News Column

"System and Method for Determining and Controlling Focal Distance in a Vision System Camera" in Patent Application Approval Process

August 28, 2014

By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventor Nunnink, Laurens (Simpleveld, NL), filed on October 1, 2013, was made available online on August 14, 2014, according to news reporting originating from Washington, D.C., by VerticalNews correspondents.

This patent application is assigned to Cognex Corporation.

The following quote was obtained by the news editors from the background information supplied by the inventors: "Vision systems that perform measurement, inspection, alignment of objects and/or decoding of symbology (e.g. bar codes) are used in a wide range of applications and industries. These systems are based around the use of an image sensor, which acquires images (typically grayscale or color, and in one, two or three dimensions) of the subject or object, and processes these acquired images using an on-board or interconnected vision system processor. The processor generally includes both processing hardware and non-transitory computer-readable program instructions that perform one or more vision system processes to generate a desired output based upon the image's processed information. This image information is typically provided within an array of image pixels each having various colors and/or intensities. In the example of a symbology (barcode) reader, the user or automated process acquires an image of an object that is believed to contain one or more barcodes. The image is processed to identify barcode features, which are then decoded by a decoding process and/or processor obtain the inherent alphanumeric data represented by the code.

"Often, a vision system camera includes an internal processor and other components that allow it to act as a standalone unit, providing a desired output data (e.g. decoded symbol information) to a downstream process, such as an inventory tracking computer system. It is desirable that the camera assembly contain a lens mount, such as the commonly used C-mount, that is capable of receiving a variety of lens configurations so that it can be adapted to the specific vision system task. The choice of lens configuration can be driven by a variety of factors driven by such factors as lighting/illumination, field of view, focal distance, relative angle of the camera axis and imaged surface, and the fineness of details on the imaged surface. In addition, the cost of the lens and/or the available space for mounting the vision system can also drive the choice of lens.

"An exemplary lens configuration that can be desirable in certain vision system applications is the automatic focusing (auto-focus) assembly. By way of example, an auto-focus lens can be facilitated by a so-called liquid lens assembly. One form of liquid lens uses two iso-density liquids--oil is an insulator while water is a conductor. The variation of voltage passed through the lens by surrounding circuitry leads to a change of curvature of the liquid-liquid interface, which in turn leads to a change of the focal length of the lens. Some significant advantages in the use of a liquid lens are the lens' ruggedness (it is free of mechanical moving parts), its fast response times, its relatively good optical quality, and its low power consumption and size. The use of a liquid lens can desirably simplify installation, setup and maintenance of the vision system by eliminating the need to manually touch the lens. Relative to other autofocus mechanisms, the liquid lens has extremely fast response times. It is also ideal for applications with reading distances that change from object-to-object (surface-to-surface) or during the changeover from the reading of one object to another object.

"A recent development in liquid lens technology is available from Optotune AG of Switzerland. This lens utilizes a movable membrane covering a liquid reservoir to vary its focal distance. This lens advantageously provides a larger aperture than competing designs and operates faster. However, due to thermal drift and other factors, the liquid lens may lose calibration and focus over time.

"One approach to refocusing a lens after loss of calibration/focus is to drive the lens incrementally to various focal positions and measure the sharpness of an object, such as a runtime object or calibration target. However, this requires time and effort that takes away from runtime operation, and can be an unreliable technique (depending in part on the quality of illumination and contrast of the imaged scene).

"It is therefore desirable to provide a system and method for stabilizing the focus of a liquid (or other auto-focusing) lens type that can be employed quickly and at any time during camera operation. This system and method should allow a lens assembly that mounts in a conventional camera base mount and should avoid and significant loss of performance in carrying out vision system tasks. The system and method should allow for focus over a relatively wide range (for example 20 cm to 2 m) of reading distance."

In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventor's summary information for this patent application: "This invention overcomes disadvantages of the prior art by providing a system and method for determining and controlling focal distance in a lens assembly of a vision system camera using an integral calibration assembly that provides the camera's image sensor with optical information that is relative to focal distance while enabling runtime images of a scene to be acquired along the image axis. In an illustrative embodiment, a system and method for determining focal distance of a lens assembly includes a variable lens located along an optical axis of the lens assembly that provides a variable focus setting. This variable lens can be associated with a fixed imager lens positioned along the optical axis between the variable lens and a camera sensor. A novel calibration assembly integral with the lens assembly is provided. The calibration assembly generates a projected pattern of light that variably projects upon the camera sensor based upon the focus setting of the variable lens. That is, the appearance and/or position of the pattern varies based upon the focus setting of the variable lens. This enables a focus process to determine the current focal length of the lens assembly based upon predetermined calibration information stored in association with a vision system processor running the focus process. The calibration assembly can be located along a side of the lens so as to project the pattern of light from an orthogonal axis approximately perpendicular to the optical axis through a reflecting surface onto the optical axis. This reflecting surface can comprise a prism, mirror and/or beam splitter that covers all or a portion of the filed of view of the lens assembly with respect to an object on the optical axis. The pattern can be located on a calibration target that is located along the orthogonal axis and is remote from the reflecting surface.

"Illustratively, the calibration target can define alternating transparent and opaque regions and can be illuminated (e.g. backlit by an LED illuminator). This target is illustratively oriented along an acute slant angle with respect to a plane perpendicular to the orthogonal axis so that pattern elements of the target (e.g. parallel lines) are each associated with a predetermined focal distance. For a given focal distance, the sharpness of a plurality of adjacent line pairs is evaluated. The sharpest pair represents the current focal distance. An intervening calibration assembly lens can resolve the light rays from the calibration target before they strike the beam splitter so as to provide the desired range of focal distances at the sensor (e.g. 20 cm-2 m (or infinity)). Illustratively the target can be illuminated by light of a predetermined wavelength (visible or non-visible), and the focus process distinguishes the predetermined wavelength so that calibration can potentially run without interfering with regular runtime imaging of objects.

"In another embodiment, the calibration assembly can include a projected 'pattern' defined generally as a spherical wave front. The lens assembly includes at least one micro lens, and generally a plurality of micro lenses each defining a portion of a wave front sensor that is oriented to project the pattern onto a predetermined portion of the camera sensor. Typically this portion is near an outer edge of the sensor so that it minimally interferes with the field of view used for runtime image acquisition.

"The system and method can further include, within the focus process, a control process that controls and/or adjusts the focus setting of the variable lens based upon the determined focal distance and a desired focal distance for the lens assembly. The variable lens can be a liquid lens, or other movable geometry lens, and more particularly can comprise a membrane-based liquid lens.


"The invention description below refers to the accompanying drawings, of which:

"FIG. 1 is a schematic perspective view of a vision system camera including a lens arrangement for determination and control of focal distance according to an embodiment herein, shown acquiring images of an object in a scene;

"FIG. 2 is a schematic diagram of a lens arrangement according to an embodiment herein including a membrane-based liquid lens and a fixed imager lens oriented along an optical axis between an object and a camera image sensor;

"FIG. 3 is a schematic diagram of a lens arrangement as shown in FIG. 2 oriented between an object and an image sensor, and including a calibration assembly based upon a slanted calibration target according to an illustrative embodiment;

"FIG. 3A is a schematic diagram of an idealized representation of the optical system of FIG. 3 showing the computation of focal distance of the liquid lens therein;

"FIGS. 3B and 3C are schematic diagrams of the ray trace of the optical system of FIG. 3 shown for the variable lens at minimum (approximately zero) and maximum optical power, respectively;

"FIG. 4 is a schematic diagram of a lens arrangement as shown in FIG. 2 oriented between an object and an image sensor, and including a calibration assembly based upon an illumination point source and Shack-Hartmann (wave front) sensor according to an illustrative embodiment;

"FIGS. 4A and 4B are a schematic diagrams illustrating the computation of the focal distance of the variable lens according to the illustrative Shack-Hartmann embodiment of FIG. 4;

"FIG. 5 is a schematic diagram of a lens arrangement as shown in FIG. 2 oriented between an object and an image sensor, and including a calibration assembly based upon redirection of part of the field of view to an integral calibration target according to an illustrative embodiment; and

"FIG. 6 is a flow diagram of an illustrative procedure for determining and controlling focal distance in a camera assembly including a variable lens and a calibration assembly according to at one of the arrangements of FIGS. 3-5."

URL and more information on this patent application, see: Nunnink, Laurens. System and Method for Determining and Controlling Focal Distance in a Vision System Camera. Filed October 1, 2013 and posted August 14, 2014. Patent URL:

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Source: Politics & Government Week

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