During WWII and the Korean War, most tanks used a direct sighting system where the gun sights and rangefinders were slaved directly to the gun's barrel but its long range accuracy depended upon the focusing abilities of the individual gunner. There are two basic types of optical rangefinders, stereoscopic and coincidence. With a stereoscopic model range determination occurred by measuring the distance from the observer to a target using the observer's capability of binocular vision. The coincidence rangefinder uses a single eyepiece. Light from the target enters the rangefinder through two windows located at either end of the instrument. The ARCOVE emphasis upon increased long range accuracy led to the incorporation of a fire control system (FCS) in the M48. The fire control system included a rangefinder, mechanical ballistic computer, ballistic drive, and gunner's sight. Collectively, these mechanical devices resembled in miniature the fire control systems used in naval gunnery. Only after the Second World War did such systems become small enough for use in combat vehicles. These mechanical fire control systems permitted tanks to engage effectively at much longer ranges than in World War II, a critical consideration for the Army, expecting to enter the European battlefield outnumbered. Instead of a gunner's sight being slaved to the gun tube, the ballistic computer and drive computed the range and elevated the gun. The gunner's primary responsibility lay in keeping the sight on the target. The mechanical ballistic computer made a more accurate computation of range possible by mathematically accounting for such factors as vehicle cant and ammunition type. Many developmental range finders based on pulses of IR light, such as the Optical Tracking, Acquisition and Ranging (OPTAR) rangefinder of the T95 continued into 1957.