Dye diffusion thermal transfer printing

A dye donor, such as a transfer ribbon, comprises a supporting substrate and a relatively thick dye layer consisting of a dye dispersed within a dye binder. A heater, such as a modulated scanning laser beam, heats selected pixel regions of the ribbon and causes dye to diffuse from the heated regions to a receiver sheet and print a number of pixels thereon which build up to form an image. In order to allow the donor to be reused, it is passed between a pair of heated rollers to cause dye in the dye layer to diffuse to an even density whereby the regions depleted of dye during the print process are replenished. Instead of the replenishment dye coming from the body of the donor, it may be supplied by a separate source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to dye diffusion thermal transfer printing, 
which should be taken to cover sublimation transfer printing, and relates 
particularly to the efficient use of dye in such printing, the term "dye" 
being taken to cover dyes, inks and other soluble colorants. 
2. Description of the Prior Art 
In diffusion thermal transfer printing, heat is applied to selected pixel 
areas of a dye donor sheet or ribbon by a suitable heat source, such as a 
series of resistive heating wires or a scanning laser beam. This heating 
causes diffusion of dye in the selected areas, and transfer of the dye to 
form printed pixels on an adjacent receiver sheet or ribbon. 
The transfer may also be by sublimation, wherein the heating of the donor 
sheet causes the dye to enter the vapour phase. The dye then crosses an 
air gap and condenses onto the surface of the receiver sheet from where it 
may then diffuse inwards. 
After printing, the dye sheet or ribbon is left with a number of dye 
depleted pixel areas where the dye has transferred to the receiver. The 
dye sheet or ribbon cannot therefore be reused, and must be discarded 
after a single print from the sheet or after the end of the ribbon is 
reached. This is wasteful, as much dye still remains on the dye sheet or 
ribbon in the regions which were not printed from. 
SUMMARY OF THE INVENTION 
The present invention aims to provide a system which is less wasteful of 
dye, and, from a first aspect, provides a dye diffusion thermal transfer 
printing system comprising dye donor means carrying an amount of thermally 
diffusible dye, receiver means for receiving dye from the donor means, and 
means for heating selected regions of the donor means to cause dye in 
those regions to transfer to the receiver means, wherein means are 
provided for replenishing, with thermally diffusable dye, regions of the 
donor means which have become depleted of dye through printing. 
From a second aspect, the invention provides a method of dye diffusion 
thermal transfer printing in which selected regions of a dye donor means 
are heated to cause dye in those regions to transfer to a receiver means, 
wherein regions of the donor means which have become depleted in dye 
through printing are replenished with thermally diffusable dye. 
The replenishment of the dye depleted regions allows the donor means to be 
used repeatedly, so that it need not be discarded after merely a single 
print run or at the ribbon end. Therefore, at least some of the dye 
remaining in the unprinted regions of the donor means is not lost, and may 
be used during further print operations. Moreover, savings are also 
provided because the dye donor means itself is re-used. For example, a dye 
ribbon may typically comprise a substrate for supporting a dye layer, a 
dye binder within which dye is dispersed to form a dye layer, and a laser 
light absorbing material either as a separate layer or dispersed with the 
dye, and, by the present invention, all of these materials are able to be 
re-used. 
In a preferred form, the depleted regions are supplied with dye from other 
regions of the donor means. In this case, means may be provided for 
supplying heat to the donor means after printing to cause dye from 
undepleted regions of the donor means to diffuse into the depleted 
regions. This replenishment heating means may heat the donor means in any 
suitable manner. It need not be of as high an intensity as the print 
heating means and, indeed, it is preferable for the replenishment heating 
means to operate at a low power level to provide a more even dispersion of 
dye. 
In this preferred self-reservoir form, the donor means may comprise a dye 
ribbon or sheet having a supporting substrate with a dye layer thereon 
which is thicker than is standard to enable sufficient dye to be held 
within the layer to replenish the dye transfer regions over a plurality of 
print cycles. The dye diffuses from the body of the layer to replenish the 
dye depleted surface regions. In this embodiment, the replenishment heat 
may be applied directly to the dye layer side of the ribbon or sheet, 
and/or through the substrate. Heating through the substrate may be 
preferable, as it promotes faster dye diffusion from regions of the dye 
layer close to the substrate to the dye depleted regions produced at the 
dye layer surface. 
The replenishment heating means may take any suitable form, and may 
comprise a radiant element over which the ribbon or sheet passes. 
Alternatively, the heating means may contact the ribbon or sheet, with the 
ribbon or sheet passing over a flat or arcuate face of the heating means. 
In one preferred form, the heating means comprises roller means, having 
one or more rollers in contact with the ribbon or sheet, and one or more 
of the rollers heated. This has the advantage that the roller means may 
also be used to wind on the ribbon or sheet during printing. Also, two 
opposed rollers provide better ribbon/sheet contact. 
If the replenishment heating means contacts the donor means surface, then 
it is preferred for the heating means to have a surface to which the donor 
means has no or very little adhesion, and which has low affinity for the 
dye. For example, the heating means may comprise a polypropylene coated 
surface, such as polypropylene coated paper against which the dye sheet or 
ribbon may be heated by a hot roll laminator. 
When using a ribbon, it may be wound on separate spools or housed within a 
reversible cassette, and, may be heated to replenish the dye depleted 
regions during a rewind operation after the whole ribbon has been printed 
from. The ribbon may also form a continuous loop, the system having a 
print station at one position along the loop and a replenishment station 
at another. 
When using a dye sheet, it may be replaceably mounted on a rigid support 
means and moved from a print station to a replenishment station, or may be 
mounted on the outer periphery of a rotating drum with print and 
replenishment stations at circumferentially spaced positions adjacent the 
drum. These arrangements allow the dye sheet to be handled easily, the dye 
sheet remaining on the support means for a number of print runs, until its 
dye concentration becomes too low and it is replaced. 
In a further self-reservoir form of the dye donor means, the donor means 
may comprise a dye pad consisting of a solid body portion throughout which 
dye may diffuse, such as a dye filled porous pad. A dye pad can be more 
robust and have better handling properties than a ribbon, and, moreover, 
may hold more dye and so last longer before needing to be replaced. 
Indeed, it may be possible to refill the pad once the dye in it has been 
used up over a number of print runs. 
The pad may be in any suitable form, and may comprise a solid block having 
a flat or arcuate printing surface which lies against the receiver means, 
in use, and which either moves between print and replenishment heating 
positions, or is stationary at the print position and is surrounded by 
heating means, and/or contains heating means within it, to ensure that dye 
continually diffuses to the pad printing surface during printing. This may 
be achieved, for example, by an elongate pad having one, preferably 
tapering, end mounted to face the receiver means, and with the heating 
means lying along and around a length of the pad. 
The pad could also be in the form of a roller having printing and 
replenishment stations around its periphery, with perhaps also heating 
means mounted within the roller to ensure that dye nearer the centre of 
the roller may diffuse toward the outer regions where dye depletion takes 
place. 
In a preferred form, a laser is used to heat selected regions of a dye pad 
which is in the form of a porous carbon roller which may be made by 
sintering or any other conventional process for forming a porous carbon 
element. Here, the carbon itself absorbs the laser energy, thus becoming 
hot and transferring the heat to the dye. 
The carbon pore sizes need to be controlled to ensure uniform heating and 
dye retention, with very small pores leading to excessive resistance to 
dye rising to the surface and very large pores giving uneven printing. 
Pore sizes of between about 0.01 and 10 .mu.m in diameter have been found 
to work well, with pore diameters of between about 0.05 and 2 .mu.m being 
preferred. 
After transfer, the roller can be heated by radiation heating (e.g. from a 
filament lamp), as discussed above, or by electrical heating, using the 
resistive properties of the carbon. 
When using a dye pad such as a carbon roller, it can be desirable to 
incorporate a carrier material into the composition, which can aid in 
transfer of the dye to the receiver medium, but whose principle use is to 
provide faster equilibrium at the surface of the pad during the 
replenishment process. It has been mentioned that the pad may from time to 
time be refilled, and the replenishment mixture will depend on the extent 
of uptake of the various components in the pad by the receiver. This need 
not correspond to the optimum concentration for the whole pad. For 
example, if carrier molecules are present and transfer more slowly than 
the dye, then a lower concentration of the molecules will be desirable in 
the replenishment mixture. 
An alternative to using a carbon roller as the dye pad is to use a thick 
coating of, for example, dye, binder and infra-red absorber on a solid 
mandrel. This coating may be up to 10 mm thick and consist of equal parts 
of the three components. Here, a radiation heating element or light bulb 
may be used to heat the surface to redistribute the dye. Again, a carrier 
material may be provided. In this embodiment, the dye concentration will 
eventually become too low to allow printing, and the pad will need 
replacing. This may be delayed by regularly removing the surface of the 
pad, thus removing the portion which has become most depleted in dye, and 
incidentally removing any contaminants that may have accumulated at or 
near the surface. 
These contaminants may be particulate in nature (e.g. dust) or may arise 
from chemical decomposition of the dye, binder or IR absorber due to 
continual thermal cycling. They could also include dyes that have been 
clawed back out of the image as printed by previous pads in a colour 
printing sequence (discussed later). 
Removal of the surface layer may conveniently be carried out by a hot blade 
or by applying a porous sheet, such as paper, to the hot surface to carry 
away the surface layer. 
In a further embodiment, the dye pad may comprise filter paper, which may 
be in the form of a sheet and may be mounted on a supporting roller. Such 
a sheet may be, for example, a millimeter or more thick. 
The replenishing dye need not necessarily be contained within the dye donor 
means, and may be held in a separate source which transfers dye to the 
donor means. This can be advantageous, since, as mentioned above, the dye 
concentration in a self-reservoir donor means may eventually fall so low 
that further printing from the donor means will not be possible. Also, the 
dye in the donor means may deteriorate over time. By providing fresh dye 
from a separate source, however, these problems may be overcome. 
This separate dye source may take the form of a heated dye reservoir, such 
as a heated porous pad, dye being transferred to the donor means during 
contact with the reservoir. Alternatively, the source could also replenish 
the dye donor means by exposing it to dye vapour. Separate heating means 
may be provided after the replenishment point, in order to ensure that the 
newly transferred dye is evenly distributed in the donor means. 
A donor means for use with this separate dye source may take the form of a 
dye transfer ribbon or sheet, similar to the ribbon and sheet 
self-reservoir arrangements mentioned above, but, in this case, the dye 
layer need not be so thick, and the ribbon or dye sheet may pass over a 
heated dye reservoir whilst contacting a surface of the reservoir from 
which dye diffusion to the ribbon or sheet takes place. This reservoir may 
be in the form of a block having a flat or arcuate surface, or in the form 
of a dye roller with perhaps an opposing pressure roller to urge the 
ribbon or sheet against the dye roller. As in the above self-reservoir 
arrangements, the ribbon may be continuous, rewindable and mounted in a 
cassette, while the dye sheet may be mounted on a support such as a 
rotating drum. 
The donor means may alternatively be in the form of a pad movable from a 
print position to a replenishment position, in contact with a dye 
reservoir, or may be in the form of a roller having print means and 
reservoir means at circumferentially placed positions about its periphery. 
The portion of the pad which carries the dye need not be as thick as for 
the above-mentioned self-reservoir reservoir pads, since the dye will 
always be supplied to and transferred from the pad surface regions. 
In all the above embodiments, be they self-reservoir or not, the print 
heating means for transferring dye from the donor to the receiver means 
may take any suitable form such as an array of resistive heating wires, an 
array of laser beams, a scanning laser beam, or even ultrasound. These 
print heating means may be arranged, as is standard, to heat the selected 
pixel regions through the dye donor means. For example, a laser beam may 
pass through the supporting substrate and heat the body of the dye layer 
before heating the dye at the desired surface transfer regions. In a 
preferred form, however, the print heating means heats the dye donor means 
through the receiver means. This may be achieved, for example, with a heat 
source of resistive heating wires, by employing a thin receiver means 
having good thermal conduction properties, or, with a laser source, by 
using a receiver means which is transparent to the laser light. By this 
arrangement, the dye in the regions of the dye layer nearest the receiver 
means may be heated first, without the heat needing to spread through the 
body of the dye layer. This then increases print speed, and is especially 
advantageous where the donor means is its own reservoir and the dye layer 
has a relatively high thickness. Moreover, the support substrate of the 
dye ribbon or sheet does not need to be transparent to a laser beam or 
thin enough to allow heat from resistive wires to penetrate efficiently, 
and so may be made from a wider range of materials and in a more rugged, 
for example thicker, form, so that it is better able to survive a number 
of print runs. Furthermore, this arrangement allows the print heat source 
to be on the opposite side of the receiver means to the donor means, which 
can simplify the construction of the apparatus when using a dye pad or 
other rigid dye carrier, as the bulk of the dye pad or carrier could 
otherwise hinder the mounting of the source and the application of heat to 
the donor means transfer regions near the receiver. 
That is not to say, however, that arrangements with the pad and print 
heating means on the same side of the receiver means are not possible. For 
example, a rotating drum having a dye sheet thereon may be hollow and have 
a peripheral wall transparent to the light from a laser source mounted 
within, but separately from, the drum. Further, a stationary dye pad may 
have a channel extending therethrough, the end of the channel being 
bridged by a thin dye donor element which is continually supplied with dye 
diffusing from the rest of the pad. A laser beam may then be guided along 
the channel to impinge on and heat dye in the donor element and provide 
dye transfer, or resistive wires could be mounted in the channel. Such a 
pad could take the form of a cylinder with the thin dye donor element 
extending across one of the cylinder ends or could comprise two or more 
separate pad portions connected together at one end by a thin bridge 
element. In either case, the pad or pad portions and replenishment heating 
means need to be arranged to ensure continuous diffusion of dye from the 
pad or pad portions to the thin dye donor element. 
The systems of the present invention may be used to print full colour 
images by forming a number of separate prints onto a single receiver 
sheet, each separate print using a dye of a different colour, for example 
yellow, cyan and magenta. A problem which may occur, however, is that dye 
already printed onto the receiver means may reverse migrate during a 
subsequent print to contaminate the next donor means. Measures may 
therefore be taken to prevent this from happening. In one method, the 
receiver means is heated after each individual dye print, so that the dye 
penetrates deeper into the receiver means. This leaves less dye at the 
receiver surface, and so there will be less reverse migration to a 
subsequent dye source. In addition or as an alternative to this, the 
receiver means may have a sublayer which is more attractive to the dye 
than is the surface layer, so that dye is pulled in to again leave less 
dye at the surface layer. 
It is also possible to fix the dye in the receiver means between each dye 
print. This may be done in several ways. For example, the dye may be fixed 
chemically by a suitable reactive species in the receiver medium, 
especially by means of acid-base reactions or by complexation (mordanting) 
reactions of suitable dyes. Such reactions are known in the art for 
diffusion thermal transfer printing practised with thermal heads and 
disposable ribbons. The fixation of the dye can impede the uptake of 
further dyes by the receiver layer, and it is sometimes necessary to 
provide separate receiver layers for each colour. Thus, after printing a 
yellow dye, for example, a new receiving layer with appropriate fixing 
properties for a magenta dye may be applied to the surface of the print. 
An alternative is to incorporate fixing agents specific to each colour 
distributed throughout the receiver layer, so that the system does not 
become saturated with one colour and reject further dye. 
A further method of reducing reverse migration is to reduce the dye 
mobility by illuminating the receiver means with ultraviolet light or 
other suitable radiation. A receiver of suitable composition becomes 
cross-linked, thus impeding the reverse migration of dye. Further receiver 
layers are then applied as above. 
A number of physical methods may also be used to prevent reverse migration. 
For example, a thin film may be laminated onto the surface of the receiver 
means after each dye print, the film being impenetrable to dye on the side 
adjacent the receiver means, but receptive to dye on its opposite side so 
that a subsequent dye may be printed onto it. 
Another measure is to print each separate dye onto separate receiver means, 
and to then laminate the receiver means together. It is preferable, in 
this case, for each receiver means to be quite thin, and they may 
therefore be mounted onto a substrate after printing for extra support. 
A further approach is to provide an air gap between the dye donor and 
receiver means, in which case dye transfer may occur by sublimation. 
Reverse transfer of dye is then reduced by the air gap, which may also act 
as a barrier to the heating of the dyes already on the receiver means. The 
air gap may be provided by microspheres protruding from the dye donor or 
receiver means' surfaces. 
The receiver means need not of course be the final article onto which a 
print is to be formed and may be an intermediate carrier which bulk 
transfers a printed image of one or more dye colours to one or more 
further receiver means. The intermediate carrier is preferably impermeable 
to the dye or dyes used so as to ensure that the print is easily 
transferrable to a further receiver means. The intermediate carrier may be 
kept warm, so that the dye is a liquid or soft solid to allow the bulk 
transfer of the printed image to a further receiver means by the 
application of pressure. In another embodiment, both heat and pressure are 
applied to produce the bulk transfer, whilst, in still another embodiment, 
the bulk transfer may be by sublimation of the dye across an air gap. 
Use of an intermediate carrier has the advantage that the diffusion 
properties of a dye are not so important at the bulk transfer stage, and 
so it is possible to print onto a wider range of receiver materials. Also, 
when producing colour prints, the intermediate carrier may transfer each 
dye colour separately and be cleaned between each transfer, so that dye 
from a previous print does not contaminate a subsequent dye source. An air 
gap could also or alternatively be used, as described above. 
A preferred form of intermediate carrier is a roller made of glass or other 
laser light transparent inorganic material. A laser beam may then pass 
through the roller to cause the dye transfer, and the roller may then 
carry the dye to the final receiver. Virtually all dye can transfer from a 
glass roller to the final receiver, because glass has very little affinity 
for dye, and so all of the dye remains on the roller surface and does not 
penetrate into the roller body. It is preferred to use an air gap, so that 
sublimation transfer takes place and a solid dye deposit forms on the 
roller surface. This gap may conveniently be defined by frosting the 
roller surface, for example by using a mechanical or chemical etching 
process to provide the desired relief. It is preferred for the depth of 
the features to be between about 0.5 and 30 .mu.m, and advantageously, 
between about 2 and 10 .mu.m. The final transfer may be advantageous 
achieved by passing the receiver medium through a nip between the glass 
roller and a heated rubber roller. 
Where the print heat source is a laser or other radiation source, the dye 
donor means must be able to absorb the radiation energy to heat the dye. 
Therefore, either the dye needs to be able to absorb the radiation, or a 
separate radiation absorber dispersed with the dye or formed as a separate 
layer needs to be provided. Where the donor means is a dye pad, it could 
itself absorb the laser energy, e.g. the carbon roller discussed above. 
Where the donor means contains its own reservoir of dye and the radiation 
absorber is in a separate layer, this layer is preferably permeable to the 
dye so that it can be arranged near the transfer surface of the donor 
means without preventing dye diffusing through this layer to the surface 
regions from deeper within the donor means. If the radiation absorber is 
the dye or transfers to the receiver with the dye, and if the radiation 
reaches the dye source through the receiver means, it is preferable for 
each separate coloured dye, or the radiation absorber used with each 
separate dye, to absorb radiation of different wavelengths, as otherwise 
the dye or radiation absorber already transferred to the receiver means 
may impede further heating of the donor means, through its absorption of 
the radiation energy. 
The dye will normally be dispersed within a suitable binder, such as 
polyvinyl butyral. In order to facilitate dye replenishment, it may be 
advantageous to use a binder which becomes somewhat fluid at the 
temperature of the replenishment heating, such as a chlorinated wax, for 
example Cereclor 70.

DESCRIPTION OF THE ENCLOSED EMBODIMENTS 
In the embodiment of FIG. 1, a dye transfer ribbon 1 comprises a supporting 
substrate 2 and a relatively thick dye layer 3 consisting of a dye 
dispersed within a dye binder. The ribbon 1 passes around a roller 4 and 
contacts a receiver sheet 5 consisting of a dye receiving layer mounted on 
a supporting substrate. The ribbon 1 and receiver sheet 5 are placed in 
contact and moved passed a laser source 6 which scans a laser beam 7 
across the width of the ribbon 1. The beam 7 is modulated as it is scanned 
to heat selected pixel regions of the ribbon 1 and cause dye to diffuse 
from these regions to the receiver sheet 5 and print a number of pixels 
which build up to form an image. The ribbon 1 then passes between a pair 
of heated rollers 8a, 8b to cause dye in the dye layer 3 to diffuse to an 
even density, so that the regions depleted of dye during the print process 
are replenished. 
The ribbon 1 may be in any suitable form, and may comprise a continuous 
loop or be housed in a reversible cassette. Instead of using a pair of 
heated rollers 8a 8b, only one of the rollers may be heated, and indeed, 
the outer roller 8b could be omitted. The receiver sheet 5 could be 
transparent to the laser beam 7, in which case the beam 7 could impinge on 
the ribbon 1 through the receiver 5, and the laser source 6 could be 
mounted on the receiver side of the apparatus. 
FIG. 2 shows an apparatus again using a ribbon 1 and laser source 6, but, 
in this embodiment, the ribbon 1 is in the form of a continuous loop, and 
the regions of the ribbon depleted through printing are replenished by 
fresh dye from a separate heated dye reservoir 9. 
The ribbon dye layer in this embodiment need not be as thick as that in the 
first embodiment, as it does not need to hold a reservoir of dye. 
Moreover, the ribbon 1 may not need to be changed as often as in the first 
embodiment, because the dye concentration can be kept constant and will 
not fall below that required for producing a print of acceptable quality. 
Again, the receiver 5 may be transparent to the laser beam 7 so that the 
laser source 6 may be mounted upon the receiver side of the apparatus and 
not interfere with the mounting of the dye reservoir 9. 
In the FIG. 3 embodiment, a dye pad 10 is used instead of a transfer ribbon 
1. The receiver sheet 5 is transparent to the laser beam 7, so that the 
beam 7 can scan across the surface of the dye pad 10 and cause diffusion 
of dye from the surface regions of the pad 10 to the receiver sheet 5. 
After a surface region of the pad 10 has been scanned by the beam 7, it is 
passed across a low-level heater 11 which causes dye in the pad 10 to 
diffuse into the regions depleted by the printing and form an even 
distribution of dye. The pad 10 may comprise a porous carbon roller having 
pores of between 0.01 and 10 .mu.m, and may be resistively heated instead 
of or in addition to the low-level heater which may be a filament lamp. As 
indicated schematically on FIG. 3, and as will be obvious to those skilled 
in the art, multi-color printing can be accomplished by transferring a 
number of separate prints onto a single receiver sheet 5, each separate 
print using a dye of a different color. Thus, the dye pads 10, 10' and 10" 
will contain different color dyes which will be caused to diffuse to the 
receiver sheet 5. As shown in the case of pads 10' and 10", it may be 
desirable to include an air gap between the individual dye donors and the 
receiver 5 to prevent reverse color migration from the receiver to the 
donors. 
As an alternative to this embodiment, the peripheral surface only of the 
pad 10 may be suitable for dye diffusion, and the pad may receive fresh 
dye from a heated dye reservoir mounted in place of the low-level heater 
11. 
FIG. 4 is also a dye pad embodiment, but, in this case, the dye pad 12 is 
stationary so that dye is transferred to the receiver 5 from the same pad 
surface regions. The pad 12 is surrounded by a heater 13 to ensure that it 
is heated sufficiently to enable dye to continually diffuse through the 
pad to the transfer regions during printing. 
In this embodiment, the receiver sheet 5 is again transparent to the laser 
beam 7 to allow the pad 12 and laser source 6 to be mounted on opposite 
sides of the receiver sheet 5 out of each other's way. It is, however, 
possible to mount the laser source 6 on the same side of the receiver 
sheet 5 as the dye pad 12, and an embodiment achieving this is shown in 
FIG. 5. 
In this embodiment, a stationary pad is made up of two separate pad 
portions 14 connected together at one end by a short thin bridge element 
15 transparent to the laser beam 7. Dye in the pad portions 14 are heated 
by heating elements 16, so that the dye diffuses across the bridge 15 
where the laser beam 7 may heat it and cause transfer to the receiver 
sheet 5. 
Instead of being formed of separate dye portions 14, the dye pad could 
comprise a single, for example cylindrical, dye pad having a channel along 
its centre axis with the transparent bridge 15 extending across one end of 
the channel. The laser beam 7 may then propagate down this channel to 
impinge on the bridge 15. 
It will be appreciated that the above are merely specific embodiments of 
the present invention, and that other variations also fall within the 
scope of the invention. For example, an array of heated resistive wires, 
or ultrasound, may be used as the heat source, instead of a laser beam. 
Also, as represented in FIG. 7, the donor may be a dyesheet 1', having a 
support substrate 1' and a dye layer 3', and may be mounted on a carrier, 
such as a rotating drum 18. Further, as shown in FIG. 8, the receiver 
sheet could be replaced by one or more intermediate carriers which 
transfer a finished printed image to a final receiver sheet, such as a 
glass roller 20 frosted to provide an air gap. As described above, the 
laser beam 7' may pass through roller 20 to cause dye transfer to the 
surface of roller 20 from dye donor pad 12'. Final image transfer to 
receiver medium 5 may be achieved by passing the receiver through a nip 
between roller 20 and a rubber roller 22 which is heated by a heater 24. 
Also, colour prints may be produced by printing a number of images onto a 
single receiver sheet, each image using a differently coloured dye, and 
means being provided to prevent reverse migration of already printed dye 
into the donor means of subsequent dyes. 
EXAMPLE 1 
The principle of a self-replenishing dyesheet is demonstrated in this 
example. It involves the use of a thick dye layer which when imaged 
becomes depleted of dye at the surface of the coating. This depleted area 
is then replenished after printing by passing the dyesheet through a hot 
roll laminator to facilitate mobilisation and redistribution of dye. 
A dye coat solution of the following formulation was coated on to S grade 
melinex: 
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Magenta Dye 
0.833 g 
IR absorbing dye 
0.197 g 
PVBBX1 0.444 g (polyvinyl butyral from Sekisui) 
ECT10 0.111 g (ethyl cellulose from Hercules) 
THF 11.11 g (tetrahydrofuran) 
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The coating was laid down with a K bar to give a dry coat thickness of 4.5 
.mu.m. 
The standard printing procedure used was as follows. The dyesheet was 
placed against a transparent receiver film and the two held together 
against an arc to retain laser focus by the application of 1 atmosphere 
pressure. The media was arranged so that the dyesheet rested underneath 
the receiver film (i.e. the incident laser light passing through the 
receive before being absorbed in the dyecoat). An SDL 150 mW diode laser 
operating at 807 nm was collimated using a 160 mm achromat lens and 
projected through the receiver film to be absorbed in the dyecoat. The 
incident laser power was about 100 mW and the full spot size (full width 
at half power maxima) about 30.times.20 .mu.m. The spot was scanned across 
the media by a galvanometer to address the laser to locations 20.times.10 
.mu.m apart to give good overlap of adjoining dots. At each location the 
laser was pulsed for a specific time to build up a block of colour on the 
receiver. Blocks of varying optical density were produced by varying the 
laser pulse times. The extent of dye transfer was determined by measuring 
the transmission optical density of the printed block on the receiver 
using a Sakura densitometer (manufactured by Konishiroku) operating with a 
green filter. 
The depletion of dye in the dyecoat was observed by repeated printing with 
a single dyesheet. In FIG. 6, the points joined by dashed lines represent 
the OD obtained from the dyesheet after 1, 2 and 3 consecutive prints at 
different laser pulse times. The diagram shows that the dyesheet becomes 
gradually more depleted particularly when printing with longer laser pulse 
times. 
Only a fraction of dye within the dyecoat is transferred during these 
printing steps, the depletion occurring in the surface region of the 
dyecoat. By allowing the dye to redistribute after the printing step, the 
depleted areas of the coating become replenished with dye thereby 
improving the optical density delivered by the dyecoat. This was 
demonstrated by carrying out a second print test with a fresh piece of 
dyesheet, but before each of prints 2 and 3, the dyesheet was removed from 
the printer and laminated against polypropylene coated paper using an 
Ozatec 350 Hot Roll Laminator operating at 150.degree. C. and 0.5 m/min in 
order to facilitate redistribution of dye within the dyecoat. (PP coated 
paper provides a surface against which the dyecoat can be laminated 
without removal of the dye coating from the substrate, and into which the 
dye will not diffuse to any great extent. In principle the dyecoat could 
be laminated against any other surface to which it has no or very low 
adhesion, and which has low affinity for the dye in the coating). 
The results from this second print test are also summarised in FIG. 6 where 
the points joined by solid lines show the OD obtained for the same laser 
pulse times as in the previous test. Comparison of the solid lines with 
the dashed lines in the figure shows the improvement in OD seen when the 
dyesheet has been heated, allowing the unused dye to replenish the 
depleted regions in the coating prior to re-use. 
EXAMPLE 2 
The principle of a porous pad (as exemplified by the use of filter paper), 
which is replenished from a separate reservoir, is demonstrated in this 
example. The following dye solution was used (masses are in grammes): 
______________________________________ 
M3 4.2 
PVBbx1 2.8 
S101743 2.8 
Tospearl 3 .mu.m 3.2 
THF 216 ml 
______________________________________ 
The dye M3 is 
3-methyl-4(3-methyl-4-cyanoisothiazol-5-ylazo)-N-ethyl-N-acetoxy-ethylanil 
ine. 
The infra-red absorber S101743 is Hexadeca-.beta.-thionaphthalene 
copper(II)phthalocyanine. 
The silicone gel spheres, Tospearl, provide an air gap between the dye and 
receiver sheets. 
The filter paper was initially dipped twice into a reservoir of the dye 
solution and allowed to dry for ca. 10 mins. before printing. 
Printing entailed imaging a block of colour 2.7.times.2.6 cm 
(1500.times.1500 pixels) onto a transparent receiver sheet, which 
consisted of a 10% solution of Vylon 200 in THF with a K5 bar and dried 
for 30 sec. at 100.degree. C. The printer delivered an energy of 3.802 
J/cm.sup.2. 
All laminations were carried out at 140.degree. C. at a speed of 0.5 m/s. 
All prints were post-heated after printing to fix the prints, as dye 
sublimation was occurring. This was done at 140.degree. C. for 1 min. 
The dyesheet was initially laminated to smooth out the surface of the 
filter paper and ensure an even distribution of dye. It was then fixed to 
the printer platen and a series of print runs were made without any 
treatment to the dyesheet between prints. The results were: 
______________________________________ 
Print No. 
Mean OD 
______________________________________ 
1 1.68 
2 1.15 
3 0.91 
4 0.8 
5 0.65 
6 0.5 
______________________________________ 
The same experiment was repeated, but, in this case, the dyesheet was 
dipped into the dye solution between prints. It was allowed to dry for ca. 
10 mins. before being laminated ready for printing again. The results 
were: 
______________________________________ 
Print No. 
Mean OD 
______________________________________ 
1 1.15 
2 2.15 
3 2.5 
4 2.65 
5 2.75 
______________________________________ 
Comparison of the results of the two experiments shows the improvement in 
OD seen when the dyesheet is replenished, and the ability of the dye sheet 
to be used more than once. 
EXAMPLE 3 
The principle of replenishment by exposure of a dyesheet to dye vapour is 
demonstrated in this example. A piece of 50 .mu.m S-grade Melinex was 
coated with a solution of binder, IRA and filler with a K3 bar, as follows 
(masses in grammes): 
______________________________________ 
PVBbx1 3.33 
S101743 1.2 
Tosp.4.5 .mu.m 
0.9 
THF 75 ml 
______________________________________ 
The coated Melinex was then stretched taut over a Petri dish the bottom of 
which was covered with a carpet of magenta M3 dye. The dish was placed in 
a vacuum oven at 150.degree. C. and the pressure reduced to 1000 mBar. The 
dish was left for two hours after which the Melinex was removed and 
subjected to laser printing. 
The (opaque) receiver consisted of a 10% solution of Vylon 200 in THF 
coated with a K5 bar. Printing entailed imaging a block of colour 
2.7.times.2.6 cm (1500.times.1500 pixels); the printer delivered 3.345 
J/cm.sup.2. The receiver was post-heated for 1 min. at 140.degree. C. to 
fix the dye. Subsequent replenishments were for the same two hour period 
with the same conditions. 
The initial heating gave a mean dyesheet OD of 4.65 in transmission, the 
first replenishment increased this to 4.94 and the OD was constant 
thereafter. The results were: 
______________________________________ 
Replenished: Non-replenished: 
Print No. 
OD (ref.) Print No. 
OD (ref.) 
______________________________________ 
1 1.2 1 1.3 
2 0.8 2 0.8 
3 1.01 3 0.7 
4 0.96 4 0.59 
______________________________________ 
Again, comparison of the results shows the ability of the replenished 
dyesheet to be reused, without degradation of the optical density.