A photocell is a type of resistor.
When light strikes the cell, it allows current to flow more freely. When dark, its resistance increases dramatically.
Photocells along with daylight control circuitry are normally factory calibrated to ensure optimum responsiveness in the nominal lighting scenario.
As such, it is widely used to provide automatic ON/OFF or dimming control of the light system. The daylight sensitivity setting is 300 Lux ± 50 Lux.