Different terms are employed in the literature to specify this method on investigations: dualbeam coherent interferometry or laser Doppler interferometry, optical coherence tomography (OCT) or optical coherence reflectometry. OCT is analogous to ultrasonic imaging, which measures the intensity of reflected infrared light rather than reflected sound waves from the sample. Time gating is employed so that the time for the light to be reflected back (or echo delay time) is used to assess the intensity of back-reflection as a function of depth. Unlike ultrasound, the echo time delay on the order of femtoseconds cannot be measured electronically due to the high speed associated with the propagation of.
