As a technology of the projection display screen in the related art, there exists a rear projection display screen including a diffusion film (for example, see Document 1, Okita et. al, Sumitomo Kagaku 1991-I, p. 37-48) for diffusing incident light from a specific angular range into a specific angular range (for example, see WO2004/034145).
This screen is different from a screen including a Fresnel lens, a lenticular lens, and a diffusion film used generally in the related art, and has very effective advantages such that the structure is simple and the cost can be reduced easily because it includes only the diffusion film for diffusing the incident light from the specific angular range into the specific angular range as shown in FIG. 10, and that a diffuse light intensity distribution characteristics are substantially uniform for the incident light from the specific angular range as shown in FIG. 11 and hence variations in brightness in a screen is low.
On the other hand, in order to reduce the thickness of an optical system for a rear projection display system, as shown in FIG. 31 for example, it is necessary to arrange a projector 20 not on a screen optical axis 10A, but at a position shifted from the screen optical axis 10A to cause a projector light to enter into a screen 10 from an oblique direction. In general, most part of the projector light entered into the screen 10 from the oblique direction does not diffuse in the front direction of the screen where an observer exists, but strongly in the direction of straight-ahead transmission of the projector light. Therefore, in order to achieve the thickness reduction, it is very important to convert the out-going direction of the projector light incoming into the screen from the oblique direction into the front direction.
In order to achieve the conversion of direction of the projector light as described above, as shown in FIG. 32 for example, a technology to cause the projector light 20A incoming from the oblique direction to pass through a prism 30 to convert the outgoing direction by the use of one or both of refraction 40 and total reflection 50 at an interface of the prism is known in the related art (for example, see Document 3, Shikama, S. et. al., SID'02 Digest, p. 1250-1253)