Photocopying – Projection printing and copying cameras
Reexamination Certificate
1999-10-01
2001-11-20
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
C355S055000
Reexamination Certificate
active
06320641
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to image acquisition using a camera and, in particular, to an apparatus and method used to set and maintain the resolution of an image with a high degree of precision.
BACKGROUND OF THE INVENTION
In certain situations, it is necessary or desirable to know and to be able to reproduce repeatedly and reliably the resolution of an image captured on a medium. In the context of an image captured on film, i.e., as a conventional photograph, the term “resolution” is analogous to the scale of the photograph and indicates how a distance or dimension measured “off of” the photograph corresponds to the distance or dimension in the scene or object that has been photographed. In the context of digital image acquisition using, e.g., a digital “photographic” or “still” camera or a digital video camera, the term “resolution” refers to the number of pixels in the image corresponding to a distance or dimension in the scene or oject being “photographed” or monitored, typically referred to as image pixels per object inch or, more conveniently, dots per inch (“dpi”).
One such application where it is necessary to know the resolution of an image (object inch per photograph inch in a film-based photograph or dots per object inch (dpi) in a digital image) relates to photographing fingerprints, e.g., for purposes of crime investigation. When fingerprints are photographed using a conventional, film-based camera, the person taking the picture typically puts an object of known size (“reference object”), e.g., a ruler or a coin, in the field of view with the fingerprint target. Dimensions within the fingerprint, e.g., from one fingerprint landmark to another, are then calculated by measuring the distance as shown in the photograph and scaling that distance (as shown in the photograph) either up or down, the scale factor being equal to the known, actual size of the reference object divided by the size of the reference object as shown in the photograph. Similarly, if a digital camera is used (either still (“photographic”) or video), the relevant distance is determined by measuring it in the image in terms of pixels, and then multiplying the number of pixels by an appropriate scale factor expressed in terms of length per pixels, the scale factor being equal to a known dimension of the reference object divided by the number of pixels in the image corresponding to that known dimension.
Calibrating the resolution of the image in this manner can be time-consuming, difficult, and therefore inaccurate. Accordingly, there is a need for an apparatus and method to facilitate recording an image with a known resolution, and doing so repeatedly and reliably.
SUMMARY OF THE INVENTION
The present invention fulfills this need. In general, the invention provides an apparatus and method by which a camera—preferably a digital video camera—can be positioned with respect to an object being imaged, with the resolution consistently and easily being maintained at a desired value. Furthermore, in preferred embodiments of the invention, the camera can be positioned such that the plane of optimal focus, typically at the center of the depth of field, is coincident with the plane of view having the required resolution.
In general, this is achieved with an apparatus in which the distance from the camera to a target object can be adjusted and then held fixed. The apparatus uses two or more light sources, e.g., lasers, collimated lights, spotbeams, slit lamps, etc., whose light axes can be pivoted such that their beams of light intersect at the surface of the target object. The light sources, which are held at fixed distances from the camera, can be locked into the particular angular orientation with respect to the camera. With this apparatus, once the resolution is set, that resolution can be regained for any subsequent object placed in front of the camera, regardless of its dimension or size, simply by moving the camera with respect to the new object until the light beams once again intersect at the surface of the new object. Alternatively, a single light source can be used and the camera is moved with respect to the new object until the point at which the light beam strikes the new object is at the same location within the field of view of the camera.
The apparatus includes a support surface which supports an object that is to be imaged, and the camera is supported at an optical distance from the support surface (i.e., from the object). The optical distance from the object to the camera is adjustable such that the resolution of an image of the object can be adjusted. The apparatus further includes a light source which projects a beam of light onto the object. The light source is configured such that the location where the beam of light strikes the object varies as the optical distance is varied.
Preferred embodiments of the invention may include one or more of the following features. Preferably, the apparatus is configured such that the beam of light is angled relative to the optical path (axis) along which the image of the object being observed is acquired. Preferably, the apparatus includes two light sources, both of which may be angled relative to the optical path such that the beams of light projected thereby can be rotated so as to intersect at the surface of the object being imaged.
Preferably, the light source or sources is or are lasers, and where two lasers are used, it is preferable for one of them to be a line laser with the other being a more conventional laser which projects a dot or spot of light. This permits the two beams to be distinguished more easily, thereby facilitating adjustment of the optical distance based on the relative positions of where the beams strike the surface of the object.
Preferably, the apparatus includes computer hardware and software for analyzing a signal generated and output by the camera. The signal can be analyzed to determine the resolution of the image acquired by the camera, which resolution may be expressed in terms of pixels per unit of length of the object being imaged (dpi).
In further contemplated embodiments, the apparatus includes a motor which adjusts the optical distance between the camera and the support surface and/or motors which adjust the angle of the light source or sources relative to the optical path. With such motors, the image can be analyzed by computer and the motors can be controlled by the computer to adjust a) the optical distance (whether to achieve a desired resolution during calibration or to return to that desired resolution subsequently); and/or b) the angle of the light beam or beams relative to the optical path such that a single beam strikes the surface of an object being imaged at a desired location (e.g., in the center of the field of view) or such that two beams converge at the surface of the object being imaged.
In still further contemplated embodiments, the apparatus may include and x-y-&thgr; support platform to facilitate precise positioning of the object being imaged. Furthermore, the light sources preferably are interlocked with the camera such that they are turned off when the image of an object is actually being acquired.
In another aspect, the invention features a method for acquiring the image of an object with a repeatable resolution. The method includes the steps of providing a camera at an optical distance from an object, the object being positioned within the field of view of the camera. First and second light sources are provided and project beams of light on the surface of the object. One or both of the light sources are adjusted such that the beams of light converge on the surface of the object. The object is then removed from the field of view and a second object is positioned within the field of view of the camera. The optical distance from the camera to the second object is adjusted, as necessary, such that the first and second beams of light converge at the surface of the second object. This guarantees the same resolution of an image of the second object as the resolution of an im
Bauer Heino G.
Schoenberger Robert B.
Siviter David J.
Adams Russell
Agris-Schoen Vision Systems, Inc.
Nguyen Henry Hung
Pillsbury & Winthrop LLP
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