Optical LOS. Line of vision. A direct imaginary line between two points, as though it were from the center of the eye to the center of the object viewed. A direct non-guided path in the form of a straight line between a transmitter and receiver, uninterrupted by physical matter other than that suspended in the atmosphere. Opaque objects such as mountains, buildings, and trees interrupt optical LOS, as does the horizon over long distances due to the natural curvature of the Earth. Optical LOS is critical in free space optics (FSO) transmission systems and high-frequency radio system. Optical LOS is always preferable in radio systems, even those operating at low frequency.