U.S. patent application number 15/197457 was filed with the patent office on 2016-10-20 for machine vision inspection devices and machine vision methods of inspection.
The applicant listed for this patent is Mettler-Toledo, LLC. Invention is credited to Timothy P. White.
Application Number | 20160309070 15/197457 |
Document ID | / |
Family ID | 50983134 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160309070 |
Kind Code |
A1 |
White; Timothy P. |
October 20, 2016 |
MACHINE VISION INSPECTION DEVICES AND MACHINE VISION METHODS OF
INSPECTION
Abstract
Machine vision inspection devices and machine vision methods for
inspecting objects, such as objects with shiny surfaces. Device
embodiments include an illumination housing with a central aperture
and a specialized aperture cover. Use of the claimed device
embodiments to inspect objects eliminates the void (dark spot)
common to known machine vision inspection methods.
Inventors: |
White; Timothy P.; (New
Boston, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mettler-Toledo, LLC |
Columbus |
OH |
US |
|
|
Family ID: |
50983134 |
Appl. No.: |
15/197457 |
Filed: |
June 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13892103 |
May 10, 2013 |
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15197457 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2021/8819 20130101;
H04N 5/225 20130101; H04N 5/2256 20130101; G01N 21/8806
20130101 |
International
Class: |
H04N 5/225 20060101
H04N005/225; F21V 33/00 20060101 F21V033/00; G01N 21/88 20060101
G01N021/88; F21V 7/00 20060101 F21V007/00 |
Claims
1. A machine vision inspection device for inspecting the surface of
an object of interest, comprising: a substantially hollow
illumination housing having an interior and an open bottom, the
interior of the housing forming a diffusely reflective surface
capable of reflecting light toward the open bottom thereof; an
illumination source located near the open bottom of the housing and
oriented to direct light onto the diffusely reflective surface
thereof; a central aperture that passes through the housing
opposite the open bottom thereof, the central aperture positioned
to provide a view, through the open bottom of the housing, of the
object of interest; an aperture cover located on the housing
exterior and over the central aperture, the aperture cover having a
discontinuous pattern of opaque but diffusely reflecting features;
and a camera located above the aperture cover and oriented so as to
be capable of receiving, through the central aperture and aperture
cover, an image of the object of interest.
2. The machine vision inspection device of claim 1, wherein the
diffusely reflective surface of the housing interior has a shape
selected from the group consisting of dome-shaped, flat-shaped and
round-shaped.
3. The machine vision inspection device of claim 1, wherein the
illumination source is selected from the group consisting of a ring
light, a plurality of linear light sources, and an array of
discrete light sources, arranged about the circumference of the
housing near the open bottom thereof.
4. The machine vision inspection device of claim 1, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover comprises a plurality of substantially
parallel and spaced apart narrow slits.
5. The machine vision inspection device of claim 4, wherein the
width of each slit is between approximately 0.01-2.0 millimeters in
minimum dimension.
6. The machine vision inspection device of claim 4, wherein the
width of each slit is between approximately 0.1-0.5 millimeters in
minimum dimension.
7. The machine vision inspection device of claim 4, wherein the
width of each slit is approximately 0.25 millimeters in minimum
dimension.
8. The machine vision inspection device of claim 1, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover comprises an arbitrary pattern, the dimension
of whose constituent features and the spaces separating them are
between approximately 0.01-2.0 millimeters in minimum
dimension.
9. The machine vision inspection device of claim 1, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover comprises an arbitrary pattern, the dimension
of whose constituent features and the spaces separating them are
approximately 0.1-0.5 millimeters in minimum dimension.
10. The machine vision inspection device of claim 1, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover comprises an arbitrary pattern, the dimension
of whose constituent features and the spaces separating them are
approximately 0.25 millimeters in minimum dimension.
11. The machine vision inspection device of claim 1, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover are selected from the group consisting of a
plurality of spaced apart circles, ovals, or other arbitrary
shapes.
12. The machine vision inspection device of claim 1, wherein the
distance of the camera to the aperture cover is such that, with a
lens aperture of the camera held sufficiently open, the material
pattern of the aperture cover will be rendered out-of-focus and
substantially invisible to the camera.
13. The machine vision inspection device of claim 1, wherein the
reflectivity of the aperture cover is greater than the reflectivity
of the diffusely reflective surface of the housing interior.
14. The machine vision inspection device of claim 1, wherein the
reflectivity of the diffusely reflective surface of the housing
interior is within about .+-.10% of the average reflectivity of the
aperture cover.
15. The machine vision inspection device of claim 1, wherein the
reflectivity of the diffusely reflective surface of the housing
interior substantially matches the reflectivity of the aperture
cover.
16. The machine vision inspection device of claim 1, wherein a
camera side of the aperture cover is coated with a non-reflective,
substantially non-reflective or light-absorbing material.
17. A machine vision method of inspecting an object surface,
comprising: (a) providing a machine vision inspection device, the
machine vision inspection device comprising: a substantially hollow
housing having an interior and an open bottom, the interior of the
housing forming a diffusely reflective surface capable of
reflecting light toward the open bottom thereof, an illumination
source located near the open bottom of the housing and oriented to
direct light onto the diffusely reflective surface so as to create
an illumination field, a central aperture that passes through the
housing opposite the open bottom thereof, the central aperture
positioned to provide a view, through the open bottom of the
housing, of the object surface, an aperture cover located on the
housing exterior and over the central aperture, the aperture cover
having a discontinuous pattern of substantially opaque but
diffusely reflective features, and a camera located above the
aperture cover and oriented so as to be capable of receiving,
through the central aperture and aperture cover, an image of an
object surface; (b) locating the camera close to the aperture
cover; and (c) holding a lens aperture of the camera sufficiently
open so as to render the material pattern of the aperture cover
out-of-focus and substantially invisible to the camera, while
simultaneously rendering the reflected image of the diffusely
reflecting features of the aperture cover out-of-focus such that
the diffusely reflecting features blend into the surrounding
illumination field.
18. The machine vision inspection method of claim 17, wherein the
discontinuous pattern of opaque but diffusely reflecting features
of the aperture cover are selected from the group consisting of a
plurality of substantially parallel and spaced apart narrow slits,
circles, ovals, or other arbitrary shapes.
19. The machine vision inspection method of claim 17, wherein the
average reflectivity of the diffusely reflective surface of the
housing interior substantially matches the average reflectivity of
the aperture cover.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of currently pending U.S.
application Ser. No. 13/892,103, filed on May 10, 2013, which is
incorporated by reference as if fully recited herein.
TECHNICAL FIELD
[0002] The present invention is directed to the field of machine
vision inspection, particularly to the machine vision inspection of
objects, such as objects with an irregular and/or specular
surface.
BACKGROUND
[0003] Machine vision inspection of objects would be generally
familiar to one of skill in the art. Systems and methods of machine
vision inspection typically use one or more cameras and,
frequently, one or more sources of illumination. Illumination may
be provided in various forms including, but not limited to, direct
illumination, back light illumination, ring illumination,
horizontal illumination, coaxial illumination, and dome
illumination.
[0004] When inspecting an object having a specular (shiny) surface,
a super-diffuse illumination environment is particularly effective.
Of the aforementioned illumination types, it has been found that
dome illumination works very well for inspecting such objects, as
well as objects having an irregular (e.g., convex, spherical,
semi-spherical) shape. A machine vision inspection device that
provides dome illumination is generally referred to as a dome
illuminator.
[0005] As illustrated in FIG. 1, known dome illuminators typically
include a hemispherical dome that overlies an object to be
inspected (imaged). An illumination source, such as a ring of LEDs,
is located within the dome. The interior surface of the dome is
typically painted or otherwise coated with a diffusely reflective
material such that light emitted by the illumination source is
reflected off the dome and onto the object to be imaged.
[0006] Dome illuminators also include a central (axial) aperture
that passes through the dome to provide a viewing window for an
associated camera. The camera is focused on the upwardly facing
surface of the object to be imaged, which surface is illuminated by
the light being reflected off of the dome interior. An unfortunate
side-effect of this design, however, is that the central aperture
appears in reflection as a dark spot on the surface being imaged.
This may result, for example, in an inability to read a code
imprinted on the surface being imaged or an inability to see other
features thereof.
[0007] A known solution to this problem is a device commonly
referred to as a Cloudy Day.RTM. illuminator (CDI). A CDI typically
includes a dome illuminator with a second co-axial light source
that further illuminates an object to be inspected by projecting
light through the central dome aperture. The brightness of the dome
and co-axial light sources are balanced until they appear of equal
brightness in reflection from the object surface being
inspected.
[0008] While a CDI is an improvement over a typical dome
illuminator, CDIs are expensive. Further, the addition of the
co-axial light source adds to the size, and mechanical and
electrical complexity of the dome illuminator, and also renders
typical CDIs quite bulky. CDIs also employ a flat or curved beam
splitter arrangement, which is usually fragile and difficult to
clean.
[0009] It can be understood from the foregoing commentary that it
would be desirable to provide a simple, cost-effective and compact
machine vision inspection device that is capable of accurately
imaging (inspecting) the entire specular surface of an object.
Embodiments of the present invention are such devices.
SUMMARY
[0010] Embodiments of the invention provide substantially the same
illumination uniformity as a CDI, but eliminate the complexity,
added cost, and size. Embodiments of the invention are also able to
eliminate the reflected dark spot problem referred to above with
respect to the use of a typical dome illuminator. This is
accomplished by making the reflected image of the central aperture
of an illumination housing effectively disappear by covering the
aperture with a diffusely reflective aperture cover.
[0011] An aperture cover according to the invention is, generally
speaking, a cover adapted for placement over the central aperture
of an illumination housing, with the cover containing an array of
diffusely reflecting features interspersed between an array of very
narrow slits that allow light to pass therethrough. The pattern of
diffusely reflecting features and gaps may be arranged in a regular
or irregular pattern.
[0012] With the cover in place over the central aperture, an
associated camera is located close to the aperture cover. If the
numerical aperture of the camera lens is held substantially wide
open, the features of the aperture cover can be made to disappear
in reflection, and the typical dark spot problem is eliminated.
Embodiments of the invention can be further optimized by matching
as closely as possible the reflectivity of the illumination housing
interior and the aperture cover. The camera side of an aperture
cover may also be coated with a non-reflective or substantially
non-reflective material.
[0013] Other aspects and features of the invention will become
apparent to those skilled in the art upon review of the following
detailed description of exemplary embodiments along with the
accompanying drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In addition to the features mentioned above, other aspects
of the present invention will be readily apparent from the
following descriptions of the drawings and exemplary embodiments,
wherein like reference numerals across the several views refer to
identical or equivalent features, and wherein:
[0015] FIG. 1 is a partially cutaway view of a typical dome
illuminator;
[0016] FIG. 2 schematically illustrates an exemplary embodiment of
a typical cloudy day illuminator;
[0017] FIGS. 3A-3C show exemplary embodiments of aperture covers
having regular and irregular patterns of the invention, for
placement over the central aperture of an illumination housing;
[0018] FIG. 4 is a simplified cross-sectional elevation view of a
machine vision inspection device of the invention with an exemplary
aperture cover located over the central aperture thereof; and
[0019] FIG. 5 is a bottom view of the machine vision inspection
system of FIG. 4, wherein slits in the sieve aperture cover can be
seen through the central aperture in the illumination housing.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)
[0020] As discussed above, known dome illumination systems are used
to inspect specular surfaces of objects of interest, but all suffer
from an inherent void in the illumination field that results from
the aperture through which the camera views the object being
inspected. This void appears as a dark feature. "Healing" this void
in the illumination field, to make it truly continuous, may be
accomplished using a known Cloudy Day.RTM. illuminator (CDI) that
makes use of a slanted or curved beam splitter illuminated by a
secondary light source, which is balanced in brightness to match
the surrounding illumination field. However, CDIs also have certain
drawbacks, as mentioned above. Examples of a typical dome
illuminator and a CDI are respectively depicted in FIG. 1 and FIG.
2.
[0021] A typical dome illuminator 5 may be observed in FIG. 1. As
shown, the dome illuminator 5 includes a substantially hollow
illumination housing 10 with an open bottom. The interior of the
illumination housing 10 forms a hemispherical diffuser 15. An
illumination source 20, such as a ring light, a plurality of linear
light sources, or an array of discrete LED light sources, is
located near the base of the illumination housing 10 and is adapted
to provide the illumination necessary to carry out an inspection
process on an object 25 located below the illumination housing 10
and diffuser 15 portion thereof.
[0022] As can also be observed in FIG. 1, light rays 30 from the
illumination source 20 are directed upward, where they are
reflected with substantially equal brightness by the diffuser 15
back toward the object to be inspected. The desired surface of the
object of interest 25 is thus indirectly illuminated 35 by the
illumination source 20. A central aperture 40 passes through the
illumination housing/diffuser 10, 15 such that the illuminated
surface of the object of interest is made visible to an associated
camera 45 located above the central aperture.
[0023] As described previously, a problem with such a dome
illuminator is that the central aperture 40 will appear as a dark
feature in the reflection of the object surfaces being imaged.
Consequently, codes and/or other features of interest on the object
surfaces being imaged may not be seen by the camera due to
inadequate contrast.
[0024] A typical cloudy day illuminator (CDI) 50 designed to
overcome the aforementioned problem with a dome illuminator is
schematically illustrated in FIG. 2. As with the dome illuminator 5
exemplified in FIG. 1, the CDI 50 also includes a substantially
hollow illumination housing 55 with an open bottom, the interior of
the illumination housing forming a hemispherical diffuser 60. An
illumination source 65, which may again be a ring light, is located
near the base of the illumination housing 55 and is adapted to
provide a portion of the illumination necessary to carry out an
inspection process on an object of interest 70 located below the
illumination housing and diffuser 60 portion thereof.
[0025] In comparison to the dome illuminator, however, the CDI 50
includes a second illumination source 75, which may be generally
referred to as an on-axis or co-axial illumination source. The
second illumination source 75 projects light through a beam
splitter 80, which directs the light through a central aperture 85
in the illumination housing/diffuser 55, 60 and onto the surface of
the object of interest 70.
[0026] As with the dome illuminator of FIG. 1, light rays 90 from
the illumination source 65 are directed upward, where they are
reflected with substantially equal brightness by the diffuser 60
back toward the object of interest 70. The desired surface of the
object of interest 70 is thus indirectly illuminated by the
illumination source 65 and directly illuminated by the second
illumination source 75. Light from the object surface is reflected
upward through the central aperture 85 to an associated camera 95
located above the illumination housing 55 where it forms part of
the image. A clear (e.g., plastic) aperture 100 may be provided in
the beam splitter arrangement to permit viewing of the object image
by the camera 85.
[0027] As should be apparent from an observation of FIG. 2 and the
above description, the additional components required to construct
a CDI also make a CDI much more expensive than a typical dome
illuminator. Furthermore, because of the beam splitter and
associated structure commonly installed on the dome of a CDI, it is
not possible to locate the inspection camera as close to the
illumination housing as might be possible without the presence of
such a secondary illumination source. This has the result of making
a typical CDI quite bulky in comparison to a typical dome
illuminator. Still further, the beam splitter arrangements commonly
employed with a CDI are fragile and/or difficult to clean.
[0028] Therefore, embodiments of the invention are directed to
machine vision inspection systems and methods that are inexpensive
and compact in comparison to a CDI, but that produces CDI-like
inspection results by eliminating the inherent void in the
illumination field associated with a typical dome illumination-type
devices. Generally speaking, this is accomplished through the use
of an inspection device having an illumination housing equipped
with a novel aperture cover that functions to fill the aperture
void (dark spot) inherent to typical dome illuminators with a
discontinuous pattern of opaque but diffusely reflecting features
of very narrow dimension. These features may be thought of as
sieve-like in nature with respect to the aperture covers and may
be, for example, a series of slits which, when viewed in reflection
from a specular surface of an object of interest, appear
out-of-focus and blend into the surrounding illumination field.
[0029] Several exemplary and non-limiting aperture cover
embodiments having various patterns of diffusely reflective
features are shown in FIGS. 3A-3C. It is to be understood that the
roughness (smoothness) of the surface being imaged, typically
expressed as a root mean squared (RMS) dimension, largely
determines the dimensions of the diffusely reflective features of
the aperture cover. That is, the roughness of the surface being
imaged will typically dictate that the diffusely reflective
features fall within some limited range of dimensions in order for
the diffusely reflective features to appear "invisible" to the
camera in reflection off the surface. Generally speaking, the
rougher the surface being imaged, the larger the diffusely
reflective features may be. However, there is also a practical
benefit to using smaller sieve aperture features--namely that
surfaces having a greater range of surface roughness can be
reliably imaged. Consequently, the diffusely reflective features of
the exemplary aperture covers depicted in FIGS. 3A-3C are stated to
fall within a particular dimension range or to be of a particular
dimension. These dimension ranges and/or dimensions have been found
to be particularly effective when used to image common surfaces to
be inspected, such as the lids of typical metal cans. Nonetheless,
other diffusely reflective feature dimension ranges or dimensions
may be more suitable to the imaging of other surfaces.
[0030] In FIG. 3A, a first exemplary aperture cover 105 is shown to
include a sheet of material 110 having a pattern of diffusely
reflective features comprising a series of parallel, closely
spaced, narrow slits 115 that pass therethrough. The number of
slits 115, the width of the slits and the spacing between the slits
may vary as explained above. In one non-limiting example, the width
of the slits 115 may be between approximately 0.01 and 2.0
millimeters, more particularly between about 0.1 and 0.5
millimeters, and even more particularly, about 0.25
millimeters.
[0031] A second exemplary aperture cover 120 is shown in FIG. 3B to
include a sheet of material 125 having a pattern of diffusely
reflective features comprising a series of regularly recurring and
substantially equidistantly-spaced holes 130 that pass
therethrough. In this particular example, the holes 130 are shown
to be substantially rectangular in shape, but various other shapes
and other patterns are also possible. For example, such a regularly
occurring pattern may take the form of a tessellation. The number
of holes 130, the dimension(s) of the holes and the spacing between
the holes may vary as explained above. In one non-limiting example,
the minimum dimension of the holes 130 may be between approximately
0.01 and 2.0 millimeters, more particularly between about 0.1 and
0.5 millimeters, and even more particularly, about 0.25
millimeters.
[0032] A third exemplary aperture cover 135 is shown in FIG. 3C to
include a sheet of material 140 having a pattern of diffusely
reflective features comprising a multitude of non-repeating,
substantially non-parallel and irregularly spaced holes 145 of
various shape that pass therethrough. In this particular example,
the holes 145 are shown to include various shapes (e.g., ellipses
and squares), but various other shapes and combinations of shapes
are also possible. The number of holes 145, the dimension(s) of the
holes and the spacing between the holes may again vary as explained
above. In one non-limiting example, the minimum dimension of the
holes 145 may be between approximately 0.01 and 2.0 millimeters,
more particularly between about 0.1 and 0.5 millimeters, and even
more particularly, about 0.25 millimeters.
[0033] The size and shape of the exemplary aperture covers 105,
120, 135 of FIGS. 3A-3B are provided for purposes of illustration
only and are not to be construed as limiting the invention to the
relative dimensions and/or shapes shown. Aperture covers of other
shapes such as, for example, circular shapes, could also be
employed. Likewise, the pattern of the diffusely reflective
features located in a given aperture cover, as well as the
dimensions of the features, may vary as long as the effect thereof
enables the aperture cover material to be rendered out-of-focus
and, hence, substantially invisible to the inspection camera as
described above and as illustrated and described in more detail
below with respect to the embodiments of FIGS. 4-5. Similarly,
aperture cover embodiments of the invention are not limited to
manufacture from a particular material. The aperture cover pattern
may also be formed directly in the dome material rather than as a
separate piece attached thereto.
[0034] A simplified cross-sectional elevation view of an exemplary
embodiment of an inspection device 150 according to the invention
is illustrated in FIG. 4. As shown, the inspection device 150 is
similar to a typical dome illuminator and includes a substantially
hollow body (illumination housing) portion 155 with an open bottom.
The interior of the illumination housing 155 forms, in this case, a
dome-shaped (e.g., hemispherical) diffusely reflective surface
(diffuser) 160. Other diffuser shapes may also be possible in other
embodiments as long as the selected shape results in a proper
reflection of light onto an object of interest. For example, the
reflective surface may be flat in certain embodiments.
[0035] An illumination source 165, such as a ring light, a
plurality of linear light sources, or an array of discrete (e.g.,
point) light sources such as LEDs, is located near the base of the
housing 160 and is adapted to provide the illumination necessary to
carry out an inspection process on an object of interest 170
located below the illumination housing 155 and diffuser 160.
[0036] As previously described with respect to the known dome
illuminator 5 of FIG. 1, light rays 175 from the illumination
source 165 are directed upward, where they are reflected with
substantially equal brightness by the diffuser 160 back toward the
object of interest 170. The desired surface of the object of
interest 170 is thus indirectly illuminated by the illumination
source 165.
[0037] As also previously described with respect to the known dome
illuminator 5 of FIG. 1, a central aperture 180 passes axially
through the diffuser 160 and illumination housing 155 to provide an
opening through which the object of interest 170 may be viewed with
a camera 185. In a traditional dome illuminator, this would result
in the central aperture appearing in reflection as a dark spot on
the specular surface being inspected.
[0038] In order to overcome this dark spot problem, it can be
observed in FIG. 4 that an exemplary aperture cover 190 of the
invention is located over the central aperture 180 of the
inspection device 150. The aperture cover 190 may be one of the
exemplary aperture covers 105, 120, 135 depicted in FIGS. 3A-3C, or
it may be of a different design. In any case, the camera 185 lens
is located close to the aperture cover 190 and is operated at a
wide aperture setting. As a result, the features of the aperture
cover 190 are rendered out-of-focus and, hence, invisible to the
inspection camera 185, despite the aperture cover material being
substantially opaque. At the same time, the diffusely reflecting
features of the aperture cover 190, when viewed in reflection from
the specular surface of the object being inspected 170, appear
out-of-focus and cause the reflected aperture 180 to blend into the
surrounding illumination field, thereby eliminating the
aforementioned dark spot problem. This allows for a more complete
and accurate inspection of the specular surface of the object of
interest.
[0039] For purposes of further illustration, an enlarged bottom
view of the inspection device 150 of FIG. 4 is illustrated in FIG.
5. In this view, a portion of the aperture cover 190 is visible
through the central aperture 180 in the illumination housing 155 of
the inspection device 150. It can be observed that this particular
exemplary aperture cover 190 is comprised of a sheet of material
195 that includes a series of parallel, closely spaced, narrow
slits 200 that pass therethrough--much like the exemplary aperture
cover 110 depicted in FIG. 3A. Such an array of narrow slits may be
replaced by any pattern, regular or irregular, wherein the size of
reflecting features and the gaps therebetween are small enough to
be rendered substantially invisible in reflection as seen by the
camera.
[0040] As previously explained, with the aperture cover 190 in
place over the central aperture 180, the associated camera 185 is
located close to the aperture cover. If the lens aperture of the
camera 185 is held sufficiently open, the pattern of slits 200 in
the aperture cover 190 may be made to disappear in reflection by
being out of focus, and the dark spot that would typically
represent the reflected image of the central aperture 180 is made
to have substantially the same brightness as the surrounding
diffuser 160 surface or to appear sufficiently similar in
brightness to allow for reliable detection and imaging of surface
features to be performed.
[0041] The results that can be produced by such an inspection
device are quite adequate for typical optical character recognition
(OCR) algorithms, and may be as good as or nearly as good as the
results provided by a more complex and costly CDI. Additionally,
inspection device embodiments of the invention can be further
optimized by closely matching the reflectivity of the diffuser and
the aperture cover. For example, the reflectivity of the diffuser
may be slightly reduced and the reflectivity of the aperture cover
material may be slightly increased, such that the apparent
brightness in out-of-focus reflection of the diffuser and the
aperture cover can be made to match as precisely as possible.
Although not essential, the camera side of an aperture cover of the
invention may also be coated with a non-reflective or substantially
non-reflective material.
[0042] It should be understood that the choice of relative dome and
aperture cover reflectivity may be flexible based on various
factors, such as those described below, but obviously reaches an
optimum when the out-of-focus apparent brightness of the aperture
cover matches the apparent brightness of the surrounding surface.
It has also been discovered that a high quality vision system using
good edge-detection algorithms is relatively insensitive to a
brightness mismatch in comparison to a typically cheaper but faster
vision system using simplified brightness-threshold-based character
segmentation algorithms. A better vision system, like a CIV, will
thus be more insensitive to the potential difference in
reflectivity between the diffuser and the aperture cover.
Consequently, when used to examine common surfaces such as the lids
of metal cans, it has been found, depending on the vision system
employed, the range of surface roughness of the lids, etc., that
the reflectivity of the diffuser may be within about .+-.75% of the
average reflectivity of the aperture cover. In some embodiments, it
may be preferable that the reflectivity of the diffuser is within
about .+-.30% of the average reflectivity of the aperture cover
and, in yet other embodiments, it may be preferable that the
reflectivity of the diffuser is within about .+-.10% of the average
reflectivity of the aperture cover.
* * * * *