A light fixture includes a light emitting module for radiating and illuminating a preset illumination area with a fixed illuminance; a light sensor unit mounted on and in electrical communication with the light emitting module, including a light guiding pipe having a first end for guiding reflection light beam from the preset illumination area into the guiding pipe and a light sensor component mounted on a second end of the guiding pipe for measuring an illuminance of the reflection light beam guided into the guiding pipe in order to modulate the light emitting module based on the measured illuminance in such a manner that the light emitting module is capable of actively automatically controlling and maintaining the preset illumination area with the fixed illuminance.