U.S. patent application number 10/941489 was filed with the patent office on 2005-05-12 for illumination system for a printing press.
This patent application is currently assigned to Quad/Tech, Inc.. Invention is credited to Seymour, John C..
Application Number | 20050099795 10/941489 |
Document ID | / |
Family ID | 32771571 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050099795 |
Kind Code |
A1 |
Seymour, John C. |
May 12, 2005 |
Illumination system for a printing press
Abstract
An illumination system for a web travelling from upstream to
downstream in a longitudinal direction in a printing press is
described. The illumination system includes a first and a second
illuminator for emitting light, each illuminator having a long axis
arranged in a lateral direction. Further, the system includes a
first and a second reflector, with the second reflector being
arranged downstream from the first reflector. Each reflector has a
surface for reflecting light from a corresponding illuminator
toward the web, wherein a cross-section of each reflecting surface
is a portion of a parabola having a focal point, or a portion of an
ellipse, having two focal points. An illuminator is located at a
corresponding focal point.
Inventors: |
Seymour, John C.; (Genesee
Depot, WI) |
Correspondence
Address: |
Michael Best & Friedrich LLP
Two Riverwood Place
Suite 200
N19 W24133 Riverwood Drive
Waukesha
WI
53188
US
|
Assignee: |
Quad/Tech, Inc.
Sussex
WI
|
Family ID: |
32771571 |
Appl. No.: |
10/941489 |
Filed: |
September 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10941489 |
Sep 15, 2004 |
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10385311 |
Mar 10, 2003 |
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Current U.S.
Class: |
362/101 |
Current CPC
Class: |
B41F 33/0045 20130101;
B41F 33/0036 20130101 |
Class at
Publication: |
362/101 |
International
Class: |
F21V 033/00 |
Claims
What is claimed is:
1. An illumination system for a web travelling from upstream to
downstream in a longitudinal direction in a printing press, with a
lateral direction substantially perpendicular to the longitudinal
direction, the system comprising: a first and a second illuminator
for emitting light, each illuminator having a long axis arranged in
the lateral direction, and a first and a second reflector, the
second reflector arranged downstream from the first reflector, each
reflector having a surface for reflecting light from a
corresponding illuminator toward the web, wherein a cross-section
of each reflecting surface is a portion of a parabola having a
focal point, and wherein a corresponding illuminator is centered at
each focal point.
2. The illumination system of claim 1, wherein light rays from each
reflector hit the web at an angle of substantially 45 degrees from
the plane of the web.
3. The illumination system of claim 1, wherein the first and second
reflectors are symmetric to each other about a plane perpendicular
to the web.
4. The illumination system of claim 3, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially away from the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially away
from the web.
5. The illumination system of claim 3, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially toward the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially toward
the web.
6. The illumination system of claim 1, wherein the first and second
reflectors are not symmetric to each other.
7. The illumination system of claim 6, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially away from the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially toward
the web.
8. The illumination system of claim 6, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially toward the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially away the
web.
9. An illumination system for a web travelling from upstream to
downstream in a longitudinal direction in a printing press, with a
lateral direction substantially perpendicular to the longitudinal
direction, the system comprising: a first and a second illuminator
for emitting light, each illuminator having a long axis arranged in
the lateral direction, and a first and a second reflector, the
second reflector arranged downstream from the first reflector, each
reflector having a compound reflecting surface for reflecting light
from a corresponding illuminator toward the web, wherein a
cross-section of each compound reflecting surface includes a first
curve that is a portion of a parabola having a focal point, and a
second curve that is a portion of a circle centered at the focal
point, and further wherein a corresponding illuminator is centered
at each focal point.
10. The illumination system of claim 9, wherein light rays from
each reflector hit the web at an angle of substantially 45 degrees
from the plane of the web.
11. The illumination system of claim 9, wherein the first and
second reflectors are symmetric to each other about a plane
perpendicular to the web.
12. The illumination system of claim 11, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially away from the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially away
from the web.
13. The illumination system of claim 11, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially toward the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially toward
the web.
14. The illumination system of claim 9, wherein the first and
second reflectors are not symmetric to each other.
15. The illumination system of claim 14, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially away from the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially toward
the web.
16. The illumination system of claim 14, wherein the reflecting
surface of the first upstream reflector reflects a portion of the
light emitted from the first illuminator that is directed
substantially toward the web, and the reflecting surface of the
second downstream reflector reflects a portion of the light emitted
from the second illuminator that is directed substantially away the
web.
17. An illumination system for a web travelling from upstream to
downstream in a longitudinal direction in a printing press, with a
lateral direction substantially perpendicular to the longitudinal
direction, the system comprising: a first and a second illuminator
for emitting light, each illuminator having a long axis arranged in
the lateral direction, and a first and a second reflector, the
second reflector arranged downstream from the first reflector, each
reflector having a surface for reflecting light from a
corresponding illuminator toward the web, wherein a cross-section
of each reflecting surface is a portion of an ellipse having a
first and a second focus, wherein the first illuminator is centered
at the first focus of the first reflector, and the second
illuminator is centered at the first focus of the second reflector,
and the second focus of the first reflector and the second focus of
the second reflector are substantially coincident.
18. The illumination system of claim 17, wherein a first angle
between the web and a line connecting the first focus and the
second focus of the first reflector is 45 degrees or slightly
greater than 45 degrees, and a second angle between the web and a
line connecting the first focus and the second focus of the second
reflector is 45 degrees or slightly greater than 45 degrees.
19. The illumination system of claim 17, wherein the first
reflector and the second reflector both include a blind spot.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/385,311, filed Mar. 10, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates generally to an illumination
system for a printed work on a web in a printing press, and more
particularly, to reflectors for reflecting light from an
illuminator to the printed work on the web.
BACKGROUND OF THE INVENTION
[0003] A web-offset printing press includes an inking assembly for
each color of ink used in the printing process. Each inking
assembly includes an ink reservoir and a plurality of hard nylon
keys or a segmented blade disposed along the outer surface of an
ink fountain roller. The amount of ink supplied to a roller train
of the press and ultimately to a substrate, such as a web of paper,
is adjusted by changing the spacing between the edge of the blade
segments or the nylon keys and the outer surface of the ink
fountain roller. The position of each blade segment or each key
relative to the ink fountain roller is independently adjustable via
an ink control system to thereby control the amount of ink fed to a
corresponding longitudinal strip or ink key zone of the
substrate.
[0004] Typically, ink is spread laterally from one longitudinal
zone to adjacent zones due to the movement of vibrator rollers,
which oscillate in a lateral direction relative to the substrate.
The amount of ink on the ink fountain roller itself is also
adjustable by changing the angle through which the ink fountain
roller rotates each stroke. This generally occurs by adjusting a
ratchet assembly, as is known in the art.
[0005] While such a printing press is running, a camera is
typically used to continually monitor the printed output and to
make appropriate ink key adjustments in order to achieve
appropriate quality control of the color of the printed image.
Specifically, the camera moves across the web to collect images of
color patches on the moving web. Each pixel of the color patch
images is then processed, and assigned a color value. Each color
value is compared against a desired color value. If the absolute
difference between the desired color value and the determined color
value is outside some predetermined tolerance, the ink key is then
controllably adjusted, thereby effecting a change in the ink flow
rate.
[0006] It is not uncommon for printed images on the web, color
patches in particular, to be corrupted by some printing artifact
such as the effect of a paper fiber on the blanket roller (commonly
known as a hickey), a droplet of ink, an indentation on the
blanket, a slime hole in the paper, a scratch on the plate, or some
other such defect. In this case, the measured color values of a
defective color patch may not accurately reflect the color within
the printed work itself. While methods for detecting a small defect
in a color patch exist in marked color control systems, they are
generally limited to eliminating small defects that do not
encompass a relatively large portion of the color patch.
Furthermore, these color control systems use techniques that assume
that the color properties of the printed work remain constant over
a defined area. However, the color properties of the print work may
not remain constant. As a result, other techniques are needed to
detect defects.
[0007] Color control systems for printing presses not requiring the
use of color patches, or markless color control systems have been
developed. Such markless color control systems measure color values
in the printed work itself. Since the color of the printed work is
measured directly in the markless systems, the correspondence
between color patches and the work is not in question. However,
these systems do not detect defects on the printed work. Even
though the marked color control systems are configured to detect
defects in the printed work, these defect detection techniques are
applied to marked color control systems only.
[0008] For example, printing presses typically include a defect
detection system as are known in the art. This type of defect
detection system scans, and acquires an image of the printed web.
The acquired image is subsequently compared to a stored digital
template image. Any discrepancy between the acquired image and the
template image beyond some tolerance is considered to be a defect.
The isolated defects are then logged in a data file. When the
systems detect a large change in color due to a change in inking
level, a non-isolated defect is reported over a large portion of
the web. When non-isolated defects are reported, an alarm will
subsequently be set off to alert an operator to take appropriate
corrective action.
[0009] Once a printed product is determined to be acceptable, the
defect detection control systems will typically establish a new
template image by scanning the acceptable printed product. The
defect detection control system is not fully functional until a
printed product is determined acceptable. While a template image
can be collected before the printed product is considered
acceptable, the template image may actually contain a defect, and
an actual defective image may be considered acceptable or good, and
therefore no corrective action is taken.
[0010] Furthermore, the printed product may have subtle defects
even when it is judged acceptable. For example, a printing plate
may have been scratched before the printing process started, or a
blanket flaw such as a hickey or indentation may have been
present.
[0011] The makeready process typically includes a visual comparison
and inspection of a print product against a contract proof. This
visual comparison and inspection process establishes that no
formatting errors are introduced into the press between making the
contract proof and putting the printing plates on press. However,
typical defect detection control systems do not allow for a
template image that has been collected based on a contract proof,
or based on a digital representation of the printed work that was
used to create the printing plate.
[0012] Traditionally, color control systems and defect detection
control systems are two separate systems operating on a printing
press. These separate systems utilize separate web scanning
mechanisms. Image processing is often duplicated in these two
control systems as well.
SUMMARY OF THE INVENTION
[0013] The invention provides an illumination system for a web
travelling from upstream to downstream in a longitudinal direction
in a printing press. A lateral direction is defined to be
substantially perpendicular to the longitudinal direction. The
system includes a first and a second illuminator for emitting
light, each illuminator having a long axis arranged in the lateral
direction. Further, the system includes a first and a second
reflector, with the second reflector being arranged downstream from
the first reflector. Each reflector has a surface for reflecting
light from a corresponding illuminator toward the web, wherein a
cross-section of each reflecting surface is a portion of a parabola
having a focal point. A corresponding illuminator is located at
each focal point.
[0014] In another aspect, the invention provides an illumination
system for a web travelling from upstream to downstream in a
longitudinal direction in a printing press. A lateral direction is
defined to be substantially perpendicular to the longitudinal
direction. The illumination system includes a first and a second
illuminator for emitting light, each illuminator having a long axis
arranged in the lateral direction. The system also includes a first
and a second reflector, the second reflector arranged downstream
from the first reflector. Each reflector has a surface for
reflecting light from a corresponding illuminator toward the web,
wherein a cross-section of each reflecting surface is a portion of
an ellipse having a first and a second focus. The first illuminator
is located at the first focus of the first reflector, and the
second illuminator is located at the first focus of the second
reflector. The second focus of the first reflector and the second
focus of the second reflector are substantially overlapping.
[0015] Other features and advantages of the invention will become
apparent by consideration of the detailed description and
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a perspective view of a portion of a printing
press;
[0017] FIG. 2 is a side view of a scanner assembly;
[0018] FIG. 3 is a perspective view of a lighting element of the
scanner assembly;
[0019] FIG. 4 is a sectional view of the lighting element with a
slit aperture;
[0020] FIG. 5 is a sectional view of an alternative embodiment of
the lighting element;
[0021] FIG. 6 is a perspective view of the lighting element
emitting light from a single point;
[0022] FIG. 7 is a perspective view of an image sensor
arrangement;
[0023] FIG. 8 is a flow chart of a control system;
[0024] FIG. 9 is a table indicating input and output rules;
[0025] FIG. 10 is a perspective view of a portion of printing press
including an alternative embodiment of the control system;
[0026] FIG. 11 is a side view of one embodiment of an arrangement
of two parabolic reflectors;
[0027] FIG. 12 is a side view of another embodiment of an
arrangement of two parabolic reflectors;
[0028] FIG. 13 is an illustration of a group of parabolic
curves;
[0029] FIG. 14 illustrates various utilization angles for different
parabolic reflectors;
[0030] FIG. 15 plots utilization angles versus focal length for
various clearance lengths;
[0031] FIG. 16 is a graph of angle versus distance;
[0032] FIG. 17 is a graph of light intensity versus distance for an
illuminator and parabolic reflector;
[0033] FIG. 18 is a side view of another embodiment of an
arrangement of two parabolic reflectors;
[0034] FIG. 19 is a graph of light intensity versus distance for
the arrangement illustrated in FIG. 18;
[0035] FIG. 20 is a side view of a compound reflector including a
circular portion and a parabolic portion; and
[0036] FIG. 21 is a perspective view of the reflector illustrated
in FIG. 20.
[0037] Before any embodiments of the invention are explained in
detail, it is to be understood that the invention is not limited in
its application to the details of construction and the arrangement
of components set forth in the following description or illustrated
in the following drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. Also, it is to be understood that the phraseology and
terminology used herein is for the purpose of description and
should not be regarded as limiting. The use of "including,"
"comprising," or "having" and variations thereof herein is meant to
encompass the items listed thereafter and equivalents thereof as
well as additional items.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] A control system 130 according to the present invention is
shown in FIG. 1. The control system 130 includes a single scanner
assembly 134 for both color control and defect detection purposes,
and a single system processor 138. The scanner assembly 134
collects image data from a web 142 moving in a direction 143. Once
collected, the acquired image data is transferred to the processor
138 for processing in a color control subsystem and a defect
detection subsystem. Such processing includes color control, such
as ink level adjustment, and defect detection. The ink level
adjustment information is then communicated to the associated
printing press to effect a change in ink level when deemed
necessary as is known in the art.
[0039] Generally, the scanner assembly 134 includes an illumination
system which illuminates the moving web 142, an image sensor which
senses reflected light from the moving web 142, and any associated
optic elements required to appropriately disperse the illumination
or direct light to the image sensor. Referring now to FIG. 2, a
preferred scanner assembly 134 is shown. The scanner assembly 134
includes a pair of light sources or lighting elements 144 located
upstream and downstream from an image sensor 145. Each lighting
element 144 further includes an illuminator 146, arranged
substantially parallel to the moving web 142 and substantially
perpendicular to the direction 143, and a reflector 150.
[0040] The illuminator 146 provides illumination to the web 142
with a pair of fluorescent bulbs, for example. As the web 142
moves, an encoder signal from the printing press drives a shutter
mechanism to trigger acquisitions of data. At each acquisition, the
image sensor 145 senses a portion of the efflux light that is
reflected from the web 142.
[0041] When high-speed web or fine resolution printing is desired,
the illuminator 146 is typically powered by a high frequency power
supply to maintain a relatively constant strength of illumination
from one image line to the next. In the preferred embodiment, the
illuminator 146 is a tube-shaped halogen bulb with a filament
running parallel to the web 142. The tube-shaped halogen bulb
typically provides illumination stability until its point of
failure, and the filament provides substantially uniform
illumination across the web 142. Other illumination devices such as
a series of conventional incandescent bulbs may also be used.
[0042] Referring now to FIGS. 2-3, the reflector 150 is shown which
is utilized to make efficient use of light. The reflector 150
extends substantially parallel to the illuminator 146. In the
preferred embodiment, the reflector 150 has a general shape of a
part of an ellipse 154, which has two foci 158, 162. The
illuminator 146 is substantially aligned at the first focus 158.
The second focus 162 is generally at a point on or just above the
web 142 and below the image sensor 145. The two reflectors 150 are
aligned such that the second focus 162 of each reflector 150 is
substantially coincident.
[0043] FIG. 4 shows another embodiment of the lighting element 144.
The illuminator 146 as shown in FIG. 4 is positioned such that a
45.degree. angle is made between the web 142 and a line 166
connecting the two foci 158, 162. A slit aperture 170 is placed
near the focus 162 to obstruct the light that impinges the web 142
at an angle substantially different from 45.degree.. The reflector
150 is designed to utilize only the reflected light that passes
through the aperture 170. The reflector 150 includes a blind spot
174. The light reflected from the blind spot 174 generally does not
pass through the aperture 170. The blind spot 174 is preferably
given a flat black finish to absorb a significant portion of the
light from the illuminator 146. If the reflector 150 is left
reflective at the blind spot 174, the light that leaves the
illuminator 146 toward the blind spot will be reflected back
through an illuminator surface. Since the reflected light does not
re-enter perpendicular to the illuminator surface, the illuminator
surface subsequently refracts and scatters the reflected light.
Thus, the blind spot 174 is preferably darkened.
[0044] The lighting elements 144 are preferably packaged in an
enclosure such that all the light emitting from the enclosure
leaves through the aperture 170. The interior walls of the
enclosure preferably have a black finish, or are baffled as
necessary to reduce stray light.
[0045] To increase the utilization of light energy, and as shown in
FIG. 5, a lens 178 is placed between the reflector 150 and the web
142 to increase the amount of light focused at the focus 162 on the
web 142. The illumination directly from the illuminator 146 at or
about 45.degree. toward the web 142 typically spreads and covers a
wide swath on the web 142. The lens 178 is placed such that the
lens focus and the focus 162 are generally coincident. The lens 178
focuses the direct illumination into the same line as the
elliptical reflected light. The size and placement of the lens are
also chosen such that there is no interference between the lens 178
and the reflected light paths.
[0046] A circular reflector 182 centered at the first focus 158 is
positioned at the blind spot 174. The illumination proceeds from
the illuminator 146 to the circular reflector 182. From the
circular reflector 182, the illumination is reflected back through
the illuminator 146 and further to the lens 178, which focuses the
illumination on the web 142.
[0047] If the distance between the circular reflector 182 and the
illuminator 146 is approximately the same as the distance between
the elliptical reflector 150 and the illuminator 146, the circular
reflector 182 and the elliptical reflector 150 can be fabricated as
a single extruded assembly. In this way, the blind spot no longer
requires darkening. Both the circular reflector 182 and the
elliptical reflector 150 are preferably mirrors, polished enough in
order to reflect nearly all the illumination as gloss, but with
bumpy surfaces on a millimeter scale such that a filament image is
not projected on the web 142.
[0048] It may be beneficial for the angle created between the web
142 and the straight line 166 formed between the foci 158, 162 to
be slightly greater than 45.degree.. As shown in FIG. 6, two light
rays 190, 194 emanate from a single point on the illuminator 146
onto the web 142 thereby defining two angles 198, 202 between the
light rays 190, 194 and the web 142. The two rays 190, 194 also
impinge a scan line 204 on the web 142 at two points 205, 206. The
first light ray 190, from the illuminator 146 to point 205, is on a
plane that is perpendicular to the illuminator 146. The first angle
198 is 45.degree., which is appropriate for the desired geometry.
The second light ray 194, from the illuminator 146 to point 206
away from point 205 of the scan line 204, is not on the plane
perpendicular to the illuminator 146. As a result, the second angle
202 is shallower than 45.degree.. That is, there is a bias toward
the light rays that impinge the web 142 at shallower angles than
the desired 45.degree.. Consequently, to achieve the 45.degree.
desired geometry on average, the angle between the web 142 and the
foci 158, 162 is increased by tilting the lighting elements 144 to
allow for angles between the web 142 and the line between the foci
158, 162 to be non-ideal, that is, slightly greater than
45.degree..
[0049] Another embodiment of the reflectors of scanner assembly 134
is shown in FIG. 11, which also illustrates two reflectors 150a,
150b with one reflector 150a downstream of the other. A reflecting
surface of each of reflectors 150a, 150b has a general shape in
cross section that is a portion of a parabola. As shown, the cross
section of the reflecting surface of the reflector on the right is
a portion of the left half of a parabola that has been rotated 45
degrees clockwise from a line parallel to line 500, which extends
perpendicular to the web 142. Similarly, the cross section of the
reflector on the left is a portion of the right half of a parabola
that has been rotated 45 degrees counterclockwise from a line
parallel to line 500. The respective focal points of the parabolas
are denoted 502a and 502b. The web 142 is illustrated as moving in
longitudinal direction 143. An illuminator (not shown) is
positioned at each respective focal point 502a, 502b. Each
illuminator radially emits light toward the surface of its
associated reflector. In particular, an appropriate illuminator for
this arrangement is a tube shaped bulb, having a radius of about 5
mm, and about four inches long. The long direction of the bulb is
parallel to the lateral direction of the web (i.e., the bulb would
extend perpendicularly to the plane illustrated in FIG. 11, similar
to the arrangement shown in FIG. 3). Each reflector 150a, 150b
extends substantially parallel to an associated illuminator. As
shown, the light reflected by each reflector is redirected in a set
of parallel rays to the web. The web 142 then reflects this
collimated light and the reflected light travels to image sensor
145 (not shown in FIG. 11), which records image data indicative of
the printed work on the web.
[0050] Another embodiment of the arrangement of parabolic
reflectors 150a, 150b is illustrated in FIG. 12. In this case, the
cross section of the reflector on the right (150a) is a portion of
the right half of a parabola that has been rotated 45 degrees
clockwise from vertical. Similarly, the cross section of the
reflector on the left is a segment of the left half of a parabola
that is rotated 45 degrees counterclockwise from vertical. The
arrangement illustrated in FIG. 12 takes more space vertically as
compared to the arrangement illustrated in FIG. 11.
[0051] However, for both arrangements, the light rays directed
toward the web are collimated and at the desired angle of 45
degrees, and the rays from both reflectors overlap in a region
denoted by 504. Such an arrangement is advantageous in that the
illumination on the web is relatively constant, despite the fact
that web weave, i.e., movement of the web up and down, may occur
from a highest web position to a lowest web position. Typical web
weave may be on the order of 0.75 inches or so. As more fully
explained below, the use of parabolic reflectors of appropriate
size and spacing from the illuminator allows the width of the
collimated light directed to the web to be approximately 15 mm and
also allows for the efficient utilization of light.
[0052] Several parabolas 506a-d, having respective focal lengths of
5, 10, 15, and 20 units, and all having a focal point at (0,0) are
illustrated in FIG. 13. Each parabola has the following general
formula, where c is the focal length: 1 f ( x ) = - x 2 4 c + c
[0053] Assuming that a bulb radius of the illuminator 146 is 5 mm,
and a desired width of collimated light is 15 mm, one important
consideration in the selection of an appropriate portion of a
parabola and its arrangement with respect to the bulb is the
utilization of light from the illuminator. In other words, only
some of the light from the illuminator is emitted in the direction
of the reflector, with the rest unused. The amount of light
utilized can be quantified by looking at the angle of light rays
that hit the reflector 150a, 150b. The greater this utilization
angle is, the higher the utilization of light. In particular, FIG.
14 illustrates various parabolic curves having respective focal
lengths (c) of 10, 15, 20, 25 and 30 mm, arranged near an
illuminator 146. The illustrated illuminator 146 has a diameter of
10 mm, with the light rays being emitted in a radial direction.
Assuming a necessary clearance between each reflector and the
illuminator of 5 mm, the parabolic curves in FIG. 14 can be used to
determine the utilization angle in order to achieve a 15 mm wide
strip of collimated light. For example, for c=10 mm, angle ABC
represents the utilization angle of the light that is collimated
into the 15 mm wide strip. Similarly, for c=15 mm, angle DBE
represents the utilization angle of the light that is collimated
into a 15 mm wide strip. For c=20, 25, and 30 mm, the similar
utilization angles are respective angles FBG, HBI, and JBK. These
utilization angles, in degrees, can be plotted as a function of
focal length c, as is shown in FIG. 15 for the curve labelled
"starts at x=10" (i.e, the portion of the parabola starts at x=10
mm).
[0054] By varying the start position of the portion of the
parabola, other curves relating utilization angle to focal length
can be generated. In particular, the illustrated curve having a
maximum value is for a parabolic portion beginning at x=5 mm, and
the middle curve is for x=7.5 mm. The graph illustrated in FIG. 15
can thus be used to optimize the arrangement and focal length of
the parabolic reflector to achieve a high light utilization amount.
FIG. 15 illustrates that the highest light utilization arises when
the leftmost edge of the parabola is at x=5 mm, or where the
reflector would be touching the side of the bulb. Because some
clearance between the illuminator 146 and the reflector 150 is
practically necessary, a clearance of x=7.5 mm is selected. FIG. 15
also illustrates that for the edge of the reflector at x=7.5 mm,
the utilization angle is at a maximum for a focal length just
greater than 9 mm. Because this curve is actually fairly flat over
this region, a value of 10 mm is selected for the focal length for
an appropriate parabolic reflector.
[0055] A second important consideration in the selection of an
appropriate parabolic reflector is to insure that the light
intensity varies as little as possible in the x-direction, in
particular from x=7.5 mm to x=22.5 mm. With uniformity, the
intensity will not vary as the web moves up and down. By relating
the direction (angle) of the light that leaves the bulb to the
lateral position that the light hits the web when the web is in the
nominal position, and differentiating this with respect to
position, one is able to determine the flux density of light.
[0056] Define .theta. to be the angle between the ray emitted from
the bulb and the x-axis. Then (k cos .theta., k sin .theta.) is a
point on this ray at a distance k from the filament of the bulb.
Inserting this into the equation for the parabola results in: 2 k
sin = - 1 4 c ( k cos ) 2 + c
[0057] Writing this in the standard form for a quadratic equation:
3 0 = ( cos 2 - 4 c ) k 2 - ( sin ) k + c
[0058] Using the quadratic equation: 4 k = sin sin 2 + 4 cos 2 4 c
c - 2 cos 2 4 c
[0059] Using cos.sup.2 .theta.+sin.sup.2 .theta.=1, this can be
rewritten: 5 k = 2 c sin 1 cos 2
[0060] The positive root is: 6 k = 2 c sin + 1 ( 1 + sin ) ( 1 -
sin ) Or : k = 2 c 1 - sin
[0061] Since x=k cos .theta., 7 x = 2 c 1 - sin cos
[0062] Solving for .theta. in terms of x: 8 = cos - 1 [ 4 cx 4 c 2
+ x 2 ]
[0063] FIG. 16 illustrates a plot of angle .theta. vs x over the
range from x=7.5 mm to x=22.5 mm. The flux density is proportional
to the derivative of .theta. with respect to x. This derivative is:
9 x = 180 ( 4 c 4 c 2 + x 2 )
[0064] This function is plotted in FIG. 17. The light intensity
from a single illuminator (with a parabolic reflector having c=10
mm, extending from 7.5 mm to 22.5 mm) will drop, at x=22.5, to
about 60% of the maximum intensity. When the two illuminators are
combined, the variation in the two lights will unfortunately not
cancel each other out. Using the arrangement illustrated in FIG. 1,
the light intensity when the web is at the highest position is only
60% of the light intensity when the web is at the lowest position.
Using the arrangement illustrated in FIG. 12, the variation is also
60%, but the maximum intensity occurs when the web is at its
highest position, rather than the lowest. This suggests a combined
approach, as illustrated in FIG. 18, with one reflector at each
orientation, such that the combined light is roughly constant with
respect to height.
[0065] FIG. 19 plots the flux density versus distance for the
arrangement illustrated in FIG. 18. The flux density ranges from a
maximum of 7.55 degrees/mm at the ends to 7.33 degrees/mm in the
middle. This is roughly a 3% variation, which is a definite
improvement compared to the flux density variation for the
arrangements of reflectors shown in FIGS. 11 and 12.
[0066] FIG. 20 illustrates a further improvement to a reflector for
an illumination system. Illustrated is a spiral type reflector 510
that incorporates a parabolic reflecting surface 512 as well as a
circular reflecting surface 514. A perspective view of this
reflector is illustrated in FIG. 21. The circular reflecting
surface 514 is designed so that the filament of the bulb is at the
center 516 of the circle, so that the light and heat exiting the
bulb will be directed directly back to the filament. The center 516
of the circle also coincides with the focal point of the parabola
defining the parabolic reflecting surface 512. The reflection from
the circular reflecting surface 514 serves to increase the
temperature of the filament so that ultimately less power is needed
to achieve the same temperature (light output). This reflector can
be fabricated from a single piece of metal.
[0067] Turning to FIG. 7, the scanner assembly 134 preferably
includes a plurality of image sensors 145 such as linescan cameras.
Each image sensor 145 generally covers a specific scan area on the
web 142. The image sensors 145 are generally arranged laterally
across the web 142. The number of image sensors 145 is generally
application dependent. For example, a single image sensor 145 may
adequately cover the web 142 in one application, but more than one
image sensor 145 may be required to span the web 142 in another. In
an application where a plurality of image sensors 145 is required,
partial overlapping of the scan areas may be necessary to ensure
complete web coverage.
[0068] Each image sensor 145 preferably includes a plurality of
independent image channels. In one embodiment, there are three
channels responsive generally to the wavelength ranges 400 to 500
nanometers, 500 to 600 nanometers, and 600 to 700 nanometers. These
three channels are referred to as the blue, green and red channels,
respectively. If the densitometric fidelity is more important than
the colorimetric fidelity in the print work, the spectral
responsivity of the three channels will be designed to comply with
the definitions of Status T or Status E as defined in ISO 5-3, or
with the German standard DIN 16536, for example.
[0069] If the colorimetric fidelity is more important than the
densitometric fidelity, the three channels would be designed to
meet the Luther-Ives condition. Spectral responsivities that meet
the Luther-Ives condition are 1) spectral responsivities that are
each a linear combination of the tristimulus functions, as defined
in ISO 15-2, and 2) spectral responsivities that span the
three-space of the tristimulus functions.
[0070] If no adequate compromise between densitometric fidelity and
calorimetric fidelity can be found with three channels, a set of
more than three channels may be necessary.
[0071] With respect to spatial resolution, requirements are
typically application dependent. Applications requiring a high
quality of inspection typically require extremely fine resolution.
Applications requiring only detection of image defects that are
readily apparent to a observer do not require extremely fine pixel
resolution. In the preferred embodiment, an image pixel resolution
of 75 DPI is chosen for example. A resolution of 75 DPI is
sufficient to detect defects that are readily apparent to the human
eye at arms length, and it is also a resolution sufficiently coarse
that halftone screens typically used on commercial print product
will not be imaged as moir patterns.
[0072] If the requirements for the defect detection subsystem and
the color control subsystem are sufficiently different, or an image
sensor with higher resolution is preferred for reasons of
availability or cost, it is possible to re-sample an image to a
different resolution for one or both of the subsystems.
Specifically, a full resolution image is first blurred in a manner
consistent with the amount of size reduction, and the image is
subsequently decimated to produce a down-sampled image. Decimation
is a process in which a set of data sampled at an original sampling
rate is down-sampled at a lower sampling rate thereby producing a
down-sampled set of data. The decimation process occasionally
introduces staircase-like aberrations on sharp slanted lines.
Increased smoothing or combining decimation with bilinear
interpolation or any other suitable interpolation procedure
typically reduces the staircase effect. Since decimation can be
performed without applying the initial blurring process to all
pixels, both decimation and blurring are combined to form a more
efficient operation.
[0073] A flowchart 300 according to the present apparatus and
method is shown in FIG. 8. The steps set forth in FIG. 8 are
modular in nature and detail one embodiment of the invention. The
operation generally includes five processes: templating,
acquisition, color control, defect detection and integration.
Depending on the application, the operations preferably run on the
processor 138, such as a conventional general purpose computer, but
can be adjusted to run completely or partially on a digital signal
processor, an application specific integrated circuit, specialized
digital hardware, pipelined array processors, systolic processors,
or the like.
[0074] Specifically, FIG. 8 includes a templating subsystem module
304, an acquisition subsystem module 308, a color control subsystem
module 312, a defect detection subsystem module 316, and a
integration subsystem module 320. Briefly, in the templating
process, a preferably digital representation is initially created
of what should ideally be printed on the web. This so-called
template image is created based on a prepress source of
information. The template image could be created from the data
files used to create the printing plate, or it could be based on a
scan of a proof, for example. When the printed work on the press is
of acceptable quality, an acquired image may also be used as the
template image. The acquisition process encompasses the collection
of an image of a complete repeat of the print, as well as
additional processing to bring this image to a standardized form.
The color control process, which is preferably a markless system,
entails comparison of the currently acquired image against the
template image. Based on this comparison, recommendations are made
for adjustments of inking levels. These recommendations may be fed
to an operator, directly to an inking level actuator, or to an
external process which is controlling inking levels via a PID loop,
an adaptive control loop, or to some model-based control system,
for example. The defect detection process entails comparison of the
acquired image against the template image. The purpose of defect
detection is to find print defects rather than to adjust inking
levels. Therefore, the processing for defect detection after the
comparison will differ substantially from the processing after
color control. The integration process receives inputs from the
color control subsystem and the defect detection subsystem. Based
on these inputs, the integration process may choose to enable or
disable the action of either the color control subsystem or the
defect detection subsystem, or perhaps choose to modify any of the
outputs.
[0075] In normal operation, the templating process will be the
first to occur. This will preferably occur in a computer located
off-press, and networked to various printing presses throughout a
plant. During the initial makeready impressions, the ink levels
will be stabilizing and the inks will be substantially out of
register. The integration subsystem module will most likely be
informed that a substantial amount of defects have been found as
compared to the template image, and that the color control
subsystem does not believe that it can adequately correct the color
yet. Based on this, the outputs from the defect detection subsystem
and from the color control subsystem will be disabled.
[0076] Eventually, the inks will all be at some nominal level and
registration will be reasonable. At this point, the defect
detection subsystem will still see a substantial amount of defects,
but the color control system will deem the color substantially
correctable. Based on this, the integration subsystem will enable
the output of the color control subsystem, but will continue to
disable the defect detection subsystem output. The color control
subsystem will work to adjust the inking levels on the web to
within target tolerances of the colors in the template image. As
this happens, the amount of defects detected will be reduced, and
the degree of color match will improve.
[0077] When the amount of defects and the degree of color match are
within a specified tolerance, the integration subsystem module will
enable the output of the defect detector subsystem. At this point,
the defect detector subsystem will apprise the operator of any
defects that have been detected. This may take the form of, for
example, an image display with an overlay highlighting the places
on the web where appreciable differences occur. These highlighted
defects may be used to diagnose the need for further adjustment of
color, or may indicate a plate scratch or composition error. These
highlighted defects may also indicate inaccuracies in the process
by which the appearance of the web is estimated from the prepress
information. Therefore, when the press has reached the "color ok"
stage, it may be desirable to obtain a more representative image of
the print on the web by capturing an image directly from the web.
At this point, the operator may choose to replace the template
image with an image collected from the web. It is possible to
reduce operating tolerances at this time in either of the color
control subsystem or the defect detection subsystem.
[0078] Turning now to the specifics of each module, in the
templating subsystem module 304, a prepress image 324 is first
derived from a digital data file 328 that is used to image a
printing plate. Some applications may require an entire repeat be
stored in the image 324, while other applications may require only
critical portions of the repeat be stored. However, when a template
image is created from an online image, it may be preferable to
store multiple repeats as the template image. Alternatively, the
prepress image 324 can also be obtained by scanning a contract
proof. Using a contract proof to generate the prepress image 324 is
preferred because defects introduced after the proofing stage may
be flagged by the defect detection system 316. In addition, the
contract proof also has an appearance agreed upon by the printer
and the print buyer. Contract proofs typically cover only a single
page of a multi-paged repeat. As a result, multiple contract proofs
are joined together in mosaic fashion to create an image of the
full repeat.
[0079] The prepress image 324 format does not always match with
that of the scanner assembly 134. Specifically, the pixel size of
the prepress image 324 does not usually match the pixel size of the
image sensors used in the scanner assembly 134. Therefore, it is
generally necessary to resample the prepress image 324 to a pixel
size equivalent to the pixel size of the scanner assembly 134 such
as in step 332. Alternately, both the prepress image 324 and an
acquired image are converted to a lower resolution in order to
reduce the computational overhead and memory requirements.
[0080] The prepress image 324 and the acquired image may not be in
the same color space, and preferably a color space that exhibits a
degree of perceptual uniformity, such as CIELAB, is utilized. For
example, the prepress image 324 may be in CMYK format, whereas the
acquired image may be in RGB format. Thus, it is generally
necessary to convert the images to a common color space as in step
336. Given the prepress image 324 as an input, the conversion step
336 effectively determines a press image estimate, that is what the
press will produce. A template image 340 is thus obtained, and
subsequently stored in template storage 344.
[0081] In the acquisition subsystem module 308, images of the web
142 are continuously acquired in step 348, such that an image of
every line of every repeat is collected using a line scanner. If
the defect detection requirements are stringent, scanning of every
portion of the web 142 may be necessary. The acquisition of an
individual line may be triggered by pulses from an encoder coupled
with the printing press, for example. As images of new lines are
being collected, the previously collected lines are processed. The
processing includes a correction step 352 for distortions inherent
to the image sensor 145 on a line-by-line basis as the lines are
collected.
[0082] The correction step 352 includes a photometric zero
subtraction in which a baseline value indicating an absence of
light is subtracted from all the pixels in a line. However, the
baseline value generally varies over time due to temperature
fluctuations, for example. Updated photometric zeros can be
obtained from periodically sampling the line scanner with the
illumination disabled, and with the ambient light adequately
isolated. Step 352 also corrects geometric distortion, such as the
geometric distortion associated with some lens designs. To correct
the geometric distortion, for each pixel in the geometrically
corrected output line, the graph or formula from the lens design,
or the lens empirical measurements can be used to determine the
location to retrieve the pixel from the input line. The retrieved
location is generally not an integer. Linear interpolation is used
to approximate the value to be stored in the geometrically
corrected line.
[0083] The imaging system as a unit will not typically respond
uniformly in all the pixels. This is due to at least three effects.
First, the intensity of the illumination may not be completely
uniform. Second, due to vignetting, the lens will capture a wider
angle of light from the center of the field of view. Third, the
sensor itself may not be equally efficient at capturing light in
all pixels due to manufacturing imperfections. To correct for such
inconsistencies, the image of a line is divided by a correction
line collected from a uniform white object. Other types of image
that may require corrections include, but not limited to, the
effects of nonlinear digitization and of scattered light, for
example.
[0084] Colorimetric values, such as CIELAB, are used in the
preferred embodiment. The conversion from the regular RGB value to
the color space or colorimetric values is performed in step 364. In
the preferred embodiment, a 9.times.3 matrix transform is used: 10
[ X Y Z ] = [ 0.868 0.046 0.115 0.042 0.074 0.084 - 0.136 0.018 -
0.037 0.425 0.527 - 0.012 - 0.059 - 0.031 0.031 0.174 - 0.014 -
0.038 - 0.017 0.064 0.976 0.031 - 0.003 0.000 - 0.039 - 0.054 0.039
] [ R G B R 2 G 2 B 2 RG RB GB ] , ( E1 )
[0085] where X, Y, and Z, are the standard precursors to the
calculation of CIELAB values.
[0086] The translation from RGB values to colorimetric values can
be performed in a variety of ways. The coefficients of the
transform matrix depend on the specifics of the spectral response
of the scanner assembly 134 and the illumination used, as well as
the reflectance spectra of the inks printed on the web 142. The
transform itself may take any number of forms.
[0087] Once step 352 is completed, most of the distortions
contributing to the dissimilarities between the acquired image and
the prepress image 324 have been corrected. What is not known is
the precise registration of the acquired image relative to the
prepress image 324. In order to compare the acquired image with the
template image 340 in subsequent steps, the two images are aligned
in step 356. Specifically, alignment may require buffering from a
plurality of lines to potentially all the lines of an entire
repeat. A number of buffered lines is preferably stored in a
memory. Once a predetermined number of lines from roughly the
appropriate area of the image has been stored in the buffer,
alignment step 356 takes place.
[0088] Alignment of the acquired image to the template image 340
can be performed in a variety of ways well known in the art. For
example, fiducial marks can be printed on the web 142 and located.
Alternately, alignment without fiducial marks may also be used. The
alignment frequency is generally dependent upon how accurate the
encoder ticks reflects the actual flow of the web 142. In the
preferred embodiment, alignment will be performed once per repeat,
although it could be performed multiple times per repeat, or only
once per multiple repeats. Note that if the lateral stretch of the
web 142 has sufficient variability compared to the pixel size of
the scanner assembly 134, it may be necessary to also perform
alignment in sections across the web 142.
[0089] After the alignment step 356 has been completed, correction
for another distortion of the scanner assembly 134 is preferably
performed in step 360. Normal fluctuations in the intensity of the
illumination of the web 142 will cause an otherwise ideal acquired
image to have a different brightness and chronia with respect to
the template image 340. Step 360 corrects the illumination
intensity by first averaging the intensities of a plurality of
preselected areas on the acquired image. Corresponding areas of the
prepress image 324 are also averaged. The entire acquired image is
subsequently scaled such that the template image average and the
acquired image average are the same. Depending on light source
stability and the web speed, the normalization process in step 360
may be performed on a line-by-line basis or on a multi-line basis,
but preferably on a repeat-by-repeat basis. Furthermore, the
pre-selected areas may be user defined or set up to include all the
pixels in a single line, multi-line section, or repeat, whether the
pixels are inked or non-inked, for example. The pre-selected areas
are preferably the non-inked portions of the web 142. Automatic
identification of these areas could be based on the prepress
information and a sensitivity matrix defined hereinafter.
[0090] Once the calorimetric values have been normalized for
illumination in step 360, the data is sent to a comparison step 368
which generates results that are shared by both the color control
subsystem module 312 and the defect detection subsystem module 316.
In step 368, the corrected and color converted acquired image is
subtracted from the template image 340.
[0091] Referring now to the defect detection subsystem module 316,
the process of defect detection begins with the subtraction of the
corrected and color converted online image from the template image
in step 368. A defect in a pixel is detected in step 376 when a
difference between the pixel value on the acquired image and the
pixel value on the template image 340 is outside a pre-specified
threshold. The threshold may be specified as an absolute difference
of either L*, a* or b* that is greater than a predetermined number,
for example, 5. Alternatively, the threshold may be specified as a
AE value that is greater than a second predetermined number, for
example, 10. In the preferred embodiment, a CMC color differencing
formula is used, with a threshold value determined by the quality
requirements of a print job and an ability of the press to maintain
the color.
[0092] The presence and (x, y) locations of these potential defects
may be all that is required for some applications. In this case,
the connectivity analysis step 380 will be minimal. The presence or
absence of a defect may be used to trigger a mechanism by which the
corresponding impression may be marked as defective, or shunted
into a different workflow from the non-defective product after the
web 142 has been cut into individual signatures. The defect
locations may be logged to a data file for statistical process
control purposes. Alternately, an acquired image with the defect
area highlighted may be displayed to a pressman.
[0093] In other applications, further discrimination of defects may
be required. In particular, the size or intensity of the defects
may be of importance. The size of a defect may be determined by
defect or connectivity analysis in step 380. The result of the
thresholding in step 376 may be considered as a binary defect
image, with a "1" in a pixel indicating a defective pixel, and a
"0" in the pixel indicating otherwise. In the connectivity analysis
step 380, adjacent defective pixels are joined into a single defect
particle. The information in the binary image will thus be reduced
to a list of defect particles, each with a plurality of defective
pixels.
[0094] If it is desired that only defects above a predetermined
size be reported, a binary morphological operation such as binary
erosion may be used in step 380. The original binary defect image
is eroded so that all defects are reduced in size, and only defects
that are larger than a single pixel remain. The erosion process may
be repeated to erode more of the eroded binary image. Each erosion
removes the outer rim of pixels from a defect. If it is desired,
for example, that only defects with a radius greater than six
pixels be reported, erosion has to be performed six times. At the
end of the erosion processes, pixels having a "1" indicate a defect
which is larger than the predetermined size. It may then be
desirable to refer back to the original binary defect image to
locate all the pixels associated with the defect.
[0095] The defect locations reported by the defect detection
subsystem 316 may be used to decide which pixels are used by the
color control subsystem 312. To this end, the color differences
computed in step 368 are sent to a pixel selection step 370. The
pixel selection step 370 passes only those pixels that have been
selected by a combination of the press operator, the original
customer of the printed work, and some automated analysis program.
Alternately, the pixel selection step 370 may make use of only the
pixels in the colorbar such as in a marked color control system.
The computational load for the color control subsystem may thus be
reduced. Additionally, the pixel selection step 370 may suppress
such pixels that are deemed defective in step 380.
[0096] The color differences are then used to determine the color
error in the color control subsystem module 312 which attempts to
minimize the color error by adjusting a set of ink metering devices
in step 372. The error minimization process first assumes that for
small changes in ink metering, the relationships in equations E2,
E3 and E4 are reasonable approximations to the actual relationships
between the variables therein. 11 L p ( x , y ) = L 0 ( x , y ) + i
k ( i , j ) F ( x , y , j ) S L ( x , y , i ) ( E 2 ) a p ( x , y )
= a 0 ( x , y ) + i k ( i , j ) F ( x , y , j ) S a ( x , y , i ) (
E 3 ) b p ( x , y ) = b 0 ( x , y ) + i k ( i , j ) F ( x , y , j )
S b ( x , y , i ) ( E 4 )
[0097] where,
[0098] (x, y) represents coordinates of a pixel in the acquired
image or the template image 340,
[0099] L.sub.o(x, y), a.sub.o(x, y), and b.sub.o(x, y) represent
the CIELAB values of the online image at location (x,y),
[0100] k.sub..DELTA.(i, j) represents a change in the amount of ink
number i (for example, with i=1 being cyan, i=2 being magenta)
metered at lateral position j, where j goes from 1 up to the number
of ink metering devices across the web 142,
[0101] F(x, y, j) represents the relative effect that ink metering
device j has on pixel (x,y),
[0102] S.sub.L(x, y, i), Sa (x, Y, i), and S.sub.b(x, y, i) are
three dimensional sensitivity matrices that estimate the amount of
change there will be in L*, a*, and b*, respectively, at a point
(x, y) for a unit change in kA (i, j), and
[0103] L.sub.p(x, y), a.sub.p(x, y), and b.sub.p(x, y) represent
the predicted CIELAB values of the acquired image at location (x,
y), after a change in the ink metering as specified by the
k.sub..DELTA. vector.
[0104] Due to the spread of ink by the vibrator rollers, an ink
metering device will typically provide ink to a somewhat wider area
than the actual width of the ink metering device. As a result, if
information of the ink spread is available during the make-ready
process, the convergence time can be improved especially when the
ink metering devices require large changes. For example, one value
for F(x, y, j) is 0.5 for pixels within the width of the ink key
metering device, and another value is 0.2 for the pixels in the
neighboring areas. The value of F(x, y, j) can be changed at color
ok to reflect no ink spread.
[0105] Equations E2, E3 and E4 are a linear set of equations in
k.sub..DELTA.(i, j). To determine the required changes in ink
metering in step 372, a residual error as shown in Equation E5 is
first set up: 12 = x , y [ ( L p ( x , y ) - L t ( x , y ) ) 2 + (
a p ( x , y ) - a t ( x , y ) ) 2 + ( b p ( x , y ) - b t ( x , y )
) 2 ] ( E5 )
[0106] where L, (x, y), a.sub.t(x, y), and b.sub.t(x, y) represent
the CIELAB values of the template image 340 at location (x, y). The
quantity being summed is the standard color difference between
corresponding pixels. The required ink changes are determined by
obtaining a vector k.sub..DELTA.(i, j) that minimizes the residual
error, . Alternatively, the changes can be determined from a
differencing formula such as the CMC color differencing
formula.
[0107] This is an overdetermined linear system. It is therefore
possible to use standard regression techniques to determine the
minimization.
[0108] In the preferred embodiment, images will be taken of every
impression. In a typical web offset printing press, a change in the
ink metering may take hundreds of impressions to be fully
expressed. A Proportional-Integral-Derivative ("PID") loop could be
tuned to deal with the long delay. The color control subsystem
module 312 will preferably wait for a number of impressions after
issuing a change in ink metering before requesting a subsequent
change. In this way, the computational load on the system is
decreased.
[0109] The sensitivity matrices, S.sub.L(x, y, i), S.sub.a(x, y,
i), and S.sub.b(x, y, i), may be estimated by analyzing the effect
of changes in inking levels. In one embodiment, estimates about the
ink composition at various points in the impression may also be
made based on knowledge of the typical color values for various
combinations of inks.
[0110] Turning now to the integration subsystem module 320, this
module enables or disables the inking control or the defect outputs
from the color control subsystem module 312 and the defect
detection subsystem module 316, respectively, depending on the
outputs of the modules 312, 316. The information from these two
modules 312, 316 determines the state of the printing press and
also the appropriateness of the enabling and disabling outputs. For
example, the defect detection subsystem is preferably disabled if
it is determined that the defects found are largely the result of
the color being incorrect. An estimate of the time that it will
take to correct the color as well as the magnitude of the defects
may be used as a basis for disabling the defect detection
subsystem. Further, by determining when color is within a given
tolerance, it is possible to tighten the defect tolerance since
spuriously detected color defects would be eliminated.
[0111] The information received by the integration subsystem module
320 from the color control subsystem module 312 may include the
residual color error, .epsilon. determined from equation E5. The
value of .epsilon. indicates how close the template image 340 and
the acquired image will be once the requested inking change has
stabilized on press.
[0112] In addition, the information received from the defect
detection subsystem module 316 may include the sum of defects,
.delta.. The sum of defects, .delta. indicates how close the
template image 340 and the current acquired image are: 13 = x , y [
( L 0 ( x , y ) - L t ( x , y ) ) 2 + ( a 0 ( x , y ) - a t ( x , y
) ) 2 + ( b 0 ( x , y ) - b t ( x , y ) ) 2 ] ( E6 )
[0113] Note that if k.sub..DELTA.=0 in equations E2, E3 and E4,
L.sub.o(x, y)=L.sub.p(x, y), a.sub.o(x, y)=a.sub.t(x, y), and
b.sub.o(x, y)=b.sub.p(x, y), and hence, .epsilon.=.delta.. Since
.epsilon. is determined from a minimization process, it follows
that .epsilon..ltoreq..delta. will always be the case.
[0114] One possible set of rules, for example, for the output
control is shown as Table 384 in FIG. 9. Table 384 uses .epsilon.
and .delta., as defined in equations E5 and E6, as inputs
respectively. Table 384 also uses "Previous prediction," which
indicates a previous value of the residual color error, .epsilon.,
with time scale taken such that any color changes would have
stabilized. If the color control was to be disabled at any step,
the next value for "Previous prediction" would preferably be set to
the current value of the residual color error .epsilon..
[0115] The rules set may be modified to include more than two
values such as, for example, "Small," "Medium," and "Large." The
rules may also include a larger number of previous states.
Implementation can be based on a state machine, a neural network,
or fuzzy logic. Similarly, the rules may be laid out explicitly as
a series of "if-then" statements.
[0116] The computations of .epsilon. and .delta., and the
application of the rules may be applied based on a full impression.
As a result, the enabling or disabling the color control output or
the defect detection output is based on the entire impression.
Alternately, the enabling and the disabling action may be applied
separately to individual alleys, or ink key zones, as required by
the application.
[0117] Furthermore, the defect detection subsystem 316 also
operates to keep the color control subsystem 312 from making
decisions simply based on defective pixels. For example, the color
control subsystem 312 will be disabled in the event of a blanket
wash, or other such severe defect such that only few inked pixels
are detected. The integration module 320 may also elect to disable
inking control outputs based on whether the compute ink key
adjustments module 372 has an adequate pixel count or ratio of
allowed pixels to possible pixels. Alternately, the color control
subsystem 312 may also be disabled based on a numerical analysis on
the stability of the solution of the linear equations representing
the system, or a condition number or a singular value decomposition
of the relevant matrices of the system. Other severe condition that
may disable the color control subsystem 312 includes a startup
condition of the printing press. Specifically, the inking levels
may be substantially off during the startup of the printing press.
When the inking levels are substantially off, the defect detection
subsystem 316 will label a large quantity of pixels defective
thereby undesirably disabling the color control subsystem 312.
[0118] As shown in FIG. 8, the pixel selection module 370 limits
the number of pixels that are suppressed to avoid undesirable
disabling of the color control subsystem 312. For example, if
suppression is required by more than half of the pixels in an
acquired image, the pixel selection module 370 then passes along
only those pixels with the smallest errors. In another embodiment,
the output of the defect analysis module 380 is fed instead to a
second compute ink key adjustment module. The second compute ink
key adjustment module will perform an actual inking control. In
this way, the defect analysis module 380 provides information for
true defect suppressions, but not the defects that cover the entire
web 142. Furthermore, the initial computation of the original ink
key adjustments in module 372 will be made based on all the pixels,
except for those requiring suppression for other reasons.
[0119] FIG. 8 also shows a single output from the defect detection
module 316. Some applications may include more outputs with
different criteria. For example, one output may be the data from
which visualizations of the defects are constructed. Another output
may indicate whether a given impression contains an error
sufficiently large to warrant diverting the corresponding
impression from the acceptable print.
[0120] Sharing of image acquisition and processing by the color
control and defect detection control systems of the present
invention reduces the overall cost of the control system, reduces
maintenance costs, as well as reduces the space needed to house the
control system.
[0121] The preferred embodiment uses prepress information in
advantageous ways. A prepress representation is first used as a
template during makeready for both the defect detection and the
color control in step 328. The sensitivity matrices are also
computed from the prepress information in step 334. Furthermore,
areas where there is no ink coverage are determined by analyzing
the prepress information in module 316. This, in turn, is used to
select pixels to be used for normalization of illumination
levels.
[0122] In the absence of the prepress information, an alternative
embodiment that does not require the prepress information can be
used. For example, the acquired image corrected in the acquisition
module 308 can be used as a template. During makeready, the defect
detection subsystem module 316 will generally not be used, and the
color control subsystem module 312 may be either disabled or based
solely upon color patches within a color bar. Therefore, there will
be enough time for an adequate acquired image to be acquired and
stored as a template image 340.
[0123] FIG. 10 illustrates an alternative embodiment of a control
system 400 according to the present invention. A printed web 404
moves passes a defect detection system scanner 408 in a direction
indicated by arrow 412. The defect detection system scanner 408
contains an array of lighting elements, such as those described
earlier, and an array of image sensors. The lighting elements and
the image sensors are generally arranged laterally across the
scanner 408 and perpendicular to the direction of the moving web
412. Depending on the application, the scanner 408, the lighting
elements, and the image receptors can be arranged differently.
[0124] The defect detection system scanner 408 scans to acquire
image data representative of the printed web 404. The scanned image
data is subsequently transferred to a defect detection system
processor 416 for further processing including a comparison of the
acquired image with a template image stored in the processor 416.
All the discrepancies between the template image and the acquired
image that are outside of some predetermined threshold or tolerance
are considered as defects, and locations at which defects are
detected are also identified. The defect detection system processor
416 then transfers the defect locations to a color control system
processor 420.
[0125] After the web 404 has moved past the defect detection system
scanner 408, the web continues to move in the same direction 412.
As the web 404 moves below the color control system scanner 424,
the color control system scanner 424 acquires image that is
representative of the printed web 404. Similar to the defect
detection system scanner 408, the color control system scanner 424
typically contains an array of lighting elements and an array of
image receptors.
[0126] The color control system scanner 424 passes the image data
to the color control system processor 420 for further processing.
Typical processing includes color value conversion which converts
the image data into its corresponding color values for an
individual pixel or a group of pixels. Other processing includes
assembling the image data into a plurality of lines and aligning
the lines with a color control image template.
[0127] Furthermore, if the defect detection system processor 416
detects no defect with a predetermined number of lines, the color
control system processor 420 performs only a comparison between the
color values and the color control image template. When a
difference is detected by the color control system processor 420,
changes in inking level are generated and sent to a press
interface.
[0128] It should be noted that preferably, the color control
subsystem 312 of the present invention is of the markless color
control type. However, the invention can be utilized with
conventional color patch color control. Furthermore, depending on
application, the present invention allows for ink key zone control
and monitoring as well as the control and monitoring of the whole
web.
[0129] Various features and advantages of the invention are set
forth in the following claims.
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