1. Field of the Invention
The present invention relates to a transparent screen, a projection display device, and an image display method for and of projecting image light onto a screen from the rear side of the screen as viewed from a viewer, so as to display an image.
2. Description of Related Art
There are projection display devices as a device which displays an image by using a combination of a Fresnel lens screen and diffusion sheets (diffusion layers). Unlike CRT (Cathode Ray Tube) and PDP (Plasma Display Panel), projection display devices are of nonluminescent type. A projection display device is provided with, as a projector, an illuminating optical system for guiding light from a light source toward a predetermined direction, a light valve to which the light guided by the illuminating optical system is applied, and for adjusting the amount of the light according to an image signal to form an image, and a projection optical system for enlarging and projecting the image formed by the light valve onto a screen thereof.
Projection display devices include rear projection type display devices each of which projects image light onto a screen from the rear side of the screen as viewed from a viewer, and front projection type display devices each of which projects image light onto a screen from the front side of the screen as viewed from a viewer. A transparent screen for use in a rear projection type display device among these devices is provided with a Fresnel lens screen for bending image light from a projector toward a viewer, and an image display element for forming the image light from the Fresnel lens screen into an image, and for providing an angle of diffusion for the image light so as to diffuse the image light.
Generally, because a Fresnel lens is formed in such a way as to have a lens pitch smaller than the size of projected pixels (e.g., one-tenth of the size of pixels), the Fresnel lens has a very thin size in its thickness direction (for example, the thickness including the prism portion is several hundreds of μm). Therefore, in order to hold the Fresnel lens, there is a necessity to provide a base having a thickness of about 1 to 5 mm. In many cases, the base is made from a resin, such as PMMA (Poly Methyl MethAcrylate), MS (Methyl methacylate Styrene), MBS (Methyl methacylate Butadiene Styrene), or PC (Polycarbonate), or glass. Furthermore, the Fresnel lens is formed directly on the base by using a light-curing resin or the like in many cases, and an element which is comprised of the Fresnel lens and the base is called a Fresnel lens screen.
The image display element is constructed in such a way as to include at least a light diffusing means and a base. The light diffusing means uses projections and depressions each having a larger surface than the wavelength of certain light (visible light having a wavelength ranging from 380 nm to 780 nm), or uses the difference between the refractive index of a diffusion layer substrate and that of particulates having a size larger than the wavelength of the certain light. When such the structure having larger fluctuations larger than the wavelength of the certain light is illuminated by light which is enlarged by a projection optical system so as to have large degree of spatial coherence, a large number of bright and dark spots (glaring) are recognized disorderly even if the light source is a lamp having small temporal degree of coherence. Generally, these light and dark spots are called speckles (strictly speaking, subjective speckles) or scintillation, and cause a problem of image degradation.
As measures against such speckles (scintillation), there have been proposed a method of arranging the diffusion layers included in the screen apart from one another (for example, refer to patent reference 1), a method of making the particle size of particulates whose refractive index difference with the diffusion layer substrate is large (Δn>0.08) be smaller than about 20 times (10 μm) as long as the wavelength of the light λ (for example, refer to patent reference 2), and so on.
[Patent reference 1] Japanese patent No. 3606862
[Patent reference 2] JP, 2004-271922,A
A problem with the above-mentioned conventional technology is, however, that in a case in which the gap between two adjacent diffusion layers is large, the image becomes blurred while the light propagates from the first diffusion layer to the next diffusion layer, and therefore the resolution decreases. A further problem is that in a case in which particulates having a large refractive index difference (a large dispersion of refractive index and a small particle size are used for the diffusion layers, the color temperature becomes low. This is because the path light follows bends at a different angle (diffuses with a different angle) according to its wavelength, i.e., color, and the path light follows bends at a larger angle (diffuses with a larger angle) as the refractive index difference becomes larger and the particle size becomes smaller. Although the reason can be explained on the basis of scattering problems for Maxwell's equations, it is generally known as the Mie scattering theory.