Device for alignment of images in a control system for a printing press

A device for alignment of images for a control system of a printing press having a device for creating targets, a device for aligning a camera, a device for finding actual dot positions on at least one of the targets, a device for calculating the desired dot positions, a device for generating transfer functions, and a device for aligning the images.

BACKGROUND OF THE INVENTION 
The present invention relates to control systems for a printing press. 
In the past, four process inks (cyan, magenta, yellow and black) have been 
used on a printing press to produce copies with a gamut of colors. To 
improve trapping and reduce ink cost, various undercolor removal 
techniques (UCR) and grey component replacement (GCR) techniques have been 
used in the color separation processing. The UCR and GCR techniques remove 
a certain amount of the cyan, magenta and yellow ink from some printing 
areas and replace them with a certain amount of the black ink. Thus, the 
black ink has been used to generate not only the text but also the color 
image, thus reducing the total volume of ink used to print. Different 
color separation equipment manufacturers offer different UCR and GCR 
techniques to determine when this black ink substitution will take place 
and what amount of inks will be substituted. 
In the past, the press room color reproduction quality control process has 
been divided into two categories: "control by target" and "control by 
image." 
In the "control by target" method, a set of color control targets is 
printed in a margin. Instruments, such as densitometers, are used to 
monitor the color attributes, such as the optical density, of these 
targets. The printing press is then adjusted based on the measured 
deviation of these control targets from a predefined attribute value. The 
application of this method for quality control creates waste and consumes 
resources in that an additional process is required to cut off this target 
from the final product. It also requires a tight material control for 
paper, ink, and other printing parameters. 
In the "control by image" method, the print image on a production copy is 
compared with the printed image on a reference copy, called a proof. The 
press is then adjusted based on the difference between the production 
image and the reference image. This system is more versatile because it 
does not require an additional target to be printed. The "control by 
image" method is also more accurate than the "control by target" method 
because in some situations although the measured attributes of control 
targets on the production and reference images are the same, the two 
images will look different. Conventionally, both the image comparing task 
and the press adjusting task are performed by a press operator. To improve 
the productivity and the color consistency, several automatic printing 
quality inspection systems have been reported recently. These systems use 
opto-electronic sensor devices, such as a spectrophotometer, or CCD color 
cameras, to measure the color reproduction quality. Currently, the 
bandwidth of these sensor devices is limited to the visible region of 400 
nm through 700 nm in wavelength of the electromagnetic spectrum. However, 
within the visible region, it is not possible for these devices to 
reliably distinguish the black ink from the process black made by the 
combination of cyan, magenta, and yellow inks, or to determine whether the 
black ink or all cyan, magenta, and yellow inks should be adjusted. 
Although these devices, such as spectrophotometers, might be able to 
measure the printed color accurately, it is difficult to use the measured 
color information to achieve the automatic control for a four-color press 
without a target due to the involvement of the UCR and GCR techniques. A 
control method without targets could require selecting the points in the 
image to be measured or a large number of measurements would have to be 
acquired. A camera system can acquire a large number of measurements 
simultaneously, giving it an advantage when targets are not printed. 
It has been found that when a four-channel camera is constructed by 
utilizing a single channel black/white camera (B/W) and a 3-channel color 
camera, the infrared image obtained from the B/W camera is misregistered 
with the red, green, and blue images obtained from the color camera. 
Geometric distortion may also be observed from both cameras. 
SUMMARY OF THE INVENTION 
A principal feature of the present invention is the provision of a device 
for aligning images in a control system of a printing press. 
The device of the present invention comprises, means for creating targets, 
means for aligning a camera, means for finding actual dot positions on at 
least one of the targets, means for calculating the desired dot positions, 
and means for generating transfer functions. 
A feature of the present invention is the provision of means for aligning 
images for the control system of the printing press. 
Another feature of the invention is that the images are automatically 
aligned. 
Still another feature of the invention is that the images are closely 
aligned. 
Yet another feature is that the device is of simplified construction and 
reduced cost. 
Further features will become more fully apparent in the following 
description of the embodiments of the invention, and from the appended 
claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, there is shown a control system generally 
designated 10 for a printing press 11 of the present invention. 
The control system 10 has a 4 channel sensor 21, a data converter 23 for 
processing information from the sensor 21, and a device 25 for controlling 
ink for the press 11. As will be seen below, the 4 channel sensor 21 
detects the energy reflected from a paper surface, such as the paper web 
for the press 11, in both the visible region and the infrared region of 
the electromagnetic spectrum. As shown in FIG. 8, electromagnetic waves in 
the infrared region have a longer wave length than the visible spectrum, 
with the wave lengths of the electromagnetic waves in the region of 
visible light being approximately 400 to 700 nanometers (nm), and the wave 
lengths of the electromagnetic waves in the infrared region, including 
near infrared, being equal to or greater than 800 nm. 
As show in FIG. 2, the control system 10 has a support 12 for placement of 
a sheet of paper 14 with image or indicia 16 on the sheet 14 in a 
configuration beneath a pair of opposed lights 18 and 20 for illuminating 
the sheet 14. The system 10 has a first color video camera or sensor 22 
having three channels for detecting attributes of the inks from the sheet 
14 in the visible region of the electromagnetic spectrum such as red, 
green and blue, or cyan, magenta, and yellow, and for sending the sensed 
information over separate lines or leads 24, 26, and 28 to a suitable 
digital computer 30 or Central Processing unit having a randomly 
addressable memory (RAM) and a read only memory (ROM), with the computer 
or CPU 30 having a suitable display 32. Thus, the three distinct color 
attributes of the inks are sensed by the camera 22 from the sheet 14, and 
are received in the memory of the computer 30 for storage and processing 
in the computer 30. 
The system 10 also has a black/white second video camera or sensor 34 
having a filter 50 such that it senses the attributes of the inks in the 
infrared region of the electromagnetic spectrum, having a wave length 
greater than the wave length of the electromagnetic waves in the visible 
region of light. The camera or sensor 34 thus senses infrared information 
from the sheet 14, and transmits the sensed information over a lead 36 to 
the computer 30, such that the information concerning the infrared rays is 
stored in and processed by the computer 30. 
The normalized percentage of infrared (IR) reflection vs. the percentage of 
dot area is show in the chart of FIG. 7. It will be seen that the infrared 
reflectance of cyan, magenta, and yellow inks show no significant change 
as a function of percentage of dot area. However, the normalized infrared 
reflectance of the black ink displays a significant change as a function 
of percentage of dot area, and changes from a normalized value of 100% IR 
reflection for 0% dot area to approximately 18% IR reflection 
corresponding to 100% dot area. Hence, the black ink may be easily sensed 
and distinguished from other color inks in the infrared region of the 
electromagnetic waves. 
As shown in FIG. 2, the sheet 14 may contain printed image or indicia 16 
which is obtained from a current press run of the press 11, termed a 
production or current copy. In addition, a sheet 38 containing printed 
image or indicia 40, termed a reference copy, from a previous reference 
press run may be placed on the support 12 beneath the cameras 22 and 34 in 
order to sense the energy reflected from the sheet 38, and send the sensed 
information to the memory of the computer 30 for storage and processing in 
the computer 30, as will be described below. 
Thus, the cameras or sensors 22 and 34 may be used to sense both the 
current copy or sheet 14 and the reference copy or sheet 38. The 
information supplied by the cameras 22 and 34 is formed into digital 
information by a suitable analog to digital converter in a frame grabber 
board on the computer 30. Thus, the computer 30 operates on the digital 
information which is stored in its memory corresponding to the information 
sensed from the sheets 14 and 34 by the cameras or sensors 22 and 34. 
Referring now to FIG. 3, there is shown a block diagram of the control 
system 10 for the printing press 11 of the present invention. As shown, 
the four inks (cyan, magenta, yellow, and black) of the four-color 
printing press 11 are first preset, after which a print is made by the 
press 11 with a current ink setting, thus producing a production or 
current printed copy, as shown. The color and black/white video cameras or 
sensors 22 and 34 of FIG. 2 serve as a four channel sensor 21 to capture 
an image of the current printed copy, and then place this information into 
the memory of the computer 30 after it has been formed into digital 
information. 
Next, an "Ink Separation Process" 23 is used to convert the red, green, 
blue and IR images captured by the four channel sensor 21 into four 
separated cyan, magenta, yellow and black ink images, which represent the 
amount of corresponding ink presented on the live copy. The "Ink 
Separation Precess" 23 may utilize mathematic formulas, data look up 
tables or other suitable means to perform the data conversion task. 
The similar processes are also applied to the reference copy. First, the 
four channel sensor 21 is used to capture the red, green, blue and IR 
images from the reference copy. Then, the "Ink Separation Process" 23 is 
utilized to obtain the cyan, magenta, yellow and black ink images, which 
represent the amount of corresponding ink presented on the reference copy. 
As shown, the ink images of the production copy are compared with the ink 
images of the reference copy by the computer 30 to detect the variation of 
ink distribution for each of the cyan, magenta, yellow and black inks. 
The determined differences in ink distribution are then processed by the 
computer 30 in order to obtain an indication for controlling the keys or 
other devices of the press 11 in an ink control process, and thus provide 
an indication of an ink adjustment to the press to obtain further copies 
which will have a closer match to the reference copy. The indication of 
ink changes may be automatically supplied to the press 11, or the operator 
may utilize the indications of ink color attributes to set the press 11, 
such as adjustments to ink input rate by using the keys. 
In the past, four process inks (cyan, magenta, yellow, and black) have been 
used on a printing press to produce copies with a gamut of colors. In 
these systems, the black ink has been used to generate not only the text 
but also the color image. In a control by image system, the print image of 
a production copy is compared with the printed image on a reference copy, 
termed a proof, and the press is adjusted based on the difference between 
the production image and the reference image. However, within the visible 
region, it is not possible to reliably distinguish the black ink from the 
process black made by the combination of cyan, magenta, and yellow inks, 
or whether the black ink or all cyan, magenta, and yellow inks should be 
adjusted. 
The four channel sensor 21 is utilized to sense not only attributes in 
three channels of the visible region, the fourth channel of the sensor 21 
senses an attribute in the infrared region in order to determine the 
correct amount of inks, including black ink, to correctly reproduce the 
proof. The printing press control system uses the four channel detector or 
sensor 21 to detect the energy reflected from a paper surface, such as the 
sheets 14 and 38, or the paper web of the press 11, with three channels 
being in the visible region and one channel being in the infrared region 
of the electromagnetic spectrum. The control system 10 has a device 23 for 
converting the output of the sensing device 21 to a set of variables which 
represent the amount of ink presented on the paper for any of the cyan, 
magenta, yellow, and black inks, and a device 25 responsive to the 
converting device 23 for adjusting the four-color printing press 11 to 
maintain the color consistency. 
In a preferred form, the bandwidth of the infrared channel may be between 
800 nm and 1100 nm, which is a portion of the near infrared region, and 
which is compatible with a regular silicon detector, although the working 
wavelength of the infrared channel may be longer than 1100 nm. At least 
three distinct channels are utilized in the visible region which may 
correspond to red, green, and blue (RGB), or cyan, magenta, and yellow 
(CMY), or other colors. The bandwidth of each channel in the visible 
region may be less than 70 nm, more than 100 nm, or any value in between, 
with channels having a multiple peak in its passing band, such as magenta, 
being also included. 
The sensor device 21 may be constructed from either a single element 
detector, a one-dimensional (linear) detector, a two-dimensional (area) 
detector, or other suitable detector structure, as will be seen below. The 
sensor device may be constructed by adding an additional infrared channel 
to existing devices, adding an infrared channel to a RGB color camera or a 
densitometer, or by extending the working band into the infrared region, 
e.g., adding infrared capability to a spectrophotometer. The light source 
18 and 20 used provides sufficient radiated energy in both the visible 
region and the infrared region, depending upon the sensor working band and 
sensitivity. 
All possible values which are output from the sensor device 21 may be used 
to form a vector space. For example, all possible values output from the 
sensor device 21 with red, green, blue and infrared channels form a four 
dimensional vector space R-G-B-IR, with the vector space being termed a 
sensor space S.sub.1, with each output from the sensor device 21 being 
termed a vector in the sensor space S.sub.1, with the minimum number of 
dimensions required by the sensor structure being 4. Thus, as shown in 
FIG. 9, a set S.sub.1 of elements e.sub.11 and e.sub.12 being given, with 
the elements e.sub.11 of the set S.sub.1 being the vectors v.sub.11 
corresponding to the output from the sensor device 21 of sensing a 
production or current printed copy, and with the elements e.sub.12 of the 
set S.sub.1 being the vectors v.sub.12 corresponding to the output from 
the sensor device 21 sensing a reference printed copy. In accordance with 
the present invention, the printed image on a production or current copy 
may be compared with the printed image on a reference copy in the sensor 
space, and if the difference between the live copy L.C..sub.s and the 
reference copy R.C..sub.s is within a predefined tolerance level delta, at 
least for all the channels in the visible region of the sensor space, such 
that, L.C..sub.s -R.C..sub.s !&lt;delta, the production or current copy is 
said to be acceptable by definition. 
A set of variables may be defined to represent the amount of ink presented 
in a given area. For example, a set of variables C, M, Y, and K can be 
defined to represent or be a function of the amount of cyan, magenta, 
yellow, and black ink in a given area. This set of variables may 
correspond to the ink volume, average ink film thickness, dot size, or 
other quantities related to the amount of ink in a given area on the paper 
surface. The vector space formed by this set of variables is termed an ink 
space S.sub.2, with the ink space S.sub.2 having a dimension of 4 for a 
four color printing press 11. Thus, with reference to FIG. 9, a set 
S.sub.2 of elements d.sub.11, and d.sub.12 are given, with the elements 
d.sub.11 of the set S.sub.2 being the vectors v.sub.j1 corresponding to 
the variables associated with the production or current copy in the ink 
space S.sub.2, and with the elements d.sub.12 of the set S.sub.2 being the 
vectors v.sub.j2 corresponding to the variables associated with the 
reference copy in the ink space S.sub.2. 
With reference to FIG. 9, there exists at least one transfer function or 
transformation phi which can map the elements d.sub.11 and d.sub.12 of the 
set S.sub.2 or the four dimensional ink space, into the elements e.sub.11 
and e.sub.12 of the set si or the four dimensional sensor space, with the 
transformation phi being termed a forward transfer function, as shown in 
FIGS. 9 and 10. It is noted that the subsets in each set S.sub.1 and 
S.sub.2 may overlap or may be the same. 
The forward transfer function may be used in a soft proof system which can 
generate a proof image which can be stored in the system as a reference or 
can be displayed on a CRT screen. 
With further reference to FIG. 9, there exists at least one transfer 
function or reverse transformation phi.sup.-1 which can map the elements 
e.sub.11 and e.sub.12 of the set S.sub.1 of the four dimensional sensor 
space into the elements of d.sub.11 and d.sub.12 of the set S.sub.2 of the 
four dimensional ink space, with the transfer function being termed a 
reverse transfer function. Thus, both the production image and the 
reference image in the sensor space or set S.sub.1 can be mapped into the 
ink space or set S.sub.2 by applying the reverse transfer function 
phi.sup.-1 point by point as shown in FIGS. 9 and 10. 
The difference between the production image and the reference image in the 
ink space S.sub.2 thus represents the difference of the ink distribution 
for each of the cyan, magenta, yellow, and black inks, as shown in FIG. 
11. The difference between the live and reference images in the ink space 
S.sub.2 indicates which printing unit should be adjusted, which direction, 
up or down, it should be adjusted, and the amount of ink which should be 
adjusted. A suitable press control formula may be developed to adjust 
press parameters, such as ink input rate in lithographic or letterpresses, 
ink consistency in flexographic or gravure presses, water input rate in 
lithographic presses, or temperature in any of the above, based on the 
differences between the production and the reference image in the ink 
space S.sub.2. 
The press adjustments can be achieved by the automatic control system 10, 
by press operator alone, or by the interaction between the automatic 
control system 10 and the press operator. Also, the sensor device 21 may 
be used to monitor the printing web of the press 11 directly, i.e., on 
press sensing, or to monitor the prints collected from the folder of the 
press, i.e., off press sensing. If the digital images from the color 
separation processing, or the film/plate images are available, the image 
of the reference copy in the sensor device 21 can be generated 
electronically by the forward transfer function phi. The electronically 
generated reference may be used to set up the press 11 in order to reduce 
the make ready time. 
The color reproduction quality can be maintained through the entire press 
run, through different press runs on different presses, or at different 
times. Thus, a closed loop automatic color reproduction control system may 
be formed without an additional color control target. The variation of 
ink, paper, and other press parameters can be compensated such that the 
printed copies have the highest possible overall results in matching the 
reference copy. 
As shown in FIG. 4, the camera or sensor 22 may be associated with a 
rotating filter member 52 having filters which only transmit the desired 
colors F.sub.1, F.sub.2, and F.sub.3, such as red, green, and blue during 
rotation, such that the camera or sensor 22 senses and records the colors 
F.sub.1, F.sub.2, and F.sub.3, sequentially or separately from the printed 
material which may be taken either from the current press run or from the 
reference press run. In addition, the filter member 52 may have an 
infrared (IR) filter F.sub.4 in order to sense and record the energy 
reflected form the printed material in the infrared region. The 
information received by the camera or sensor 22 from the filters may be 
recorded in the computer or CPU for use in forming the desired data to 
control the inks, as previously discussed. 
In another form as shown in FIG. 5, the camera or sensor 22 may comprise a 
charge coupled device (CCD) with built in filters which converts light 
energy reflected from the printed material into electric energy in a video 
camera, i.e. F.sub.1, F.sub.2, F.sub.3, and F.sub.4, (IR), such as the 
distinct colors red, green, and blue in the visible region, and the near 
infrared energy in the infrared region, in order to supply the information 
to the computer 30 for storage and processing, as previously discussed. 
Another embodiment of the camera or sensor 22 of the present invention is 
illustrated in FIG. 6, in which like reference numerals designate like 
parts. In this embodiment, the camera or sensor 22 has a beam splitter in 
order to separate the incoming light reflected from the printed material 
into an infrared beam for a first CCD 1, F.sub.1 such as red for a second 
CCD 2, F.sub.2 such as green for a third CCD 3, and F.sub.3 such as blue 
for a fourth CCD. In this embodiment, suitable prisms, lenses, or mirrors 
may be utilized to accomplish the beam splitting of light in order to 
obtain the desired color attributes in the various charge coupled devices 
to supply the information to the computer 30 for storage and processing in 
the computer 30, in a manner as previously described. Of course, any other 
suitable camera or sensing device may be utilized to obtain the desired 
colors. 
Thus, a control system 10 for a printing press 11 is provided which 
ascertains three distinct attributes, such as colors, in the visible 
region of electromagnetic waves and an attribute in the infrared region of 
the electromagnetic spectrum for the printed inks. The control system 10 
utilizes these four attributes in a four channel device to indicate and 
control the ink colors for use in the press 11. 
Thus, the colors may be sensed from a sheet taken during a current press 
run, and from a sheet taken during a reference press run, after which the 
sensed information is utilized in order to modify ink settings of a press 
11 in order to obtain repeatability of the same colors from the reference 
run to the current press run. In this manner, a consistent quality of 
colors may be maintained by the printing press 11 irrespective of the 
number of runs after the reference run has been made, and may be 
continuously used during a press run if desired. 
It has been found that when a four-channel camera is constructed by 
utilizing a single channel black/white camera (B/W) and a 3-channel color 
camera, the infrared image obtained from the B/W camera is misregistered 
with the red, green, and blue images obtained from the color camera. 
Geometric distortion may also be observed from both cameras. 
As previously discussed, a four channel camera is utilized having a 
black/white (B/W) camera and a color camera. At least one of the cameras 
is equipped with a zoom to adjust the image size. Also, the cameras are 
provided with at least one rotational adjustment plus two additional 
adjustments between the two cameras. The two adjustments can be 
translation or rotation. This can be accomplished by mounting one of the 
two cameras, for example the B/W camera, on an adjustment device such as a 
3-axis rotation stage. The two cameras are mounted along with the 
adjustment device in such manner so that both cameras point to the center 
of the imaging area. 
First, two targets are printed using an ink containing carbon black, which 
can be seen in both the B/W and color cameras. The first target is printed 
as a grid pattern, and the second is printed as an array of evenly spaced 
dots forming columns and rows. 
Second, the grid pattern is placed under the camera field of view. An image 
is displayed from the B/W camera and an image from one channel of the 
color camera together on a monitor as separate colors. For example, the 
red image might correspond to the B/W camera image, and a superimposed 
green image could be obtained from the red channel of the color camera. 
The zoom lens is adjusted along with the adjustment device so that these 
two images are aligned as close as possible on the monitor. 
Third, the dot pattern target is placed under the camera field of view and 
images are captured from the B/W and a single channel of the color camera. 
The device is used to find the actual X and Y positions for each dot in 
each of the two images. 
Fourth, the average X position is calculated for each column and then Y 
position of each row of dots. From these numbers the average spacing 
between columns and rows and the center point of the dot pattern is 
determined. The desired column and row spacing is calculated by one of the 
two methods: 
a) The desired column and row spacing equal the averaged column and row 
spacing so that there is no aspect ratio modification of the captured 
images. 
b) Either the desired column or row spacing equals the averaged column or 
row spacing found from the captured images. The other spacing is 
determined so as to maintain the aspect ratio of the original dot pattern 
object. 
The grid coordinates are calculated using the desired column and row 
spacing. The grid coordinates are adjusted so that the center point of the 
grid is at the center point of the dot pattern. These calculated 
coordinates are the desired dot positions. 
Fifth, for each of the two images, transfer functions are developed which 
map the actual dot positions in that image to the desired dot positions 
described in step 4. A transfer function is developed for each group of 
four dots forming a rectangular shape. An example of such a transfer 
functions is a bi-linear transfer function. 
Since the red, green, and blue images are already aligned inside the color 
camera, the transfer function developed for a single channel of the color 
camera is also applicable to the two remaining color images. 
Sixth, an image is captured under the camera setup described in step 2. A 
geometric transfer operation is performed for each of the four images 
based on the individual transfer functions developed for that image. 
Seventh, steps 4-6 introduce a way to translate the four images from the 
two cameras so that the geometric distortion can be corrected. The aspect 
ratio can also be corrected if the step of 4b is used. If the geometric 
distortion is tolerable in at least one camera image, the number of images 
to be translated can be reduced. This can be accomplished by using the 
camera without distortion as a reference and translating only the image or 
images from the other camera. For example, if the color camera is selected 
to be the reference, only the B/W camera would have to be translated. In 
this case, the actual dot positions obtained from the single channel from 
the color camera would be used as the desired dot positions to develop the 
transfer functions. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood, 
as modifications will be obvious to those skilled in the art.