for establishing a third waveform signal of known phase and having a third frequency equal to the difference of the first and second frequencies; modulating the second electromagnetic radiation signal by the second waveform signal to form a third electromagnetic radiation signal; receiving the third electromagnetic radiation signal at a detector for establishing a detector output waveform signal having a shifted phase at the third frequency; and comparing the known phase of the third waveform signal and the shifted phase for establishing a phase-shift between the first and second electromagnetic radiation signals.