Description
Detection of Surface Defects of an Object
The present invention relates to an arrangement and method
for detecting surface defects of an object.
Optical scanners are generally used for automatic inspection
of objects for fault detections. These systems are generally
fast and accurate, as they have a high degree of resolution
and can detect the kind of surface defects which cannot be
detected visually by the user.
As an example case, in the field of continuous sheet material
manufacture and, high speed machines manufacture and process
paper is continuous webs. These papers are then etched with
markings for different requirements.
Generally, high speed machines manufacture and process
objects in continuous webs. These objects may be then etched
with different markings or other substrates for use in
various applications. But these etching may get adversely
affected as the process requires cutting blades, instruments
and fixtures that may cause surface defects. The surface
defects such as streaks or scratches along the major axis of
object are impossible to detect during processing. This
necessitates the surface of the object to be inspected using
an online mechanism to detect even small breakages or
markings on the surface.
Conventionally, line scan cameras are used to check these
types of surface defects on the paper. However, the cost of a
line scan based vision system is prohibitively high.
Alternatively, area-scan cameras can be used for larger

imaging of the area of the paper. But, if every bit of paper
is to be inspected for faults then the field of view of the
camera need to be wide. Correspondingly the vertical field of
vision of the object oriented parallel to the camera may also
be large which in-turn causes loss of resolution of the image
in the vertical direction. This loss of resolution in the
vertical direction of the image decreases the ability of the
image processing algorithm to detect flaws.
The object of the invention is to magnify the surface width
of an object for continuous inspection of surface defects.
These problems are solved by the optical inspection method
according to claim 1 and an optical arrangement according to
claim 14.
By placing two cylindrical lenses of different focal lengths
perpendicular to each other increases the magnification of
the surface region of the object placed in parallel to the
imaging device in the vertical direction and also retains the
field of vision required for continuous inspection of
surface, thereby providing better measurement accuracy of the
surface defects with simple construction and cheaper cost.
According to an embodiment herein, the first cylindrical lens
and second cylindrical lens are placed at a distance to focus
the object to sensor of the imaging device. This helps to
attain a sharp image of the object with different
magnifications in the vertical and horizontal directions.
The distance between the first cylindrical lens and second
cylindrical lens is equal to focal length of the first

cylindrical lens. This helps to retain the radiant energy of
the light rays passing through the lenses, so as to focus the
light rays to sensor of the imaging device.
According to another embodiment herein, the first cylindrical
lens and the second cylindrical lens are movably mounted to
traverse along the length of the elongated object. This helps
to increase the cumulative field of view to cover the entire
width of the surface region to be inspected as a part of
quality assurance.
The lenses are mounted such that they are movable along the
length of the elongated object enables to selectively set the
distance between the lenses for adjusting size of the image.
This alignment increases the integration of light rays at the
imaging device, which in turn increases the quality of the
image.
According to another embodiment, a series of images of
different surface regions of the elongated object moving
along its longitudinal axis is obtained. This enables
inspection of different surface regions of the moving object
for surface defects.
The elongated object is moved translational such that the
series of images covers the entire surface region of the
object. This enables to inspect even very long objects in a
continuous fashion.
The focal lengths of the lenses are adjusted such that the
outer proportion of the image is adapted to the surface outer
proportion of the elongated object. This image proportion

matches the proportion of the object to use the resolution of
the imaging device more efficiently.
According to an embodiment herein, the imaging device
disclosed is an area scan camera. This helps to image a
larger area of the surface region of the object at a
comparatively lower cost with high precision.
According to another embodiment, the image information is
analyzed using an image processing algorithm to detect the
surface defects. This enables continuous online inspection of
image to detect flaws on the surface region precisely in real
time.
According to yet another embodiment, separation between the
first cylindrical lens and the imaging device equals the
predetermined focal length of the first cylindrical lens and
separation between the second cylindrical lens and the
imaging device equals the predetermined focal length of the
second cylindrical lens. This helps to direct the light rays
from the object to the imaging device without refraction.
The present invention is further described hereinafter with
reference to illustrated embodiments shown in the
accompanying drawings, in which:
FIG 1 illustrates a block diagram depicting an optical
arrangement for detecting surface defects of an
object according to an embodiment herein,

FIG 2 illustrates a schematic diagram depicting a top
view of the optical arrangement of FIG.l, according
to an embodiment herein,
FIG 3 illustrates a schematic diagram depicting a side
view of the optical arrangement of FIG.l, according
to an embodiment herein,
FIG 4 illustrates an exemplary depiction of magnification
of glue lines on a cigarette paper using the
optical arrangement disclosed herein, and
FIG 5 illustrates a flow diagram depicting an optical
inspection method for detecting surface defects of
an object according to an embodiment herein.
Various embodiments are described with reference to the
drawings, wherein like reference numerals are used to refer
to like elements throughout. In the following description,
for purpose of explanation, numerous specific details are set
forth in order to provide a thorough understanding of one or
more embodiments. It may be evident that such embodiments may
be practiced without these specific details.
FIG 1 illustrates a block diagram depicting an optical
arrangement 10 for detecting surface defects of an object 11
according to an embodiment herein. The arrangement 10
comprises of an elongated object 11, a first cylindrical lens
13, a second cylindrical lens 14 and an imaging device 15
placed in a linear fashion. The first cylindrical lens 13 and
second cylindrical lens 14 have different focal lengths f1
and f2 and are aligned perpendicular in accordance with each

other and separated by a distance. The imaging device 15 is
an area scan camera. The imaging device 15 comprises sensors
16 for receiving light rays 12 emitted from the object's 11
illuminated surface region. The illuminated surface region of
the object 11 sends light rays 12 to the first cylindrical
lens 13 positioned relative to the object 11 horizontally.
The first cylindrical lens 13 directs the light rays 12 from
the object 11 to the second cylindrical lens 14 arranged
perpendicular to the first cylindrical lens 13. The second
cylindrical lens 14 directs the lights rays 12 exiting from
the first cylindrical lens 13 to sensor 16 of the imaging
device 15. The light rays 12 exiting from the second
cylindrical lens 14 converge at a point 17 on the sensor 16.
The imaging device 15 on reception of the homogenized light
rays 12 from the second cylindrical lens 14 images the
surface region of the object 11 to obtain an image of the
object 11. The perpendicular alignment of first cylindrical
lens 13 and second cylindrical lens 14 increases the
magnification of the image 18 of the surface region. The
magnified image 18 of the object 11 thus obtained is
processed using an image processor 19 to detect surface
defects and to measure related surface characteristics.
The lenses 13 and 14 are positioned in such a way that the
object 11 positioned at a certain distance is bought to focus
at the sensor 16 of the imaging device 15. The distance
between the first cylindrical lens 13 and the second
cylindrical lens 14 is selected such that the second
cylindrical lens 14 is within the range which can be covered
by the focal length f1. Here, the separation between the
first cylindrical lens 13 and the imaging device 15 equals
the predetermined focal length f1 of the first cylindrical

lens 13 and the separation between the second cylindrical
lens 14 and the imaging device 15 equals the predetermined
focal length f2 of the second cylindrical lens 14. The focal
lengths f1 and f2 of the lenses 13 and 14 are adjusted such
that the outer proportion of the image 18 is adapted to the
surface outer proportion of the elongated object 11.
The first cylindrical lens 13 and the second cylindrical lens
14 are movably mounted to traverse along the length of the
elongated object 11. The elongated object 11 is moved
translational such that a series of images 18 of different
surface regions of the elongated object 11 moving along its
longitudinal axis is obtained. This series of images 18
covers the entire surface region of the elongated object 11
to be inspected for surface defects.
The cylindrical lenses 13 and 14 bends light rays 12 in only
one axis and have the ability to re-converge light from an
object 11 to its focus in one direction. The light rays 12 in
the other direction are not focused and is blurred just like
as if there is no lens. By combining the first cylindrical
lens 13 and the second cylindrical lens 14, the image 18 can
be focused in both vertical and horizontal directions. This
avoids the image blur and helps to attain a sharp image 18
with different magnification in the vertical and horizontal
directions. The cylindrical lens optics herein thereby
increases the resolution of the image 18 in the vertical
direction while maintaining the Field of Vision (FOV)
required for continuous inspection of the surface region
unaffected.

The image processor 19 herein uses an image processing
algorithm to process the image 18 to detect the surface
characteristics of the object 11. An encoder (not shown) in
the image processor 19 tracks the movement of the elongated
object and trigger an image 18 acquisition at the imaging
device 15 at abrupt intervals. At every trigger, the image 18
is processed using the image processing algorithm to check
the surface characteristics to find the surface defects such
as breakages, thickness of the substrate, if any etc. Here,
the image processor 19 can be a digital signal processor or a
computer system capable of executing the image processing
techniques.
In the preferred embodiment the surface inspected is a strip-
like surface or a sheet-like surface, for example glue lines
on a cigarette paper. The optical arrangement facilitates
detection of surface defects such as glue-line breakage due
to uneven application of glue along the glue line, etc and
also precision measurement of dimensions of the glue line,
presence of all glue lines or distance between the glue lines
in the cigarette paper.
FIG 2 illustrates a schematic diagram depicting a top view of
the optical arrangement of FIG 1, according to an embodiment
herein. The illuminated surface region of the object 11 to be
inspected emits light rays 12 to the first cylindrical lens
13 which is positioned horizontal to the object 11. The first
cylindrical lens 13 exhibits the properties of a glass slab,
and hence there is a slight deviation of the light rays 12 at
a point (a). The light rays 12 from the first cylindrical
lens 13 are then traced to the second cylindrical lens 14
which is positioned vertically. The second cylindrical lens

14 then converge the light rays 12 from the object 11 by lens
refraction and focuses the light rays 12 to be incident on a
single point 17 on the sensor 16. The magnification of the
light rays 12 received on the sensor 16 can be approximately
written as:

Here, u is the distance between the object 11 and the first
cylindrical lens 13, d is the distance between the first
cylindrical lens 13 and second cylindrical lens 14 and v is
the distance between the second cylindrical lens 14 and the
point 17 of converging of the light rays 12 on the imaging
device 15. The focal length of first cylindrical lens f1 and
second cylindrical lens f2 is selected to satisfy the
equations:

The focal length of first cylindrical lens f1 equals the
separation between the first cylindrical lens 13 and the
imaging device 15 and the separation between the second
cylindrical lens 14 and the imaging device 15 equals the
focal length f2 of the second cylindrical lens 14. The lenses
13 and 14 can be mounted such that they are movable along the
length of the elongated object 11 so as to selectively set
the distance between the lenses 13 and 14 for adjusting size
of the image 18.

Here the combination of cylindrical lenses 13 and 14 obtains
a sharp image 18 with different magnification in the
horizontal and vertical directions. The arrangement 10
increases the magnification of the surface region in the
image 18 in the vertical direction while the horizontal
magnification is unaffected. Also the required FOV is
maintained for continuous inspection of the surface. Further,
a plurality of images 18, each image 18 picturing different
surface regions of the object 11 can be obtained for
continuous inspection of surface defects by movably mounting
the lenses 13, 14 so as to traverse along the length of the
elongated object.
FIG 3 illustrates a schematic diagram depicting a side view
of the optical arrangement 10 of FIG.l, according to an
embodiment herein. In the side view, the first cylindrical
lens 13 is positioned horizontal and the second cylindrical
lens 14 is positioned vertical to the object 11. The light
rays 12 discharged by the object 11 incidents on the first
cylindrical lens 13 positioned horizontal and draws the light
rays 12 together like any other lens. The light rays 12 then
incidents on the second cylindrical lens 14 positioned
vertically. The cylindrical lens 14 in vertical position
exhibits optical properties in the vertical direction similar
to a glass slab. As a result, the light rays 12 incident on
the lens gets slightly deviated at a point (b). The diverged
rays 12 further meet at the point 17 on the imaging device
15. The magnification in vertical direction can be
approximately written as:


Here, u is the distance between the object 11 and the first
cylindrical lens 13, d is the distance between the first
cylindrical lens 13 and second cylindrical lens 14 and v is
the distance between the second cylindrical lens 14 and the
point 17 of converging of the light rays 12. The focal length
of first cylindrical lens f1 and second cylindrical lens f2
is selected to satisfy the equations:

The focal length of first cylindrical lens f1 equals the
separation between the first cylindrical lens 13 and the
imaging device 15 and the separation between the second
cylindrical lens 14 and the imaging device 15 equals the
focal length f2 of the second cylindrical lens 14. The lenses
13 and 14 can be mounted such that they are movable along the
length of the elongated object 11 so as to selectively set
the distance between the lenses 13 and 14 for adjusting size
of the image 18.
Here the combination of cylindrical lenses 13 and 14 obtains
a sharp image 18 with different magnification in the
horizontal and vertical directions. The optical arrangement
10 increases the magnification of the surface region on the
image 18 in the vertical direction while the horizontal
magnification is unaffected. Also the required FOV is
maintained for continuous inspection of the surface. Further,
a plurality of images 18, each image 18 picturing different
surface regions of the object 11 can be obtained for

continuous inspection of surface defects by movably mounting
the lenses 13, 14 so as to traverse along the length of the
elongated object 11.
FIG 4 illustrates an exemplary depiction of magnification of
glue lines 21 on a cigarette paper 20 using the optical
arrangement 10 disclosed herein. The glue lines 21 are
necessary for wrapping the paper 20 around the tobacco and
the width of the glue lines 21 should be maintained large
enough for correct application of glue on the paper 20. The
image acquisition time of an area scan camera operating at a
speed of 30 frames per second is approximately 30
milliseconds. The paper 20 propelling speed of the machines
is 400 meters per minute, i.e. 6.6mm per ms. On an account,
the paper 20 converges a distance of 30*6.6mm = 200mm during
each image acquisition. Then, the field of view (FOV) of the
imaging device 15 is required to be 200mm wide to inspect
every bit of paper for glue line 21 faults.
An area scan camera with the optical arrangement 10 disclosed
herein can be used for glue line 21 breakage detection and
precision measurement of dimensions of the glue lines 21 and
the gap between glue-lines on the cigarette paper 20. The
glue line 21 of the paper 20 is imaged with a magnification
with a field of view of 200mm so that breakages of even less
than 1 mm can be detected.
The glue-line 21 of the paper 20 being illuminated emits
lights rays 12 to the first cylindrical lens 13. The first
cylindrical lens 13 is positioned horizontal to the glue line
21. The first cylindrical lens 13 exhibits the properties of
a glass slab, and hence there is a slight deviation of the

light ray 12. The light rays 12 from the first cylindrical
lens 13 are then traced to the second cylindrical lens 14
which is positioned perpendicular to the first cylindrical
lens 13. The second cylindrical lens 14 then converge the
light rays 12 from the object 10 by lens refraction and
focuses the light rays 12 to be incident on a single point 17
on the sensor 16. This increases the resolution of the image
18 in the vertical direction.
The cylindrical lenses 13 and 14 bends the light in only one
direction and hence the magnification of the glue lines 21
can be attained in the vertical direction. The alignment of
the cylindrical lenses 13 and 14 with different focal length
enables to focus the image 18 in both directions to avoid
image blur, thereby obtaining a sharp image 18 with different
magnification in the horizontal and vertical directions. Thus
the imaging device 15 camera with the cylindrical lens
arrangement can now be positioned at a distance from the
paper 20 so that 200mm of the paper can be visualized in each
frame and can retain good measurement resolution in the
vertical direction for continuous inspection of glue lines
21. An encoder (not shown) can be used to track the movement
of the cigarette paper 20 and trigger an image acquisition
from the imaging device 15 at every 200mm intervals. At every
trigger the image is processed to check for presence of all
glue lines 21, to measure the distance between the glue lines
21 and the thickness of each glue line 21. This can be
achieved by processing the image using a digital signal
processor in case of a smart camera or by processing on a
computer using image processing techniques in case of
computer based systems. The results from each inspection can
be displayed on to a monitor (not shown).

FIG 5 illustrates a flow diagram depicting a method of
detecting surface defects of an object 11 according to an
embodiment herein. A first cylindrical lens 13 and a second
cylindrical lens 14 are arranged between an object 11 and an
imaging device 15 in a linear manner in step 51. The first
cylindrical lens 13 is positioned horizontally relative to
the object 11 so as to receive the light rays 12 from the
illuminated surface region of the object 11 according to step
52. The second cylindrical lens 14 positioned perpendicular
to the first cylindrical lens 13 directs the lights rays 12
exiting from the first cylindrical lens 13 to sensor 16 of
the imaging device 15 in step 53. The light rays 12 exiting
from the second cylindrical lens 14 converge at a point 17 on
the sensor 16. The imaging device 15 on reception of the
homogenized light rays 12 from the second cylindrical lens 14
images the surface region of the object 11 to obtain image
information for analysis in step 54. The perpendicular
alignment of first cylindrical lens 13 and second cylindrical
lens 14 increases the magnification of the image 18 of the
surface region. The magnified image 18 of the object 11 thus
obtained is processed using an image processor 19 to detect
surface defects and measure surface characteristics in step
55.
The image processor 19 herein uses an image processing
algorithm to process the image 18 to detect the surface
characteristics of the object 11. An encoder (not shown) in
the image processor 19 tracks the movement of the elongated
object and trigger an image 18 acquisition at the imaging
device 15 at abrupt intervals. At every trigger, the image 18
is processed using the image processing algorithm to check
the surface characteristics to find the surface defects such

as breakages, thickness of the substrate, if any etc. Here,
the image processor 19 can be a digital signal processor or a
computer system capable of executing the image processing
techniques.
Here, the first cylindrical lens 13 and second cylindrical
lenses 14 are of different focal length and is arranged at a
distance to bring the object 11 to the focus at the sensor 16
of the imaging device 15. The imaging device 15 used here is
an area scan camera. The iterative positioning of the
cylindrical lenses 13, 14 aid to focus the image 18 in both
vertical and horizontal directions thereby providing a sharp
image 18. The optical arrangement 10 increases the width of
the surface region to provide better measurement accuracy
without altering the horizontal magnification.
The arrangement described herein enables continuous online
inspection of the surface region of an object for surface
defects. Further, due to the increase in resolution of the
image, the width measurement of the surface region of
interest can be more accurately measured. Moreover, the
embodiment herein can be extended for reading or recording
any other invisible marks on a paper or reading a bar code
etc, by imaging through an area scan camera which has a
superior performance and cheaper manufacturing cost, without
the need for using expensive lenses.
While this invention has been described in detail with
reference to certain preferred embodiments, it should be
appreciated that the present invention is not limited to
those precise embodiments. Rather, in view of the present
disclosure which describes the current best mode for

practicing the invention, many modifications and variations
would present themselves, to those of skill in the art
without departing from the scope and spirit of this
invention. The scope of the invention is, therefore,
indicated by the following claims rather than by the
foregoing description. All changes, modifications, and
variations coming within the meaning and range of equivalency
of the claims are to be considered within their scope

We claim:
1. An optical inspection method for detecting surface defects
of an elongated object (11), said method comprising steps of:
- arranging a first cylindrical lens (13) having a first
predetermined focal length and a second cylindrical lens (14)
having a second predetermined focal length between said
object (11) and an imaging device (15) in a linear mode;
- positioning said first cylindrical lens (13) relative to
said object (11), wherein said first cylindrical lens (13)
directs light rays (12) from said object (11) to said second
cylindrical lens (14);
- positioning said second cylindrical lens (14) perpendicular
to said first cylindrical lens (13), said second cylindrical
lens (14) directing said lights rays (12) exiting from said
first cylindrical lens (13) to said imaging device (15);
- said imaging device (15) receiving said light rays (12)
projecting from said second cylindrical lens (14) for imaging
said surface region of said object (11) to obtain an image
(18) of said object (ll);and
- processing said image (18) to detect surface defects of
said object (11).

2. The method according to claim 1, wherein said first
cylindrical lens (13) and said second cylindrical lens (14)
are placed at a distance to focus said object (11) to a
sensor (16) of said imaging device (15).
3. The method according to claim 1 or 2, wherein said
distance between said first cylindrical lens (13) and said
second cylindrical lens (14) is equal to said focal length of
said first cylindrical lens (13).

4. The method according to one of the claims 1 to 3, wherein
said first cylindrical lens (13) and said second cylindrical
lens (14) are movably mounted to traverse along the length of
said elongated object (11).
5. The method according to claim 4, wherein mounting said
lenses (13, 14) such that they are movable along the length
of said elongated object (11) enables to selectively set the
distance between said lenses (13, 14) for adjusting size of
said image (18).
6. The method according to one of the claims 1 to 5, wherein
a series of images (18) of different said surface regions of
said elongated object (11) moving along its longitudinal axis
is obtained.
7. The method according to claim 6, wherein said elongated
object (11) is moved translational such that the series of
images (18) covers the entire surface region of said object
(11) .
8. The method according to one of the claims 1 to 4, wherein
the focal lengths of the said lenses (13, 14) are adjusted
such that the outer proportion of said image (18) is adapted
to the surface outer proportion of said elongated object
(11).
9. The method according to claim 8, wherein the focal lengths
of the said lenses (13, 14) are adjusted such that said image
(18) covers the entire surface area of said elongated object
(11).

10. The method according to one of the claims 1 to 9, wherein
said imaging device (15) is an area scan camera.
11. The method according to one of the claims 1 to 9, wherein
said image (18) is processed using an image processing
algorithm to detect said surface defects.
12. The method according to one of the claims 1 to 11,
wherein the separation between said first cylindrical lens
(13) and said imaging device (15) equals said predetermined
focal length of said first cylindrical lens (13).
13. The method according to one of the claims 1 to 12,
wherein the separation between said second cylindrical lens
(14) and said imaging device (15) equals said predetermined
focal length of said second cylindrical lens (14).
14. An optical inspection arrangement (10) for detecting
surface defects of an elongated object (11), said arrangement
comprising:
- a first cylindrical lens (13) having a first predetermined
focal length;
- a second cylindrical lens (14) having a second
predetermined focal length, said second cylindrical lens (14)
being positioned perpendicularly to said first cylindrical
lens (13);
- an imaging device (15);
- wherein said lenses (13, 14) and said imaging device (15)
are arranged in a linear mode;
- wherein said first cylindrical lens (13) is arranged such
that it directs light rays (12) from said object (11) to said
second cylindrical lens (14);

- wherein said second cylindrical lens (14) is arranged such
that it directs said light rays (12) exiting from said first
cylindrical lens (13) to said imaging device (15) to generate
an image (18) of said object (11); and
- a means for processing said image (18) for detecting said
surface defects of said object (11).
15. The optical arrangement (10) according to claim 14,
wherein said means for processing said image (18) for
detecting said surface defects is an image processor device
(19) .
16. The optical arrangement (10) according to claim 14,
wherein said image processor (19) includes an image
processing algorithm to detect said surface defects of said
object (11) from saiid image (18).
17. The optical arrangement (10) according to one of the
claims 14 to 16, wherein said imaging device (15) is an area
scan camera.
18. The optical arrangement (10) according to claim 14,
wherein said first cylindrical lens (13) and said second
cylindrical lens (14) are placed at a distance to focus said
object (11) to a sensor (16) of said imaging device (15).
19. The optical arrangement (10) according to claim 18,
wherein said distance between said first cylindrical lens
(13) and second cylindrical lens (14) is equal to said focal
length of said first cylindrical lens (13).

20. The optical arrangement (10) according to claim 14 to 19,
wherein said first cylindrical lens (13) and said second
cylindrical lens (14) are movably mounted to traverse along
the length of said elongated object (11).
21. The optical arrangement (10) according to claim 20,
wherein mounting said lenses (13, 14) such that they are
movable along the length of said elongated object enables to
selectively set the distance between said lenses (13,14) for
adjusting size of said image (18).
22. The optical arrangement (10) according to claims 14 to
21, further comprising a controller for controlling imaging
of said elongated object (11) such that a series of images
(18) of different surface sections of said elongated object
(11) moving along its longitudinal axis is obtainable.
23. The optical arrangement (10) according to claim 22,
wherein said elongated object (11) is movable translational
such that said series of images (18) covers the entire
surface region of said object (11).
24. The optical arrangement (10) according to one of the
claims 14 to 19, wherein the focal lengths of said lenses
(13, 14) are adjusted such that the outer proportion of said
image (18) is adapted to the surface outer proportion of said
elongated object (11).
25. The optical arrangement (10) according to one of the
claims 13 to 24, wherein the separation between said first
cylindrical lens (13) and said imaging device (15) equals

An optical inspection arrangement and method for detecting
the surface defects of an elongated moving object is
disclosed. The method comprising arranging a first
cylindrical lens having a first predetermined focal length
and a second cylindrical lens having a second predetermined
focal length between the object and an imaging device in a
linear mode, positioning the first cylindrical lens relative
to the object, wherein the first cylindrical lens directs
light rays from the object to the second cylindrical lens,
positioning the second cylindrical lens perpendicular to the
first cylindrical lens, the second cylindrical lens directing
the lights rays exiting from the first cylindrical lens to
the imaging device, the imaging device receiving the light
rays projecting from the second cylindrical lens for imaging
surface region of the object to obtain an image of the
object, and processing the image to detect surface defects of
the object.