Photosensitive microcapsule imaging system having improved gray scale

A photosensitive material includes a support having microcapsules and a color correction dye on its surface. The microcapsules include first, second and third sets of microcapsules, each set containing an internal phase. The first set of microcapsules is sensitive to red light and is associated with a cyan image-forming agent, the second set of microcapsules is sensitive to green light and is associated with a magenta image-forming agent, and the third set of microcapsules is sensitive to blue light and is associated with a yellow image-forming agent. The cyan, magenta and yellow image forming agents are respectively soluble in the internal phases of the first, second and third sets of microcapsules whereas the color correction dye is insoluble in the internal phase. The first, second and third sets of microcapsules are present in one or more layers containing a binder on the surface of the support. The color correction dye absorbs at least one of red, green, and blue light. The color correction dye is present in the binder of the one or more layers of microcapsules or is present in a separate layer containing a binder such that the film speed of at least one of the first, second and third sets of microcapsules is reduced and thereby the photosensitive material exhibits improved gray scale.

BACKGROUND OF THE INVENTION 
The present invention relates to a panchromatic microcapsule imaging system 
which is useful in forming full color images using the imaging processes 
described in U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead 
Corporation. More particularly, the present invention relates to a 
panchromatic imaging material employing one or more layers of 
photosensitive microcapsules and a color correction dye. The microcapsules 
include three or more sets of microcapsules for different colors whereas 
the color correction dye absorbs light of a certain wavelength band to 
which at least one of the different sets of microcapsules is sensitive. 
The color correction dye is incorporated in a binder present in the one or 
more layers of microcapsules or in a separate layer, thereby reducing the 
film speed of at least one of the different sets of microcapsules such 
that the imaging material exhibits improved gray scale. 
U.S. Pat. No. 4,842,976 to The Mead Corporation describes a photosensitive 
material useful in full color and panchromatic imaging, comprising a 
support having on its surface a layer of microcapsules. The microcapsules 
individually contain cyan, magenta and yellow color formers and 
photosensitive compositions having distinctly different sensitivities in a 
visible region. More particularly, the photosensitive composition 
encapsulated with the cyan color former is sensitive to red light, the 
photosensitive composition encapsulated with the magenta color former is 
sensitive to green light, and the photosensitive composition encapsulated 
with the yellow color former is sensitive to blue light. A uniform mixture 
of the microcapsules is distributed over the surface of the support. The 
photosensitive material is image-wise exposed to visible light, and 
thereafter, it is subjected to a uniform rupturing force, such as 
pressure, which causes the microcapsules in the underexposed and unexposed 
areas to rupture and release the color formers. The color formers then 
react with a developer material which is present on the same support or on 
a different support and produces a full color image. 
Distinctively different sensitivities of photosensitive microcapsules are 
principally enabled by encapsulating with the color formers different 
photoinitiators as respective constituents of the photosensitive 
compositions. However, these different photoinitiators often induce 
difference in film speed between microcapsules for different colors. This 
difference can be a critical drawback, particularly, in panchromatic 
imaging since it can result in images with poor gray scale. To compensate 
difference in film speed and to thereby form images with good gray scale, 
optical filters must be used upon exposure. Alternatively, it is necessary 
to separate the red, green blue components of the original, and the 
intensity of or exposure time for each component must be adjusted in 
accordance with the film speed of microcapsules for the complementary 
color. 
Addition of radiation absorbers or dyes to adjust photographic properties 
of an imaging system is known in the art. 
U.S. Pat. No. 4,840,866 to the Mead Corporation discloses a microcapsule 
imaging system which employs a radiation absorber. In this system, the 
radiation absorber is incorporated i the internal phase of at least one of 
two sets of microcapsules for the same color to control gamma and dynamic 
range. Control of these photographic properties is possible using this 
technique during preparation of microcapsules. 
U.S. Pat. No. 4,576,891 to the Mead Corporation discloses a full color and 
panchromatic microcapusle imaging system which utilizes a dye. According 
to this patent, the dye can be contained, to prevent cross-talk between 
microcapsules for different colors, in a binder for microcapsules, in the 
internal phase of microcapsules or in the walls of microcapsules. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a panchromatic imaging system 
which can exhibit improved gray scale is obtained by incorporating a color 
correction dye (a visible light absorber) in the binder of one or more 
layers of light-sensitive microcapsules or in a separate layer such that 
the film speed of microcapsules for at least one color is reduced and 
thereby the quantity of difference in film speed between different color 
microcapsules is reduced. This invention is useful in three-color system 
and in four-color systems. According to the present invention, a color 
correction dye is selected which has a spectral absorption characteristic 
such that the color correction dye absorbs light within the spectral 
sensitivity range of microcapsules whose film speed is faster than that of 
other microcapsules. For example, where the film speed of magenta 
microcapsules is faster than cyan and yellow microcapsules, a color 
correction dye is selected which has an absorption spectrum within the 
spectral sensitivity range of the magenta microcapsules. One of the 
advantages of the present invention is that selection and incorporation of 
such a color correction dye can be achieved after all the microcapsules 
necessary for full color imaging and prepared or even after microcapsules 
are coated on a substrate to form the microcapsule layer(s). Thus, the 
color balance is adjusted without modifying the internal phase of the 
microcapsules. 
Accordingly, one manifestation of the present invention resides in a 
photosensitive material comprising a support which has on a surface 
thereof microcapsules and a color correction dye. The microcapsules 
include first, second and third sets of microcapsules. The first set of 
microcapsules is sensitive to red light and is associated with a cyan 
image-forming agent. The second set of microcapsules is sensitive to green 
light and is associated with a magenta image-forming agent. The third set 
of microcapsules is sensitive to blue light and is associated with a 
yellow image-forming agent. The cyan, magenta and yellow image-forming 
agents are soluble in the internal phase of the microcapsules whereas the 
color correction dye is insoluble in the internal phase. The different 
sets of microcapsules can be present in one or more layers on the surface 
of the support. This layer or each of the layers contains a binder. The 
color correction dye is capable of absorbing at least one of red, green, 
or blue light. The color correction dye is present in the binder of at 
least one of the one or more layers of microcapsules or in a separate 
layer such that the film speed of at least one of the first, second and 
third sets of microcapsules is reduced and thereby the photosensitive 
material exhibit improved gray scale. More particularly, where the first, 
second and third sets of microcapsules are present in one layer, the color 
correction dye can be contained in the binder of the one layer or in the 
separate layer which is coated on the one layer of microcapsules. Where 
the first, second and third sets of microcapsules are present in two or 
more layers, the color correction dye can be contained in the binder of at 
least one of the layers, in the separate layer which, preferably, is 
interposed between two of the microcapsule layers, or in both the separate 
layer and the binder of at least one of the microcapsule layers. 
Another manifestation of the present invention resides in a method of 
controlling the film speed of a photosensitive material which employs a 
support having microcapsules on its surface. The microcapsules of the 
photosensitive material include first, second and third sets of 
microcapsules. The first set of microcapsules is sensitive to red light 
and is associated with a cyan image-forming agent. The second set of 
microcapsules is sensitive to green light and is associated with magenta 
image-forming agent. The third set of microcapsules is sensitive to blue 
light and is associated with a yellow image-forming agent. The cyan, 
magenta and yellow image-forming agents are soluble in the internal phase 
of the microcapsules. The first, second and third sets of microcapsules 
are present in one or more layers on the surface of the support. The 
method in accordance with the present invention comprises preparing one or 
more coating compositions in which the first, second and third sets of 
microcapsules are dispersed, adding a color correction dye to at least one 
of the one or more coating compositions or a separate coating composition, 
and applying the one or more coating compositions to the support to form 
the one or more layers of microcapsules. A proper color correction dye for 
use in this method is a dye which is insoluble in the internal phase of 
the microcapsules and can absorb at least one of red, green, and blue 
light. Such a dye is added in an amount such that the film speed of at 
least one of the first, second and third sets of microcapsules is reduced 
to allow the photosensitive material to exhibit improved gray scale. By 
being insoluble in the internal phase, any transfer of the color 
correction dye by the internal phase is reduced. 
DEFINITION 
The term "spectral sensitivity range" as herein used refers to the spectral 
region in which a photosensitive composition or microcapsule responds to 
light. 
The term "broad band white light" as used herein refers to energy at 
wavelength of visible light. 
The term "reduce the film speed" or "decrease the toe (or shoulder) speed" 
as herein used refers to increasing the amount of energy required to 
expose the microcapsules. 
The term "increase the shoulder speed" as herein used refers to decreasing 
the amount of energy required to expose the microcapsules.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is useful to control the film speed of photosensitive 
microcapsules and is thereby useful to enhance gray scale in microcapsule 
imaging, particularly, in full color and panchromatic imaging. 
Full color and panchromatic imaging systems are described in U.S. Pat. Nos. 
4,399,209; 4,440,846; and 4,842,796. These systems can be modified in 
accordance with the present invention to include a color correction dye to 
control the film speed of microcapsules and to thereby improve gray scale. 
As explained in the aforementioned references, these systems are 
preferably single exposure systems which employ three sets of 
microcapsules, namely, a set of cyan microcapsules, a set of magenta 
microcapsules, and a set of yellow microcapsules. The cyan microcapsules 
contain a cyan color precursor and a first photosensitive composition. The 
magenta microcapsules contain a magenta color precursor and a second 
photosensitive composition. The yellow microcapsules contain a yellow 
color precursor and a third photosensitive composition. Each of first, 
second and third photosensitive compositions is primarily sensitive in a 
different wavelength band such that the three sets of microcapsules can be 
individually exposed with minimum cross-talk. More specifically, the cyan, 
magenta and yellow microcapsules are respectively sensitive to red, green, 
and blue light. 
In designing full color, single exposure imaging systems, it is critical 
that the three sets of microcapsules have similar film speeds. For 
example, the H and D curves should be as nearly identical as possible. 
Otherwise, the imaging process using the imaging system will require 
means, such as optical filters, for adjusting the intensity of light 
source to provide good gray scale or good color reproduction. In 
accordance with the present invention, the three sets of microcapsules are 
distributed over a support or substrate to form one or more layers, and a 
color correction dye is incorporated in at least one of the one or more 
layers of microcapsules or a separate layer containing a binder. More 
particularly, each of the one or more layers of microcapsules contains a 
binder, and the color correction dye is present in the binder of the one 
or more layers or in the separate layer. The incorporated color correction 
dye is a dye which can absorb light within the spectral sensitivity range 
of at least one of the three sets of microcapsules, of which the film 
speed(s) is faster than that of the other set(s). Such a dye is 
incorporated in an amount which reduces the film speed(s) of the faster 
set(s) of microcapsules so that better gray scale is exhibited by the 
imaging system. This is illustrated in FIG. 1 wherein Curve A is an H and 
D curve for an imaging sheet prepared using a faster set of microcapsules 
without a color correction dye, and Curve B is an H and D curve for an 
imaging sheet prepared using the same set of microcapsules and a color 
correction dye which is capable of absorbing light within a spectral 
sensitivity range of that set of microcapsules. 
Preferably, the amount of the color correction dye added is such that the 
film speed(s) of the faster set(s) of microcapsules approaches or more 
closely approximates the film speed(s) of the other slower set(s) of 
microcapsules. For example, where curve A in FIG. 1 represents the H and D 
curve of magenta microcapsules whose film speed is faster than that of 
cyan and yellow microcapsules, Curve B can be modified similar to the H 
and D curves for the cyan and yellow microcapsules. 
In addition, in order to further modify or finely adjust the H and D 
curves, particularly, the shoulder and toe speeds, photoinitiators and 
polymerization inhibitors can be utilized. For example, where magenta 
microcapsules of a modified film speed (Curve B in FIG. 1) are slightly 
slower in shoulder speed than cyan and yellow microcapsules, a 
photoinitiator can be added to the internal phase of the magenta 
microcapsules to increase the shoulder speed. This can typically be 
achieved by increasing the amount of a photoinitiator used in the 
photosensitive composition in the microcapsules whose shoulder speed is 
intended to be increased. While, where magenta microcapsules are slightly 
faster in toe speed than cyan and yellow microcapsules, a polymerization 
inhibitor can be added to the internal phase of the magenta microcapsules 
in order to decrease the toe speed without decreasing the shoulder speed. 
Another method to increase the shoulder speed of the microcapsules is 
selective bump exposure as taught in U.S. Pat. No. 4,482,624. 
While it is difficult to match the H and D curves exactly, gray scale is 
optimized by approximating the H and D curves for the three sets of 
microcapsules as closely as possible or even by slightly shifting the H 
and D curve(s) of one or two faster sets of microcapsules toward the H and 
D curve(s) of the other slower set(s) of microcapsules. In comparing and 
approximating the H and D curves for the three sets of microcapsules, it 
is important that those H and D curves are the ones exhibited upon 
exposing the three sets of microcapsules to a light source which generates 
broad band white light. 
Persons skilled in the art will appreciate that the color correction dye 
can, alternatively, be incorporated in the microcapsule wall to control 
the film speed of the microcapsules in a similar manner to that of the 
present invention. Persons skilled in the art will also appreciate that 
the present invention described above can also reduce cross-talk between 
the different sets of microcapsules. Imaging systems and methods for 
preventing cross-talk is described in U.S. Pat. No. 4,576,891 to The Mead 
Corporation, which is hereby incorporated by reference. 
In a preferred embodiment, the mixture of three sets of microcapsules are 
homogeneously distributed on a substrate to form a microcapsule layer 
containing a binder. One or more color correction dyes are incorporated in 
the binder of the microcapsule layer. The color correction dye or dyes 
have an absorption spectrum or spectrums within the spectral sensitivity 
range or ranges of one or two of the three sets of microcapsules. The dye 
or dyes absorb light intended for exposing the faster microcapsules and 
shield them from the light. This reduces the film speed of the faster 
microcapsules. If the color correction dye or dyes are in the binder of 
microcapsule layer or layers, it is important that the binder covers the 
microcapsules so that the dye or dyes have a satisfactory shielding 
effect. Instead of being in the microcapsule layer, the color correction 
dye(s) can be present in a separate layer coated over the microcapsule 
layer. 
In another embodiment, three sets of microcapsules are provided in the form 
of at least two binder-containing layers on a substrate, and one or more 
color correction dyes are present in the binder of at least one of the 
microcapsule layers. FIG. 2 shows an example of an imaging sheet in 
accordance with this embodiment, wherein a set of magenta microcapsules 
(M), the fastest set of the three sets of microcapsules, is formed into a 
first layer 10 containing a color correction dye 12 which can absorb light 
within the spectral sensitivity range of magenta microcapsules (M), and 
the other sets of microcapsules, i.e., sets of cyan and yellow 
microcapsules (C and Y) are formed into a second layer 14 coated over the 
first layer 10. Reference numeral 16 designates a substrate on which the 
first and second layers 10 and 14 are provided. 
Persons skilled in the art will understand that many other arrangements 
which are suitable for enhancing gray scale are also possible in which, 
for example, three sets of microcapsules are respectively formed into 
different layers, and two different color correction dyes which absorbs 
light in different wavelength bands are incorporated in two or more 
layers. 
In a still another embodiment, three sets of microcapsules are present in 
at least two layers on a substrate, and one or more color correction dyes 
are incorporated in one or more separate layers on the same substrate. 
Examples of imaging sheets according to this embodiment are illustrated in 
FIGS. 3 and 4. In FIG. 3, a separate layer 18 is interposed between a 
first layer 10 including magenta microcapsules (M) and a second layer 14 
including cyan and yellow microcapsules (C and Y), and a color correction 
dye 12 which can absorb green light is incorporated in the separate layer 
18. In FIG. 4, a separate layer 18 is interposed between a first layer 10 
including magenta microcapsules (M) and a second layer 20 including cyan 
microcapsules (C), and another separate layer 22 is interposed between the 
second layer 20 and a third layer 24 including yellow microcapsules (Y). A 
color correction dye 12 in the separate layer 18 is a green-light absorber 
whereas a color correction dye 26 in the separate layer 22 is a red-light 
absorber. In either FIG. 3 or FIG. 4, the one or more color correction 
dyes may also be present in the binder of at least one of the microcapsule 
layers. 
In either of the latter two embodiments, the microcapsule layers, or the 
microcapsule layers and the one or more separate layers may be arranged in 
any order which is of utility to improve gray scale. However, in order to 
effectively reduce the film speed of a set of microcapsules without 
affecting the film speed of another set of microcapsules, a layer 
containing the microcapsules of which the film speed is intended to be 
reduced should be laid under a layer containing microcapsules of which the 
film speed is not intended to be reduced. At the same time, a color 
correction dye by which the film speed is intended to be reduced should be 
contained in the underlying microcapsule layer or in a separate layer 
which is interposed between the overlying and underlying microcapsule 
layers. In this sense, a layer containing a set of microcapsules of a 
fastest film speed, typically, is laid under a layer containing a set of 
microcapsules of a slower film speed. 
The imaging material according to the foregoing embodiments can be prepared 
by adding the color correction dye(s) to a binder solution or solutions 
after the microcapsules are prepared and by applying the solution or 
solutions to a substrate to form the microcapsule layer(s) or the separate 
layer(s). Where the color correction dye(s) is added to a binder solution 
or solutions for forming the separate layer(s), this or these binder 
solutions may be applied to the substrate after one or more microcapsule 
layers are formed on the substrate. Stated differently, the color 
correction dye(s) can be incorporated into the imaging material even after 
the microcapsule layer(s) is formed on the substrate. Where two or more 
solutions are prepared to form the microcapsule layers, or the 
microcapsule layer(s) and the separate layer(s), the solutions may be 
applied in any order which results in any of the foregoing embodiments. 
One of the preferred color correction dyes for use in the present invention 
includes a water soluble dye. 
Preferred examples of water soluble dyes useful to reduce the film speed of 
magenta microcapsules are dyes with absorption peaks in the 550 nm region, 
which includes Eosin B, Eosin Y, Erythrosin B, Acid Fuchsin, Phloxine, 
Phloxine B, Rhodamine B, and Rose Bengal. In the above-mentioned examples, 
Erythrosin B is particularly preferred. Other water soluble dyes useful in 
association with magenta microcapsules include Acid Red 4, Acid Red 8, 
Acid Red 37, Acid Red 40, Acid Red 88, Acid Red 106, Acid Red 151, Basic 
Red 29, Congo Red, Cresol Red, Cresol Red Sodium Salt, D&C Red No. 33, 
Disperse Red 13, Ethidium Bromide, Mordant Red 19, and Neutral Red. 
Water soluble dyes with absorption peaks in the 650 nm region, which are 
useful in association with cyan microcapsules include Acid Alizarian 
Violet N, Acid Blue 25, Acid Blue 29, Acid Blue 40, Acid Blue 41, Acid 
Blue 45, Acid Blue 80, Acid Blue 92, Acid Blue 120, Acid Blue 129, Acid 
Blue 161, Acid Violet 5, Acid Violet 7, Acid Violet 17, Alcian Blue GX, 
Alizarian Violet 3R, Aniline Blue, Aniline Blue W. S., Basic Blue 41, 
Basic Blue 47, Basic Blue 66, Basovit Blue 665 E, Brilliant Blue G, 
Brilliant Blue R, Brilliant Cresyl Blue, Bromophenol Blue, Celestine Blue, 
Chicago Sky Blue 6B, Coomassie Brilliant Blue G-250, Coomassie Brilliant 
Blue R-250, Direct Blue 71, Direct Violet 51, Disperse Blue 1, Disperse 
Blue 3, Disperse Blue 14, Eriochrome Blue Black B, Ethyl Violet, Evans 
Blue, Indigo Carmine, Luxol Fast Blue ARN, Meldola's Blue, Methyl Violet 
3B, Mordant Blue 13, Naphthol Blue Black, Oxazine 4 Perchlorate, Oxazine 
170 Perchlorate, Prussian Blue, Reactive Blue 2, Reactive Blue 4, Remazol 
Brilliant Blue R, Solar Blue RCLL Supra Powder, Trypan Blue, for reducing 
the film speed of cyan microcapsules include Basic Blue 3, Brilliant 
Cresyl Blue ALD, DOTC Iodide, HDITC Perchlorate, HITC Iodide, HITC 
Perchlorate, Luxol Fast Blue MBSN, Methylene blue, New Methylene Blue N, 
Nile Blue A, Oxazine 1 Perchlorate, Patent Blue VF, and Reactive Blue 15. 
GL. 
Water soluble dyes useful to reduce the film speed of yellow microcapsules 
are dyes with absorption peaks in the 450 nm range. Examples of such 
yellow dyes include AAA Yellow Atlas 85, AAOA Yellow Solar Set, Acid 
Yellow 42, Acid Yellow 65, Acid Yellow 76, Brilliant Yellow, Bromothymol 
Blue Reagent ACS, Coumarin 102, DCM, Direct Yellow 62, Disperse Orange 1, 
Disperse Orange 11, Disperse Orange 13, Fast Blue B Salt, Fast Dark Blue 
R, Fast Yellow GC Salt, Fluorol Yellow 088, Flourescein, Mordant Yellow 
10, Nitrazine Yellow, Palatine Fast Yellow BLN, Saffron, Thiazol Yellow G, 
Thymol Blue Reagent ACS, and Variamine Blue RT Salt. Preferred examples of 
water soluble yellow dyes include Acid Yellow 17, Acid Acid Yellow 40, 
Acridine Yellow G, Auramine 0, Basic Yellow 11, Cibacron Brilliant Yellow 
3G-P, Direct Yellow 8, Direct Yellow 27, Direct Yellow 29, Direct Yellow 
50, Disperse Orange 3, Disperse Yellow 3, Disperse Yellow 7, Disperse 
Yellow 9, Flavazin L, Fast Garnet GBC Base, Intraplast Yellow 3R, Naphthol 
Yellow S, Rosolic Acid, Thioflavin S, and Thioflavin T. Tartrazine is the 
most preferred yellow dye. 
The aforementioned water soluble dyes are particularly useful where the 
binder in the microcapsule layer(s) or in the separate layer is a 
hydrophilic binder. Oil soluble dyes are also useful in the present 
invention where the binder is a hydrophobic binder. 
The amount of the color correction dye is selected to produce the desired 
adjustment in gray scale. The amount can be easily calculated in the 
manner described hereinafter. First, a three-color test imaging sheet is 
prepared which includes cyan, magenta and yellow microcapsules coated 
thereon, and H and D curves (white light sensitometry) for three sets of 
microcapsules are plotted with respect to the test imaging sheet. Second, 
on the basis of the H and D curves, energy of light (i.e., loss in speed) 
is determined which is necessary for a dye to absorb in order to match or 
approximate the film speed(s) of slower microcapsules with that of faster 
microcapsules. The amount of dye can then be calculated from the 
determined loss in film speed on the basis of the following equation: 
EQU E=K C 
where E is loss in film speed, K is a coefficient empirically determined 
from experiments, and C is the concentration of a dye. 
One of the advantages of the invention is that for any two sets of 
microcapsules having different film speeds, the amount of the color 
correction dye can be calculated and added to faster set of microcapsules 
to produce better gray scale. 
One of the preferred binders useful in the present invention is a 
hydrophilic binder which can be soluble in water with the water soluble 
dyes and can form a liquid coating composition or coating solution 
applicable to a suitable substrate. Hydrophobic binders can also be useful 
in the present invention as stated above. It is important that the 
binder(s) in which the color correction dye(s) is incorporated can 
dissolve the color correction dye(s) when being in the form of coating 
solution. Examples of hydrophilic binder include gelatin, polyvinyl 
alcohol, polyacrylamide, polyvinyl acetate, organosilanes (Dow Additive 
25), and acrylic lattices. 
The photosensitive material of the present invention can be used in 
conjunction with various light-sensitive materials and image-forming 
agents to produce images by different techniques. 
For example, positive working photohardenable or negative working 
photosoftenable light sensitive compositions can be contained in the 
internal phase of the microcapsules. Photohardenable compositions such as 
photopolymerizable and photocrosslinkable materials increase in viscosity 
or solidify upon exposure and yield positive images. Photosoftenable 
compositions, such as some photodecomposable or photodepolymerizable 
materials, decrease in viscosity and result in negative images. 
Ethylenically unsaturated organic compounds are useful photohardenable 
materials. These compounds contain at least one terminal ethylene group 
per molecule. Typically, they are liquid. Polyethylenically unsaturated 
compounds having two or more terminal ethylene groups per molecule are 
preferred. All examples of this preferred subgroup are ethylenically 
unsaturated acid esters of polyhydric alcohols, such as trimethylol 
propane triacrylate (TMPTA) and dipentaerythritol hydroxypentaacrylate 
(DPHPA) (see U.S. patent application Ser. No. 420,632 filed Oct. 5, 1989). 
Initiators useful in the present invention includes homolytic initiators 
which generate free radicals upon photochemical cleavage caused by visible 
light such as certain benzoin ethers and initiators which function via 
hydrogen abstraction. Preferred initiators are soluble in the 
light-sensitive composition. Xanthones, thioxanthones, polycyclic quinons, 
benzoin alkyl ethers are particularly useful. Specific examples of useful 
initiators are benzophenone, Michler's ketone, benzoin methyl ether, 
2,2-dimethoxy-2-phenylacetophenone, isopropylthioxanthone, ethyl 
para-dimethylaminobenzoate, 3-cinnamoyl-7-diethylamino coumarins, etc. A 
potentially useful red initiator is N,N,N',N'-tetra-n-butylthionine; a 
potentially useful green initiator is 
N,N,N',N'-tetramethyl-4'-dodecylsafranine, and a potentially useful blue 
initiator is phenanthraquinone. Another initiator systems useful in a red 
or green light-sensitive microcapsule is described in U.S. Pat. No. 
3,495,987. Most preferred photoinitiators for use in the present invention 
include ionic dye-counter ion compounds described in U.S. Pat. No. 
4,772,541. A preferred class of ionic dye-counter ions is cationic dye 
borates and still more particularly cyanine dye-borates. Typically, the 
borate is a triphenylalkyl borate such as a triphenylbutyl borate. Other 
dye complexes such as Rose Bengal iodonium and Rose Bengal pyryllium 
complexes may also be used. It is also preferred to use disulfide with 
dye-borates (see U.S. patent application Ser. No. 321,257 filed Mar. 9, 
1989). Silver halide systems described in U.S. Patent Nos. 4,772,531 and 
4,767,690 is also useful in the present invention. 
Various color precursors as well as other image-forming agents can be used 
in the present invention. Where color precursors are used, images can be 
formed by the interaction of color precursors and color developers of the 
type conventionally used in the carbonless paper art. Images can also be 
formed by the color producing interaction of a chelating agent and a metal 
salt or by the reaction of certain oxidation-reduction reaction pairs, 
many of which have been investigated for use in pressure-sensitive 
carbonless papers. Other useful image-forming agents include an oil 
soluble dye. Images can be formed by transfer the oil soluble dye to plain 
or treated paper. The internal phase of the microcapsules itself has its 
own image-forming capability. For example, it is known that toners used in 
the xerographic recording process selectively adhere to the image areas of 
an imaging sheet exposed and developed as in the present invention. 
Furthermore, the image-forming agent can be provided inside the 
microcapsules, in the microcapsule wall, or outside the microcapsules in 
the same layer as the microcapsules or in a different layer. In all the 
cases, it is important that the image-forming agent is soluble in the 
internal phase of the microcapsules. In the latter two cases, the internal 
phase picks up the image-forming agent (e.g., by dissolution) upon being 
released from the microcapsules and carries it to the developer layer or 
an associated treated or untreated sheet. Also in all the aforementioned 
cases, it is preferred that the internal phase does not dissolve the color 
correction dye upon release. The color correction dye can otherwise be 
carried together with the image-forming agent and can affect a resultant 
image. Oil soluble image-forming agents and hydrophobic internal phase are 
therefore preferred where water soluble color correction dyes are 
employed. 
Typical color precursors useful in the present invention includes electron 
donating type compounds. Representative examples of such color precursors 
include substantially colorless compounds having in their partial skeleton 
a lactone, a lactam, a sulfone, a spiropyran, an ester or amido structure 
such as triarylmethane compounds, bisphenylmethane compounds, xanthene 
compounds, fluorans, thiazine compounds, spiropyran compounds and the 
like. Preferred yellow color precursors are described in U.S. Pat. No. 
4,908,447. Crystal Violet Lactone and Copikem X, IV, and XI (products of 
Hilton-Davis Chemical Co.) and Reakt Yellow (a product of BASF) are often 
used alone or in combination as color precursors in the present invention. 
Illustrative examples of color developers useful in conjunction with the 
embodiment employing the aforesaid color precursors are clay minerals such 
as acid clay, active clay, attapulgite, etc.; organic acids such as tannic 
acid, gallic acid, propyl gallate, etc.; acid polymers such as 
phenol-formaldehyde resins, phenol acetylene condensation resins, 
condensates between an organic carboxylic acid having at least one hyroxy 
group and formaldehyde, etc.; metal salts or aromatic carboxylic acids 
such as zinc salicylate, tin salicylate, zinc-2-hydroxynaphthoate, 
zinc-3,5-di-tert-butyl salicylate, oil soluble metal salts or 
phenol-formaldehyde novolak resins (e.g., see U.S. Pat. Nos, 3,672,935; 
3,732,120; and 3,737,410) such as zinc modified oil soluble 
phenol-formaldehyde resins as disclosed in U.S. Pat. No. 3,732,120, zinc 
carbonate etc. and mixture thereof. A preferred class of glossable 
developer is designed in commonly assigned U.S. Pat. No. 4,859,561. 
The most common support for imaging sheet in accordance with the present 
invention is transparent film or paper. The paper may be a commercial 
impact raw stock, or special grade paper such as cast-coated paper or 
chrome-rolled paper. The latter two papers are preferred when using 
microcapsules having a diameter between approximately 1 and 5 microns 
because the surface of these papers is smoother and therefore the 
microcapsules are not easily embedded in the stock fibers. Transparent 
supports such as polyethylene terephthalate (PET) or translucent support 
can also be used in the present invention. Another preferred support for 
the microcapsules is aluminized PET as described in U.S. Pat. No. 
4,910,117. 
It has been found that a uniform re-exposure or a co-exposure can be used 
to improve the sensitivity of the imaging sheet by generating radicals 
which react with the oxygen present in the microcapsules to prevent it 
from inhibiting the imaging photochemistry during the imaging exposure. 
The details are described in U.S. Pat. No. 4,482,624. This technique is 
also useful in the present invention. 
The microcapsules used in the present invention can be produced using known 
encapsulation techniques including coacervation, interfacial 
polymerization, polymerization of one or more monomers in an oil, etc. 
Representative examples of suitable wall-formers are gelatin materials 
(see U.S. Pat. Nos. 2,370,456 and 2,800,457 to Green et al.) including gum 
arabic, polyvinyl alcohol, carboxy-methyl-cellulose; 
resorcinol-formaldehyde wall formers (see U.S. Pat. No. 3,775,190 to Hart 
et al.); isocyanate wall-formers (see U.S. Pat. No. 3,914,511 to 
Vassiliades); isocyanatepolyol wall-formers (see U.S. Pat. No. 3,796,669 
to Kiritani et al.); urea-formaldehyde wall-formers, particularly 
urea-resorcinol-formaldehyde in which oleophilicity is enhanced by the 
addition of resorcinol (see U.S. Pat. Nos. 4,001,140; 4,087,376; and 
4,089,802 to Forris et al.); and melamine-formaldehyde resin and 
hydroxypropyl cellulose (see commonly assigned U.S. Pat. No. 4,025,455 to 
Shackle). Melamine-formaldehyde capsules are particularly preferred (see 
U.S. Pat. No. 4,608,330 and U.S. patent application Ser. No. 370,103 filed 
Jun. 22, 1989 (allowed)). The mean microcapsule size used in the present 
invention generally ranges from about 1 to 25 microns. 
The imaging sheets of the present invention can be constituted such that 
they are sensitive to visible light. The exposure alone may cause a 
sufficient change in viscosity of the internal phase to control imaging. 
Otherwise, exposure can be used to initiate or advance the photochemistry 
in the exposed areas and a subsequent heat treatment can be used to 
enhance the image. 
The present invention is further illustrated by the following non-limiting 
examples. 
EXAMPLE 1 
Melamine-Formaldehyde Prepolymer Preparation 
1. Into 5 gallon reactor, 7,300 g water is added. 
2. While mixing at 550 rpm, 710 g melamine and 1,374 g formaldehyde are 
added to the reactor. 
3. The pH is adjusted to 8.5 using 5% Na.sub.2 CO.sub.3. 
4. The preparation is heated to 65.degree. C. and is held for one and a 
half hour during which melamine-formaldehyde prepolymer is prepared. 
Capsule Preparation 
1. Into a 38 liter stainless steel vessel, 17,200 g water, 320 g sodium 
polystyrene, 506 g pectin, 9.6 g NaHCO.sub.3, and 343 g melamine are 
weighed. The vessel is stirred at 3,450 rpm for two hours. 
2. The pH is adjusted to 6.0 using a 20% solution of NaOH. 
3. The preparation (continuous phase components) are then added to a 10 
gallon emulsification vessel and mixed at 4,320 rpm. 
4. The internal phase is added over a period of 1 minute. Emulsification is 
continued for 20-40 minutes. 
5. After the desired particle size (4-10 microns) has been achieved, the 
emulsion is pumped to a reactor and is heated to 60.degree. C. When 
60.degree. C. is reached, the melamine-formaldehyde prepolymer prepared as 
described above is added. 
6. The pH is stabilized for 5-15 minutes and is then adjusted to 6.0 using 
5 to 20% phosphoric acid. 
7. The preparation is heated to 70.degree. C. and is held for a one hour 
cure time during which the capsule walls are formed. 
8. 1,700 g of a 50% urea solution is added, and the preparation is 
continued to be heated for another 40 minutes. 
9. 300 g of 20% sodium hydride is added and then the capsule preparation is 
cooled to 25.degree. C. 
Three batches of microcapsules were prepared as above for use in a full 
color imaging sheet using the three internal phase compositions set forth 
below. 
______________________________________ 
Internal Phase A (460 nm) 
TMPTA 6,000 g 
Reakt Yellow (BASF) 720 g 
7-diethylamino-3-cinnamoyl coumarin 
32 g 
DIDMA 60 g 
N-100 (Desmodure Polyisocyanate Resins) 
400 g 
Internal Phase B (550 nm) 
TMPTA 4,800 g 
DPHPA 1,200 g 
Copikem Mead Magenta 1,440 g 
(Hilton Davis Chemical Co.) 
1,1'-di-N-heptyl-3,3-3'-3'- 
48 g 
tetramethylindocarbocyanine 
triphenylbutylborate 
DIDMA 40 g 
N-100 400 g 
4-nitro-benzyl-alcohol 40 g 
Internal Phase C (650 nm) 
TMPTA 4,200 g 
DPHPA 1,800 g 
Copikem Mead Cyan 720 g 
(Hilton Davis Chemical Co.) 
1,1'-di-N-heptyl-3,3-3'-3'- 
100 g 
tetramethylindodicarbocyanine 
triphenylbutylborate 
DIDMA 60 g 
N-100 400 g 
______________________________________ 
Each of the above internal phase compositions was a composition having been 
prepared in accordance with the steps described below. 
Internal Phase Preparation 
1. TMPTA and DPHPA are added to 5 gallon vessel, and are mixed at 350 rpm. 
2. The mixture is heated to 90.degree. C. to 105.degree. C. 
3. The color former is added to the mixture, and is held until dissolved. 
4. the preparation is cooled to 50.degree. C. (90.degree. C. for Internal 
Phase A). 
5. The photoinitiator is added to the preparation, and is mixed until 
dissolved. 
6. DIDMA is added to the preparation, and is mixed at 50.degree. C. 
(90.degree. C. for Internal Phase A) for 5 minutes. 
7. 4-nitro-benzyl-alcohol is added, and is mixed for 5 minutes for Internal 
Phase B. 
8. N-100 is added, and is mixed for 5 minutes. 
9. The internal phase composition is ready to add to the emulsification 
vessel. 
Internal Phase A provides a yellow image-forming agent and is sensitive at 
460 nm. Internal Phase B provides a magenta image-forming agent and is 
sensitive at 550 nm, and Internal Phase C contains a cyan image-forming 
agent and is sensitive at 650 nm. 
Full Color Coating Preparation 
1. Into a 30 liter vessel, 4,014.1 g cyan microcapsule solution (one of the 
three batches of microcapsules containing Internal Phase C), 2,221.1 g 
magenta microcapsule solution (the batch of microcapsules containing 
Internal Phase B), and 3,818.4 g yellow microcapsule solution (the batch 
of microcapsules containing Internal Phase A) are added, and the vessel is 
stirred. 
2. 648 g Dow Additive 25 (binder), Triton X-100 (surfactant), and 474.9 g 
Vinol 205 (binder) are added to the preparation, and are mixed. 
3. 1,280 g of 1% Rose Bengal (green light absorber) is added to the 
mixture, and is mixed. 
4. 3,543.5 g water is added to dilute the mixture to a 18% (solid content) 
coating solution. 
5. The resultant coating solution is mixed thoroughly for one hour. 
6. The coating solution was coated on an aluminized PET film, and then the 
coating is dried to provide a full color imaging sheet. 
The film speed of the prepared imaging sheet was measure The light used was 
broad band white light (54,111.8 1m/m.sup.2). The densitometer used was a 
X-Rite 404 with an optic head status A (product of X-Rite Incorporated). 
The exhibited H and D curves (Equivalent Neutral Density curves) for the 
three batches of microcapsules are shown in FIG. 5 wherein a H and D curve 
for the yellow microcapsules is represented by the dot-and-dash line, a H 
and D curve for the magenta microcapsules is represented by the broken 
line, and a H and D curve for the cyan microcapsules is represented by the 
solid line. 
COMATIVE EXAMPLE 
A microcapsule coating solution was prepared in a similar manner to that in 
Example 1 except that Rose Bengal was not added. A full color imaging 
sheet was prepared from this coating solution in the same manner as in 
Example 1, and the film speed was measured. The light and the densitometer 
used for measuring the film speed are the same as in Example 1. 
The exhibited H and D curves (Equivalent Neutral Density curves) for the 
three batches of microcapsules are shown in FIG. 6 where H and D curves 
for the yellow, magenta and cyan microcapsules are represented by the 
dot-and-dash, broken, and sold lines, respectively. 
As can be seen by comparing FIGS. 5 and 6, addition of Rose Bengal 
approximates the H and D curves for the three batches of microcapsules. 
Having described the invention in detail and by reference to the preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the invention defined in 
the appended claims.