Television – Camera – system and detail – Optics
Reexamination Certificate
1999-07-01
2003-05-20
Moe, Aung S. (Department: 2612)
Television
Camera, system and detail
Optics
C348S351000
Reexamination Certificate
active
06567126
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the correction of planar focus in an electronic still or video camera.
BACKGROUND OF THE INVENTION
Desktop flat bed scanners are very common in office imaging applications. Although these are relatively inexpensive and work well, a disadvantage is that these invariably take up a significant amount of desk space, which is always at a premium.
Digital camera products are becoming common in many areas of still and motion photography, and as a result are becoming ever less expensive. However such cameras are still used almost exclusively for photography of people or places, and have yet to be adapted for use in office imaging applications. This is because electronic cameras, which normally employ two dimensional CCD arrays, have insufficient resolution to image a complete A4 page at 300 dots per inch, the minimum that is conventionally believed necessary for reasonably high quality reproduction. Whilst larger CCDs are available in electronic cameras, they are much too expensive for a mass-market office imaging product.
A camera mounted over the desk and looking directly downwards could image documents on the desktop, avoiding this permanent waste of space. In order to obtain sufficient resolution with the limited number of available pixels, the camera would need to be directly above the document to be imaged, so that all of the document was in best focus. This, however, is inconvenient, requiring either that a user lean directly over the document or that a frame is provided over the desk on which the camera would be mounted. An overhead camera could obstruct a user's headroom or be inadvertently knocked.
Alternatively, if the camera were not directly above the document but, say, held or mounted at near an edge of the desktop, then not all of the document would be in focus simultaneously because the document would not be at right angles to the optical axis of the camera. Owing to the loss of resolution due to the camera's limited depth of field, only a part of the document would be captured with sufficient resolution in one frame. This is inconvenient, because a user or some type of mechanical actuator would then have to manually pan and tilt the camera to take a number of overlapping pictures, each with a magnification high enough to obtain sufficient resolution, and subsequently “stitch” these pictures together using software.
There is also the additional problem that the image would be distorted owing to the oblique viewing angle, an effect, referred to herein as “keystone distortion”. Although such perspective distortion can readily be rectified using well-known image processing techniques, this does result in non-optimal resolution over the portions of the image where a printed character is spread over fewer pixels.
Conventional approaches to addressing these problems suffer from other limitations. Greater depth of field can be provided by reducing the camera aperture but this lowers the amount of light at the image plane, which raises noise levels. A greater exposure time will not work with hand-held photography, owing to camera shake. Active or electronic image stabilizers add to cost, and are not fully effective at eliminating the effect of camera shake. A frame and mechanical mounting to hold and pan/tilt the camera can eliminate camera shake, but with a significant penalty in terms of mechanical complexity and cost. Even with a small aperture, or longer shutter time, it is difficult to obtain the necessary depth of field to operate down to an angle of less than 45 degrees, as may be desirable with an electronic camera held or mounted at the edge of a desktop.
For many years, photographers have understood that under certain circumstances as defined by the Scheimpflug condition, it is possible simultaneously to focus on several things at different distances from the camera, providing that all the objects of interest lie on a flat plane. When the Scheimpflug condition is satisfied, the object plane, image plane, and a plane passing through the lens, all intersect along a line. Whilst this condition is necessary for correct focus, it is not, on its own, sufficient. Setting up a camera to focus on a tilted object plane has up to now been regarded as requiring a tripod mounted large format view camera and the skill of a professional photographer.
Professional photographers using such large format cameras can, with practice and an intuitive understanding of three dimensional geometry, achieve the Scheimpflug condition, and adjust the angle of the lens and/or the image plane in order to focus on a scene that has an area of interest in a plane at an oblique angle to the optical axis. Conventionally, this is done by viewing the image projected on a ground glass screen in the camera, whilst adjusting the relative orientation of the lens and image plane to focus on the object plane of interest.
Although this principle has been known for many years, Scheimpflug cameras have remained manually operated and large format. One attempt to partially automate a Scheimpflug camera is described in patent document U.S. Pat. No. 4,564,277. This however, describes a camera suitable for professional film photography and which may have a ground glass screen for viewing an image. The process is only semi-automated, requiring a photographer to move an image plane perpendicular to the optical axis to at least two locations where different areas of the screen are in focus, and enter (x,y,z) position data for these locations into a calculator, which from the known focal length of the lens, can then calculate the correct orientation of the lens or image plane to meet the Scheimpflug condition. Such a camera and process are not sufficiently inexpensive, quick or convenient for a mass-market desktop imaging product.
It is an object of the present invention to provide an electronic camera that addresses these problems.
BRIEF DESCRIPTION OF THE INVENTION
Accordingly, the invention provides an electronic, camera, comprising: a detector array; an objective lens arranged to direct optical radiation from an object plane onto the detector, the lens defining an optical axis of the camera, and the object plane being at an oblique angle to the optical axis; movement means to change the relative orientation of the detector with respect to the lens so that the detector and lens may be moved relatively toward or away from each other along the optical axis and also tilted with respect to each other with at least one degree of freedom; and focus detection means connected to the detector to detect when a portion of an image falling on the detector is in focus, characterized in that the camera includes a processor means to control the movement means according to the detected focus, the processor means bringing the image into focus on the detector by first changing the relative orientation of the lens and detector until a first portion of the image is in focus, and then holding said first portion in focus whilst continuing to change the relative orientation of the lens and detector until a second portion of the image is also in focus.
The term “lens” as used herein is not restricted to a single lens element and includes lenses with compound optical elements.
The focus detection means may be any of the known automatic focusing techniques that are found in digital or film-based still or video cameras. These include range finding techniques based on the transmission and reception of reflected infrared pulses. Additional focus means may also be employed, for example those relying on reflected ultrasonic pulses. Other known techniques that involve some sort of statistical or frequency domain analysis of the detected image, are particularly appropriate when the object is imaged electronically, rather than with photographic film, as with video or digital still cameras.
Acoustic and, particularly, infrared range finders are very directional. It would be possible to fit three of these to the camera pointing in different directions within the field of view. The position of a
Roe Malcolm David McKinnon
Slatter David Neil
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