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PROCESS AND APPARATUS FOR THE MANUF.A(.'TLTRE OF A
PHOTOGRAPHIC PRIl~T
FIELD OF THE INVENTION
The invention relates to a process and appalrat:us for the manufacture of a
photographic print, especially a copy of an original present in electronic
form, by
strip-wise exposure of the image information ohthe original onto a
photographic copy
material.
BACKGROUND ART
Digital print producing apparatus on photographic basis, so called digital
photographic printers, produce prints or copies by exposure of the image
information
of the underlying original stored in electronic form onto a photosensitive
copy
material. One possibility herefore consists in that the image information of
the
original is image;-wise optically reproduced by way of a suitable electro-
optical
converter operating pixel-by-pixel to produce an optical representation of the
original,
and to project this optical representation of the original onto the copy
material to
thereby expose it thereon. Suitable electro-optical converters can thereby be
acti~.~e
(self illuminating) as well as passive (modulating) electro-optical
arrangements.
Typical examples are cathode ray tubes, liquid crystal cell arrays operated in
transmission or reflection, light-emitting diode arrays, electro-luminescence
cell
arrays and lately also so-called digital micro mirror arrays.
One of the deciding factors for the quality of photographic prints or copies
produced in this way is the resolution (pixel number) of the electro-optical
converter
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used. While sufficiently highly resolving small converters are available at
economically acceptable prices, the development of economically producible and
commercially useful large surface converters of corresponding resolution is
still in its
beginnings. Thus, only relatively small forniat prints can be produced with
satisfactory quality with the available converters.
Theoretically, photographic prints of any sizf:. can be produced by line by
line
or strip wise exposure in direction transverse to the lines or strips. Only
strip-shaped
portions of the original which in their longitudinal direction cover the whole
original
are hereby successively optically reproduced by way of the converter and
successively exposed in a corresponding spatial relationship onto the copy
material.
The correct spatial positioning of the exposed strips is thereby achieved by
displacement of the copy material relative to the exposure light path. The
latter can be
achieved by an advance of the copy material or by a correspondingly moveable
imaging optic. The relative displacement is of course carried out
synchronously with
the change of the exposed strips of the onginal. If the strips are several
lines wide,
neighboring strips can also overlap. Since; the copy material is thereby
multiply
exposed depending on the degree of overlap, this must be correspondingly
considered
for the adjustment of the amount o.f copy light in the individual exposure
steps. This
exposure process is generally known under the term TIG (tine integration gray
scale).
Comparatively cheap linear converters can be used for this process of the line
or strip wise exposure. Rectangular converter element arrangements (arrays)
are
understood hereunder, the width of whioh is significantly smaller than the
length. In
the extreme case, such a linear converter has only a sin<~le row (line) of
converter
elements, typically however up to several hundred rows. Ln longitudinal
direction (per
row or line] such a linear converter in contrast typically includes 1,000 or
more
CA 02416178 2003-O1-13
converter elements. Of course, wider converters (with a higher number of rows)
can
also be used, whereby not all rows (which means not the whole width) need be
used.
With the above described methods, qualitatively satisfactory photographic
prints can only be manufactured with commercially available converters when
the
format of the print to be produced in longitudinal direction of the exposed
strips is
relatively small. However, for larger image formats in the other dimension,
the
resolution of the commercially available and economically acceptable
converters is in
many cases insufficient.
EP-A-0 9~6 243 discloses an improved process in which the resolution of the
produced photographic print is achieved by a lengthwise division of the
individual
strip-shaped portions of the original into two or more sections and by a
con~esponding
piece-wise exposure. Since the Full length of the converter is hareby
respectively only
used for the reproduction of a section, a doubling or multiplying of the
resolution
results - in longitudinal direction of the ships - according to the number of
sections, so
that very large format images of satisfactory quality can be produced even
with
conventional converters.
SU11~IMARY OF THE INVENTION
It is an object of the invention to now provide an alternative approach for
improving a process and an apparatus in such a way that larger format images
can be
produced with satisfactory quality in an economical mariner without large
additional
technical cost. In particular, this object is preferably achieved using
commercially
available and economically acceptable converters, which means no special,
higher
resolution or larger, and correspondingly more expensive, converters should be
required.
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This object of the invention is achieved in an apparatus for the production of
a
photographic print from an original present in electronic form by strip-wise
exposure
of the image information of the original onto a photo;~rraphic copy material,
including
a memory for receiving the image information of the original, an electz-o-
optical
converter operating pixel-by-pixel and organized into orthogonal rows and
colurrms
and an associated control for generating an optical representation of strip-
shaped
portions of the original stored in the memory, with a projection optic for
projecting
the optical representations of the strip-shaped portions in a strip-shaped
exposure
region onto the copy material, and with drive-means for moving the strip-
shaped
exposure region and the copy material with essentially constant speed in a
direction
transverse to the longitudinal extent of the strip-shaped exposure region and
relative
to one another.
According to the invention, this object is achieved by rotating the con~~erter
in
the exposure light path by 45E so that the strip-shaped optical image
representation of
the strip-shaped portions of the original extends along tine diagonal (of the
usable
region) of the optical converter. By way of this rotated orientation of the
optical
converter in accordance with the invention, a by a factor of 2'i= (~l .4)
higher
resolution results in both dimensions compared to the classical arrangement
and
thereby the possibility to achieve larger print foumais with conventional,
commercially available converters without a loss in quality.
BRIEF DESCRIPTION OF 'THE DR~~'I:NGS
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The invention will now be further described by way of example only and with
reference to the attached drawings, wherein
Figures 1-3 respectively illustrate a schematical illustration ofthe generally
known principle of the strip-shaped exposure also used in the process in
accordance
with the invention;
Figures 4 and ~ shorn two schematic illustrations of the process in accordance
with the invention; and
Figure 6 is a schematical illustratiotr of the apparatus in accordance with
the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following is based on the asswnption that the original of which a physical
photographical print or copy is to be made is present in electronically stored
format.
The image information composed of the totality of ali brightness and color
information for each individual image point of the original to be copied is
thereby
stored in a memory 1 (see Figure 6j from which it can be recal led pixel-by-
pixel and
possibly separated according to color proportions by way of a control 2 (see
Figure 6).
,analogously, the image information of a strip shaped portion of the origilzal
is
understood to represent the totality of the brightness and color information
of'those
image points of the original which belong to the corresponding strip-shaped
portion of
the original.
Figures I-3 illustrate the principal of the line-by-line or strip-wise
exposure
known, for example, from EP-A-O 986 ~43 and described in great detail therein.
The
original represented by the totality of all stored brightness and color
information for
each individual one of its pixels is referred to by ~'. The control apparent
from Figure
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6 reads out the image information of a first strip-shaped portion A of the
original V
and controls thereby a pixel by pixel operating electro-optical converter ~,
which
produces on the basis of the signals received an optical image representation
D of the
strip-shaped portion A. The elec.tro-optical converter 3 can, for example. be
formed
by a light-emitting diode array with, for example, 1230 x X00 individual
diodes or
prel:erably, as further described below, by a digital micro-mirror array with
a
corresponding or larger number of individual mirror<.~. The also strip-shaped
optical
representation D of the strip-shaped portion A of the original ~' produced by
the
electro-optical converter 3 is now projected by way of a here not illustrated
projection
optic in a (in this example stationary) strip-shaped exposure region E onto a
photographic photographic copy material P and ther;.by exposed thereonto
(Figure 1 ).
Subsequently, the next strip-shaped portion AN is read out, an optical
representation
DN is produced therefrom and the latter exposed onto the copy mater ial P (Fi
lure ? )
which was simultaneously advanced by a corresponding distance relative to the
exposure region E. This whole process is now repeated until the whole original
has
peen captured and the last strip shaped porrtion ANN of the original was read
out, an
optical representation DNN was produced therefrom and the latter was exposed
onto
the copy material P (Figure 3).
As is apparent, the strip-shaped portions A of the original do not lie
seamlessly
side-by-side, but overlap one another to a large de~-ee (transverse to their
.longitudinal
direction). This results in that the strips exposed onto the copy material P
also overlap,
so that the copy material P is multiply exposed depending on the degree of
overlap.
This multiple exposure is taken into consideration in that the brightness
values of the
individual pixels of the optical representations D of the portions A are
correspondingly reduced by the control (possibly c:.olor selective) so that
the sum of
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the copy light amounts impacting the copy material at the respective image
points are
again correct. This exposure method is known under the term TIG (time
integration
gray scale).
As already mentioned, it is necessary for the :grip-wise exposure that a
relative
movement occurs between the strip-shaped exposure region E and the copy
material
P. This is achieved in the above example by a cortesponding advance of the
copy
material P. Alternatively, and especially practically, the relative movement
can also
be achieved by a moveable construction of the projection optic, whereby the
copy
material remains stationary during the whole exposure. The principal of such a
moveable exposure optic is apparent from Figure 6 and dese.ribed in detail in
the
already mentioned EP-A-0 986 243.
The electro-optical converter 3 caa~ principally be any active or passive type
converter operating pixel-by-pixel. Examples are, as already mentioned,
cathode ray
tubes, light emitting diode arrays, electro-luminescence arrays or liduid
crystal arrays.
However, especially advantageous are so-called digital micro-mirror ~Lrrays
(DMD =
digital mirror device), as are used, for example, also in large image
projection
apparatus. A typical digital micro-mirror array suitable for the purpose of
the
invention includes on one chip an array of 1280 ~ 1024 mirrors, which can be
selectively flipped back and forth by electneal control between two defined
tilt
positions.
Such micro-mirror arrays are natw-ally operated in reflection, which means
they are passive. In practical use, as also apparent from Figure (> algid
illustrated and
described in detail in the already mentioned EP-.A-0 986 243, they are
positioned in
such a way in front of the pupil of a projection Tans that the micro-mirrors
direct the
light impinging thereon in the first tilt position directly into the
projection lens and in
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the other tilt position past the pupil of the projection lens. The intensity
modulation of
the reflected light is thereby carried out on the basis of the stored image
information
of the original by intermittent operation of the micro-mirrors with a
corresponding
duty cycle. Construction, control technologies and possible uses of such micro-
mirror
an-ays are described in detail in the relevant publications of the
manufacturer
companies, for example the company Texas Instruments, Houston, Texas, U.S_A.,
and
are not the subject of the present invention.
Up to this point, the process in accordance with the invezotion corresponds to
the prior art and therefore does not require any further explanation.
As is apparent, the length of the strip-shaped portions .A which cover the
whole width of the original comespomds in the conventional process to the
length (of
the useful surface) of the electro-optical converter 3. The resolution in
longitudinal
direction of the strip is therefore deterniined by the number of the
individual pixels of
the converter 3. However, the resolution achievable at larger enlargement
scales
(large format pictures) is in many cases qualitatively insufficient. This is
where the
invention starts.
According to the most basic object of t:he invention, the converter 3, as
illustrated in Figure 4, is positioned in the projection 1ig11t path in a 4~E
rotated
orientation so that the strip-shaped optical image representation D of the
strip-shaped
portions A of the original V are orientated in the. diagonal (of the usable
region) of the
optical converter 3. In other words, not a rectanguhu- pixel region of the
converter 3
defined by a certain number of~who(e lines is used for the image conversion,
but a
pixel region which essentially W eludes all available rows, but thereby uses
only a pat-t
of the available pixels from each roc~-~, so That the pixels used for the
representation
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together cover a strip-shaped region ~~hich lies in the diagonal of the
converter. This
is illustrated in Figure ~.
The rotated position in accordance with the invention of the optical converter
3, results, compared to the classical positioning, in a resolution, which in
both
dimensions is by about a factor of 2'r'_> (~-1.41 higher. For example, for a
commercially
available converter with 1024 rows of 1280 pixels each, a reproduction region
D
results ~rith 192 rows of 1919 pixel each. When tl~e projection region is of
lesser
height (which means with less rows), the number of usable pixels per row is
even
higher.
As is illustrated in Figure ~, the imaging region D of the converter 3
strictly
speaking is not rectangular for the image wise reproduction of the strip-
shaped
portions A of the original V, but has the shape of an elongated parallelogram
~lhicll is
composed of individual rectangles. However, for the generally common use of
the
TIG exposure method (time integration gray scale) this is not important.
With the rotated orientation of the optical converter 3 in accordance with the
invention, the transport direction (direction of the relative movement between
the
strip-shaped exposure region E and the copy material P) is of course also
rotated by
45E relative to the rows of the optical converter 3. The parallelogram shape
of the
pixel region used for the image reproduction, the location of the pixel used
on the
usable surface of the converter and the transport direction rotated by 45E
require a
somewhat more involved control of the individual pixels of the cormerier, but
is
readily understood and realizable by ~he person skilled in the art.
A principal schematic of the apparatus in accordance with the invention is
shown in Figure 6. The apparatus includes the already mentioned memory 1 and
the
already mentioned control 2 as well as an illumination source 4, the already
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mentioned converter 3 in the form of a micro-mirror array, an imaging optic
consisting of a Lens 5 and 3 redirecting mirrors 6, ',' and 8. and drive means
symbolized by arrows 9a to 9c for the redirecting mirrors and the lens. The
two
redirecting mirrors 6 and ? are stationary relative io one another and are at
a right
angle to one another so that they redirect the beam by 180E. The redirecting
mirror 8
is positioned parallel to the redirecting mirror ? and redirects the beam by
90Eonto the
copy material P. The redirecting minor 8 moves in the same direction as the
two
redirecting mirrors 6 and 7 but with twice the speed to that the optical
distance
between the Iens and the copy material P remains constant independent of the
position
of the redirecting mirrors. The strip-shaped exposure region E is moved over
the
(stationary) copy material P by the movement of the redirecting mirrors in the
manner
described. By adjustment of the lens 5 in combination with a corresponding
adjustment of the redirecting mirrors 6 to e, the reproduction scale can be
changed.
Vvith the exception of the positioning of the micro-mirror array 3 rotated by
4~E, the
apparatus completely corresponds to the apparatus described, for example, in
the
already mentioned EP-A-0 986 243, and therefore does not require any further
explanation. Of course, the control is correspondingly adapted.
By way of the higher resolution achieved in accordance with the invention,
large format pictures of satisfactory quality can be produced 4~~ithout large
additional
technical cost and in an economical manner. This is possible especially with
the use
of commercially available and economic<dly acceptable converters, which means
no
special, higher resolution or larger and correspondingly more expensive
converters
are required.
