An illumination device comprising a light source which emits a laser beam and a light guide which guides the laser beam from the light source to a display panel has been well known as a backlight of a display device such as a liquid crystal display device. To obtain, for example, a white light beam in the illumination device using a laser beam, laser devices emitting red, green, and blue light beams need to be prepared and the laser beams of the primary colors need to be mixed.
If the white laser beam is applied to the light guide, an optical system including optical axes of laser beams of different colors is required. Since space for this optical system is required, downsizing the illumination device is difficult.
In contrast, if laser light beams of primary colors are directly applied to respective light guides, making the brightness on the emission surfaces of the light guides uniform is difficult due to a narrow angle of divergence and the like. In addition, if the laser beams of different colors are repeatedly reflected inside the light guide and mixed, the light use efficiency is reduced since the light beams are absorbed inside the light guide. Furthermore, the laser beam has a high degree of polarization, but the degree of polarization is lowered when the light beam passes inside the light guide even if the light guide is formed of a material having small birefringence.