Abstract: An optical tennis line sensing system employs a double beam laser for projecting only a pair of collimated beams of light along the entire outer boundary and base lines of a tennis court with one beam positioned on the line slightly above the playing surface and the remaining being positioned slightly outside the lines. A detector circuit including photo multiplier light sensors detects interruptions of the light beam in a timed sequence for generating a signal indicating when a tennis ball is out of bounds. The detector circuit discriminates between momentary interruption of one of the light beams by a tennis ball and interruption of one or both of the light beams by the player's foot. Display means coupled to the detector circuit displays only out-of-bounds shots which are marginal and which cannot be accurately called by the line judges.