An image-forming element comprising a substrate, and an image-forming medium comprising (a) a compound absorbing at a first wavelength in the UV/blue region and (b) a dye absorbing at a second wavelength which is longer than the first wavelength, irradiation at said second wavelength bleaching absorption of said compound at said first wavelength, said element being free of thermally unstable urea and/or carbamate molecules.

FIELD OF THE INVENTION 
The invention relates to UV absorbing media which are bleachable by IR 
irradiation to provide images suitable for use as masks. 
BACKGROUND TO THE INVENTION 
There is continuing interest in image-forming media suitable for address by 
lasers, particularly media which require no wet processing. Such media are 
particularly useful in medical imaging, recording the output of digital 
radiographic equipment, CAT scanners, magnetic resonance scanners and 
ultrasound scanners on film or paper, and graphic arts imaging, producing 
contact films, colour proofs and printing plates. While some laser 
addressable media have been described, relatively few examples have been 
commercialised. 
In the graphic arts industry, productivity is an issue, since economics 
dictate that the same scanner should be used to output three conventional 
types of media, namely films, proofs and plates. The ability to provide 
two out of three of these media simultaneously would clearly increase 
productivity. 
It is desirable to provide medical imaging films that are a neutral black 
colour in image areas with minimal visible absorption in background areas 
and continuous tone capability to facilitate inspection and interpretation 
by the human eye. Graphic arts films are intended to be used as contact 
masks for the subsequent imaging of plates, proofs and other films. 
Subsequent imaging through masks is normally performed by flood exposure 
to UV/blue light. The image areas should show a high absorption of UV/blue 
radiation while the background areas are essentially transparent at these 
wavelengths for such masks to be effective. The image areas must also 
absorb over at least a portion of the visible spectrum to allow visual 
inspection of the image. A neutral black colour is preferred. A high 
contrast suitable for half tone imaging is also desirable. 
There are a number of techniques for the direct generation of monochrome 
images from laser exposure. A well known approach described in U.S. Pat. 
No. 3,962,513 and U.S. Pat. No. 5,171,650 is the ablation of a pigmented 
layer, for example, one containing carbon black, from a transparent 
substrate by laser irradiation. In a variation of this method, the exposed 
areas of the pigmented coating are transferred to a receptor held in 
intimate contact with the coating layer, as described in WO 90/12342, U.S. 
Pat. No. 5,171,650 and IBM Technical Disclosure Bulletin Vol. 18, No. 12, 
May 1976, p. 4142. EP-A-0465 727 discloses the transfer of UV absorbing 
dyes or pigments as a method of forming a graphic arts film. The 
partitioning of a pigmented layer between two substrates as a result of 
laser exposure is disclosed in WO 93/04411, WO 93/03928, WO 88/04237 and 
U.S. Pat. No. 5,352,562. These materials are inherently high contrast 
materials and are not adapted to continuous tone imaging. 
U.S. Pat. No. 4,981,765 and 5,262,275 and EP-A-0,488,530 disclose the 
thermal transfer of dyes or pigments directly onto the photosensitive 
coating of a printing plate or similar element so as to form an integral 
mask. 
U.S. Pat. No. 4,826,976, 4,720,449, 4,960,901 and 4,745,046 and WO 90/00978 
disclose imaging materials which develop a colour in response to heat, 
which may be supplied through absorption of laser radiation. Yellow, 
magenta and cyan colours are disclosed. 
U.S. Pat. No. 4,602,263 and U.S. Pat. No. 4,720,450 disclose imaging 
systems which bleach under the action of heat by fragmentation of 
thermally unstable carbamate and urea derivatives, respectively. The heat 
may be supplied through absorption of laser irradiation. 
Colour proofs may be formed by transferring, by laser exposure, successive 
images of different colours from appropriate donors to a common receptor, 
as described in WO 90/12342. Although this publication discloses that the 
individual donor sheets, following image transfer, are effectively film 
masks containing image information for their respective colours, such 
materials are of limited use as graphic arts films because many dyes and 
pigments commonly used in colour proofing tend to have insufficient 
UV/blue absorption. 
Laser transfer of a pigmented layer from a transparent donor to a receptor 
having a surface adapted for lithographic printing, such as a grained 
aluminium foil, and thereby producing simultaneously a film mask and a 
printing plate is disclosed in U.S. Pat. No. 3,962,513. Single-sheet 
materials that function as both a film and a plate are not disclosed. 
Photoredox processes involving dyes are well known in the art. A 
photoexcited dye may accept an electron from a coreactant, the dye acting 
as a photo-oxidant, or donate an electron to a coreactant, the dye acting 
as a photoreductant, depending on the properties of the dye and of the 
coreactant(s). Such processes have frequently been exploited for imaging 
purposes, but in general it is the bleaching of the dye that forms the 
image. Examples of this type of imaging include the photobleaching of 
cationic dyes in the presence of organoborate ions (U.S. Pat. No. 
4,447,521 and 4,343,891) and the photobleaching of pyrylium dyes in the 
presence of allylthiourea derivatives (J.Imaging Sci. & Technol. 1993 
(37), 149-155). Alternatively, the products of the photoredox reaction may 
participate in further reactions that lead to image formation. 
Photoredox processes in which a dye acts as a photo-oxidant, i.e., in its 
photoexcited state it accepts an electron from a suitable reductant, are 
of particular relevance to the preferred embodiments of the present 
invention. 
The products of the photoredox reaction may initiate polymerisation (as 
described in EP-A-515133, J.Org.Chem. 1993 (58), 2614-8, and 
J.Am.Chem.Soc. 1988 (110), 2326-8, or inhibit polymerisation (as described 
in U.S. Pat. No. 4,816,379). There are no known references to photoredox 
imaging processes involving a dye and a reducing agent in which bleaching 
of the reducing agent is the primary image forming process. 
The ability of dihydropyridine derivatives to transfer an electron to a 
photoexcited Ru(III) complex is disclosed in J.Amer.Chem.Soc. 1981 (103), 
6495-7. The reactions were carried out in solution and were not used for 
imaging purposes. 
British Patent Application No. 9508027 laser addressable thermal imaging 
media comprising a photothermal converting dye and a dihydropyridine 
derivative. Bleaching of the photothermal converting dye occurs during 
laser exposure, and the effect is exploited for the production of 
uncontaminated colour images. 
BRIEF SUMMARY OF THE INVENTION 
The present invention provides image-forming media, comprising (a) a 
compound absorbing at a first wavelength in the UV/blue region and (b) a 
dye absorbing at a second wavelength which is longer than the first 
wavelength, irradiation of the media at the second wavelength bleaching 
absorption of the compound (a) at the first wavelength, said media being 
free of thermally unstable urea and/or carbamate molecules. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferably the second wavelength is in the red or near infrared region, and 
irradiation is carried out by a laser. 
The image-forming media of the invention, in the form of a coating on a 
transparent substrate, may be used as a mask for the imagewise exposure of 
a UV sensitive material such as a printing plate, photoresist or proofing 
element. Alternatively, the image forming media of the invention may be 
incorporated into an imaging element possessing a further image forming 
medium sensitive to UV/blue radiation. Such an element could comprise a 
first imaging medium comprising components (a) and (b) defined above and a 
second imaging medium sensitive to UV/blue radiation. Initial exposure to 
radiation at said second wavelength generates an image in the first 
imaging medium which acts as a mask for exposure of the second imaging 
medium to UV/blue radiation. The second imaging medium may conveniently be 
in the form of a printing plate as part of the imaging element. 
In the context of this invention, "UV/blue region" refers to a portion of 
the spectrum ranging from about 340 nm to about 480 nm. "Bleaching" refers 
to a decrease in optical density either by elimination of the relevant 
absorption bands or by their shift to shorter wavelengths. 
Imaging media in accordance with the invention are initially opaque to 
UV/blue light, but become transparent on exposure to light of the 
appropriate (longer) wavelength, i.e., they are positive-acting. They are 
well suited to digital address by lasers, and require no processing to 
develop or fix the image. No ablation or transfer of colourant is involved 
in the simplest embodiments, and there are no waste materials requiring 
disposal. 
The resulting pattern of UV-opaque and TV-transparent areas is used as a 
mask for the imagewise exposure of a UV-sensitive material and according 
to a further aspect of the invention there is provided an imaging method 
comprising the steps of: 
(i) providing a first imaging element comprising a transparent substrate, a 
compound absorbing at a first wavelength in the UV/blue region, and a dye 
absorbing at a second wavelength which is longer than said first 
wavelength; 
(ii) exposing said first element to a pattern of radiation of the said 
second wavelength to bleach absorption at said first wavelength; 
(iii) assembling said first element in contact with a second imaging 
element which is sensitive to light of the first wavelength; and 
(iv) exposing the assembly to a source of light at the first wavelength 
with the first imaging element closest to the source. 
It will normally be necessary to subject the second imaging element to one 
or more processing steps to develop and/or fix the final image, depending 
on the constitution of said element. 
The "first wavelength" is intended to reflect a range of wavelengths rather 
than a single precise wavelength. For example, said compound may absorb at 
wavelengths between 340 and 410, with maximum absorption at 390 nm. The 
exposure of step (iv) may be made at any wavelength within the first 
absorption range, not merely at the maximum absorption wavelength within 
that range. 
When the imaging medium of the present invention is used as an integral 
mask, the imaging element comprises a substrate, a first imaging medium 
and a second imaging medium, said first imaging medium comprising (a) a 
compound absorbing at a first wavelength in the UV/blue region and (b) a 
dye absorbing at a second wavelength which is longer than said first 
wavelength, irradiation of the medium at said second wavelength bleaching 
absorption of the compound (a) at the first wavelength, and said second 
imaging medium being sensitive to light of said first wavelength, the 
first imaging medium being placed (optically and physically) such that an 
image formed in said first imaging medium may act as an exposure mask for 
the second imaging medium. 
This aspect of the invention further extends to a method of forming an 
image comprising the steps of: 
(i) providing an imaging element as described in the preceding paragraph; 
(ii) exposing the element to a pattern of radiation at the said second 
wavelength; 
(iii) exposing the element to a source of radiation of the said first 
wavelength; and 
(iv) processing the element to develop an image in said second imaging 
medium. 
The imaging media of the invention comprise two key ingredients, namely (a) 
a compound absorbing at a relatively short wavelength and (b) a dye 
absorbing at a longer wavelength, and these must be selected such that 
light absorption by the latter ultimately causes bleaching of the former, 
and hence the identity of one of these ingredients may govern the identity 
of the other. A variety of mechanisms may bring about the desired 
bleaching. For example, the compound (a) absorbing at the shorter 
wavelength may decompose or rearrange thermally as a result of heat 
generated through absorption of laser radiation by the dye. Alternatively 
(or additionally), the compound (a) absorbing at shorter wavelengths may 
be acid-sensitive, and photoirradiation of the dye may generate a strong 
acid. Suitable compounds which bleach under the action of heat and/or acid 
include those of formulae I and II: 
##STR1## 
in which each R is independently selected from an alkyl group (preferably 
of up to 20 carbon atoms) or an aryl group of up to 10 (preferably 6) ring 
carbon atoms, or both R groups together complete a non-aromatic ring (such 
as morpholine or pyrrolidine); 
each Y is independently selected from an electron-attracting group such as 
CN, COR.sup.1, CO.sub.2 R.sup.1, SO.sub.2 R.sup.1 where R.sup.1 is alkyl 
or aryl, or both Y groups together represent the atoms selected from C, O, 
N, and S required to complete a ring with electron-attracting properties 
(such as barbituric acid, isopropylidene malonate, dimedone etc.); 
each R.sup.2 is independently selected from an alkyl group (preferably of 
up to 20 carbon atoms) or an aryl group of up to 10 (preferably 6) ring 
carbon atoms; and 
M.sup.+ represents a monovalent cation (such as Na.sup.+, K.sup.+, 
NH.sub.4.sup.+). 
Specific examples of compounds of formulae I and II include: 
##STR2## 
A further mechanism for bleaching of compound (a) involves a photoredox 
reaction between the dye and the compound (a) giving rise to the short 
wavelength absorption, in particular photoreduction of the dye by the 
compound (a). In other words, compound (a) transfers an electron to the 
dye when the latter is in a photoexcited state, and in the process 
compound (a) undergoes bleaching. A preferred class of compounds absorbing 
in the UV/blue region, having the formula III below, are believed to 
operate by this mechanism. 
##STR3## 
In formula III, R.sup.3 is selected from H, alkyl, aryl, alicyclic or 
heterocyclic; 
R.sup.4 is an aryl group; each R.sup.5 and each R.sup.6 is independently 
selected from alkyl, aryl, alicyclic and heterocyclic; and Z represents O 
or a covalent bond. 
In this context, "alkyl" refers to alkyl groups of up to 20, preferably up 
to 10, and most preferably up to 5 carbon atoms. "Aryl" refers to aromatic 
rings or fused ring systems of up to 14, preferably up to 10, most 
preferably up to 6 carbon atoms. "Alicyclic" refers to non-aromatic rings 
or fused ring systems of up to 14, preferably up to 10, most preferably up 
to 6 carbon atoms. "Heterocyclic" refers to aromatic or non-aromatic rings 
or fused ring systems of up to 14, preferably up to 10, most preferably up 
to 6 atoms selected from C, N, O and S. All of the above groups may be 
substituted by one or more atoms or groups such as hydroxyl, alkoxy, 
halogen and nitrile. 
As is well understood in this technical area, a large degree of 
substitution is not only tolerated, but is often advisable. As a means of 
simplifying the discussion, the terms "nucleus", "groups" and "moiety" are 
used to differentiate between chemical species that allow for substitution 
or which may be substituted and those which do not or may not be so 
substituted. For example, the phrase "alkyl group" is intended to include 
not only pure hydrocarbon alkyl chains, such as methyl, ethyl, octyl, 
cyclohexyl, iso-octyl, t-butyl and the like, but also alkyl chains bearing 
conventional substitutents known in the art, such as hydroxyl, alkoxy, 
phenyl, halogen (F, Cl, and I), cyano, nitro, amino etc. The term 
"nucleus" is likewise considered to allow for substitution. The phrase 
"alkyl moiety" on the other hand is limited to the inclusion of only pure 
hydrocarbon alkyl chains, such as methyl, ethyl, propyl, cyclohexyl, 
iso-octyl, t-butyl etc. 
British Patent Application No. 9508027 discloses the bleaching of certain 
dyes by means of their photoirradiation in the presence of compounds of 
formula III in what is believed to be a photoredox process, but it is now 
apparent that an effect of this process is that the compounds of formula 
III are themselves bleached in the process, and this effect may be 
exploited for the imaging purposes described herein. Compounds of formula 
III absorb strongly in the UV/blue region, but when formulated with a 
suitable dye and irradiated at wavelengths absorbed by the dye, the 
UV/blue absorption shifts to shorter wavelengths. Although it is rather 
unusual for organic compounds to suffer bleaching as a result of their 
oxidation (the reverse is often the case - cf. the conversion of leuco 
dyes into dyes), it is surmised that the compounds of formula III are 
oxidised by photoexcited dye molecules, because formulations comprising a 
dye and a compound of formula III bleach readily under condition of 
photoirradiation, but not when heated in the dark. Also, there are reports 
in the academic literature of similar compounds acting as reducing agents 
towards photoexcited species (J.Amer.Chem.Soc. 1981 (103), 6495-7). 
A metal salt stabiliser may be used in conjunction with compounds of 
formula III, e.g., a magnesium salt, as this has been found to improve the 
thermal stability of the system without affecting the photoactivity. 
Quantities of about 10 mole% based on the compound of formula III are 
effective. 
In formula III, R.sup.3 is preferably H or phenyl (optionally substituted), 
R.sup.4 is preferably phenyl (optionally substituted), R.sup.5 is 
preferably lower alkyl (of 1 to 4 carbon atoms, esp. methyl) and R.sup.6 
is preferably lower alkyl (e.g., ethyl). Compounds in which Z represents a 
covalent bond show enhanced absorption in the blue region, and hence are 
preferred for many applications. Compounds of formula III may be 
synthesised by co-condensation of an aldehyde, an amine and two 
equivalents of a beta-ketocarbonyl compound in an adaptation of the well 
known Hantsch pyridine synthesis: 
##STR4## 
The other key ingredient of imaging elements of the invention is a dye (b) 
which absorbs at relatively long wavelengths. Since the elements are 
primarily intended for laser address, preferred dyes absorb in the red 
and/or near infrared region to be compatible with the commonly used diode 
lasers and YAG lasers etc. As indicated earlier, the mechanism by which 
the short wavelength absorption of the compound (a) is bleached may 
influence the choice of dye (b). If a purely thermal bleaching mechanism 
operates, then the only restriction on the dye (b) is that it should act 
as an efficient photothermal converter, i.e., it should absorb the laser 
output and generate heat as a result. A vast range of red and infrared 
dyes are known to be suitable for this purpose, including cyanines, 
phthalocyanines, polymethines, oxonols, squarylium dyes, croconium dyes 
and diamine dication dyes. 
On the other hand, if the bleaching process requires the presence of acid, 
then the dye (b) preferably generates acid via laser photolysis. 
(Alternatively, a thermal source of acid may be used in combination with a 
photothermal converting dye as described above). Examples of IR-absorbing 
dyes having acid-generating properties include the tetra-arylpolymethine 
(TAPM) dyes, which are described, for example, in U.S. Pat. No. 5135842. 
Preferred examples have a nucleus of general formula IV: 
##STR5## 
where Ar.sup.1 -Ar.sup.4 are aryl groups which may be the same or 
different such that at least two of Ar.sup.1 -Ar.sup.4 have a tertiary 
amino group in the 4-position, and X is an anion. Examples of tertiary 
amino groups include dialkylamino groups, diarylamino groups, and cyclic 
substitutents such as pyrrolidino, morpholino or piperidino. The tertiary 
amino group may form part of a fused ring system, e.g., one or more of 
Ar.sup.1 -Ar.sup.4 may represent a julolidine group. 
Preferably the anion X is derived from a strong acid (e.g. HX should have a 
pKa of less than 3, preferably less than 1). Suitable identities for X 
include ClO.sub.4, BF.sub.4, CF.sub.3 SC.sub.3, PF.sub.6, AsF.sub.6 or 
SbF.sub.6. Such dyes are believed to form the acid HX on irradiation, and 
the effect appears to be particularly strong when not all of Ar.sup.1 
-Ar.sup.4 are identical. Preferred dyes of formula IV include the 
following: 
##STR6## 
Another preferred class of acid-generating dye is the amine cation radical 
dyes, also known as immonium dyes, having counterions derived from strong 
acids, described for example in W90/12342 and JP51-88016. These include 
diamine di-cation dyes (exemplified by the commercially available 
Cyasorb.TM. IR165 (Glendale Protective Technologies, Inc.)), which have a 
nucleus of general formula V and absorb in the YAG laser wavelength range: 
##STR7## 
in which Ar.sup.1 -Ar.sup.4 and X have the same meaning as before. 
When bleaching is effected by a photoredox process, particularly when the 
dye acts as a photo-oxidant, the preferred dyes are cationic dyes (i.e., 
they have a positive charge associated with the chromophore). Examples 
include polymethine dyes, pyrylium dyes, cyanine dyes, diamine di-cation 
dyes, phenazinium dyes, phenoxazinium dyes, phenothiazinium dyes and 
acridinium dyes, but the most preferred dyes are the TAPM dyes of formula 
IV and the diamine di-cation dyes of formula V. 
The imaging elements of the invention may be prepared by dissolving the dye 
and the bleachable compound in a suitable solvent and coating the mixture 
on a transparent substrate. The substrate must be transparent from the 
near UV to the near infrared, and should be flexible, dimensionally stable 
and heat resistant. Conventional polyester base (e.g., 
polyethyleneterephthalate and polyethylenenaphthalate of thickness 20-200 
microns) is suitable, and may optionally be treated (e.g., by corona 
discharge, or application of subbing layers) to modify its wettability 
towards subsequent coatings. Any conventional coating method may be used, 
such as spin coating, bar coating, roller coating or knife coating. 
The relative quantities of the ingredients may vary with the extinction 
coefficients of the materials themselves and the intended use. The dye 
should be present in sufficient quantity to provide an optical density of 
at least 0.75, preferably at least 1.0 at the output wavelength of the 
laser source. The bleachable compound should be present in sufficient 
quantity to provide an optical density of at least 1.0, preferably at 
least 1.5, at the shorter wavelength. 
Higher imaging speeds are obtained when the dye and bleachable compound are 
coated on the substrate without the addition of a binder. The preferred 
materials described above form good quality coatings in the absence of 
binders, e.g., when vapor deposited or coated as solutions of about 2-15 
wt % solids in typical organic solvents and solvent mixtures, such as 
alcohols, ketones, esters and ethers. However, binders may be used if 
necessary, including vinyl resins, acrylate resins, cellulose esters and 
polycarbonates. Surfactants and/or other coating aids may be added if 
necessary. 
Binderless coatings of the preferred materials are surprisingly durable, 
but greater durability can be obtained by the use of a transparent 
topcoat. In order that the application of the topcoat does not disrupt the 
imaging layer, it is preferable that the topcoat be coated from a solvent 
that is incompatible with the imaging layer, e.g., water. Hence 
hydrophilic materials such as gelatin or polyvinylalcohol are particularly 
suitable as topcoats. 
The elements of the invention may be imaged by conventional techniques. The 
element may be mounted on a suitable stage, e.g., by vacuum hold down, and 
scanned by a suitable laser. The element may be imaged by any of the 
commonly-used lasers, depending on the dye used, but address by near 
infrared emitting lasers such as diode lasers and YAG lasers, is 
preferred. 
Good results are obtained from a relatively high intensity laser exposure, 
e.g., of at least 10.sup.23 photons/cm.sup.2 /sec. For a laser diode 
emitting at 830 nm, this corresponds approximately to an output of 0.1 W 
focused to a 20 micron spot with a dwell time of about 1 microsecond. In 
the case of YAG laser exposure at 1064 nm, a flux of at least 
3.times.10.sup.24 photons/cm.sup.2 /sec is preferred, corresponding 
roughly to an output of 2W focused to a 20 micron spot with a dwell time 
of about 0.1 microsecond. 
Any of the known scanning devices may be used, e.g. flat-bed scanners, 
external drum scanners or internal drum scanners. In these devices, the 
element to be imaged is secured to the drum or bed (e.g., by vacuum 
hold-down) and the laser beam is focused to a spot (e.g., of about 20 
microns diameter) on the absorbing layer. This spot is scanned over the 
entire area to be imaged while the laser output is modulated in accordance 
with electronically stored image information. Two or more lasers may scan 
different areas of the element simultaneously, and if necessary, the 
output of two or more lasers may be combined optically into a single spot 
of higher intensity. 
As a result of the above imaging process, the media in accordance with the 
invention become transparent to UV/blue light in exposed areas, but remain 
opaque to UV/blue light in non-exposed areas. In some embodiments, the 
laser absorbing dye will also bleach in exposed areas, as described in our 
copending application of even date, but this is of secondary importance to 
the present invention. No further processing is needed to develop or fix 
the image. The resulting imaged elements may be used as contact masks, in 
the same way as conventional graphic arts films, for the subsequent 
duplication of the image information (in positive or negative mode) in 
another medium. Thus, they may be used for the contact exposure (in a 
conventional printing frame, for example) of silver halide contact or 
duplicating films, lith films, colour proofing materials, printing plates, 
or indeed any material adapted for exposure to UV/blue radiation via a 
contact mask. 
In an alternative embodiment, the imaging medium of the invention may be 
incorporated in an imaging element sensitive to UV/blue radiation in a 
manner that enables formation of an integral mask for the ultimate flood 
exposure of the element to UV/blue light. Elements in accordance with this 
aspect of the invention generally comprise a substrate, a second imaging 
medium sensitive to UV/blue light, and a first imaging medium which is of 
the type described above. The first imaging medium of the invention must 
be present as one or more layers that are separate from the second imaging 
medium, and positioned so that an image formed in the first imaging medium 
can act as a mask for exposure of the second imaging medium. The second 
imaging medium may take the form of any conventional photosensitive 
medium, such as a photoresist of the type used to form printing plates or 
proofing elements. Such resists may be positive-acting (photosolubilising) 
or negative-acting (photoinsolubilising), or may be susceptible to 
peel-apart development as described below. In use, the element is first 
exposed as described above to form an image in the first imaging medium. 
This is followed by flood exposure from a suitable source (e.g., a metal 
halide lamp or mercury lamp). The exposure may also be effected by laser 
scanning at the appropriate wavelength, with the entire mask surface 
scanned by the laser. Areas of the first imaging medium which did not 
receive exposure in the first step remain opaque and block the passage of 
light during the flood exposure. Conversely, areas that did receive 
exposure in the first step become transparent, and permit passage of light 
in the flood exposure. Thus the flood exposure results in the second 
imaging medium undergoing an imagewise irradiation that duplicates the 
pattern formed in the first imaging medium by the first exposure. The 
final step is processing by any method suitable for developing and fixing 
the image formed in the second imaging medium. For example, the exposed 
(or unexposed) areas may be washed off by a suitable developer, or a 
peel-apart process may reveal the final image. 
This embodiment has the advantage of a single-sheet construction, with no 
need for a vacuum frame in the final exposure. Because the mask image may 
be generated in close proximity to the UV/blue sensitive medium, high 
resolution imaging of the latter is possible, without the optical 
artifacts that may arise from contacting two sheets. 
The exact spatial relationship of the substrate, first imaging medium and 
second imaging medium may vary depending on whether the substrate is 
opaque or transparent. When the substrate is opaque, as is the case of a 
printing plate with an aluminium base, for example, the first (mask 
forming) imaging medium must be coated on top of the photosensitive layer 
of the plate, and exposure must take place from that side. To prevent 
intermixing of the two imaging chemistries, particularly at the coating 
stage, it may be necessary to coat a barrier layer on top of the 
conventional plate coating prior to coating the mask forming chemistry. 
Water-soluble polymers such as gelatin or polvinylalcohol are preferred 
barrier materials, e.g., at a dry thickness of 1-5 microns. 
When the substrate is transparent, as is the case of a colour proofing 
element, the mask-forming chemistry may be positioned above the 
conventional photosensitive layer, or between the substrate and the 
conventional photosensitive layer, or on the back side of the substrate. 
Depending on the direction of flood exposure, any of these positions may 
be suitable. 
In the case of proofing elements intended for the assembly of an integral 
colour proof, the mask-forming chemistry must be either on the back side 
of the substrate, or between the substrate and the photosensitive layer. 
Conventional proofing elements of this type may comprise (in sequence) a 
transparent substrate, a photosensitive layer, a coloured layer and an 
adhesive layer. The photosensitive and coloured layers may be combined in 
a single layer, and other layers, such as barrier layers or release layers 
may also be present, depending on the particular construction. In use, 
conventional materials of this type may be first laminated to a reflective 
base, such as white paper or card, then flood exposed through a contact 
mask before or after removing the transparent substrate. Strippable or 
non-strippable antihalation layers may also be associated with the 
photosensitive medium. 
Development of the image either involves selective wash off of exposed or 
unexposed areas, or alternatively the action of peeling the transparent 
substrate subsequent to exposure may selectively remove exposed or 
unexposed areas of the coloured layer. The latter process is known as 
peel-apart development, and is described in European Application. No. 
93309507.7 and WO92/15920. The entire process is repeated using different 
coloured elements until a full colour image is assembled on the base. 
Proofing elements in accordance with this aspect of the invention differ 
from their conventional counterparts only in that the mask-forming 
chemistry is coated either on the back side of the transparent substrate 
or between the transparent substrate and the photosensitive layer. After 
lamination to a reflective base, the transparent substrate is left in 
place and the mask is generated by long wavelength exposure as described 
previously. After flood exposure through the mask, the transparent 
substrate is peeled away. In a peel-apart element, this develops the 
image, otherwise conventional wet development is carried out. 
From a manufacturing stand point, it is more convenient to coat the 
mask-forming chemistry on the back of the transparent substrate, as 
existing products may be modified with the minimum of disruption to the 
manufacturing process. On the other hand, various advantages may stem from 
coating the mask-forming chemistry between the substrate and 
photosensitive layer. Firstly, the mask is generated immediately adjacent 
the photosensitive layer, which makes for high resolution and eliminates 
optical distortions caused by the thickness of the substrate. Secondly, 
the Dmin areas of the mask seldom bleach to zero optical density, and the 
residual absorption can provide an antihalation effect, leading to 
improved resolution, dot gain control and exposure latitude, as described 
in EP-A-165030. 
The images produced by laser exposure of the media of the invention, in 
their simplest embodiments, appear rather faint to the naked eye because 
the image density is largely in the UV. This does not affect their utility 
as contact masks as described above, although a higher visual contrast is 
desirable for the user's convenience. If other applications, such as 
medical imaging, are contemplated, then a more visible image, preferably a 
neutral black, is essential. In the preferred embodiments of the 
invention, which comprise a compound of formula III and a dye of formula 
IV, this may be achieved by incorporating one or more visible-absorbing 
oxonol dyes in addition to the aforementioned ingredients. The oxonol dyes 
provide optical density in the visible region (400-700 nm), and by 
combining oxonols with appropriate absorption characteristics, a neutral 
black can be obtained. The oxonols bleach cleanly during laser exposure 
along with the compound of formula III. Surprisingly, there is no 
significant speed loss due to the additional presence of the oxonols. The 
extent of bleaching varies with the intensity and/or duration of laser 
exposure, and so continuous tone imaging is possible. 
The mechanism of the oxonol bleaching is not well understood. Coatings 
comprising a dye of formula IV or V and one or more oxonols undergo 
partial bleaching on laser exposure, but the additional presence of a 
compound of formula III provides much more rapid and complete bleaching. 
Suitable oxonol dyes are of the type disclosed in U.S. Pat. No. 4,701,402, 
and specific examples include: 
##STR8## 
A second manner in which the basic construction may be elaborated to 
provide broader utility involves the inclusion of a thermally transferable 
colourant layer, and use of the resulting material as a laser addressable 
thermal transfer donor. The generation of colour images by transfer of a 
colourant from a donor to a receptor in response to laser irradiation is 
well known. By successive transfer of yellow, magenta, cyan and black 
images from the appropriate donors to a common receptor, full colour 
images of high quality, suitable for proofing, may be obtained (as 
described in WO90/12342, U.S. Pat. No. 5,171,650, and EP-A-0602893. In 
systems involving mass transfer of colourant (i.e., where there is either 
0% or 100% transfer of image density depending on whether the input energy 
exceeds a given threshold), it has been recognised that the imaged donor 
sheet represents a negative of the image formed on the receptor, and could 
potentially be used as a mask for the contact exposure of other media. 
However, various problems limit the usefulness of this approach. Firstly, 
the dyes and pigments commonly used as colourants in the proofing industry 
do not absorb strongly in the UV/blue region, and hence the mask formed by 
the donor has insufficient image density. Use of an inert UV absorber in 
addition to the colourant may solve this problem, but unless the added UV 
absorber is completely colourless, the transferred image will be degraded. 
Secondly, mass transfer media do not necessarily transfer all of the 
colourant from exposed areas of the donor. In particular, when transfer 
occurs by the "melt-stick" mechanism, there is a tendency for a 
significant residue to remain on the exposed parts of the donor. This does 
not affect the quality of the image on the receptor, but if the colourant 
is UV-absorbing, or an inert absorber has been added, then use of the 
imaged donor as a mask will be hampered by a high Dmin. 
These problems do not arise when the materials of the present invention are 
used in conjunction with a thermally transferable colourant. The presence 
of a compound such as those of formulae I-III ensures an adequate optical 
density in the UV/blue region regardless of the properties of the 
colourant itself, and because the compound bleaches during laser exposure, 
the Dmin in exposed areas is low even when a residue of colourant remains 
on the donor. Likewise, the bleaching process avoids the problem of 
contamination of the transferred image by cotransfer of the UV-absorbing 
compound. It is therefore possible to form high quality colour proofs made 
up of individual colour separation images (yellow, magenta, cyan etc) at 
the same time (and as a direct result of the same series of laser scans) 
as producing matching film masks corresponding to the colour separation 
information, thereby maximising the productivity of the expensive scanning 
equipment. 
Therefore, according to a further aspect of the invention there is provided 
an imaging element comprising a substrate and an imaging medium comprising 
(a) a compound absorbing at a first wavelength in the UV/blue region and 
(b) a dye absorbing at a second wavelength which is longer than said first 
wavelength, irradiation of the medium at said second wavelength bleaching 
absorption of the compound (a) at the first wavelength, said imaging 
medium additionally comprising a thermally transferable colourant. 
There is further provided an imaging method comprising the steps of: 
(i) assembling in contact with a receptor a first imaging element of the 
type described in the previous paragraph; 
(ii) exposing the assembly to a pattern of radiation of the said second 
wavelength so as to cause thermal transfer of colourant to the receptor in 
irradiated areas and to bleach absorption of the compound (a) at the said 
first wavelength; 
(iii) separating said first imaging element from the receptor and 
assembling said first imaging element in contact with a second imaging 
element which is sensitive to radiation of the first wavelength; and 
(iv) exposing the assembly to a source of radiation of the first 
wavelength. 
Essentially any of the known mass-transfer colourant systems may be used in 
this embodiment of the invention, including waxy pigmented layers 
(JP63-319192), ablative materials (WO90/12342, U.S. Pat. No. 5,171,650) 
and binderless layers of vapour deposited dyes or pigments (International 
Patent Application PCT/GB92/01489). The preferred transferable colourant, 
however, comprises a dispersion of pigment particles in a binder together 
with a fluorochemical additive, as described in EP-A-0602893. Materials in 
accordance with the present invention may be prepared simply by adding a 
bleachable UV/blue absorber to the laser transfer imaging media described 
therein. The UV/blue absorber may be present in the colourant layer or in 
a separate layer, but for optimum bleaching it should be in the same layer 
as the laser-absorbing dye. 
A further embodiment of the present invention, providing enhanced 
productivity, combines the functions of a film and a printing plate. 
EP-A-0652483 describes laser addressable printing plates requiring no 
dissolution processing which comprise a substrate bearing an 
infrared-sensitive coating, which coating becomes relatively more 
hydrophillic on exposure to infrared radiation. In preferred embodiments, 
the coating comprises an infrared absorber and a polymer having pendant 
hydrophobic groups which react under the action of heat and/or acid to 
form hydrophillic groups. Most preferably, the infrared absorber generates 
an acid by absorption of radiation and/or an additional source of acid is 
present in the coating. The preferred reactive polymer is a copolymer of 
tetrahydropyranyl methacrylate and a vinyl-functional alkoxysilane. 
Suitable substrates include transparent polyester film. The coating is 
initially hydrophobic and hence readily accepts inks commonly used in 
lithographic printing. On exposure to infrared radiation, the coating 
becomes relatively more hydrophillic, so that in the presence of aqueous 
press fountain solutions the exposed areas are ink-repellent, i.e., the 
plate is positive-acting. The preferred source of IR radiation is a laser 
diode, and so the plates have the unique combination of digital 
addressability and no requirement for wet processing. 
The acid-generating dyes preferred for use in the printing plate described 
above are identical to the dyes of formulae IV and V preferred for use in 
the present invention. Hence, by adding a bleachable UV/blue absorber 
(such as a compound of one of formulae I-III) to the plate formulation, 
there is obtained a laser-addressable imaging element that combines the 
functions of a film and a printing plate (provided the formulation is 
coated on transparent base). The bleach chemistry does not interfere with 
the plate-forming chemistry, and vice-versa. 
Thus, a film/plate of the invention may be imaged in the normal way by a 
laser scanner, and the resulting imaged element used as a mask for the 
contact exposure of a colour proof, for example. Thereafter, the imaged 
element may be mounted on press without further processing and used to 
print multiple copies in the normal way. Once again, the productivity of 
the expensive scanning equipment is maximised.

The invention is hereinafter described in more detail by way of example 
only with reference to the accompanying figure which shows the absorption 
spectra of a film following exposure at different scan rates. 
In the examples, the following materials are used; 
Dyes D1 and D4 --polymethine dyes absorbing at ca. 830 nm, structures as 
follows: 
##STR9## 
IR165 --"Cyasorb IR165", an IR absorbing (ca. 1060 nm) diamine di-cation 
dye supplied by Glendale Protective Technologies. 
Compounds (1) and (2)--heat and/or acid bleachable UV absorbers of 
formulae: 
##STR10## 
Compounds (3) and (4)--visible absorbing oxonol dyes of formulae: 
##STR11## 
Compounds (5) and (6)--UV absorbing dihydropyridines of formulae: 
##STR12## 
MEK--methyl ethyl ketone (2-butanone). 
VAGH and VYNS--vinyl resins supplied by Union Carbide. 
FC--N-methylperfluorooctanesulphonamide. 
THP homopolymer--poly(tetrahydropyran-2-yl methacrylate), prepared as 
described in WO92/09934. 
Butvar.TM. B76--polyvinylbutyral, supplied by Monsanto. 
DRC(TM) Film--a negative-acting, high contrast graphic arts contact film 
supplied by 3M. 
EXAMPLE 1 
This example demonstrates direct formation of UV masks by media in 
accordance with the invention. 
The following formulation was coated on 102 micron unsubbed polyester at 12 
micron wet thickness and air dried to produce Element 1 : 
______________________________________ 
Butvar .TM. B76 (10 wt % in MEK) 
5.5 g 
Dye D1 0.125 g 
Compound (1) 0.20 g 
______________________________________ 
The resulting coating had an Optical Density (OD) of 2.8 at 380 nm and 1.2 
at 830 nm. It was mounted on an external drum scanner and imaged by means 
of a laser diode (830 nm, 100 mW, 20 micron spot) scanned at 200 cm/sec. 
In the exposed areas, the UV absorption was bleached to OD 0.6. The imaged 
element was placed in contact with a piece of DRC film and exposed to a 5 
kW UV source in the conventional manner. After conventional processing, 
the DRC film bore a negative replica of the original image. 
Elements 2 and 3 were prepared by the same method using the following 
formulation: 
______________________________________ 
Butvar .TM. B76 (10 wt % in MEK) 
5.5 g 
Dye D1 0.1 g 
Bleachable Compound 0.6 g 
______________________________________ 
Compound (5) was used in Element 2, giving OD 1.8 at 350 nm, and Compound 
(6) in Element 3, giving OD 1.5 at 350 nm. When subjected to the above 
imaging process, the OD in both cases fell to 0.6, and the resulting masks 
were used successfully for the exposure of DRC film as before. 
EXAMPLE 2 
This example demonstrates direct formation of UV masks by binderless media 
in accordance with the invention. 
The following formulations were coated on unsubbed polyester base (102 
micron) at 12 micron wet thickness and allowed to dry, to produce Elements 
3-8: 
______________________________________ 
Element 4 5 6 7 8 
______________________________________ 
MEK 5 5 5 5 5 
EtOH 0.2 0.2 0.2 0.2 0.2 
Dye D1 0.1 -- 0.1 -- -- 
Dye D4 -- 0.1 -- -- 
IR165 -- -- -- 0.1 0.1 
Compound (1) 
0.1 -- -- 0.1 -- 
Compound (2) 
-- 0.1 -- -- -- 
Compound (5) 
-- -- 0.6 -- 0.6 
______________________________________ 
(all quantities are parts by weight) 
Elements 4-7 were mounted on an external drum scanner and imaged by a laser 
diode (830 nm, 110 mW, focused to 20 micron spot), scanned at 200 cm/sec 
to write tracks on the media. Element 8 was mounted on an internal drum 
scanner and imaged by a YAG laser (1060 nm, 2 W, focused to 26 micron 
spot), scanned at 6400 cm/sec, to write tracks on the media. The optical 
densities at 360 nm before and after exposure are recorded in the 
following table: 
______________________________________ 
OD(initial) 
OD(final) 
______________________________________ 
Element 4 1.8 0.6 
Element 5 1.8 0.6 
Element 6 2.0 0.3 
Element 7 1.8 0.6 
Element 8 1.8 0.3 
______________________________________ 
The results demonstrate efficient bleaching of the UV absorption in all 
cases. 
EXAMPLE 3 
This example demonstrates the use of oxonol dyes to provide a neutral black 
coating which is bleachable by laser exposure. Element 9 was prepared as 
before, using the following formulation: 
______________________________________ 
MEK 3.5 g 
Ethanol 0.5 g 
Dye D4 0.05 
Compound (3) 0.025 g 
Compound (4) 0.05 g 
Compound (5) 0.3 g 
______________________________________ 
The mixture was roll-milled in a brown bottle under dim light for 30 
minutes prior to coating. 
The resulting film was neutral black in appearance, and had the absorption 
spectrum denoted by curve A in FIG. 1 which represent a plot of optical 
density against wavelength. Curves B and C of FIG. 1 denote the spectrum 
after laser exposure at scan speeds of 400 and 200 cm/sec respectively 
(830 nm laser diode, 116 mW, 20 micron spot). Clearly, bleaching occurs 
across the spectrum in proportion to the degree of exposure, indicating 
that the film is suitable for continuous tone imaging. 
Another sample of the same film was imaged with a half tone dot pattern via 
laser scanning at 200 cm/sec, and the resulting mask was used successfully 
to image DRC film as before. 
EXAMPLE 4 
This example demonstrates the simultaneous generation of matched film and 
proof images in accordance with the invention. 
Magenta and black millbases were prepared by dispersing 4g of the 
appropriate pigment chips in 32g MEK using a McCrone Micronising Mill. The 
pigment chips were prepared by standard procedures and comprised VAGH 
binder and either blue shade magenta pigment or black pigment in a weight 
ratio of 2:3. 
Element 10 was prepared by coating the following formulation as in previous 
examples: 
______________________________________ 
Magenta millbase 5.5 g 
MEK 5.5 g 
Ethanol 0.5 g 
Dye D1 0.165 g 
Compound 5 0.85 g 
Magnesium nitrate 
0.05 g 
FC 0.025 g 
______________________________________ 
A sample of the resulting donor sheet was assembled in face to face contact 
with a receptor sheet (VYNS coated paper) and mounted on an external drum 
scanner. Line scans were made at 200, 400, 600, 800 and 1000 cm/sec using 
an 830 nm laser diode (100 mW, 20 micron spot diameter), and transfer of 
magenta pigment was observed for scans at 600 cm/sec or less, although a 
residue of pigment remained in the exposed areas. The imaged donor was 
then used as a mask for the exposure of DRC film as before, and an 
accurate replica of the image formed on the receptor was obtained. 
Elements 11 and 12 were prepared in identical fashion from the following 
formulations: 
______________________________________ 
Element 11 Element 12 
______________________________________ 
Magenta millbase 
-- 5.5 g 
Black millbase 
2.75 g -- 
MEK 2.5 g 2.0 g 
Ethanol 0.5 g 1.0 g 
IR165 0.06 g 0.2 g 
Compound (5) 
0.35 g 0.6 g 
FC 0.125 g 0.025 g 
______________________________________ 
Element 11 was imaged in the same way as Element 10, with pigment transfer 
observed at scan speeds of 400 cm/sec or less. The imaged donor was again 
used successfully as a mask for the exposure of DRC film. 
Element 12 was contacted face to face with "Rainbow" (TM) thermal transfer 
imaging receptor (supplied by 3M) and imaged via a YAG laser as described 
for Element 8 in Example 2. A magenta image was formed on the receptor in 
response to the laser exposure (line width 17 microns). The imaged donor 
was again used to image DRC film, and the line width of the resulting DRC 
film image matched that of the receptor image. 
EXAMPLE 5 
This example demonstrates simultaneous generation of film and plate images 
by media in accordance with the invention. 
Element 13 was prepared as for previous examples from the following 
formulation: 
______________________________________ 
THP homopolymer (10 wt % in MEK) 
5.5 g 
Dye D1 0.15 g 
Compound (1) 0.20 g 
______________________________________ 
The resulting coating was pale pink in colour and showed an intense UV 
absorption band (OD&gt;3.0). Laser imaging was carried out as before at scan 
rates in the range 200-800 cm/sec. Bleaching of both the IR and UV 
absorptions was seen in exposed areas. The imaged element was then used as 
a mask for the UV exposure of a positive acting peel-apart colour proofing 
element of the type described in EP-A-0601760. The result was a positive 
reproduction of the image present on Element 13. 
The imaged Element 13 was then mounted on an Apollo web-fed printing press, 
wiped with fountain solution (Mander Kidd), and inked with Van Son Black 
40904 (rubber based) ink. The laser-exposed areas repelled the ink, 
whereas the non-exposed areas inked up cleanly within a few revolutions of 
the plate cylinder. 100 impressions on newsprint were taken without signs 
of wear or background toning. The printed images were an exact replica of 
the colour proof image.