Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
1998-09-22
2001-08-14
Le, Thien M. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S455000
Reexamination Certificate
active
06273338
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of illumination, and in particular to methods and systems for providing a large solid angle of high intensity illumination of an observed object.
DESCRIPTION OF THE RELATED ART
Electronic machine vision apparatuses are commonly employed in conjunction with automatic manufacturing, machining, assembly and inspection apparatuses, particularly of the robotics type. Observing apparatuses, such as television cameras, are commonly employed to observe the object being machined, assembled, or inspected, and the image received and signal transmitted by the camera can be compared to a standard image or signal stored in a database to determine if the observed article is properly machined, oriented, or assembled. Also, machine vision is widely used in inspection and flaw detection applications whereby inconsistencies and imperfection in both hard and soft goods can be rapidly ascertained and adjustments or rejections instantaneously effected.
When the object being observed has a shiny specular surface, reflections of non-uniformities in the local lighting environment may create misleading visual features that interfere with the accuracy of the inspection task such as the appearance of a reflected shadow on a laser etched letter “I” causing it to appear to the machine vision apparatus as the letter “T”. In the inspection of soldered circuits such as those used with printed circuit boards, the highly reflective nature and uneven surface geometry of the solder makes it very difficult to obtain an accurate electronic signal, and the same is true when machine vision is used to inspect laser etched metal surfaces, reflective packaging, and other objects having shiny surfaces, particularly irregular shiny surfaces.
In order to view a mark, image contrast is necessary between the mark and the underlying material. Specular surfaces require a specific illumination geometry to achieve the required image contrast for the features of interest, which is determined by the angle of viewing and the surface's geometry relative to the optical axis between the surface and the viewer. For normal viewing of a flat specular surface, i.e., a surface in which the optical axis is perpendicular to the surface being imaged and the surface is substantially a plane, the light source must have a width equal to at least twice the size of the object field of view plus the diameter of the camera aperture for a normal lens if the light source is integrated with the camera. This relationship is independent of distance from the light source to the surface being observed.
Uneven specular surfaces require a large solid angle of uniform illumination to appear uniformly illuminated, depending on the degree of surface unevenness. A large solid angle of illumination is characterized by light striking the surface to be viewed over a large continuous range of incident angles. A solid angle of front illumination of 160° allows a specular surface with approximately +/−40° of unevenness to appear uniformly illuminated.
Illumination systems exist that produce illumination that is continuous and uniform in nature and is free of dark, bright or void portions capable of generating erroneous vision signals. Examples of such systems are disclosed in U.S. Pat. Nos. 5,684,530 and 5,461,417, each of which discloses a continuous diffuse illumination (“CDI”) method and apparatus. The disclosure of each of such U.S. Patents is incorporated by reference herein. CDI illumination provides dramatically improved results when machine vision is used to view shiny, irregular objects.
FIGS. 1-6
depict various illumination geometries that have been traditionally used in machine vision systems along with their associated incident angle brightness histograms. For example, in
FIG. 1
, a coaxial illumination system
1
is employed to illuminate an object
2
as it is viewed by an electronic machine vision camera
3
. As can be seen from the incident angle brightness histogram shown in
FIG. 2
, this coaxial illumination system provides a uniform extended illumination zone
4
with a desirable incident illumination level that coincides with a zero angle of incidence off of the observation axis but is substantially devoid of any illumination as the angle of incidence deviates from zero.
FIG. 3
depicts an off-illumination axis diffuse dome lighting system
5
illuminating an object
2
to be observed by electronic machine vision camera
3
through an observation window
6
, which can be an opening or orifice or even a zone of material that appears transparent to a machine vision camera, such as clear plastic or the like. This illumination system creates the uniform diffuse illumination zone
4
shown in FIG.
4
. While the incident illumination level is substantially uniform as the angle of incidence of the light increases away from a zero angle of incidence off of the observation axis, the on-observation axis region
7
, which has an angle of incidence approaching zero degrees off-axis, is substantially devoid of any illumination.
A conventional ring illumination system and its corresponding incident angle brightness histogram, as depicted in
FIGS. 5 and 6
respectively, provides a uniform diffuse illumination zone
4
with a substantially uniform incident illumination level that corresponds to substantially the same shape as the ring illuminator
8
being employed.
FIGS. 7
,
8
,
9
, and
10
show four illumination systems and methods and their respective incident angle brightness histograms. First,
FIG. 7
shows a continuous diffuse illumination system that is comprised of a combination of the coaxial illumination system
1
of FIG.
1
and the off-illumination axis diffuse illumination system
5
of FIG.
3
. The combination of these two illumination components results in a lighting environment with the incident angle brightness histogram shown in FIG.
8
. This environment is characterized by a diffuse illumination zone
4
with a substantially uniform incident illumination level irrespective of the angle of incidence.
When utilizing machine vision techniques, it is common to employ complicated fighting systems for illumination the object being observed. Some such systems eliminate shadows, highlights, reflections and other lighting characteristics caused by shiny convex surface objects. Other systems provide increased contrast to images printed on dull, flat surfaces. Examples of complex lighting systems for use with machine vision apparatus are shown in U.S. Pat. Nos. 4,677,473; 4,882,498; 5,051,825; 5,060,065 and 5,072,127. The disclosure of such patents is incorporated by reference herein. The devices shown in these patents are capable of generating improved lighting characteristics. However, such devices may in some instances be too complex or expensive to manufacture relative to the benefit they provide. Also, some devices may require a relatively intense, illumination source. Accordingly, a simple to manufacture device that provides adequate illumination of uneven specular surfaces at high-efficiency is desirable.
FIGS. 11 and 12
depict a side view and a bottom view, respectively, of a fresnel lens
12
. A conventional fresnel lens includes a series of concentric grooves
12
, shaped onto a flat, thin piece of material. A fresnel lens may be formed from a variety of light transmitting materials through conventional manufacturing techniques. For example, it may be molded from plastic or acrylic. The fresnel lens
10
may be used to replace the curved surface of a conventional lens. The concentric grooves
12
act as refracting surfaces, bending parallel rays to a common focus. Different fresnel lenses may have concentric grooves of different angles, so that lenses of a variety of focal lengths may be provided.
SUMMARY OF THE INVENTION
Provided herein is a low-cost high-efficiency illumination method and apparatus, which may be a method and apparatus for illuminating an object, wherein illumination of the object is by an illumination source, a partially reflective reflecto
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