Abstract: The present invention provides a method and an apparatus for detecting a position of a light spot in a light distribution that can include stray light components (e.g. from other lasers, ambient lighting etc.) The apparatus includes a continous response position sensitive detector (CRPSD, e.g. lateral effect photo-iode) for determining a first centroid of the light distribution and a discrete response position sensitive detector (DRPSD, e.g. multiplexed array) for determining a second centroid of the light distribution within a reading window defined about the first centroid and within the light distribution. The second centroid represents the position of the light spot in the light distribution. This multiple stage approach exploits the high resolution and speed offered by traditional CRPSDs together with the accuracy under variable lighting conditions offered by traditional DRPSDs.

Abstract: The apparatus generates two optical beams which are directed to a relatively moving solid surface where an interference pattern, having substantially parallel fringes, is formed. The light is scattered and thereby modulated by irregularities on the surface. This modulated light is received and the signal is processed to indicate the length of the surface or its relative speed. The signal processor converts the modulated light signal to a pulse train, and adds pulses to the pulse train when a drop-out in the signal is detected, i.e. when the time between pulses in the pulse train is greater than a factor times the average period of the pulse train. The corrected pulse train is a function of the length of the surface or the relative motion between the apparatus and the surface.