FFT-based cross-covariance processing of optical signals for speed and length measurement

The optical sensor is composed of two eyesafe parallel laser beams operating at two different wavelengths (see Figure 1). When a vehicle passes in front of the sensor, the laser light is diffused on the surface of the vehicle. The diffused light is then focused on two photodetectors, one for each laser. We thus obtain two signals, x(t) and yet), which represent the fluctuations in time of the intensity of the light diffused by the vehicle. Since the laser beams are separated by a distance D, one of the signals is a delayed version of the other.