|Wednesday, 06 September 2006|
In the operation of an image projection system, an image is projected from an image source to a screen, and the projected image is viewed by a viewer in front of the screen. Projected light may be used to display images on large surfaces, such as large computer displays or television screens. In a front projection system, an image beam is projected from an image source onto the front side of a reflection-type, angle transforming screen, which then reflects the light toward a viewer positioned in front of the screen. In a rear projection system, the image beam is projected onto the rear side of a transmission-type, angle transforming screen and transmitted toward a viewer located in front of the screen. Projection screens in rear projection systems is called transmissive projection screen. A transmissive projection screen is intended for projecting an image from a position behind the screen and viewing the image from a position in front of the screen. Such screens comprise a diffusion layer for scattering the light projected by the projector from behind on the diffusion layer into more directions on the front side. In the front projection screen, an image is projected from a projector onto the front projection type screen and displayed thereon. A size of the front projection type screen is much greater than the rear projection type screen. In contrast, the rear projection type screen, which is usually adopted in typical projection televisions exhibits, provides superior image quality compared to the front projection type. The difference in the image quality between the front projection type screen and the rear projection type screen lies in a difference in structure of each screen. The rear projection type screen uses a dark tint screen providing high contrast and enabling high brightness by decreasing an angle of visibility in a vertical direction and increasing an angle of visibility in a horizontal direction. The rear projection type screen used in the image display device is usually provided with a Fresnel lens sheet and a lenticular lens sheet. The Fresnel lens sheet converts an enlarged projection image into light which is substantially parallel or light which faces somewhat inwards.
A projection screen is an optical device which does not create an image but provides a required field of view in the vertical and horizontal directions of viewer space. By reducing the field of view in the vertical direction, the screen creates the effect of increasing the brightness of the image within the viewing area, an effect which is referred to in the art as gain. The projection screen may be configured to reflect the output of the projector such that it is viewable by one or more people. For example, a projection screen in a movie theater environment may reflect light that is projected by a movie projector to be viewable by people in the movie theater. Typical examples of projection screens for use in projection systems include white-colored paper or cloth materials, and plastic films coated with inks that scatter white light. Besides, high-quality projection screens that comprise scattering layers containing beads, pearlescent pigments, or the like, capable of controlling the scattering of imaging light, are now commercially available. In front screen projection systems, a reflecting screen is placed behind the real object to directionally reflect a projected scenery image back to a camera which records the composite image. An image beam is projected from an image source onto the front side of a reflection-type, angle transforming screen, which then reflects the light toward a viewer positioned in front of the screen. In the front projection projector, a reflective type screen is located on a vertical wall in a room, and a projection unit, including liquid crystal display panels and a projection lens, is arranged at the center in the room, whereby the modulated light is projected by the projection lens onto the screen for producing an image thereon. Observers see the light reflected from the screen as an image. In front projection devices an image is projected directly onto the front face of a screen from the front face. With front projection screens, the image is produced on the side of the projection screen facing the spectator, whereby the images to be reproduced are projected on to the projection plane from a projector placed at a distance from the projection screen. Front projection screens for displaying images projected by a projector include white matrix screens, chemical etching screens, and high-luminance bead screens.
Generally, front projection screens are either reflective, light scattering, or refractive. Reflective screens have surfaces exhibiting behavior governed primarily by the law of reflection; that is, the angle of incidence of light equals the angle of reflection. Refractive screens are coated with tiny glass balls and light projected at an angle to the screen is returned along the path of incidence. Refractive screens are particularly useful when the projector is positioned low relative to the screen because the light is returned along the path of incidence toward a seated audience. This type of screen provides greater gain than is available with light scattering screens and, therefore, better visibility in areas of more intense ambient light. The surface of light scattering screens scatters the incident light in all directions rather than reflecting it at a discrete angle. Light scattering projection screens provide a wide viewing angle both horizontally and vertically but the gain of the screen is low and images are visible only under dim lighting conditions. A reflective type screen comprising a reflective layer which reflects light from a projector and a diffusion layer which diffuses the reflected light has been used as a screen for overhead projectors, slide projectors, and movie projectors. Of the reflection type screens, some are generally accommodated in a housing mounted on a ceiling or a side wall, and are electrically or manually stretched out as occasion demands. Hanging screens or screens with a tripod are used by hanging or locating them at a desired place. The most commonly used projection screens are manufactured of fabric, painted wood, metal, plastic, or other solid material. A well-known example of such a solid projection screen is the movie screen. Portable screen assemblies are today available in which a screen is wound around a roll housed in a housing. When such screen assemblies are in use, the housing is initially installed at a desired place, and the screen is subsequently stretched out by. Although roller screens are somewhat portable in small sizes, there was and is a demand for larger size projection screens for use by staging and projection professionals in remote locations for events such as sales meetings, seminars, and entertainment.
The reflective type screen is required to have a good SG (screen gain) value, a large horizontal light amount ratio, a large peripheral light amount ratio, and good optical contrast between the dark and bright portions of the projected image. A screen useful for displaying a projected image may be as simple as a reflective surface. However, the appearance of a projected image on such surfaces may be pale or washed-out, typically due to ambient light. The environment in which the projector and screen are utilized may have an effect on the visual quality of the image that is displayed on the screen. While image projection provides a large, quality image, the visibility of projected images can be greatly impacted by the amount of ambient light in the room or area where the image is projected. Generally, a projected image is easier to see in low ambient light and more difficult to see in relatively bright ambient light. As some other ambient environment light sources exist, lights such as the sunlight or the room light from any given lamps outputted from these ambient environment light sources are also reflected by the screen to viewers. When an image is projected on such a screen by a projector in a well-lighted room, the image cannot be displayed clearly due to the reduction of contrast or the loss of color balance. The quality of the projected image may be improved by employing a screen having a somewhat reflective surface. Such screen surfaces may incorporate a reflective material whose reflectance decreases rapidly as the angle to the projected image increases, so that light directed directly to the screen surface is strongly reflected toward the viewer, while ambient light impinging on the screen surface at an angle is reflected away from the viewer. One measure of the visual quality of a projected image is contrast ratio. Contrast ratio is the ratio of the brightness of a white pixel, i.e. picture element, to the brightness of a black pixel.
There are many applications that need a screen to have projection images displayed thereon, for the slide show, presentation, or movies, through devices such as overhead projectors, slide projectors, movie projectors, and video projectors. Video projectors incorporate with controlled environment lights so as to simulate a home theater. Video projectors also can display still images such as texts or graphics in any given environment with the room light for presentation purpose. Front projection screens are used in other applications in which the projectors must be placed forward and spaced from the screen such as for example in systems used for projection screens that are greater than eight feet diagonal. The brightness of images produced by liquid crystal projectors, in particular, can be relatively low because light of only one polarization state is projected onto the screen due to the nature of the liquid crystal displays used to form the images. As a result, liquid crystal projectors are used primarily in areas with low levels of ambient light, such as rooms in which the windows are covered with curtains and/or in which artificial lighting is dimmed, to limit the contrast reducing effects of the ambient light. For the purpose of having a better image projection effect on a large screen, images used in movie theaters inherently have wide ranges of light intensities and evenly require an environment having a comparatively dark light projected on the projection screen.