A ratio expressing the luminous efficacy of a light source. Typical lamp efficacies: Thomas Edison's first lamp - 1.4 lpW; Incandescent lamps - 10-40; Halogen incandescent lamps - 20-45; Fluorescent lamps - 35-105; Mercury lamps - 50-60; Metal halide lamps - 60-120; High-pressure sodium lamps - 60-140; (Note: The values above for discharge lamps do not include the effect of the ballasts, which must be used with those lamps. Taking ballast losses into account reduces "system" or lamp-ballast efficacies typically by 10-20% depending upon the type of ballast used.)