Method and apparatus for wide-angle illumination in line-scanning machine vision devices

• US 6,170,973 B1
• Filed: 11/26/1997
• Issued: 01/09/2001
• Est. Priority Date: 11/26/1997
• Status: Expired due to Fees

• Current Assignee: Ametek Incorporated
• Original Assignee: Cognex Corporation
• Inventors: Benedict, George J.
• Primary Examiner(s): O'Shea, Sandra
• Assistant Examiner(s): Neils, Peggy A
• Application Number:
  US08/979,261
• Time in Patent Office: 1,140 Days
• Field of Search: 362/551, 362/554, 362/560, 362/297, 362/298, 362/347, 362/346, 362/16, 362/17, 385/115, 385/119, 385/121, 358/509, 358/510, 348/131
• US Class Current: 362/554
• CPC Class Codes: G01N 21/8901 Optical details; Scanning details per se G02B
• CPC Class Codes: G02B 6/4298 coupling with non-coherent light sources and/or radiation detectors, e.g. lamps, incandescent bulbs, scintillation chambers
• CPC Class Codes: H04N 23/56 provided with illuminating means

Abstract

A wide-angle illuminator for machine vision applications comprises a fiber optic bundle having a tail end defining a long narrow illumination line adjacent the workpiece and a head end defining a rectangular inlet adjacent the light source. The light source comprises a lamp located within a concave, elliptical cross-section reflector that is elongated in one direction. The elliptical cross-section is the same across the entire direction of elongation, thus defining a partial elliptical cylinder, which is preferably capped by flat, reflective surfaces at each end. The lamp, particularly the filament of the lamp, is aligned with one focus of the ellipse. The head inlet of the fiber optic bundle is aligned with the second focus of the ellipse so that the light emitted from the lamp is reflected to the second focus for transmission through the fiber optic bundle. The resultant light pattern projecting at the tail exhibits an angular light distribution that is weighted toward light that projects at an angle to the optical plane of the tail-end fibers. Light projecting directly perpendicular to the optical plane of each fiber is, likewise, minimized. The resulting distribution increases light at the edges of the field of view, improving camera response across the entire field.

Claims

1. A line illuminator for projecting a line of illumination light on a workpiece for receipt by a wide-angle camera comprising:

an optical fiber bundle having a head end and a tail end, the tail end defining an elongated, narrow outlet of optical fibers located adjacent the workpiece and the head end defining a rectangular inlet of optical fibers;

a concave reflector defining a partial elliptical cylinder shape extending in a direction of elongation, with a constant partial elliptical cross-section taken along the direction of elongation, the partial elliptical cross-section defining having a major axis and defining on the major axis a first focus adjacent the reflector surface and a second focus more remote from the reflector surface;

an illumination source located within the concave reflector adjacent the first focus and wherein the rectangular inlet is located adjacent the second focus, whereby light is transmitted from the illumination source and converges at the rectangular inlet for transmission to the outlet with a predetermined angular light output distribution by the optical fiber bundle; and

wherein the outer perimeter of the inlet defines a rectangular outer perimeter having a width, taken along a line parallel to the direction of elongation that is approximately equal to a width of the concave reflector taken along the direction of elongation, and wherein the rectangular outer perimeter substantially overlaps the concave reflector.

2. The line illuminator as set forth in claim 1 wherein the illumination source comprises a lamp having a filament, the filament being positioned approximately at the first focus.

3. The line illuminator as set forth in claim 2 wherein the lamp is centered at an approximate midpoint along the direction of elongation.

4. The line illuminator as set forth in claim 1 wherein the partial elliptical cross-section includes a minor axis transverse to the major axis wherein the partial elliptical cross-section is symmetrical about the major axis and includes edges approximately aligned with the minor axis.

5. The line illuminator as set forth in claim 1 wherein the inlet includes an outer perimeter that is centered with respect to the major axis and defines an optical plane transverse to the major axis.

6. The line illuminator as set forth in claim 1 further comprising at least one end cap having a reflective surface located at an end of the concave reflector for projecting a virtual image of the illumination source at the inlet.

7. The line illuminator as set forth in claim 1 further comprising an end cap having a reflective surface located at each of opposing ends of the concave reflector for projecting a pair of respective virtual images of the illumination source at the inlet.

8. A method for projecting a line of illumination light on a workpiece for receipt by a wide-angle camera comprising the steps of:

providing an optical fiber bundle having a head end and a tail end, the tail end defining an elongated, narrow outlet of optical fibers and the head end defining an inlet of optical fibers, including locating the outlet adjacent the workpiece so that light is projected from the outlet onto the workpiece as an elongated illumination line;

providing a concave reflector defining a partial elliptical cylinder shape extending in a direction of elongation for a first distance, with a constant partial elliptical cross-section taken along the direction of elongation, the partial elliptical cross-section defining having a major axis and defining on the major axis a first focus adjacent the reflector surface and a second focus more remote from the reflector surface;

locating a lamp having a filament within the concave reflector including supporting the filament on two base wires that extend from a base located on the reflector so that the filament extends over a second distance, less than the first distance, at an approximate midpoint of the first distance along the direction of elongation and approximately parallel to the direction of elongation at the first focus, and locating the inlet adjacent the second focus, whereby light is transmitted from the illumination source and converges at the inlet for transmission to the outlet with a predetermined light output distribution by the optical fiber bundle;

projecting light from the outlet as a line onto a workpiece with the predetermined light output distribution; and

receiving light from the workpiece with the predetermined light output distribution by the wide-angle camera along a camera field of view that extends from an axis line perpendicular to an optical plane of the wide-angle camera out to an edge of the field of view, wherein the predetermined light output distribution is arranged to reduce angular light output aligned with the central line and increase angular light output aligned along a non-perpendicular angle away from the central line, thereby smoothing an intensity of viewed light across the field of view.

9. The method as set forth in claim 8 wherein the step of providing the concave reflector includes defining the partial elliptical cross-section to include a minor axis transverse to the major axis wherein the partial elliptical cross-section is symmetrical about the major axis and includes edges approximately aligned with the minor axis.

10. The method as set forth in claim 8 further comprising centering an outer perimeter of the inlet with respect to the major axis wherein an optical plane defined by the inlet is transverse to the major axis.

12. The method as set forth in claim 8 further comprising variably positioning the camera at a plurality of locations with respect to the workpiece so that the camera is aligned with a plurality of locations along the elongated illumination line.

11. A method for projecting a line of illumination light on a workpiece for receipt by a wide-angle camera comprising the steps of:

providing an optical fiber bundle having a head end and a tail end, the tail end defining an elongated, narrow outlet of optical fibers and the head end defining an inlet of optical fibers, including locating the outlet adjacent the workpiece so that light is projected from the outlet onto the workpiece as an elongated illumination line;

providing a concave reflector defining a partial elliptical cylinder shape extending in a direction of elongation, with a constant partial elliptical cross-section taken along the direction of elongation, the partial elliptical cross-section defining having a major axis and defining on the major axis a first focus adjacent the reflector surface and a second focus more remote from the reflector surface;

locating an illumination source within the concave reflector adjacent the first focus including locating the inlet adjacent the second focus, whereby light is transmitted from the illumination source and converges at the inlet for transmission to the outlet with a predetermined light output distribution by the optical fiber bundle;

centering an outer perimeter of the inlet with respect to the major axis wherein an optical plane defined by the inlet is transverse to the major axis; and

wherein the step of providing the optical fiber bundle includes defining the outer perimeter to be rectangular in shape having a width, taken along a line parallel to the direction of elongation that is approximately equal to a width of the concave reflector taken along the direction of elongation, and wherein the rectangular outer perimeter overlaps the concave reflector.

13. In a machine vision system having a camera with a wide-angle lens for viewing a predetermined width of a workpiece based upon a predetermined field of view, an illumination assembly that improves resolution of a viewed image by the camera at remote, opposing edges of the field of view, the opposing edges being oriented at a predetermined wide-angle to a line perpendicular to an optical plane of the lens, the illumination assembly comprising:

an illumination source that includes a concave reflector having a light element located within an enclosed portion of the reflector, wherein the light element includes a filament that extends over a first width a between two base wires;

a fiber optic cable having a head end defining an inlet, the head end located to receive reflected light from the reflector, the head end defining first ends of a densely packed bundle of optical fibers formed into a first shape having a first width and a tail end, defining opposing, second ends of the bundle of optical fibers formed into a second shape having a second width greater than the first width, the tail end being constructed and arranged to focus a narrow elongated line of light on a workpiece; and

wherein the reflector defines a partial elliptical cylinder elongated in a direction of elongation over a second width that is greater than the first width, and defining a substantially constant partial elliptical cross-section taken transversely to the direction of elongation, the partial elliptical cross section defining a major axis, a minor axis an adjacent focus point on the major axis and a minor focus point on the major axis and the light element is located on the major axis with the two base wires extending from a base on the reflector so that the filament extends between the two base wires approximately parallel to the direction of elongation at the adjacent focus point and at an appproximate midpoint along the second width, and the inlet is located about the major axis at the remote focus point so that the inlet overlaps the reflector, the reflector and the light element being constructed and arranged to generate the narrow elongated line of light having a characteristic angular distribution in which a greater intensity of light is oriented at an angle adjacent the predetermined wide angle and a lesser intensity of light is oriented at an angle adjacent the line perpendicular to the optical plane of the wide-angle lens.

14. The illumination assembly as set forth in claim 13 wherein the inlet defines optical plane and the optical plane is oriented perpendicularly to the major axis.

15. The illumination assembly as set forth in claim 14 wherein the partial elliptical cylinder includes a pair of end caps each having a reflective surface, each being located at and covering an opposite end of the partial elliptical cylinder and being substantially flat.

17. The illumination assembly as set forth in claim 13 wherein the line of light includes edges and a center region relative to the edges and wherein the camera is positioned so that an optical axis perpendicular to an optical plane of the camera passes through the line at a location along the line remote from the center region.

16. In a machine vision system having a camera with a wide-angle lens for viewing a predetermined width of a workpiece based upon a predetermined field, of view, an illumination assembly that improves resolution of a viewed image by the camera at remote, opposing edges of the field of view, the opposing edges being oriented at a predetermined wide-angle to a line perpendicular to an optical plane of the lens, the illumination assembly comprising:

an illumination source that includes a concave reflector having a light element located within an enclosed portion of the reflector;

a fiber optic cable having a head end defining an inlet, the head end located to receive reflected light from the reflector and a tail end located to focus a narrow elongated line of light on a workpiece;

wherein the reflector defines a partial elliptical cylinder elongated in a direction of elongation and defining a substantially constant partial elliptical cross-section taken transversely to the direction of elongation the partial elliptical cross section defining a major axis, a minor axis an adjacent focus point on the major axis and a minor focus point on the major axis;

wherein the light element is located on the major axis, centered along the direction of elongation at the adjacent focus point and the inlet is centered about the major axis at the remote focus point and the inlet defines an optical plane and the optical plane is oriented perpendicularly to the major axis, the reflector and the light element being constructed and arranged to generate the narrow elongated line of light having a characteristic angular distribution in which a greater intensity of light is oriented at an angle adjacent the predetermined wide angle and a lesser intensity of light is oriented at an angle adjacent the line perpendicular to the optical plane of the lens; and

a pair of end caps each having a reflective surface, each being located at and covering an opposite end of the partial elliptical cylinder and being substantially flat, wherein the end caps are located adjacent each of opposite edges of the inlet whereby light transmission into the inlet is maximized.