Optical: systems and elements – Projection screen – Rear projection screen
Reexamination Certificate
2000-12-27
2003-10-21
Adams, Russell (Department: 2851)
Optical: systems and elements
Projection screen
Rear projection screen
C359S453000, C359S454000, C359S459000
Reexamination Certificate
active
06636355
ABSTRACT:
The present invention is directed generally to a rear projection screen and more particularly to a rear projection screen that incorporates microstructures to waveguide the light passing through the screen and a light absorptive layer to enhance contrast.
BACKGROUND
Rear projection screens are generally designed to transmit an image projected onto the rear of the screen into a viewing space. The viewing space of the projection system may be relatively large (e.g., rear projection televisions), or relatively small (e.g., rear projection data monitors). The performance of a rear projection screen can be described in terms of various characteristics of the screen. Typical screen characteristics used to describe a screen's performance include gain, viewing angle, resolution, contrast, the presence of undesirable artifacts such as color and speckle, and the like. It is generally desirable to have a rear projection screen that has high resolution, high contrast and a large gain. It may also be desirable that the screen spread the light over a large viewing space. Unfortunately, as one screen characteristic is improved, one or more other screen characteristics often degrade. For example, the horizontal viewing angle may be changed in order to accommodate viewers positioned at a wide range of positions relative to the screen. However, increasing the horizontal viewing angle may also result in increasing the vertical viewing angle beyond what is necessary for the particular application, and so the overall screen gain is reduced. As a result, certain tradeoffs are made in screen characteristics and performance in order to produce a screen that has overall acceptable performance for the particular rear projection display application.
SUMMARY OF THE INVENTION
Some rear projection screens include microstructures to waveguide light passing through the screen toward a viewer, with the space between the waveguides filled with a light absorptive material to enhance contrast. In such a construction, however, some of the light to be internally reflected in the waveguides can be lost due to absorption in the light absorptive material. One reason for this can be that the angle of incidence of light in the waveguide is less than the critical angle necessary for total internal reflection in the waveguide given the index of refraction difference between the waveguide material and the light absorptive material.
In the present invention, a rear projection screen is provided that incorporates tapered waveguides and a light absorptive layer between the waveguides at the front of the screen that creates interstitial spaces between the waveguides that can be comprised of air or another low index medium. This allows for a larger index of refraction difference at the waveguide/interstitial region interface than when the entire interstitial region is filled with absorptive material. A larger index of refraction difference can be used to increase screen efficiency as well as allow for screen designs that further enhance contrast, provide for larger viewing angles, and the like.
In one aspect, the present invention provides a rear projection screen that includes a plurality of waveguides disposed on a transmissive substrate, the waveguides tapered from a wider back portion near the substrate to a narrower front portion away from the substrate, and a light absorptive layer disposed between adjacent waveguides and located at or near the front portions of the waveguides, thereby defining interstitial regions between adjacent waveguides and the light absorptive layer, the interstitial regions having a lower refractive index than the waveguides.
In another aspect, the present invention provides a method for making a rear projection screen that includes disposing a plurality of waveguides on a transmissive substrate, the waveguides being tapered from a wider back portion near the substrate to a narrower front portion away from the substrate, and disposing a light absorptive layer between adjacent waveguides at or near the front portions of the waveguides, thereby defining interstitial regions between adjacent waveguides and the light absorptive layer, the interstitial regions having a lower refractive index than the waveguides.
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U.S. patent application Ser. No. 09/348,809, “Rear Projection Screen Using Internal Reflection” 49 pages + 16 pages of drawings.
Cullen Peter F.
Moshrefzadeh Robert S.
Nelson John C.
Pokorny Richard J.
Rhyner Steven J.
3M Innovative Properties Company
Adams Russell
Cruz Magda
Pechman Robert J.
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