1. Technical Field
The present invention relates to an illumination device and a projector.
2. Related Art
An illumination device including an excitation light source, such as a blue laser, and a phosphor layer that is excited by blue light emitted from the excitation light source to produce fluorescent emission has been known. For example, JP-A-2012-4009 discloses an illumination device including an excitation light source, a polarizing beam splitter (PBS), a condensing optical system, a fluorescent member including a phosphor layer and a reflective film, and a ¼-wave plate.
In this illumination device, S-polarized excitation light emitted from the excitation light source is incident on the PBS and reflected by the PBS. The excitation light reflected by the PBS is illuminated on the fluorescent member through the ¼-wave plate and the condensing optical system, and fluorescence in a different wavelength band from the excitation light is emitted from the phosphor layer. In the fluorescent member, the phosphor layer absorbs a portion of the excitation light to emit the fluorescence including red light and green light, while the reflective film reflects the remaining excitation light (blue light). Due to this, white light obtained by combining the fluorescence with the excitation light is emitted from the fluorescent member. The ¼-wave plate converts the S-polarized excitation light reflected by the PBS into circularly-polarized light, and also converts the circularly polarized excitation light emitted from the fluorescent member into P-polarized light. The condensing optical system condenses the excitation light emitted from the ¼-wave plate to a spot of a predetermined diameter on the phosphor layer, and also converts the white light emitted from the fluorescent member into parallel light.
In the illumination device disclosed in JP-A-2012-4009 as described above, light not contributing to wavelength conversion in the phosphor layer out of the excitation light is used as light for display. In order to obtain a predetermined amount of white light, the phosphor layer has to be irradiated with a large amount of excitation light. In that case, a thermal load applied to the phosphor layer is increased, so that the temperature of the phosphor layer rises. As a result, a problem of a reduction in the conversion efficiency of the phosphor layer occurs. Moreover, in the illumination device disclosed in JP-A-2012-4009, both the excitation light and the fluorescence emitted from the fluorescent member are extracted through the common condensing optical system. A focal position for the wavelength of the excitation light and a focal position for the wavelength of the fluorescence are shifted from each other due to axial chromatic aberration of the condensing optical system, and therefore, there is a problem of a reduction in condensing efficiency.