Card – picture – or sign exhibiting – Photographic transparency viewer – e.g. – x-ray viewer
Reexamination Certificate
1997-06-04
2001-08-07
Green, Brian K. (Department: 3628)
Card, picture, or sign exhibiting
Photographic transparency viewer, e.g., x-ray viewer
C040S548000, C345S087000
Reexamination Certificate
active
06269565
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally related to the field of transparency viewing apparatus, and more specifically to the field of transparency viewing apparatus using a liquid crystal array display surface.
BACKGROUND OF THE INVENTION
The use of liquid crystal arrays (LCA) as the display surface for a transparency has been suggested in the past. Such systems, in order to be optimally effective, require that the portion of the array which underlies the transparency (or the portion thereof) being viewed has sufficient brightness to enable viewing of the image and that the contrast ratio between the bright and dark portions of the array (namely the area outside the transparency or an image region of the transparency) be high.
The use of active matrix, passive matrix and direct drive technologies for LCAs have been described in the art. Each of these technologies, as they are used in the art, have substantial drawbacks when applied to a transparency viewer. Active matrix arrays have the drawback of high cost for the array because of the additional process steps required for the active elements and the difficulty in achieving large size arrays due to the limitations of semiconductor thin film technology.
Direct addressing systems have large spacings between the elements for the passage of the large number of conductors needed for this system, thus interrupting the continuity of the active regions of the LCA and reducing the contrast of the system. The large number of drivers required increases the cost of the associated electronics especially for large arrays.
Passive matrix systems have a much lower contrast and viewing angle than do active matrix systems because of the scanning limitations of the multiplex drive systems which are generally required for such systems. This is especially true when a number of different areas are bright and dark in systems which have the flexibility to provide a large number of such areas. On the other hand, both the passive matrix arrays and their associated electronics are more available and less expensive as compared to direct addressing systems. However, while such passive matrix systems do have smaller inactive stripes between the LCA elements than the direct addressing matrix LCAs, in some cases even these stripes can be troublesome to the viewer.
Another problem with LCA viewers is caused by the fact that the degree of contrast in LCAs is a strong function of the angle of incidence of the light which illuminates the LCA, with maximum contrast being available for near normal incidence. Since back illumination is by its nature relatively isotropic, the contrast of the viewing surface is further degraded.
SUMMARY OF THE INVENTION
It is a purpose of one aspect of the present invention to simplify the detection of the edges of films on viewboxes.
A preferred embodiment of the present invention utilizes the polarization of the light emitted by some types of viewboxes and the depolarizing or retarding effect that most types of film used for transparencies have on polarized light. It should be appreciated that film does not generally turn polarized light into diffuse light; however, for the purpose of some aspects of the invention, it is only necessary that film transform linearly-polarized light into elliptically-polarized light.
Some embodiments of the invention, related to edge detection, are not limited to any specific type of viewbox, as long as the viewbox can be adapted to emit polarized light, such as by placing a linear sheet polarizer on its display surface.
The aspects of the invention relating to edge detection are typically applied to a viewbox with a display surface adapted to hold a film thereon. A camera views the display surface and is connected to a controller. The controller acquires images from the camera and uses an edge detection process to locate the edges of the film on the display surface. Once the edges are found, the viewbox lighting is controlled so that backlighting intensity outside the film is significantly reduced. This backlighting is typically linearly polarized, especially when it is controlled using LCAs (Liquid Crystal Arrays).
One of the problems with the edge detection process as described above is the amount of glare, caused by reflected ambient light, in the image acquired by the camera. Since the camera is typically placed at an oblique angle to the display surface, the display surface and the film thereon reflect ambient light toward the camera. Usually, when light strikes a semi-reflecting surface at an oblique angle, a large portion of the light is reflected. However, this light is preferentially polarized perpendicular to a plane formed by the light source, the camera and the reflection point, so that it can be partially attenuated by a linear polarizer.
It should be appreciated that, since the polarizer views each point on the surface of the viewbox at a different angle, different parts of the viewbox require different orientations for the polarizer to effectively attenuate the reflected light. However, if the polarizer is far enough away from the viewbox, the viewing angles, and therefore the polarizer orientations, are similar for all the areas of the viewbox.
In accordance with a first preferred embodiment of the invention, a linear polarizer is placed between the camera and the film, preferably close to the camera. The polarizer is aligned so that its polarizing axis is substantially perpendicular to the average major polarization axis of the reflected light. Since the reflected light is preferentially polarized, it is attenuated by the polarizer, thereby reducing glare. If the viewbox emits polarizing light, the camera, polarizer and polarization axis of the viewbox are configured so that the polarization axis of the light emitted from the viewbox is essentially parallel to the polarization axis of the polarizer.
In this first embodiment, using a viewbox which emits polarized light, the light passing through the film is attenuated by the polarizer because the film turns linearly polarized light into elliptically polarized light. Conversely, the light from the uncovered portions of the viewbox is substantially unaffected, because the light is polarized parallel to the polarization axis of the polarizer.
If it is desired to keep a constant brightness difference between the film and the display surface, the camera, polarizer and polarization axis of the viewbox are configured such that the polarizer attenuates polarized light from the viewbox and light passing through the film in substantially equal proportions.
It should be noted that different configurations of glare reduction and polarization matching generally involve different locations of the camera relative to the viewbox.
In accordance with a second preferred embodiment of the present invention, preferred when the film has substantially transparent edges, the camera and the polarization axis of the viewbox are configured so that the polarizer's polarization axis is substantially perpendicular to the polarization axis of the light emitted by the viewbox. In this configuration, light that is partially depolarized by passing through the film is substantially less attenuated by the polarizer than polarized light coming directly from the viewbox. Therefore, light passing through clear portions of the film, i.e. portions with low or zero optical density, is ultimately attenuated less than light not passing through the film. If the polarizer is also configured such that its polarization axis is perpendicular to the average major polarization axis of the reflected light, the ambient light glare reduction effect is also achieved.
In accordance with a third preferred embodiment of the invention, in a configuration similar to the second preferred embodiment, an LPR (Light Polarization Rotator), preferably an LC (Liquid Crystal), is placed between the linear polarizer and the display surface, preferably close to the camera. The LPR has two operational states: 90°, wherein the LPR rotates the polarization axis of incident light by 90°; a
Inbar Dan
Levi Ben-zion
Natanzon Alex
Shaffir Eran
Wolf Hanan
Fenster & Company Patent Attorneys Ltd.
Green Brian K.
SmartLight Ltd.
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