Efficiency describes the ratio between useful output and input. For lighting products, the more specific comparison is usually luminous efficacy, expressed in lumens per watt (lm/W). Power consumption, light pattern, optical losses, and thermal performance also affect how effectively a work light illuminates the required area. A high lm/W value means that more visible light is produced for each watt consumed, but it does not by itself guarantee useful illumination. A well-designed lamp must place light where the operator needs it, withstand the environment, and control glare. Comparisons should use values measured for the complete product under comparable operating conditions.