A method and apparatus provide for non-contact optical measurement of the level of a fluid stored in a tank or container, the surface of the fluid being possibly agitated. The method processes numerically the digitized
signal waveforms generated by a
lidar apparatus based on a pulsed time-of-flight modulation scheme. A key step of the numerical
processing is the computation of a waveform in which each
data point is obtained from a statistical estimator of the variability of the amplitude
signal echo measured at the distance from the
lidar apparatus that corresponds to the rank of the
data point in the waveform. The statistical estimator is preferably the standard deviation. By using a statistical estimator of the variability of the captured
signal amplitude, the specific signal echo returned from an agitated fluid surface can be greatly amplified as compared to the signal echoes returned from any obstacle or medium that could be present along the path of the
optical beam radiated by the
lidar apparatus. The method then allows for an efficient retrieval of the useful signal echo from which the level of the fluid surface can be reliably measured with greater accuracy, particularly in situations where the useful signal echo would be buried in a strong signal echo returned from any optically scattering or absorbing medium that would fill in the volume of the tank above the fluid surface.