A room measuring 21 m by 18 m and 13 m high has to be lighted by four lighting fixtures suspended 3 m from ceiling. The ceiling points are symmetrically placed on the two diagonals so that each point is 7 m from the center of the ceiling. An illumination of 12 lux apart from any effect due to the walls and ceiling is required on the center of a table 3 m high placed at the center of the room. What luminous intensity must each unit give in direction necessary to produce this result?