Card – picture – or sign exhibiting – Photographic transparency viewer – e.g. – x-ray viewer
Reexamination Certificate
1997-10-30
2001-01-30
Knight, Anthony (Department: 3628)
Card, picture, or sign exhibiting
Photographic transparency viewer, e.g., x-ray viewer
C040S367000, C040S564000
Reexamination Certificate
active
06178672
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 sheet polarizers.
BACKGROUND OF THE INVENTION
The use of backlit viewboxes for viewing transparencies, particularly for medical transparencies, is known in the art. It is also known in the art to use viewboxes which incidentally emit polarized light. Examples of viewboxes which emit polarized light are shown in U.S. patent application Ser. No. 08/348,958 filed Nov. 28, 1994 and in a PCT publication PCT/WO91/10152, titled “Self Masking Transparency Viewing Apparatus” and published on Jul. 11, 1991, the disclosures of which are incorporated herein by reference.
An important issue in transparency viewing is minimizing the amount of light from areas not covered by the transparency. Owing to certain psychophysical properties of the human visual system, it is more difficult for a viewer to discern low contrast details at lighting levels different from those to which the viewers eye is adapted. Since the eye adapts to all the light entering the eye and not just to light that is at the view point, reduction in extraneous light enables the eye to better adapt to lighting levels in the transparency. This adaptation enables the full range of sensitivity of the eye to be applied to the low contrast details in the transparency. It can also be appreciated why transparencies are usually viewed in relatively dark rooms.
A previous application, “Self Masking Transparency Viewing Apparatus,” published as PCT/WO91/10152 on Jul. 11, 1991 and assigned to the same assignee, describes a system using a CCD camera and a controller to detect at least the edges of a film transparency placed on an LCA (Liquid Crystal Array) based viewbox. In operation, images are acquired through the camera and then analyzed by the system. The controller then masks the backlighting to those portions of the film or of the display surface that are of no interest to the viewer. A similar and more advanced device is described in unpublished U.S. patent application Ser. No. 08/348,958 filed Nov. 28, 1994.
However, such devices are relatively complex and usually require replacing existing viewboxes with new viewboxes.
Transparencies for X-ray imaging are typically manufactured by depositing light sensitive materials on a plastic sheet and then cutting the sheet down to the required transparency size. Usually, the plastic sheet is taken from a continuous roll of stretched plastic and the transparencies, which are usually rectangular, are cut so that their edges are parallel to the edges of the plastic sheet.
SUMMARY OF THE INVENTION
It is an object of some preferred embodiments of the present invention to provide an improved method for viewing transparencies which does not require major changes in existing equipment. When viewing transparencies using some preferred embodiments of the present invention, light from portions of the viewbox not covered by transparency is substantially attenuated compared to light from portions covered by the transparency. Thus, the transparency can be viewed by a viewer without his being dazzled by extraneous light.
It is known in the art that when two linear polarizers are placed so that their polarization axes are at about 90° to each other, very little light passes through them. It is also known to use such crossed polarizers for analyzing stress in transparent objects.
The inventors have found that when a transparency, such as those used for X-ray imaging, is placed between two crossed polarizers, light passes through portions of the crossed polarizers containing the transparency. Preferably, the transparency is placed so that its edges are at 45° to the polarization axes of the polarizers. The inventors have found that since most transparencies are manufactured from stretched plastic, usually polyethylene, they do not have an isoradial effect on polarized light. Instead, it appears that stretched plastic acts as a retardant layer, with an ordinary and an extraordinary optical axis. These axes are oriented parallel and perpendicular to the stretch direction. The term preferred retardation axis, as used herein, refers to either the ordinary or the extraordinary retardation axis. When polarized light, having a polarization axis which is not substantially parallel or perpendicular to the preferred retardation axis of the plastic, passes through the plastic, it is converted into elliptically polarized light. Elliptically polarized light is only partially attenuated by the second polarizer, which is why some light passes through portions with overlap between the transparency and the polarizers.
Most transparencies are rectangular and are cut parallel to the stretch direction. This means the edges of most transparencies are parallel or perpendicular to the preferred retardation axis. Thus, if transparencies are oriented so that their edges are at 45° to the polarization axes of the polarizers, the transparency's preferred retardation axis is also generally at 45° to the polarization axes of the polarizers.
The inventors have also found that in order to reduce moire patterns caused by uneven stress within the polarizers and the transparencies, it is useful to apply jitter to one of the polarizers. Preferably, the jittering has a rotational component so that the angle between the polarization axes of the two polarizers changes. Alternatively, both polarizers are jittered relative to the transparency. Alternatively, the polarization axis of the polarized light passing through one of the polarizers is jittered using an LPR (Light Polarization Rotator), preferably, an LC (Liquid Crystal) without face polarizers.
An alternative method of reducing moire patterns is to use polarizers which allow some light to pass even if it polarized perpendicular to the polarization axis of the polarizers. One way to achieve this effect is to use imperfect polarizers. A small but significant amount of unpolarized light washes out the moire patterns. However, while this light does reduce the contrast between the light passing through the transparency and the light that passes only through the polarizers, the resultant contrast reduction appears to be acceptable. Alternatively, a diffuser is placed between the transparency and one of the polarizers, since a diffuser partially depolarizes polarized light.
A preferred embodiment of the present invention uses a viewbox which has a first polarizer on its display surface (behind the film) and a second polarizer which is mounted on eye-glasses worn by viewers. Alternatively, other head mounted or hand held polarizers are used.
In another preferred embodiment of the present invention the second polarizer is mounted on a transparent cover, so that transparencies can be placed on the inside surface of the cover or on the display surface when the cover is open. When the cover is closed, the transparency is located between two polarizers. The cover can, for example, have a horizontal or vertical hinge or comprise a sliding cover.
In yet another preferred embodiment of the present invention, the second polarizer is mounted in front of the display surface and forms a narrow space between the polarizer and the display surface. Preferably, transparencies are attached to a mounting device and slipped in from the top of the narrow space. The mounting is also used to remove the transparencies. Alternatively, the bottom of the narrow space is movable when removal of the transparencies is desired. When the bottom is moved, the transparencies drop out of the space. Another alternative is to slip the transparencies into the narrow space from its side, either by hand or on a sliding mount.
In a further preferred embodiment of the present invention a retractable sliding mount is used to insert transparencies into the narrow space. Preferably, the sliding mount slides into the narrow space through its bottom opening. Preferably, the sliding mount is retractable with a spring or counter weights. Alternatively, a mot
Inbar Dan
Shaffir Eran
Teltsch Giora
Wolf Hanan
Fenster & Company Patent Attorneys Ltd.
Hewitt James M.
Knight Anthony
SmartLight Ltd.
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