An apparatus for measuring speed with respect to an object comprises first to Nth sensors (13) sensitive to energy received from first to Nth consecutive regions, respectively, of the object. Signal delay means (21-27) are provided to delay the signals from the second to Nth sensors, the delay increasing with sensor number. A subtractor (16) is provided to form the difference between the sum of the signals of the even-numbered sensors and the sum of the signals of the odd-numbered sensors.