Disclosed is an illuminating device, including first and second light sources, first and second light receiving elements, a display module, and a projection module. The first and second light sources are for generating first and second light beam respectively. The first and second light receiving elements, located on the optical paths of the first and second light beams respectively, are for converging the first and second light beams respectively. The display module, located on the optical paths of the first and the second light beams, receives the first and second light beams and generates corresponding first and second image beams respectively. The projection module, located on the optical paths of the first and second image beams, projects the first and second image beams. A first etendue of the first light beam is greater than a second etendue of the second light beam.