Spectral efficiency refers the spectral effectiveness of the delivered spectral power distribution (SPD) compared to an optimum spectrum for the function of the light. The SPD of a light source should overlap the optimum spectrum for the activity or intent of the lighting, e.g. visual acuity, color rendition, engagement of physiological responses, etc. The concept of spectral efficiency was introduced with the advent of solid state lighting, which offers the new capability of precise SPD control. The SPD of an LED lighting can be tailored, and, in some products, can be actively controllable for various applications, such as human centric lighting and horticulture lighting.