1. A light-based touch screen, comprising: a housing for a display screen; a plurality of infra-red light emitting diodes (LEDs), fastened on said housing, for generating light beams; at least one LED selector, fastened on said housing and connected with said plurality of LEDs, for controllably selecting and deselecting one or more of said plurality of LEDs; a plurality of photodiode (PD) receivers, fastened on said housing, for measuring light intensity; at least one PD selector, fastened on said housing and connected with said plurality of PD receivers, for controllably selecting and deselecting one or more of said plurality of PD receivers; an optical assembly, fastened on said housing, for projecting light beams emitted by said plurality of LEDs in substantially parallel planes over said housing; and a controller, fastened on said housing and coupled with said plurality of PD receivers, (i) for controlling said at least one LED selector, (ii) for controlling said at least one PD selector, and (iii) for determining therefrom position and velocity of an object crossing at least one of the substantially parallel planes, based on output currents of said plurality of PD receivers.