Illumination – Light fiber – rod – or pipe – With optical fiber bundle
Reexamination Certificate
1997-11-26
2001-01-09
O'Shea, Sandra (Department: 2875)
Illumination
Light fiber, rod, or pipe
With optical fiber bundle
C361S017000, C361S310000
Reexamination Certificate
active
06170973
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for providing an illumination line to a workpiece that is viewed by a wide-angle line scan camera, and more particularly to an illumination line that enhances line scan camera performance.
BACKGROUND OF THE INVENTION
Moving webs and other workpieces are often scanned for defects or known patterns using a machine vision device. Machine vision devices typically employ an illumination system to concentrate a bright light into a narrow line along the workpiece. A line scan camera is directed at the illuminated area to receive light from the web. The camera can be oriented to view a direct reflection or transmission of the image as produced by the illumination light. This is termed a so-called “bright field” image. Conversely, the camera can be oriented off-axis relative to the reflected or transmitted is image light, resulting in a so-called “dark field” image.
FIG. 1
schematically illustrates a machine vision system
20
adapted for use with a wide moving web
22
. Width is taken in the direction perpendicular to the page and the web moves in the direction of the arrow
23
. The web
22
in this embodiment is a transparent or translucent web in which light is transmitted from one face of the web material to an opposing face. The concepts described are also applicable to an opaque web with certain modifications to be described below. The web
22
is typically wide (about four or more feet wherein width is taken in a direction into the page). A single system
20
can view a web section approximately four feet wide. For web widths greater than four feet, a plurality of identical machine vision systems are arranged side-by-side along the web width. A lamp
24
, with an associated power supply
26
, generates a bright illumination light that is represented as a series of rays
28
. The lamp
24
can be any acceptable bright illumination source such as a metal-halide lamp, a high-pressure sodium lamp, a filament lamp, a laser or other coherent light source, a gas discharge lamp, or an LED array or other semiconductor light source. A 750-watt quartz-tungsten-halogen (QTH) lamp is used in one known illumination arrangement. While not shown, a reflector or shield may be provided so that light emanates only from a predetermined region of the lamp
24
. A condenser lens assembly
30
is provided adjacent the lamp
24
. In this embodiment, a condenser assembly
30
comprises two or more counterfacing plano-convex lenses
32
. The lenses have a circular perimeter, forming a focused beam that projects a circular pattern. At the desired focal point
34
of the rays
28
is positioned the head or “entrance face” of a fiber optic cable
38
.
With further reference to
FIG. 2
, the fiber optic cable assembly is shown in further detail. The head
36
of the cable
38
reveals a fiber optic bundle
40
constructed from a large number of small-diameter fiber optic strands with flat, polished ends. The cable
38
typically comprises a resilient outer covering that encases over three-hundred-thousand 0.002-inch diameter optical fibers. Typically, the fibers are packed to substantially fill the cross-sectional area of the cable
38
. The diameter D of the head
36
is approximately 1.35 inches, and is approximately equal to the diameter of the spot generated by the rays
28
. In other words, the head
36
is located relative to the condenser lens assembly
30
so that the illumination spot approximately covers the head
36
. Note that the amplitude of the distribution within this spot may not be uniform.
At the opposing tail or “exit face” of the cable
38
is located an illumination line housing
42
. The line housing
42
includes an elongated rectangular opening
44
that reveals the polished tail ends of the cable's individual optical fibers. The line housing opening
44
and a typical implementation has a width W of four feet and a height H of 0.030 inch. In general, ten to twenty fibers can be aligned vertically side-by-side within the confines of the height H.
The long, narrow line of bundled optical fibers within the line housing
42
generates a corresponding narrow elongated line of light rays
46
that are typically refocused by an elongated focusing lens
48
. Randomization of the fibers between the head and tail ends of the cable generally tends to improve the uniformity of the resulting illumination line. In fact, it is common to twist and braid the fibers within a cable to achieve greater uniformity in transmitted light distribution. However, the system may rely on a certain rotational orientation of the head end relative to the illumination source (e.g. the light bulb), and rotation of the head end relative to the source at a later time may disturb the system's settings. In addition, the particular distribution of light produced may limit the ability to subsequently move the camera relative to the image line. Thus the randomization can be both advantageous and problematic.
A variety of lens constructions can be used such as a linear convex cylinder lens or an acrylic rod lens approximately one inch in diameter. An off-axis elliptical reflector can also be employed. The term “lens” should be taken broadly to include a focusing reflector. The lens
48
, focuses the rays
46
onto the web
22
to generate the desired narrow illumination line across the width of the web
22
. Based upon this illumination line, light is transmitted through the web
22
to a camera assembly
50
that comprises a wide-angle focused lens
52
and an electro-optical pick up assembly, such as a commercially available CCD camera unit
54
. The lens
52
is focused on the web
22
using the focus ring
55
and a desired lens aperture is provided using the aperture adjustment ring
57
. Images are converted by the CCD camera into electronic signals for transmission to an appropriate data acquisition device
56
. The device can comprise a microcomputer having pattern-recognition software for analyzing the surface of the viewed on the web. A monitor
58
can also be provided for real-time of viewing of the web surface or for viewing data derived by the software.
One problem encountered with the machine vision system described above is that the quality of the substantially uniformly illuminated image viewed by the wide-angle lens tends to degrade near the edges of the field of view of the lens. This degradation is due largely to the inherent optical characteristics of wide-angle lenses.
FIG. 3
illustrates the camera lens response measured by the CCD element
54
for a 35 mm Nikon Nikkor™ lens. The specified field of view of this lens is ±32° from perpendicular (0°, directly beneath the camera) which translates into approximately four feet of web width at a focal length of approximately 3.2 feet from the web face to the camera'optical plane
53
. The measured response curve
52
registers 100 percent intensity at a 0° field of view. In other words, maximum intensity is viewed directly beneath the lens. As the viewing angle increases, the viewed intensity drops off sharply. The decrease in intensity occurs as a function of the cosine to the fourth power of the camera's field of view angle. At maximum viewing angles the intensity of the viewed image is about 52 percent of the maximum value. This uneven response can lead to viewing errors since illumination at the edges of the field of view may be insufficient to acquire a proper image of the web. Another associated problem with the prior art illumination arrangement is that the camera operates most effectively when it is aligned near the center of the illumination line. Placing the camera “off-axis” can result in a significantly degraded acquired image.
There are several techniques currently employed to overcome the imaging limitations of wide-angle lenses. One technique entails providing additional illumination near the edges of each camera's field of view. Cameras can be placed closer together so that the full field of view of each camera is not utilized. This and
Cognex Corporation
Loginov William A.
Neils Peggy A
O'Shea Sandra
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