Source: {"pile_set_name": "USPTO Backgrounds"}

As the light source of a projector, a discharge lamp such as an ultra-high pressure mercury lamp has been widely used. However, in recent years, a projector, in which a solid state light source such as a LD (Laser Diode) is used instead of a discharge lamp, have been gaining attention. For example, Patent Document 1 discloses a projector that uses a solid state light source that emits linearly polarized light.
The solid state light source has a high durability and has an advantage that a time required for stabilizing the quantity of light is short, as compared with the discharge lamp. Further, since the solid state light source can output light that does not contain UV light rays, it is possible to prevent degradation of optical parts on which light enters.
Meanwhile, a projector in which polarized light is projected has been proposed recently. The projector of this kind is applied to, for example, a stereoscopic image display apparatus that makes the user view stereoscopic images, a secure display that makes only a particular user visually recognize particular information, and the like.
The stereoscopic display apparatus alternately projects two images taken from different points of view (image for right eye and image for left eye) onto the screen, by using two light beams whose polarization states are different from each other. When the user who wears polarized glasses, through which different polarized light is given for left and right eyes, views the projected image on the screen, the image light for right eye enters the right eye of the user and the image light for left eye enters the left eye of the user. As a result, the user can recognize the displayed image three-dimensionally.
On the other hand, the secure display projects, on the screen, an image that indicates particular info nation and its reversed image, each of which uses light having a different polarization direction. When the user who wears polarized glasses, that only allow light of a particularly polarization direction to pass, views the projected image on the screen, the user can visually recognize the particular information. On the other hand, since the user who does not wear the polarized glasses observes the state in which the particular information and the reversed image are overlapped, the user cannot visually recognize the particular information.