1. A method for determining the dimension of an object moving on a conveyor, comprising the steps of: providing a first triangulation based sensor unit, including a light source projecting a zone of light on the surface of said object and a TV camera, said first sensor unit oriented substantially such that the plane including the light source projection axis and the camera axis is in the direction of conveyor motion; providing a second triangulation based sensor unit, including a light source projecting a zone of light on the surface of said object and a TV camera, said second sensor unit oriented substantially such that the plane including the light source projection axis and the camera axis is in the direction of conveyor motion; positioning said sensor units so that said first sensor unit views the object moving toward it due to the conveyor motion, while the second sensor unit views the object moving away from it; analyzing the outputs of said sensor units; and from said analysis determining at least one dimension of said object.