Illumination device and projector

An illumination device comprising:

• a light source device;
• a rotary diffusion plate, which includes a first surface, a second surface, a diffusion section disposed on the first surface, and a detection section disposed on at least one of the first surface and the second surface, and to which light from the light source device is input;
• a light collection optical system to which light from the diffusion section is input;
• a detector adapted to detect light from the detection section;
• and
• a control device adapted to control the light source device in accordance with a signal output from the detector,
• wherein:
• the detection section includes a plurality of detection patterns,
• the detection section is disposed at a position different from a position where the light from the light source device enters the diffusion section, and
• the control device is adapted to determine a state in the rotary diffusion plate by whether the signal is periodic or not,
• the light source device includes a first light source and a second light source,
• light from the first light source enters the diffusion section, and
• light from the second light source enters the detection section.