1987-10-01
1988-12-20
Wintercorn, Richard A.
G03B 2156
Patent
active
047922095
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to projection screens and more particularly to projection screens having a surface which compensates for projected variations in light intensity levels which are imperceptible to the human observer but are detectable by video reproduction of an image on the screen. The invention also relates to a method for making such projection screens, as well as methods of projecting an image onto such screens.
The projection of slide images on screens is well known. A common problem with such slide projection is that the image projected on the screens tends to be brightest at the center of the screen and dimmest on the edges.
This phenomena is caused by the geometry and optics of conventional slide projectors in that the projector light source projects outward with a beam of light which is in the shape of a sector of a sphere. Since the screen intersects this sphere sector of light with a flat plane, the center of the screen is closer to the light source than the edges. This results in the brightest image portion at the center, with decreasing brightness radially outward therefrom. This phenomena is sometimes termed the "halo" effect.
To the human observer, the halo effect light intensity difference on the screen is imperceptible because the human brain has the capacity to adjust for light intensity variations greater than the differential between the center of the screen and the edges of the screen. Thus, although the human eye can discern the light variations as well as, if not better than, conventional video cameras can, the human brain adjusts for these light variations and the human observer does not perceive these variations. However, reproduction of these projection slide screen images by a video camera detects and records these varying light intensity levels as these video cameras have less capacity to adjust to the varying light levels than the human brain. Therefore, these video cameras will reproduce an image which shows the variation in light intensity to an extent that the video image then includes variations that are Perceptible to the human observer of the video images. Depending on various adjustments, the video image might appear to be of a normal light intensity at the center an appear dark at the edges or might appear to be normal at the edges and excessively bright at the center.
Although the human brain can adjust or accommodate for some of these video recorded light intensity variations, the quality and uniformity of the video signals, as perceived, is lower than would be the case if these slide projector induced light intensity variations were not existent in the video images.
Various methods are presently available which approach the varying light intensity problem by adjusting the light intensity before the light beam reaches the screen. Some methods involve placing a light filter in the path of the light beam between the projector and the screen. These methods require attachments to or adjustments in the video equipment filming the projected image on the screen. Further, these existing processes could easily add few thousand dollars to the cost of a professional quality video recording. A much less expensive approach for reproducing a uniform light intensity video image from a projection screen would be to adjust for the problem on the projection screen itself. Having a projection screen which adjusts for the varying light intensity problem would allow professional quality video recording by a normal video recorder for an indefinite period of time.
A translucent type screen, which adjusts for the varying light intensity problem is shown in U.S. Pat. Nos. 2,242,567 and 2,368,099. In these screens, still or motion pictures are projected on one side of a screen and viewed or photographed on the opposite side of the screen. A transparent base is first coated with a uniform layer of a material which includes microscopic light polarizing crystals. To compensate for the increased intensity at the center of the screen, successive
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"The Stereo Black Screen", Film 79, by Paul McGurk.
Dow Ronald G.
Laine Curtis S.
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