Illumination – Light source and modifier – Including selected wavelength modifier
Reexamination Certificate
1999-08-31
2002-01-29
Quach-Lee, Y. My (Department: 2875)
Illumination
Light source and modifier
Including selected wavelength modifier
C362S030000, C362S019000, C362S583000
Reexamination Certificate
active
06341878
ABSTRACT:
FIELD OF INVENTION
This invention relates to illuminators and more particularly to illuminators for use in machine vision systems for inspection of irregularly shaped surfaces.
BACKGROUND OF THE INVENTION
It is often desired to inspect objects during manufacture for imperfections, and to verify proper size, shape and surface characteristics. Such inspections can be part of an overall quality control process undertaken during manufacture. The inspection procedure can be performed using automated equipment, or by an operator who visually inspects each part as it passes through an inspection station. Parts are typically transported to and from the inspection station by a moving conveyor belt, robot arm or other transport mechanism. Surface imperfections, and other irregularly shaped formations on a part, may extend transversely to the flat surface under view-thereby requiring specialized illumination to be properly viewed. In general, such specialized illumination is generated by a diffuse lighting source.
FIG. 1
shows a generalized illumination system for inspecting irregularly shaped parts according to the prior art. The system
10
includes a base
12
upon which a part to be inspected
14
is located. The upper supporting surface
15
of the base
12
can have a variety of shapes, colors and reflectivity depending upon the inherent optical characteristics of the part
14
. In many systems, the surface
15
is matte black in reflectivity and color. The base
12
can be a moving surface such as a conveyer belt or other transport mechanism for carrying parts through the inspection system
10
, as noted above.
The part
14
is located on the surface
15
directly beneath a combination light source/reflector assembly
16
. The clearance C between the bottom rim of the reflector assembly and the surface
15
is usually sufficient to enable the part to pass thereunder without requiring axial (up and down) movement of the reflector assembly
16
or base
12
. The inner surface
18
of the reflector assembly
16
defines the interior of a hemisphere in a typical system arrangement. The surface
18
is coated with a matte white coating to generate a highly diffuse reflected light from any light striking the surface. The illumination light is, itself, provided by a ring illuminator
25
constructed integrally within the reflector assembly. Specifically, the bottom edge
20
of the reflector assembly
16
includes a well
22
with an inner upstanding wall
24
. This well, defining an annular channel, houses the light elements of ring illuminator
25
. The ring illuminator
25
is generally formed from a series of LEDs
26
arranged at even intervals around the perimeter of the edge
20
. Fiber optic tips or other light sources can also be employed. The ring illuminator
25
generates a series of rays (shown schematically as arrows)
30
that strike the part
14
in an indirect manner. As such, the exposed surface of the part is illuminated effectively with a largely diffuse, indirect light.
During inspection, the illuminated part
14
is viewed through an aperture
32
, located at the apex of the hemisphere. In the typical system a camera assembly
34
is mounted at an appropriate focal length above the aperture for remote viewing of the part. The camera assembly
34
can comprise any acceptable, commercially available solid-state camera unit usually adapted to transmit real-time image data of viewed subjects. The camera assembly
34
can include a telecentric lens arrangement
35
according to a conventional design. This type of lens selects light rays that are oriented generally parallel to the axis (dashed line
37
) of the camera
34
. The lens can include a zoom function and inherent magnification as well as internal aperture (F-stop) control.
A typical camera assembly for use in the illustrated system employs a solid state CCD element that generates an image signal for transmission via a data line
36
. This image signal is carried over the line
36
to an image processor/frame grabber
38
. The image processor/frame grabber
38
typically acquires a single image or set of frames showing the viewed part, and may be programmed to provide digital enhancements to various portions of the image. The captured image is displayed on a video display
40
. Various input/output (I/O) peripherals can be provided to control the image processor/frame grabber
38
. These can include various microprocessor functions accessed through a keyboard, mouse or graphical user interface. As noted above, the process can be automated.
The above-described inspection system
10
has the disadvantage that the ring illuminator
25
is located at a significant distance (clearance C) above the part
14
. In addition, the upstanding wall
24
of the illuminator housing further blocks illumination light at lower angles of incidence with respect to the part
14
and underlying surface
15
. The higher-angle rays
30
shown are less capable of fully illuminating side edges (for example edge
44
). As such, structures and edges on the part that require low-angle illumination for proper definition are less well-defined.
A further disadvantage of the illuminator according to
FIG. 1
is that the size and shape of the reflector assembly
16
limits the variety of illumination structures that can be employed as well as the color and intensity of the illuminator. In addition, the reflector must include various electrical connectors and leads when incorporating an illuminator therein. These overhead connections and leads can be cumbersome, and can sometimes impede movement of parts and equipment about the manufacturing floor.
Accordingly, it is an object of this invention to provide a method and apparatus for illuminating parts with a uniform diffuse light that enables lower-angle light rays to strike the part. This invention should decrease the complexity of the reflector by removing various elements for integral illuminators and should enable a wider range of illumination colors, intensities and light characteristics to be employed.
SUMMARY OF THE INVENTION
This invention overcomes the disadvantages of the prior art by providing an illuminator in the form of a backlight unit upon which the part is directly supported while a hemispherical-shaped reflector is positioned above the part, free of a ring illuminator thereon, so that light is reflected onto the part at a lower angle with respect to the supporting base. This more-fully illuminates side edges and upright surfaces on the part for enhanced viewing of these features. An aperture is located at the apex of the reflector, overlying the part. A camera or other viewing device is positioned over the aperture. Through the use of a plane polarizer on the backlight and on the camera lens, any light transmitted directly from the backlight to the camera is filtered-out, while indirect light bounced from the reflector back onto the part is substantially fully viewed.
In a preferred embodiment, the edges of the reflector are brought as close as possible to the level of the part without obstruction by the upstanding walls of an integral illuminator. As such, lower-angle incident light is effectively transmitted onto the part for viewing by a camera. This camera is mounted above an aperture at the apex of the reflector. The reflector can define a hemisphere with a matte white inner surface for providing a highly diffuse light. Alternatively, a different geometry can be employed and/or a different reflector surface characteristic is contemplated. The backlight unit, according to a preferred embodiment, comprises a commercially available structure having an upper diffuser plate and a grid of upturned fiber optic tips mounted therebeneath to generate a wide field of diffuse, uniform light. In particular, the tips are formed on the ends of a series of respective fibers that are collected into a cable bundle. The cable is itself connected to a high-intensity light source. Various filters, controllers and selected bulb intensities can be used to vary the characteristic of light provided
Cognex Corporation
Loginov Williams A.
Weinzimmer Russ
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