Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF CHANGING THE DENS ITY OF IMAGE ON S IMPLE
COLOR P~OOF AND A ~SK USED THEREFOR
. . DETAILED DESC~IPTION OF THE INVENTION
Field of the Invention
The present invention relates to a method of
changing the density of an image on a simple color proof
used in color printing, and to a mask for use in that
method.
Backqround Art
Simple color proofs can be prepared by various
methods, of which the overlaying process and the
surprinting process are by far the most common. Simple
color proofs are commercialized under such trade names as
Color Key (3M~, Chromalin tdU Pont), Transfer Key (3M) and
Color Art (Fuji Photo Film).
One of the greatest problems with the
conventional methods of making simple color proofs is the
difficulty in changing the densities of color proofs at
will. This difficulty arises from the fact that the
concentrations of colorants (e.g. dyes and pigments)
incorporated in color proofs are predetermined. For
instance, the maximum density (solid density) of image
areas in a color proof are uniquely determined by the
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density of the colored film or colorant used. In order to
change the density of the color proof, the density of the
colored film or colorant used must be changed but this is
combersome in actual practice. Therefore, it has long
been desired to develop a method capable of easy change in
the density of a color proof prepared by the simplified
color proofing process.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention
is to solve the above described problem.
In particular, an object of the present
invention is to provide a simple method of changing the
density of an image on a color proof.
It has been found that the density of an image
on a color proof can be readily changed by using a special
mask in the image exposure step.
In one aspect of the present invention, there is
provided a method of obtaining a color proof, comprising
the steps of:
(A) exposing a plurality of light-sensitive layers which
are provided on respective ~ases, to respective color-separated
images of a color image, wherein said light-sensitive layers
comprise:
(i) a photolytic polymer selected from the group consisting
of photocrosslinkable polymer as a light-sensitive material,
(ii) a colorant, and
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(ii;~ a binder, and wherein each of gaid light-sensltive
layers contains a different colorant which produces a color
selected from the group consisting of yellow, magenta, cyan and
black;
(B) developing said light-sensitive layers; and
(C) transferring said developed light-sensitiYe layers
successively in registration onto an image-receiving material
comprising an image-receivinq layer provided on a base; and
(D) transferring the resulting image-receiving layers of
step (C) to a printing paper to obtain a color proof; wherein
said exposing includes exposing at least one of said
liqht-sensitive layers through a randomly dotted half-tone mask
having dots of a first transmissivity on a background of a
second transmissivity, wherein said dots are randomly at a
density on the randomly dotted half-tone mask in the range of
lO - lO 4 per cm2 and have diameter~ in the range of 5 - 300
micrometers and wherein said color s~parated images are
negative color-separsted images and said first transmissivity
of said dots is less than said second transmissivity of said
background and wherein said exposing step includes exposing
said light-sensitive layers to said respective color-separated
images through said randomly dotted half-tone dot mask.
The term "halftone image~ as used in this
specification shall be construed as applying to both
color-separated halftone photography and alphanumeric
images.
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BRIEF DESCRIPTION OF THE DRAWI~GS
Figs. l(A) and (B) show two embodiments in which
a color separation negative film is used in the exposure
step performed in accordance with the present invention;
and
Figs. 2(A) and (B) show one embodiments in which
a color separation positive film is used in the exposure
step performed in accordance with the present invention. -
DETAILED DESCRIPTION OF THE PREFERR$D EMBODIMENTS
The present invention is hereunder describedspecifically with reference to the case where it is
applied to the making of a simple color proof from a
light-sensitive material containing a colorant. The
process of preparing the proof includes the step of
transferring the image to an image-receiving material. It
should however be understood that the method of the
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present invention may be extensively used for the purpose
of changing the density of a color proof which is being
prepared by the simplified color proofing process
including the step of exposing a color-separated halftone
image to a light-sensitive material from which the color
proof is to be made~
(1) Exposure step
Four color-separated films are made from the
original. They have yellow (Y), magenta (M), cyan (C) and
black (B or K) colors, respectively. The films are then
contact-exposed to light-sensitive materials constituting
a color-proof which are dyed to yellow, magenta, cyan and
black, respectively.
Each of the color-proof constituent light-
sensitive materials has a light-sensitive layer dyed to
one of the colors, yellow, magenta, cyan and black, which
is formed on a plastic film base. The base is typically
made of PET (polyethylene terephthalate). A release
layers is inserted between light-sensitive layer and the
base. The colored light-sensitive layer contains not only
a light-sensitive material (e.g. photopolymer) and a
colorant (e.g. dye or pigment) but also other necessary
components such as a binder. A resin that will harden
upon illumination by light is used as the light-sensitive
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material in a negative-acting color-proof constituent
light-senstive material which is to be exposed from a
color-separated film having a negative halftone image. On
the other hand, a resin which will become soluble under
illumination by light is preferably used in a positive-
acting color-proof constituent light-sensitive material
which is to be exposed through a color-separated film
having a positive halftone image).
(2) Development step
The four exposed light-sensitive materials are
then developed (for example, with an alkali solution or an
organic solvent) to provide positive relief images which
are colored yellow, magenta, cyan and black, respectively.
(3) Transfer step
The so formed yellow, magenta, cyan and black
colored images are successively transferred onto an image-
receiving material which is formed of an image-receiving
layer on a plastic film base (temporary support),
typically made of PET. Transfer may be effected by
heating under pressure with an appropriate transfer
apparatus such as a laminator. As a result of transfer, a
full-colored image is formed on the receiving layer.
In the next phase, the image-receiving material
is superimposed on printing paper such as wood-free paper
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with the image-receiving layer facing down, and the
temporary support is separated from the image-receiving
layer to leave the image-receiving layer on the printing
paper. The printing paper may be given any necessary
post-treatments to provide a finished color proof.
Details of these procedures and the materials
used therefor may be found in Japanese Unexamined
Published Application No. 97140/1984.
In the method described above, the color image
formed on the image-receiving material is transferred to
printing paper as a result of transfer of the image-
receiving layer itself. This means that the color density
of the transferred image is substantially fixed and color
proofs of essentially the same tone will be obtained
irrespective of whether the printing paper is art paper,
wood-free paper, or woody (ground wood) paper.
However, in the actual case of printing, the
color density obtained will depend on the type of printing
paper even if the same type of ink and printing conditions
are employed. Stated more specifically, color printing on
art paper will produce a different color density than when
printing is done on wood-free paper or woody paper under
the same conditions. In the latter case, lower yellow,
magneta, cyan and black color densities result because of
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such factors as the unevenness of the paper surface and
ink penetration. For instance, if color printing on art
paper produces color densities which are approximately
Y = 0.94, M = 1.30, C = 1.54 and B = 1.79, printing on
wood-free paper which is performed under the same
conditions will provide lowered densities which are
approximately Y = 0.90, M = 1.23, C = 1.43 and B = 1.64.
Therefore, in the making of color proofs, the
final color density is required to have different values
as between the case where art paper is used as the
printing paper and the case where wood-free paper or woody
paper is used. However, as already mentioned, the color
proofs prepared by the prior art techniques are by no
means appropriate since they have substantially the same
color densities.
The simplified color proofing process
presupposes the use of printing paper of the class of art
paper as a standard. So, in order to avoid the
aforementioned problem, the ma~ing of a color proof with
wood-free paper or woody paper being used as the printing
paper must be so performed that the density of the ~inal
color image is lowered to a level which is suitable for
actual printing on wood-free paper or woody paper.
This requirement can be readily met by using the
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special mask of the present invention in the exposure step
I1) in one of the processes which are described below with
reference to the accompanying drawings.
Figs. l(A) and (B) show embodiments in which
exposure is performed with a color-separated negative
film. As shown in Fig. l(A), a printer glass plate 1
carries thereon a yellow, magentar cyan or black
separation negative film 2, a negative mask 3 (dark dots
on a transparent background) of the present invention, and
a light-sensitive material 4 for making a color proof.
Exposure is made through the printer glass plate 1 under
illumination by a light source 5. This operation is
repeated for each of the required color separation
negative films using the light-sensitive materials for
making the intended color proofs.
Each of the color separation negative films has
a halftone image corresponding to the separated color. If
exposure to these negatives is made using the mask of the
present invention which has preselected size and number of
light-shielding (non-transmitting) dots in accordance with
the type of printing paper, a color proof having a desired
color density can be finally obtained.
For instance, if a negative mask prepared by the
method which will be specifically described below is used
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during exposure, a satisfactory color proof is obtained
whose tone reproduction is substantially the same as that
of a print on wood-free paper.
Therefore, in the present invention, proper
adjustment (change) of the density of a color proof can be
made in accordance with the type of printing paper
selected and the color proof obtained has a tone which is
faithful to the color image actually formed on that
printing paper.
Fig. l(B) shows another embodiment of using a
color separation negative film in the exposure step
perfor~ed in accordance with the present invention. This
embodiment differs from the one shown in Fig. l(A) in that
the negative film 2 and the mask 3 of the present
invention are reversed in position.
Fig. 2 shows an embodiment in which a color
separation positive film, rather than a negative film, is
used in the exposure step performed in accordance with the
present invention. As shown in Fig. 2(A), a printer glass
plate 1 carries thereon a positive mask 6 (transparent
dots on a dark background) o~ the present invention and a
positive-acting light-sensitive material 7 for making a
color proof, and exposure is made through the printer
glass plate by illumination under a light source 5. In
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the next phase, the mask-exposed light-sensitive material
7 and a color separation positiv~ film 8 are stacked on
another printer glass plate 1 as shown in Fig. 2(B) and
illuminated under a light source 5. In this embodiment,
two exposures are made but the order of steps (A) and (B)
is not critical. By following the procedures described
above, a color proof can be made from a color separation
positive film, with the color density of the proof beiny
properly adjusted (changed) by using the positive mask of
the present invention.
The special masks of the present invention have
the characteristics described above and are preferably in
the form of a plastic or glass sheet. Any o the known
methods may be employed to prepare the special mask of the
present invention. For example, a random array of dots
are formed on a light-sensitive film with a dot generator,
a grained cont;act screen is printed onto a light-sensitive
film. Alternatively, more than 300 line rulings are
formed.
One method of forming a negative-acting mask is
described below. A random array of dots with a covering
of 90~ dot area are generated with a commercial scanner.
The dots are printed onto a commercial scanner film, which
is developed by standard techniques. The random dot film
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obtained is subjected to three exposures in a commercial
daylight printer so as to prepare the desired negative-
acting mask.
A mask having a desired number of dots and a
desired degree of uniformity in dot array can be obtained
by properly adjusting the dot area (e.g. 60 - 99%), the
time of multiple exposures, and the number of exposures.
A positive-acting mask may be readily formed by
"reversing" the negative mask by routine procedures.
The dots, whether dark or transparent, should
have a density on the mask within the range of I0 - 104
per cm2 and should have diameters in the range of 5 - 300
micrometers. Preferably the range of diameters is 5 - 100
micrometers.
As will be apparent from the foregoing
description, the procedures for preparing a simple color
proof according to the present invention may be the same
as those employed in the prior art process except that the
special mask of the present invention is used in the
exposure step.
While several preferred embodiments of the
present invention are explained above, it should be noted
that various other embodiments are possible by employing
any of the known techniques with respect to the type and
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number of color separations, the type of the light-
sensitive material used for making a color proof, the type
of image-receiving material, the method of coloring, the
method of processing, and the method of transfer.
The mask of the present invention to be used in
the exposure step may be the same for all of the color
separation films or, alternatively, masks having different
characteristics may be selectively used for the individual
color separation films.
In accordance with the present invention, a
simple color proof can be prepared by readily changing the
desired color density such that it has a tone which is
faithful to the color image to be formed in actual
printing.
