An illumination device includes a light source generating initial light beams, a lens assembly disposed on an axis, and a reflector comprising a reflecting surface. The initial light beams include a first and second reference light beams traveling along a first and second positions, respectively. The first reference light beam passes through the lens assembly to form a first predetermined light beam traveling away from the light source. A first angle is substantially formed between the first position and the axis. The second reference light beam is reflected by the reflective surface to form a second predetermined light beam traveling away from the light source. A second angle is formed between the first and second positions. The first angle is less than or equal to the second angle. The initial light beams are guided by the lens assembly and the reflector to emit the light beams in a projecting mode.