Luminous efficacy (sr) measures how efficiently a light source produces visible light, measured in lumens per watt within a solid angle.

1 Kcd = 683.002 s^3*cd*sr/m^2/kg (#7 of 7 SI Defining constants); The "kcd" is a unit of luminous efficacy measuring the efficiency of visible radiation. It is the 7th of 7 SI defining constants.