Illumination system

An illumination system for a projection apparatus includes a light source module, an integrating rod and an optical detecting module. The light source module is capable of emitting an illuminating light beam. The integrating rod has a light entrance surface facing toward the light source module. The optical detecting module includes a light guiding device and an optical detecting device. The light guiding device has a first end surface and a second end surface. The first end surface is farther away from the light entrance surface of the integrating rod than the second end surface. The optical detecting device is disposed adjacent to the first end surface of the light guiding device and electrically connected to the light source module. The light source module is capable of adjusting intensity of the illuminating light beam according to randomly emitted light detected by the optical detecting device.