U.S. patent application number 09/308433 was filed with the patent office on 2002-01-31 for exposure device for printing plates and method of controlling same.
Invention is credited to LUELLAU, FRIEDRICH, MAYER, CLAUS.
Application Number | 20020012110 09/308433 |
Document ID | / |
Family ID | 8166468 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012110 |
Kind Code |
A1 |
LUELLAU, FRIEDRICH ; et
al. |
January 31, 2002 |
EXPOSURE DEVICE FOR PRINTING PLATES AND METHOD OF CONTROLLING
SAME
Abstract
The invention relates to a method for triggering an exposure
device for the photomechanical production of structured surfaces as
a copy of an electronically memorized model, in particular for
exposing printing plates, in which the exposure device includes at
least one light source (3), one picture generating unit (8)
comprising movable, electronically individually triggerable micro
mirrors arranged in a grid, and one copying optical element (9). To
shorten the total exposure time without a loss of quality, it is
provided that the model is broken down electronically into
two-dimensional partial images; that the partial images are shown
in succession, but each partial image is shown with its entire area
at the same time, by means of the picture generating unit (8), and
successive partial exposures of the surface to be structured are
made, in that a exposure head (1), provided with the light source
(3), the picture generating unit (8) and the copying optical
element (9), moves between each two partial exposures from one
exposure position to the next and stops there for the exposure, so
that the individual partial copies are combined again into a total
copy of the electronically memorized model.
Inventors: |
LUELLAU, FRIEDRICH;
(BARDOWICK, DE) ; MAYER, CLAUS; (SCHARNEBECK,
DE) |
Correspondence
Address: |
STRIKER STRIKER & STENBY
103 EAST NECK ROAD
HUNTINGTON
NY
11743
|
Family ID: |
8166468 |
Appl. No.: |
09/308433 |
Filed: |
May 19, 1999 |
PCT Filed: |
December 31, 1996 |
PCT NO: |
PCT/EP96/05869 |
Current U.S.
Class: |
355/65 |
Current CPC
Class: |
G03F 7/2057
20130101 |
Class at
Publication: |
355/65 |
International
Class: |
G03C 005/00 |
Claims
1. A method for triggering an exposure device for the
photomechanical production of structured surfaces as a copy of an
electronically memorized model, in particular for exposing printing
plates, in which the exposure device includes at least one light
source (3), one picture generating unit (8) comprising movable,
electronically individually triggerable micro mirrors arranged in a
grid, and one copying optical element (9), characterized in that
the model is broken down electronically into two-dimensional
partial images; that the partial images are shown in succession,
but each partial image is shown with its entire area at the same
time, by means of the picture generating unit (8), and successive
partial exposures of the surface to be structured are made, in that
a exposure head (1), provided with the light source (3), the
picture generating unit (8) and the copying optical element (9),
moves between each two partial exposures from one exposure position
to the next and stops there for the exposure, so that the
individual partial copies are combined again into a total copy of
the electronically memorized model.
2. The method of claim 1, characterized in that by means of an
evaluation circuit or evaluation software, the image content of
each partial image is checked for whether picture information is
contained therein, and as a function of this checking, copying of
the partial image on the surface to be exposed takes place.
3. The method of claim 1 or 2, characterized in that each partial
image is assigned a first data set for its image content and the
second data set for its position, and that only data sets belonging
together with image content are used to generate a succession of
control data sets for transfer to a numerical controller.
4. The method of claim 3, characterized in that the quantity of the
second data sets is sorted in such a way that the lowest possible
sum of individual spacings between the positions of the partial
images results, and the data sets are transferred in this order to
the numerical controller.
5. The method of one of claims 1-4, characterized in that at least
one electric linear drive is used as the drive for generating a
motion of the picture generating unit.
6. The method of one of claims 1-5, characterized in that before
each copying of a partial image, a measurement of the spacing of
the picture generating unit (8) from the surface to be exposed is
made, and deviations from a previously input value are
automatically corrected.
7. The method of one of claims 1-6, characterized in that for
copying a light source (3) is used whose light flux is measured,
and deviations from a predetermined light flux are automatically
corrected.
8. The method of claim 7, characterized in that the correction is
done by varying the exposure time.
9. The method of claim 7, characterized in that the correction is
done by varying an electrical supply to the light source (3).
10. The method of one of the foregoing claims, characterized in
that the picture generating unit (8) is copied at a reduced
size.
11. The method of one of the foregoing claims, characterized in
that a positioning of the partial images is done with an accuracy
of better than 5 .mu.m, and in particular better than 2 .mu.m.
12. The method of one of the foregoing claims, characterized in
that by using a plurality of exposure heads (1), a plurality of
partial images are simultaneously copied.
13. The method of claim 12, characterized in that a relative motion
of a plurality of copies with respect to the support to be exposed
is effected synchronously.
14. The method of claim 12 or 13, characterized in that a spacing
between two simultaneously exposed copies is variable.
15. The method of one of the foregoing claims, characterized in
that the partial images are copied overlapping one another.
16. An exposure device for performing a method of one of the
foregoing claims, having a movably embodied exposure head (1),
which includes a light source (3), one picture generating unit (8)
comprising movable, electronically individually triggerable micro
mirrors arranged in a grid, and one copying optical element (9),
characterized in that a measuring and regulating device for
measuring and regulating the spacing of the picture generating unit
(8) or of the exposure head (1) from the surface (2) to be exposed
is provided.
17. The device of claim 16, characterized in that a sensor for
measuring the light flux and a regulating circuit for correcting
deviations in the light flux from a set-point value are
provided.
18. The device of claim 17, characterized in that an adjusting
device for varying an exposure time is provided in the regulating
circuit.
19. The device of claim 17 or 18, characterized in that an
adjusting device for varying the light flux is provided in the
regulating circuit.
20. The device of one of claims 16-19, characterized in that a
shutter or switching the light flux is provided.
21. The device of claim 20, characterized in that the picture
generating unit (8) is embodied such that it acts as an exposure
shutter.
22. The device of one of claims 16-21, characterized in that the
driver for generating the motion of the exposure head (1) is
embodied as a linear drive.
23. The device of one of claims 16-22, characterized in that a
plurality of exposure heads (1) are disposed on one axis.
24. The device of claim 23, characterized in that each exposure
head (1) is assigned its own linear drive, and for the other axis a
common linear drive for all the exposure heads is provided.
Description
[0001] The invention relates to a method for triggering an exposure
device for the photomechanical production of structured surfaces as
a copy of an electronically memorized model, in particular for
exposing printing plates, in which the exposure device includes at
least one light source, one picture generating unit comprising
movable, electronically individually triggerable micro mirrors
arranged in a grid, and one copying optical element.
[0002] The invention also relates to an exposure device suitable
for performing the method, having a movably embodied exposure head,
which includes a light source, one picture generating unit
comprising movable, electronically individually triggerable micro
mirrors arranged in a grid, and one copying optical element.
[0003] Even today, up to 90% of offset printing plates are exposed
via fill models with the aid of contact copy technology or in
isolated cases with projection systems. This means that before an
offset printing plate can be exposed to light, a film model is
made. This is done with film exposers and film developing machines
developed especially for this purpose.
[0004] The method step for preparing the film model
disadvantageously increases the time and expense involved in
putting images on offset printing plates.
[0005] From German Patent Disclosure DE 41 21 509 A1, a device
suitable for exposing printing plates is known, which has an
elongated light source in the form of a linear arc lamp, a
collimator lens, an elongated light modulator comprising
electronically triggerable movable micromirrors, and a copying
optical element, all these elements being disposed in stationary
fashion. The elongated linear form of the light source is used here
to attain a higher light yield than in point-type light sources.
Consequently, however, the stationary exposure arrangement can
expose only a very narrow strip of a printing plate. To expose the
complete printing plate, the plate is therefore displaced
continuously relative to the stationary exposure arrangement. To
assure that the copied data remains stationary relative to the
exposed material, the transmission of the data within the mirror
array must also be displaced in synchronism with the motion of the
printing plate.
[0006] The known apparatus has the disadvantage that because of the
continuous motion of the printing plate, it can expose only in
strips and can therefore utilize only a narrow region of the
micromirror array. This leads to excessively long total exposure
times. Also, because of the limited length of the micromirror
array, as a rule it is not possible to expose the entire length of
the printing plate simultaneously. Instead, the printing plate must
be exposed column by column and moved back and forth for the
purpose. This in turn, however, requires that the entire heavy
table on which the printing plate is spread out be displaced with
accuracy in the micrometer range. Because of the mass to be moved,
this cannot be done arbitrarily quickly with the requisite
precision. The result is a further lengthening of the total
exposure time.
[0007] From International Patent Disclosure WO 95/22787, an
apparatus for photomechanically making structured surfaces, in
particular for exposing printing plates, is known that has a
movable exposure head with a light source, a picture generating
unit, and a copying optical element. The movable exposure head is
compact in design and relatively light in weight. As a result, it
can be positioned with micrometer accuracy. The picture generating
unit comprises a liquid crystal screen that is disposed between two
polarizers. From this reference an exposure method is also known in
which the electronically memorized models are broken down into
partial images, and the partial images are shown in succession on
the liquid crystal screen and copied onto the printing plate in
such a way that they combine into a total copy of the model. To
that end, the exposure head is moved with extreme precision from
one exposure position to the next between each two partial
exposures. For exposing the entire printing plate, however, a great
number of partial images have to be exposed. This can result in a
very long exposure time.
[0008] In principle, the exposure time can be shortened by using
higher light intensities for the exposure. Given the necessarily
narrow design of the exposure head, however, higher light
intensities lead to a no longer tolerable heat burden, especially
since for the requisite polarization of the light entering the
liquid crystal screen, a polarization foil is used. Such
polarization filters admit the portion of the incident light that
has the "correct" direction of polarization, while the remainder
with the "wrong" direction of polarization is absorbed. A
considerable amount of heat is thus created in the polarization
filter at high light intensities and this heat must be dissipated
if destruction of the polarization filter is to be averted. In the
liquid crystal screen itself and in the second polarization foil
that follows as well, heat from absorbed light occurs.
[0009] Particularly in producing offset printing plates for
newspaper printing, the sequential exposure in known methods and
apparatuses leads to disadvantageous bottlenecks in terms of
time.
[0010] The object of the invention is to disclose a method and a
device of the type defined at the outset that produce exposure
results of satisfactory quality at reduced total exposure
times.
[0011] In a method of this generic type, this object is attained in
that the model is broken down electronically into two-dimensional
partial images; that the partial images are shown in succession,
but each partial image is shown with its entire area at the same
time, by means of the picture generating unit, and successive
partial exposures of the surface to be structured are made, in that
a exposure head, provided with the light source, the picture
generating unit and the copying optical element, moves between each
two partial exposures from one exposure position to the next and
stops there for the exposure, so that the individual partial copies
are combined again into a total copy of the electronically
memorized model.
[0012] The invention adopts the known method, in conjunction with a
precision-controlled movable exposure head, which is based on the
production of flat partial images using a liquid crystal screen, to
a stationary exposure arrangement that is based on linear picture
generation by means of micromirrors. For that purpose, in a first
step of the invention, the stationary exposure device is embodied
as a movable exposure head, and in a second step of the invention
the quasi-linear micromirror array is replaced by a genuinely
two-dimensional array, and in a third step of the invention the
electronic process control for the motion of the exposure head and
the exposure of the partial images is transferred. In this respect,
the teaching provided in the first paragraph of the background
section of German Patent Disclosure DE 41 21 509 A1 actually leads
one skilled in the art away from using two-dimensional light
modulators. If conversely one skilled in the art takes as his point
of departure the apparatus and the associated method known from WO
95/22787, then by simply adopting the stationary exposure
arrangement with micromirrors known from DE 41 21 509 A1, he
certainly does not arrive at the subject of the invention. Nor does
simply replacing the liquid crystal screen with the linear
micromirror array lead directly to the invention. That would
require a genuinely flat-area, two-dimensional micromirror array,
which furthermore must be triggered quite differently from the
linear array. Furthermore, the micromirror array cannot simply be
used to replace the liquid crystal screen, because it reflects the
light and does not transmit it. In addition, an absorber for the
unusable reflected light must be provided, while the polarizers are
no longer needed.
[0013] It can happen that not all the partial images can actually
contain picture information. According to the invention, only
partial images that have picture contents are therefore transmitted
to the surface to be exposed. Because of the reduced number of
partial images, the total exposure time is reduced approximately in
proportion to the total number of partial images without any
picture content. The exposure head arrives at each position of a
partial image, along a meandering path.
[0014] The total exposure time still required for the remainder of
the partial images can be reduced still further if each partial
image is assigned a first data set for its image content and the
second data set for its position, and that only data sets belonging
together with image content are used to generate a succession of
control data sets for transfer to a numerical controller. As a
result, the times required for changing position between two
exposure sites between the individual exposure times can also be
reduced.
[0015] The time for producing the offset printing plates can be
reduced by a further distance reduction in that the quantity of the
second data sets is sorted in such a way that the lowest possible
sum of individual spacings between the positions of the partial
images results, and the data sets are transferred in this order to
the numerical controller. In this way, the total image is produced
over the shortest possible travel distance.
[0016] High travel speeds of the exposure head are attained if at
least one electric linear drive is used as the drive for generating
a motion of the picture generating unit. The exposure times can
directly follow these thus-shortened positioning times without any
waiting time, since the effects of elasticity, play and friction
and natural vibrations are largely avoided by means of the electric
linear drive.
[0017] Since offset printing plates can have some waviness that is
sometimes greater than 2 mm, and these uneven features are also
unevenly distributed, an adequate depth resolution of the copy must
be attained by means of suitably small selected apertures. The low
light yield, however, lengthens the exposure times for the partial
image. To shorten these exposure times, it is provided that before
each copying of a partial image, a measurement of the spacing of
the picture generating unit or of the exposure head from the
surface to be exposed is made, and deviations from a previously
input value are automatically corrected. In this way, with a
surprisingly low depth resolution, even with wavy plates, a high
copy quality can be attained at short exposure times. The
correction can preferably be made by moving the entire exposure
head. In this way, even offset printing plates of various thickness
can be exposed. The focal length can also be corrected accordingly,
although this can lead to copying errors.
[0018] A uniform copy quality over the entire surface area of the
model is attained if for copying a light source is used whose light
flux is measured, and deviations from a predetermined light flux
are automatically corrected.
[0019] This correction cab be done either if the correction is done
by varying the exposure time or if the correction is done by
varying an electrical supply to the light source.
[0020] Even the most demanding print models in terms of quality can
be produced by this method if the picture generating unit is copied
at a reduced size. The reduction in size makes resolutions of 2540
dpi, for instance, possible. However, then the model must also be
broken up into a corresponding number of partial images that are
exposed in succession and then combined into the total model.
Because the number of individual exposures is thus increased, the
total exposure time rises considerably.
[0021] If a positioning of the partial images is done with an
accuracy of better than 5 .mu.m, and in particular better than 2
.mu.m, the individual partial images succeed one another without
gaps.
[0022] The total exposure time of a model can be reduced by
providing that by using a plurality of exposure heads, a plurality
of partial images are simultaneously copied.
[0023] Collisions of different exposure heads are avoided by
providing that a relative motion of a plurality of copies with
respect to the pattern carrier is effected synchronously. The
spacing of the partial images lined up with one another and copied
simultaneously advantageously remains constant then. Since the
likelihood that both exposure heads simultaneously lack any picture
content drops, and an exposure position can at the same time be
skipped, it is advantageous if within a predetermined limit region
the exposure heads can move independently of one another.
[0024] Because a spacing between two simultaneously exposed copies
is variable, models with different resolutions can be exposed using
the same exposure device. The spacing between two simultaneous
copies amounts to an integral multiple of the individual picture
dimensions.
[0025] Greater uniformity of the transitions is attained if the
partial images are copied overlapping one another. In the region of
the overlaps from the edge of the pictures to the interior, the
density is adapted to rise linearly, so that by the superposition,
a distribution corresponding to the original image is obtained.
[0026] An exposure device that is especially suitable for
performing the method of the invention is distinguished in that a
measuring and regulating device for measuring and regulating the
spacing of the picture generating unit or of the exposure head from
the surface to be exposed is provided. Before each exposure, by
means of the measuring device, the spacing from the surface to be
exposed is ascertained. By way of example, known laser measuring
devices are suitable as the measuring device. The spacing is
determined by interferometric length measurement. However, the
space measurement can also be done by known acoustical or optical
measuring methods. By comparisons with a predetermined set-point
value, a controlling variable is determined that then readjusts the
spacing via a suitable final control element. By way of example,
this can also be done by adjusting the focal length or by
motor-driven adjustment of the height of the exposure head or parts
thereof. Because of the regulation provided according to the
invention, light sources can be used that sweep a wide angle in
space, so that because of the high light yield, short exposure
times are possible.
[0027] Especially high speeds of motion and accelerations are
attained by providing that the printing plate together with a
required heavy plate to which it is fastened is not moved and
instead only the exposure head is moved. In this way, more than ten
partial images per second can be positioned and exposed.
[0028] The entire model is copied with constant quality over the
entire surface if a sensor for measuring the light flux and a
regulating circuit for correcting deviations in the light flux from
a set-point value are provided. As a result, even slow changes in
the light yield cannot have any influence on the outcome of the
work.
[0029] For example, changes that make themselves felt over a
relatively long period of time can be corrected by providing that
an adjusting device for varying an exposure time is provided in the
regulating circuit. The sensor for measuring the light flux is
preferably disposed in the vicinity of the surface to be exposed,
so that the incident light flux is detected once and for all before
the exposure of the partial images begins.
[0030] Transient changes, caused for instance by fluctuations in
the voltage from the power grid, can be compensated for if
[0031] an adjusting device for varying the light flux is provided
in the regulating circuit. Here the light sensor is disposed
preferably in the vicinity of the light source, or the supply
voltage is measured directly. By means of known circuits, the
supply voltage can thus be kept constant, for instance.
[0032] It is possible to avoid turning the lamp on and off
frequently if a shutter or switching the light flux is provided.
This lengthens the service life of the lamp.
[0033] If the picture generating unit itself is embodied such that
it acts as an exposure shutter, then it is advantageously possible
to dispense with a separately embodied exposure shutter.
[0034] An especially advantageous dynamic behavior of the overall
device is obtained if the driver for generating the motion of the
exposure head is embodied as a linear drive. Such drives have high
positioning precision of better than 2 .mu.m.
[0035] The entire model can be finished in an even shorter time is
a plurality of exposure heads are disposed on one axis.
[0036] An especially favorable construction is obtained if each
exposure head is assigned its own linear drive, and for the other
axis a common linear drive for all the exposure heads is provided.
In return, the locking speed in one axis is somewhat less than that
of the two heads in the other direction. This affects the total
exposure time only slightly. Because each exposure head has its own
linear drive in the other direction, it is possible to switch
briefly over to different resolutions, by moving the exposure heads
in such a way as to create a larger or smaller interstice.
[0037] The invention will be described in terms of a preferred
embodiment in conjunction with the drawing, from which other
advantageous details can be learned.
[0038] The sole drawing FIGURE is a section through an exposure
head of an exposure device according to the invention shown
schematically.
[0039] In the drawing, an exposure head 1 can be seen, which is
disposed on a support 2 so that it can be moved by motor by means
of some device, not shown. The exposure head 1 has a light source
3, which comprises a metal halide lamp 4, a reflector 5 and a
collimator 6. The collimator 6 includes an optical lens element,
not shown, which aligns the light, intended for the exposure,
substantially parallel. The exposure head 1 also includes a picture
generating unit 8 and a copying optical element 9, by means of both
of which the picture model generated in the picture generating unit
8 can be projected onto a printing plate, not shown, on a reduced
scale on the support 2 and copied.
[0040] For easier manipulability, the exposure head 1 is disposed
substantially horizontally above the support 2. The light used for
the exposure originally has a beam path with a horizontal optical
axis 10. For copying on the support 2, an optical axis 11 of the
copying optical element 9 is vertically oriented. For the
deflection of the beam out of the horizontal to the vertical, a
reflective surface 12 of the picture generating unit 8 is provided.
Adequate heat dissipation is provided by a fan 20 mounted laterally
on the housing of the exposure head 1.
[0041] A UV-permeable filter or a UV-permeable optical element
could advantageously be disposed in the beam path downstream of the
metal halide lamp 4; it extracts a substantial portion of the light
intensity, in the non-usable spectral range, from the beam path and
thus reduces the heat burden on the downstream optical
elements.
[0042] The picture generating unit 8 comprises a microchip, which
electronically individually triggerable movable micromirrors
disposed in a grid. The mirrors may be oriented by electronic
triggering either in such a way that they deflect the beam path of
the incident light out of the horizontal to the vertical and thus
into the copying optical element 9, or such that the reflected
light is deflected just past the side of the copying optical
element 9. By individual alignment of the micromirrors in certain
area regions, a picture model can be created which can be projected
by the copying optical element 9 onto the printing plate. Nearly
all the light not used for the copying is deflected out of the beam
path in the process.
[0043] A model to be copied onto the printing plate or the support
2 is stored digitally in memory in a computer not shown in the
drawing. By means of suitable programs, the model is broken down
into partial images and carried in a known manner, via a control
line, on to the micromirror array that here acts as the picture
generating unit 8.
[0044] For the exposure, the light source 4 is briefly supplied
with sufficient current for the requisite light quantity.
[0045] This mode of operation, using suitable flashbulbs, has the
advantage of less power loss compared with continuous
operation.
[0046] Alternatively, however, a shutter can also be provided that
with a suitable control unit determines the exposure time.
[0047] After the exposure of one partial image, the exposure head 1
is displaced relative to the support 2 by the grid amount
corresponding to the portion of the partial image. The exposure of
the next partial image is done in such a way that the partial
images follow one another without gaps and add up in terms of their
picture contents to form the total image.
[0048] In this way, it is possible for models produced with the aid
of modern data processing systems to be copied directly onto a
print medium. The costs for producing a film and the attendant time
consumed are dispensed with.
[0049] The metal halide lamp 4 used as a light source 3 must be
operated for a certain length of time until it reaches its full
power. The light flux originating at the light source 3 can
therefore be measured by a light sensor and transmitted via a
signal line to an electronic triggering means. Depending on the
measured intensity of the light flux, the exposure time is
specified, and a shutter is opened for the duration of the exposure
time. After a first partial image has been exposed onto the
printing plate, the exposure head 1 is displaced by one image
division, and another partial image is then exposed.
[0050] From a spacing measurement, embodied for instance as a laser
distance measuring device, the electronic triggering means learns
the current spacing from the printing plate. In the event of
deviations from a previously established set-point value, a
corresponding control command for the function group for spacing
regulation or focal length regulation is then generated, so that
the spacing or focal length is then corrected accordingly to
achieve the greatest possible sharpness of the copy.
[0051] Accordingly, the result of an exposure measurement is
compared with a preestablished set-point value, and a suitable
correction signal to a shutter regulator or light source regulator
is generated. The instantaneous position of the exposure head can
be transmitted by a measurement head.
LIST OF REFERENCE NUMERALS
[0052] 1 Exposure head
[0053] 2 Support
[0054] 3 Light source
[0055] 4 Metal halide lamp
[0056] 5 Reflector
[0057] 6 Collimator
[0058] 7
[0059] 8 Picture generating unit
[0060] 9 Copying optical element
[0061] 10 Optical axis
[0062] 11 Optical axis
[0063] 12 Surface
[0064] 13
[0065] 14
[0066] 15
[0067] 16
[0068] 17
[0069] 18
[0070] 19
[0071] 20 Fan
* * * * *