The SPAR-1 is a precision light meter that measures photosynthetic Active Radiation otherwise known as PAR light. When the SPAR-1 is used, the controller can "actively" automatically control your lighting to provide the EXACT amount of light you select. The PAR light level is important in order to ensure you are providing the correct amount of light without wasting energy, and to ensure enough light is provided for the plant’s current stage of growth. Alternately the SPAR-1 sensor can be used “passively” to constantly measure light levels and allow the user to manually control their lights based on the PAR light level measured by the SPAR-1.