Optical: systems and elements – Projection screen – Embedded particles
Reexamination Certificate
2001-03-30
2003-10-07
Mahoney, Christopher (Department: 2851)
Optical: systems and elements
Projection screen
Embedded particles
C359S453000
Reexamination Certificate
active
06631030
ABSTRACT:
FIELD
This invention relates generally to methods for conveniently and efficiently making a projection screen and more particularly to projection screens constructed according to such methods.
BACKGROUND
Recycled glass is generally glass that has been previously used for a particular purpose and then discarded. Examples include automobile windshields, beverage bottles and windows that have been broken into small pieces. It is known to form recycled glass into low quality microspheres. Recycled glass microspheres are available from Flex O Lite of Muscatine, Iowa, Paris, Texas and St. Thomas, Canada; and Potters of Valley Forge, Pa. Recycled soda lime glass has a refractive index of about 1.5.
FIG. 4
illustrates an example of glass microspheres constructed from recycled glass. Recycled glass microspheres are considered to be lower quality microspheres than similar microspheres generated from substantially pure raw materials. Note the glass has several non-spherical defects. Glass microspheres constructed from recycled glass are typically used as inexpensive filler materials to modify the flow characteristic of a material, or for peening processes, or as an reflective component in a reflective article such as a road marking product. When used in these processes, a large number of defects (e.g., cullet, opaque particles and bubbles) may be tolerated. Glass microspheres constructed from recycled glass are available in a wide range of sizes. The standard deviation of the size of such recycled glass spheres is typically quite large.
A rear projection screen is a sheet-like optical device having a relatively thin viewing layer that is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by a projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by a projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
Front projection systems are also known in the art. They comprise a projector designed to project an image on a surface (e.g., the wall of a conference room or a screen). Overhead projectors are an example of a front projection system. Front projection systems have less capacity to absorb ambient light than rear projection systems.
A well-known type of rear projection screen is a thin, light diffusing layer such as a frosted or translucent glass surface, which may be produced by etching, sandblasting, or otherwise roughening a smooth glass surface. The translucent surface limits the visibility of objects behind the screen. The screen must, however, be sufficiently light transmissive to allow the projected image, which is focused precisely on the translucent surface, to be viewed from the front side of the screen. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent suffer, however, from a tendency to strongly reflect ambient light incident on the front side, thereby causing fading, or washout, of the projected image. This problem is particularly severe if the background or ambient light is bright.
An approach to reducing the effects of ambient light while still maintaining an acceptable level of projected image light is to attach an array of closely packed glass microspheres (i.e., beads) to a substrate by an opaque polymeric binder. The glass microspheres and substrate are both light transmissible (e.g., transparent). The glass microspheres act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the microspheres. The foci are approximately in the areas where the microspheres contact the front support layer.
Because the transparent microspheres contact the front of the substrate, they exclude most of the opaque binder material from the space between the microspheres and their contact areas on the substrate. This forms an optical aperture between each microsphere and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the microsphere interstices. As a result, ambient light incident in these areas is absorbed. Thus the front side of the screen appears black, except for the light transmitted through the microspheres.
U.S. Pat. No. 2,378,252 (Staehle) describes projection screen displays, such as rear projection screen displays, based on glass microspheres embedded in an opaque matrix. These types of screens have been growing in popularity for various uses, such as in large format televisions. Rear projection screens with glass beads are also disclosed in U.S. Pat. Nos. 5,563,738 and 5,781,344. Additional beaded screens and methods of making such screens are disclosed in commonly assigned patent application PCT WO 99/50710.
The appearance of such screens is highly sensitive to the quality and placement of the glass microspheres used. Excessive amounts of microspheres that are of incorrect size, are not spherical, or are broken, nicked, scratched, or otherwise defective can create a variety of visible defects, variously called graininess, scintillation, sparkles, speckle, punch through, or simply spots. For example, U.S. Pat. No. 5,563,738 (Vance) states that it is “necessary to eliminate out-of-round, wrong-sized, and discolored microspheres” in order to obtain a uniform appearance.
PCT WO 98/45753 states that glass bead rear projection screens suffer from limitations, especially when they are used to project high quality images that are to be viewed from short distances. The appearance of such screens is highly sensitive to the quality and placement of the glass beads used. Beads that are of incorrect size, are not spherical, or are broken, nicked, scratched or otherwise defective can create a variety of visible defects, variously called graininess, sparkles, punch through or simply spots. These defects are particularly troubling when the screen is used, for example, as a computer monitor, where the need to see a high level of detail is likely to lead the user to scrutinize the screen closely, from a short distance, for long periods of time.
Sparkle is of particular concern for a rear projection system. Sparkle in a rear projection screen is a random pattern of bright points of light that change with the position of the viewer and results from defective beads.
U.S. Pat. No. 6,172,814 (Wantanabe et al.) discloses a rear projection screen having a plurality of glass minute transparent balls. Wantanabe et al. discloses several embodiments. In those embodiments, glass minute transparent balls having a refractive index of n=1.5, n=1.6, n=1.7, n=1.8, n=1.9 and n=2.1 are disclosed. Wantanabe et al. stated that glass minute transparent balls having a refractive index of n=1.5, n=1.9 and n=2.1 were mass produced, generally available and at low cost. Wantanabe et al. also stated that glass minute transparent balls having refractive indexes of n=1.6, n=1.7 and n=1.8 are comparatively expensive.
U.S. Pat. No. 6,204,971 (Morris et al.) discloses glass microspheres for use in rear projection screen displays and methods of making such microspheres. In a preferred embodiment, Morris et al. tolerates low levels (e.g., less than 15%) of defects. In a preferred embodiment, the defect level was measured by counting the microspheres and the respective defects of a microsphere sample under an optical microscope. The percent defects referred to the total number of microspheres that contained bubbles, possessed odd shapes, were opaque or formed cullet.
A number of processes have been devi
Morris Geoffrey P.
Stevenson James A.
3M Innovative Properties Company
Buckingham Stephen W.
Mahoney Christopher
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