For most home phototherapy products, you can choose a timer controller or dosimetry controller.  A timer controller provides the treatment “dose” based only on time of exposure.  While this is the most common and most economical option, it does not account for warm-up time of the lamps, lamp age, and other factors.  A dosimetry controller measures the actual energy output of the phototherapy device and adjusts the treatment time to provide a precise dose.  Note that the precision that dosimetry offers is usually not necessary so in most cases a timer controller is adequate, but both options are offered.