Spatial filter type speed measuring apparatus

A speed measuring apparatus comprising:

• a light projecting unit comprising a light source for emitting light, an optical projection system arranged to project a light from the light source onto a relatively moving object, and a first polarizer disposed in the optical projection system for allowing the passage of one polarized light component of the light from the light source as projected light;
• a light receiving unit comprising a second polarizer arranged to allow the passage of polarized light in a direction at right angles to the direction of polarization of the projected light reflected from the relatively moving object, and spatial filter means for taking out a predetermined spatial frequency component of the polarized light passed through the second polarizer and outputting an electrical signal representing the frequency component;
• and
• means for calculating the relative speed of the relatively moving object on the basis of the signal from the spatial filter means.