Abstract: A light screen device for measuring the velocity of a projectile comprising laser disposed opposite to one end of a first plane reflecting mirror and inclined with respect thereto to cause a laser beam to strike the first plane reflecting mirror and in turn to reflect the beam to a second plane mirror and successively reflecting the beam back and forth vertically to higher points on each mirror surface forming a criss-cross rectangularly shaped light grid network therebetween. A light detector is disposed axially in line with a reflected beam to intercept and detect the continuing presence thereof and to generate a pulse signal in response to the projectile passing through the light screen to stop a counting circuit.