APPARATUS, RECEIVER MEDIUM AND METHOD FOR DYE SUBLIMATION TRANSFER PRINTING

Abstract: An image printed onto a receiver sheet (6) by dye sublimation transfer printing is fixed in the receiver sheet (6) by a flash of infrared rich light from e.g. xenon or krypton flash tubes (2). The receiver sheet (6) is mounted on a glass support plate (4). A reflector (3) directs the light towards the receiver sheet (6), and a reflector (5) reflects back infrared light which has passed through the receiver sheet (6). Instead of or in addition to using infrared light, a flash of ultraviolet and/or any other invisible electromagnetic radiation could be used.

Description: Apparatus, Receiver Medium and Method for Dye Sublimation Transfer Printing The present invention relates to a method, a receiver medium, and apparatus for dye sublimation transfer printing. In dye diffusion thermal transfer printing, a dye donor sheet and a receiver sheet (or other such dye donor and receiver mediums) are held in intimate contact with one another, and selected regions of the dye sheet are heated by for example a modulated scanning laser beam or thermal head. This causes dye from the selected regions to diffuse into the receiver sheet to form a corresponding image therein. An alternative method of dye transfer printing is by sublimation. This is the method of interest in the present invention. In sublimation transfer, there is a gap, typically of a few microns, between the donor and receiver sheets. Heating of the donor sheet causes the dye to enter the vapour phase. The dye then crosses the gap, and condenses onto the relatively cool surface of the receiver sheet. This is therefore a true sublimation process and should not be confused with diffusion transfer (described above) , which is sometimes erroneously referred to as "sublimation transfer" . Typically, the gap is provided by spacer particles, such as microbeads, mounted in the surface of the donor and/or receiver sheets. An advantage of sublimation in a laser transfer process is that it requires less energy for a given amount of dye transfer than does laser diffusion transfer. Further, it is less prone to image defects caused by dust particles between the donor and receiver sheets, because of the gap. A problem with sublimation transfer, however, is that the receiver may not become hot enough to allow the transferred dye (or dyes, if colour printing) to diffuse adequately into the receiver. The dye may therefore be vulnerable, and easily wiped off. The method may also be ineffective at providing colour, as all of the dye is clustered together into microcrystals rather than being molecularly dispersed in the receiver. To overcome these problems, the dye is usually fixed into the receiver after printing. One method of fixing is to heat the receiver in an oven, so that the dye then diffuses adequately into the receiver's body. This method, however, is time-consuming and thermally inefficient. A further method is to expose the receiver to solvent vapours as for example described in US 5162291. This is, however, a difficult process, and ventilation and fire precautions are usually needed, as solvent vapours can be toxic and flammable. The present invention aims to provide an improved method, receiver medium and apparatus for sublimation transfer printing which addresses the problem of fixing an image in a receiver. Viewed from one aspect, the present invention provides a method of dye sublimation transfer printing, in which an infrared absorber is included in the receiver medium, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of infrared light to fix the transferred dye therein (the absorber being heated by the infrared light to heat the dye on the receiver medium and cause the dye to diffuse into the receiver medium) . Viewed from a further aspect, the present invention provides a method of fixing a dye or dyes into a receiver medium, in which an infrared absorber is included in the receiver medium, and in which the receiver medium with the dye thereon is exposed to a flash or burst of infrared light. The invention extends to a receiver medium incorporating an infrared absorber therein in amounts and arrangements suitable for fixing dye transferred to the receiver medium, when irradiated by a flash or burst of infrared radiation. The invention further extends to an apparatus for dye sublimation transfer printing including an infrared light source which provides a flash or burst of infrared radiation for fixing an image on a dye receiver medium after the receiver medium has been printed to. From another aspect, the invention provides apparatus for fixing a dye or dyes into a receiver medium, after it has been printed to, which includes means for exposing the receiver medium to a burst or flash of infrared light. By the present invention, the transferred dye is heated through the absorption of the infrared light by the infrared absorber in the receiver medium. The heated dye diffuses into the body of the receiver medium from the surface to fix the image, and to thereby protect the image from damage and help to maximise the effectiveness of the dye absorption. A flash provides an efficient and fast method of fixing the dye. To the extent that a flash delivers the energy quickly, the heat does not diffuse through the receiver substrate to any significant degree, and so the heating is localised near the surface and the energy is used efficiently. Further, the dye does not spread out laterally, and so a sharp image may be produced without loss of resolution. In this latter regard, the original image should preferably be produced without any grain or other unevenness that would otherwise be hidden by the greater spread of the dye occurring during for example oven fixing. The absorber provides a substantially uniform absorption of infrared energy throughout the receiver medium, and so uniform fixing of the image. Without the absorber, the receiver medium might not receive the necessary amount of heat from the flash to adequately fix the dye, especially in regions of the receiver medium of low dye concentration. Any suitable infrared source may be used which is able to emit the electromagnetic radiation in a short high-intensity burst. The source may for example be a flashlamp, and may comprise a photographic studio flashlamp. The source preferably comprises a xenon or krypton tube, which may discharge a capacitor. Preferably, more than one discharge tube is used to provide the infrared flash, as this helps to provide a more even illumination of the receiver medium. Preferably, six to ten tubes are used, with eight being particularly preferred. The tubes are preferably longer than the length of the portion of the receiver medium which is to be flashed for fixing, as there tends to be a reduction in the intensity of the light emitted at the ends of the tubes, which is partly due to the presence of the electrodes in the tubes, and partly due to the fact that all of the points along the tube contribute to the illumination. The longer length tubes thus provide a more uniform illumination over the fixing area, and mitigate against the problem of lower illumination intensities at the edges of the fixing area. The flash tubes are preferably fed independently, e.g. they may be fed independently through inductors or resistors from a single charge storage capacitor. With such an arrangement, the energy discharge through the different tubes can be varied to compensate for any residual weakness of illumination at the edges of the array. For example, the tubes at the edges of an array of tubes may be controlled to produce brighter flashes than those at the centre, and the tubes may be controlled so that the intensities of the outputs from the tubes increase with the nearness of the tubes to the edges. A further way of increasing the uniformity of the intensity of the light over the fixing area is to move the flash source further away from the receiver medium. In this case, there are increased losses in the intensity of the flash, which should be compensated for by increasing the total energy output of the tubes. In one preferred embodiment, means are provided between the flash source and the receiver medium to diffuse, refract or scatter the light and produce a more uniform illumination front. This means may be in the form of e.g. a frosted window between the flash source and receiver medium, or a window having a series of shielding stripes thereon which are designed to reduce the intensity of the brightest regions of the illumination front from the tubes. The stripes could for example be frosted portions or filter elements on the window. In one embodiment, the means could comprise an arrangement of lenses for providing an even illumination, e.g. a series of cylindrical lenses. The flash may have a duration of from about 0.1 μs to about 500 ms, and is preferably between about 50 μs and about 5 mε. More preferably, the pulse length is between about 0.15 and about 1 ms, with 0.5 ms being especially preferred. The energy delivered by the flash at the surface of the receiver medium may be between about 0.5 and about 10 Jem" 2 , preferably between about l and 4 Jem" 2 (as for example measured by a broad band pulse energy meter) . The upper limit on the duration of the flash may be determined by the ability of heat to diffuse away from the area where it is absorbed. A long flash time means that the energy is used inefficiently, as a substantial thickness of the receiver is heated by the energy, rather than having all of the heat concentrated initially in the surface layer where diffusion is desired. The lower limit may be set by two main factors. Firstly, at short pulse times, the energy may be put in so fast that extremely high temperatures may be instantaneously generated which may cause significant resublimation of the deposited dye, so that the resublimed dye is lost from the image. High energies at short pulse times can also cause the temperature to rise so high that decomposition of the materials sets in, causing dark patches to appear due to carbonisation of the materials. Secondly, as the pulse time is shortened, the balance of the light emitted by a flash tube tends to shift to shorter wavelengths, so that the coupling to the infrared absorber may be less efficient. Also, due to this shift to shorter wavelengths, at short pulse durations at least as much of the total radiation emitted may be in the visible and ultra-violet regions of the spectrum, as in the infra-red region. This can cause large quantities of energy to be absorbed directly by the dye, which may increase the amount of loss by sublimation. The relative amount of infra-red emission may be increased, as said, by increasing the pulse duration, and also by increasing the area of the discharge. Both factors lead to a cooler discharge, which emits preferentially at longer wavelengths, although the efficiency (radiation out/electrical energy in) may be reduced. Thus, the optimum pulse duration may be determined by a compromise between the overall radiation efficiency, the proportion of the emission occurring in the infrared, whether or not an ultraviolet absorber is additionally incorporated into the receiver, and the susceptibility of the dyes to resublimation and decomposition at high temperatures. The lower limit of energy delivered may be determined by the quantity needed to provide a sufficiently high fixing temperature that allows the dyes to diffuse into the receiver. The upper limit of energy may be determined by a need to limit the cost of the system by not wasting energy, and also by the danger of causing unwanted side effects at high energies. For example, high energies would be expected to cause excessive deformation of the receiver substrate as the bulk of it became heated. A major advantage of short flash times is the coupling of the localisation of the heat with the great temperature dependence of dye diffusion. For oven fixing, a time of 1 minute at 140 °C is generally required, whereas, by the present invention, fixing can take place in less than 1 ms, which is consistent with temperatures of about 300°C. Reflecting means may be placed behind the infrared source, e.g. a flash tube or tubes, to direct the light forward. The reflecting means may be a planar reflector, but preferably includes side elements for reflecting back light from the side edges of e.g. an array of discharge tubes, which light would otherwise escape and make the edges of the illumination front dimmer than the centre. These side elements could be perpendicular to the planar back portion of the reflector means, or for example angled outwardly to the perpendicular to concentrate more light nearer the edges, which might otherwise be less illuminated than the centre. Reflecting means, e.g. mirrors, may be provided at the ends of the discharge tubes. These should be pierced to allow the tubes to penetrate them. The reflecting means may be made from a blank folded into a five-sided box having openings in two opposed sides to accommodate the discharge tubes. The reflecting means may for example be comprised of aluminium, an aluminised reflective sheet or a white diffusive reflector. If a receiver medium is used which is partially transparent to infrared light, then a further reflecting means may be placed on the opposite side of the receiver medium to the infrared source to reflect back residual light which may have passed through the receiver medium. This reflector may take the form of e.g. an aluminium or aluminised reflective sheet or a white diffusive reflector. Alternatively, the receiver medium itself may have a substrate designed to reflect back the light. This may be in the form of a white substrate on which a layer of dye receiver material is formed. For good performance, there should be a good wavelength match between the energy emitted by the infrared source, and the absorption of the infrared absorber in the receiver medium. Preferably, there should be a strong infrared peak in the infrared source, preferably in the near infrared, and a corresponding absorption in the infrared absorber. Any suitable absorber may be used, preferably one that is substantially transparent in the visible region of the spectrum. Filters may be used to modify the characteristics of the light from the infrared source, and for example a filter may contain dyes to alter the spectral distribution of the energy. Filters may be used to reduce the proportion of visible and/or ultra-violet radiation in the light. This helps to ensure that only the receiver medium is significantly heated, and prevents large amounts of heat from being absorbed directly by the dye, which can tend to cause loss of the dye from the image by resublimation. The use of such a filter may require a higher energy fixing flash, as some of the infrared energy may also be filtered out. Such filters are particularly advantageous when short flash durations are used, as flash tubes generally emit a higher proportion of visible light at shorter flash durations. As said, the absorber preferably absorbs at a peak wavelength of the infrared source, e.g. at about 800 nm for a xenon flashlamp, as this ensures efficient conversion of the light energy to heat. So as not to interfere with the final image, the absorber also preferably gives low visible Optical Densities, e.g. the average optical density in the visible region is preferably less than about 0.3 for transparencies, and preferably about 0.15 or less. It is further preferable to neutralise any colour that the absorber may give to the receiver medium in the visible region of the spectrum by adding appropriate dyes to the receiver medium in appropriate concentrations. For example, a yellowish colour may be compensated for by adding cyan and magenta dyes to the receiver medium. This may then give the receiver medium a neutral grey appearance. Typical absorbers may be substituted phthalocyanine dyes, squarylium dyes, cyanine dyes, or other dyes known in the art to provide strong absorption at around 800 nm with little absorption in the visible region of the spectrum. Dyes added to compensate for the visible colour of the receiver medium may be azo-dyes or anthraquinone dyes or any other type of soluble dye, provided only that they are used in such combinations and quantities that any colour bias caused by the absorber is neutralised. Both the infrared absorbers and the dyes can be water or solvent soluble depending on the structure of the receiver layer of the receiver medium. The absorber may be distributed throughout the receiver medium, either at a set concentration or at a varying concentration which for example is preferably greater towards the print receiving surface of the receiver medium. The absorber is preferably provided in a layer close to or on the surface of the receiver medium, as this allows the heat to be supplied directly to the desired location, i.e. the dye, and prevents energy from being wasted in heating unnecessary portions of the receiver medium. The receiver medium may take any suitable form. A typical receiver may comprise a coating of a soluble polyester incorporating the infrared absorber and neutralising dyes, on a substrate of for example biaxially oriented polyester such as Melinex (produced by ICI) . The coating is typically between about 0.5 and about 5 μm in thickness, preferably between about 1 and 4 μm. In one preferred embodiment, a lower energy pre- flash is applied before the main fixing flash. The pre- flash may then aggregate the dye somewhat, so that it is not vaporised by the main fixing flash. The pre-flash and full flash may be provided in any suitable manner, for example by operating the flash source at different power outputs, by varying the distance between the lamp and the substrate, or by placing one or more filters between the source and the receiver. In the latter case, a combination of filters may be used to provide the pre-flash whilst a single filter may be used to provide the full flash. When colour printing (e.g. by making three passes over the receiver medium, each time printing to it with a different coloured dye, such as cyan, magenta and yellow) , it may be desirable to fix the dyes to some extent between each print run. The present invention is particularly useful in this situation, in that the fixing is quickly and easily carried out using compact and simple equipment. These intermediate fixing stages may be carried out at lower energies than the energy used at a final fixing stage at which all of the dyes are present. The lower fixing energies help to reduce thermal distortion of the substrates that could otherwise lead to misregistration between the colours. The intermediate fixing flashes preferably deliver an energy at the surface of the receiver medium which is between about 1.2 and 1.8 Jem "2 . If the substrate supporting the receiver layer is transparent to infrared radiation, then the radiation may be directed at the printed or unprinted side of the receiver. Irradiation through the unprinted side may be advantageous and for example may allow a printing set-up in which the receiver medium is held print side up on a transparent support plate (for example a glass sheet) through which the radiation is passed. This provides a compact and simple arrangement in which the printed side of the receiver is not damaged by contact with the support plate. Irradiation through the unprinted side may not however be as energy efficient as irradiation from the printed side, as any energy absorbed by the receiver substrate may not be efficiently transferred to the receiver layer. Also the bottom of the receiver layer will tend to absorb more energy than the top (because of attenuation of the beam as it passes through the absorbing layer) . Fixing need not be carried out simultaneously over the whole of the receiver medium, and may for example be carried out progressively, with successive regions of the print being fixed one at a time. Thus, the receiver medium may be exposed to multiple flashes, as, for example, it is advanced across a flash area, or as the flash area is advanced across it. The invention may be used with any type of receiver medium, e.g. a transparency, and is particularly useful in printing requiring high resolutions. This is especially so for the printing of 35 mm slides, which have to be magnified for viewing, and so show up even small defects. The invention may be used in any dye sublimation transfer system. For example, the dye donor medium may be heated by laser beams, such as from laser diodes, by LEDs, by ultrasound, or by an incandescent source or flash (e.g. through a mask or spatial light modulator) . As said above, the use of flash infrared light is especially advantageous, and, although it is preferable to use an absorber in the receiver, this is not always necessary. From a further aspect, therefore, the invention provides a method of dye sublimation transfer printing in which an image is fixed in a receiver medium by exposing the medium to a flash of infrared light. The invention also extends to dye sublimation transfer apparatus incorporating a source of infrared light for providing a flash of light to fix an image in a receiver medium. From a still further aspect, the invention provides a method of fixing a dye or dyes into a receiver medium, in which the receiver medium with the dye thereon is exposed to a flash or burst of infrared radiation. Although the above description emphasises the use of an infrared absorber in the receiver medium and the use of a flash rich in infrared (which is the most preferred method of carrying out the present invention) , the invention also extends to flash fixing using a flash of any suitable wavelengths of electromagnetic radiation (preferably ones which are invisible to the eye) in combination with a suitable absorber in the receiver medium. Therefore, viewed from a further aspect, the present invention provides a method of dye sublimation transfer printing, in which an absorber is included in the receiver medium for absorbing, preferably invisible, electromagnetic radiation, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of said electromagnetic radiation to fix the transferred dye therein. The invention also extends to a receiver medium incorporating such an absorber, and to apparatus for such a method. In one preferred form, the electromagnetic radiation may be ultra-violet radiation, and the receiver medium may include a suitable ultra-violet absorber therein. Generally, such absorbers will not impart any significant colouration to the receiver medium, and so further dyes will not generally be needed to produce a receiver medium of neutral colour. The source of the UV flash may again be a flash tube, as discussed above, which may be operated for shorter flash times, and/or may use a gas therein of lower pressure, in order to increase the amount of UV light produced. In a further form, the receiver medium may have both UV and IR absorbers therein, in order to utilise both the UV and IR light produced by a flash tube. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which Fig. 1 shows schematically a flash fixing apparatus according to one embodiment of the present invention,- Fig. 2 shows a perspective view of an alternative flash source; and Fig. 3 shows the reflector of Fig. 2 folded flat. Referring to Fig. 1, the fixing apparatus 1 comprises a multiplicity, in this case a pair, of xenon (or krypton) flash tubes 2, a reflector 3 behind the tubes 2, a thin glass support plate 4, and a further reflector 5. The xenon tubes 2 are caused to flash by the momentary application of a high voltage pulse of electricity, which triggers the discharge of a capacitor, initially charged to e.g. 250 V. This flash technology is established art and is used in photographic flash guns, such as those built into compact cameras. The reflector 3 ensures that as much light as possible from the flash is directed towards the glass plate 4. The reflector 3 may, for example, be made from an aluminium, silver or gold coating on a suitable substrate. A receiver sheet 6, which has infrared absorber material therein and which has been printed to by a dye thermal transfer method, is placed on the glass support plate 4, between the plate 4 and the second reflector 5. If the receiver sheet 6 is transparent to infrared light, then either its printed or unprinted surface may be placed onto the glass plate 4. If the receiver sheet 6 is opaque to infrared light, the printed surface should be next to the glass plate 4. When the receiver sheet 6 is transparent to infrared light, residual energy may pass through it. To increase energy efficiency, therefore, the second reflector 5 is used to reflect this residual light back towards the receiver sheet 6. The reflector 5 may, for example, be a mirror surface, such as an aluminium, silver or gold coating on a suitable substrate, or may be a white scattering surface. Once the receiver sheet 6 is placed over the glass plate 4, the discharge tubes 2 are flashed using a suitable energy input so as to produce a flash of light rich in infrared light. The infrared light is absorbed by the infrared absorber in the receiver sheet 6 to heat the sheet and cause the dye previously printed onto the receiver sheet 6 to become fixed. Such a fixing apparatus and method provides a simple, controllable and fast fixing system. It is only one preferred embodiment, and modifications and variations on this embodiment are of course possible. For example, in one embodiment, the tubes are first flashed at a lower energy to pre-fix the dye, and then flashed at a higher energy to fully fix them. The pre¬ fix helps to prevent the dyes from resubliming and leaving the receiver sheet 6 during the higher energy flash. Also, instead of a static system as shown, the apparatus may advance the receiver sheet 6 over an area illuminated by the discharge tubes, so that the sheet 6 is progressively fixed, e.g. by fixing successive regions of the sheet 6 one at a time. Alternatively, the glass plate 4 may remain stationary, and the discharge tubes may be moved instead. In a further modification, filters may be used to modify the characteristics of the flash reaching the receiver sheet 6. A filter may be used to reduce the proportion of visible and/or ultra-violet radiation, to thereby ensure that only the receiver sheet 6 is significantly heated, and to prevent large amounts of heat from being absorbed directly by the dye, which can tend to cause loss of the dye from the image by sublimation. The use of such a filter may require a higher energy flash, as some of the infrared energy will also be filtered out. Such filters are particularly advantageous when short flash times are used, as the flash tubes 2 emit a higher proportion of visible light at shorter flash times . A combination of filters may be used to provide a pre-flash at low energy followed by a flash at full energy to provide the above-mentioned pre-fixing. Flash fixing according to the present invention is especially useful in colour printing, in which it is sometimes desirable to fix the fragile dye layer between the different colour print runs. This intermediate fixing can easily be carried out in practice because flash fixing is a rapid process that can be conducted close to the part of the apparatus where the dye is transferred. The intermediate fixing may be carried out an energies lower than those of the final full fix which is carried out once all of the coloured dyes have been printed onto the receiver sheet. The apparatus may include means for dispersing, e.g. scattering or refracting, the light from the tubes 2 to provide a more even light intensity distribution. For example, the glass plate 4 could be frosted or include shielding (e.g. frosted or filtering) striped areas which reduce the intensity of the light in the brightest regions. The present invention is especially useful in fixing 35 mm slides, and Figs. 2 and 3 show an arrangement for a discharge tube source which is especially suited for such slides. Eight discharge tubes 2 are used in order to provide an even illumination over the fixing area. Suitable tubes are xenon tubes produced by Tec-West USA, Inc., which are 50 mm long with a 4 mm diameter, and which produce a spectrum in the range of 200-950 nm, with peaks at 250 nm, 450 nm, 825 nm, and 900 nm (the tubes have a maximum energy rating of 45 J, anode voltage of 200 VDC Min, 300 VDC Nominal, 400 VDC Max, and a trigger voltage of -4 KV) . In a preferred arrangement, they are operated at about 18 J per tube (energy discharged through each tube) . The discharge tubes 2 are mounted in a five-sided box-shaped reflector 7 for reflecting the light from the tubes towards the flash fixing area which the receiver medium is to occupy. The box 7 is formed from a folded aluminium blank 8. Two opposed sides 9a, 9b of the blank 8 have openings 10 therein for accommodating the ends of the discharge tubes 2. The side edges 9c, 9d may be angled slightly outwardly, e.g. as shown in Fig. 1, to reflect more light towards the edge regions of the fixing area. The tubes 2 may be fed independently, so that their individual energy outputs can be varied to provide a suitably even illumination front, e.g. the tubes towards the edges of the array may flash brighter. The above apparatus may also be used to flash fix receiver sheets using UV light or other electromagnetic radiation which is invisible to the eye, the receiver sheet itself incorporating a suitable absorber material therein corresponding to the electromagnetic radiation used. For example, the flash tubes shown in the figures could be of a kind suitable for providing a flash rich in UV light. This may be achieved by using tubes similar to those for the IR flashes but operated at shorter flash durations and/or using a gas therein of lower pressure. The receiver sheet may then have therein a suitable UV absorber, which may be for example one such as from the range of Tinuvin™ absorbers produced by Ciba-Geigy. The receiver sheets could have more than one absorber therein for absorbing two or more different wavelengths of electromagnetic radiation. For example, the receiver medium could include both infrared and ultraviolet absorbers. Both the IR and UV light from the flash tubes would then be used in the fixing of the dye in the receiver sheet. Example 1 Two receivers were used in this example, each was a transparency comprising a mixture of polyester resin, infrared absorbing dye and other coloured dyes. The absorbing properties of the transparencies can be modified by changing the amount of infrared dye. It should be noted that higher levels of infrared dye in the transparencies lead to unwanted absorption in the visible region of the spectrum. The infrared absorber used has a maximum absorption at about 800 nm, but with some residual absorption in the visible part of the spectrum, which imparts a dull yellow colour to the material. The other dyes were incorporated in order to correct the absorption to a visually neutral grey. The two formulations were: 1) Hand coated formulation using Meyer bars: SC101743 = hexadeca (2-thionaphthalene) copper II phthalocyanine. C2 = CI Solvent Blue 63 M3 = N-2-acetoxyethyl-4- (4-cyano-3- methylisothiazol-5-ylazo) -N-ethyl-3-methylaniline The coating was about 3.8 μm in thickness, and the OD (optical density) at 790 nm was 0.44. The OD in the visible region of the unprinted receiver was 0.06 and 0.09 as measured by a Sakura PDA65 (Konica) and Macbeth TR 1224 meter respectively. 2) Machine coated formulation: The coating was about 3.2 μm in thickness, and the OD at 790 nm was 0.45. The OD in the visible region of the unprinted receiver was 0.06 and 0.09 as measured by a Sakura and Macbeth meter respectively. The transparencies above had incorporated therein spacer particles, and were printed to by laser sublimation transfer printing from a dye sheet of OD 2.88 having a coat thickness of 1.05 μm and a base thickness of 23 μm. The solution used to produce the dye sheet coating was : The printed transparencies were then fixed using an electronic flash gun produced by Bowens International Ltd (an Esprit 125 model modified by using a xenon- filled flash tube of smaller dimensions than standard in order to provide a more concentrated light source) . The xenon-filled tube delivered approximately 2 Jem "2 of energy at the samples, with 60% of the energy delivered in 0.25 ms . The energy density was determined by means of a Scientech 365 power and energy meter, which records the incident energy averaged over a disc of about 4 cm 2 area. As the optics of the reflector on the flash gun was not optimised for this use, there was some variation in the energy density over the samples, and the edges and especially the corners of the samples were in consequence less well fixed than at the centre. A comparison was made between transparency prints fixed as above, and oven fixed transparency prints. Effectiveness of the fixing was measured by first measuring the ODs right across a square print after the flash fixing, and then applying adhesive tape to the surface of the prints, removing the tape, and remeasuring the ODs to find any OD loss . Each transparency print was printed as three equal area regions, a left, centre and right block, with different optical densities. Before being fixed, the ODs of the transparency prints used in each of the following six regimes of example 1 were roughly the same, as they were printed under identical conditions, although they did vary slightly due to dye sheet and focus variations. The average OD values for each block for each of the prints in the following six regimes were: Block 1 = 1.02 ± 0.04 Block 2 = 0.54 ± 0.02 Block 3 = 0.16 ± 0.01 After fixing, the ODs of each print were measured at twenty seven positions, nine in each of the three blocks across the print. The results of six different fixing regimes are shown below. The results shown are the maximum and average OD obtained after fixing for each block, and the maximum and average percentage OD loss after the tape was applied (the average was taken as the mean of several readings over each block of the print) . This procedure was used to indicate the effectiveness and uniformity of fixing, as the actual OD measurements fluctuated across the blocks due to dye sheet and focus variations. OD measurements were made with a Sakura densitometer. The results shown are for the machine-coated receiver defined above, similar results are obtained with the lead-coated receiver. 1) Oven fixing - 1 minute at 140 "C 2) One flash with transparency print adjacent the flash plate (Energy density = 1.8 - 1.9 Jem "2 ) 3) One flash with transparency print 5 mm from the flash plate (Energy density = 1.55 - 1.65 Jem "2 ) 4) One flash with transparency print 10 mm from the flash plate (Energy density = 1.3 - 1.4 Jem "2 ) 5) Two flashes with transparency print on flash plate 6) One flash with transparency print on flash plate, but with the non-printed side of the transparency facing the flash plate As can be seen from the above results, block 2 was generally better fixed than blocks 1 and 3. This was because it was the middle block, and was over the highest light intensity part of the flash tube. This may be compensated for by, for example, optimising the optics of the flash tube to provide a more even energy spread over the fixing region, and/or by advancing the transparencies across the flash region during fixing, so that each portion of the transparencies is flashed by the main flash region of the flash tubes. Also, it can be seen that although there was only a relatively small difference between a 5 mm spacing and no spacing, there was a big difference between the 5 mm and 10 mm spacing, which implies that there is a sharp threshold of energy for fixing to be effective. There was no improvement in OD when the image was exposed to two flashes rather than one flash, but there was an improvement in stability (roughly a two-fold improvement in % OD loss) . This shows that the flash used was not optimised for fully fixing the images in one flash. This may be corrected by increasing the energy of the first flash. The OD and stability were both worse when the image was fixed through the receiver (i.e. with the printed side of the receiver sheet facing away from the flash gun) , which may be due to the delivery of the light energy preferentially to the lower side of the receiver layer. Example 2 A flash gun was used in which the flash duration (to half maximum intensity) was about 100 μs. The energy density at the sample was varied by the use of spacers. The dyesheet used was similar in formulation to that in Example 1. Two receiver formulations were used in two series of experiments. The formulation used in the first series of experiments was the same as that of the hand- coated formulation of example 1. The formulation used in the second series of experiments was similar to that of the hand-coated formulation of example 1, but had an additional quantity of a second infrared absorber 0.171 g of Pro-jet 900 NP (Zeneca Ltd) - thus matching the quantity of SC101743. The values recorded are the average figures over block l as defined in Example 1. Fixing results with SC101743 only: Fixing results with the addition of Pro-jet 900 NP: These results show that under these conditions of short flash times, there is a sharply defined optimum energy density for maximum OD, and that dye loss (by sublimation or decomposition) can limit the OD achieved at high input energy densities. At lower input energy densities, not enough energy is supplied to give complete fixing. The optimum energy density for the single IR absorber is 1.22 Jem "2 , whereas for the combination of IR absorbers (which gives greater total energy absorption in the IR) the optimum is 1.04 Jem "2 . Example 3 This example used the dye sheet and hand-coated formulation for the transparency of Example 1 with filters in front of the flash lamp. The flash was of approximately 100 μs duration, and gave an energy of 2.4 Jem" 2 , which caused dye loss and discolouration when used without filters. When a KB4 (pale blue, absorbence 0.8 at 590 nm) filter was placed between the flash lamp and the transparency, the transmitted energy was reduced to 1.18 Jem "2 , and when a f urther filter, giving a sharp cut-off at wavelengths shorter than 430 nm was combined with the KB4, the energy was reduced to 0.74 Jem" 2 . A transparency printed to as in Example 1 was fixed first through the combination of the two filters, and then through only the KB4 filter. The first flash (through the two filters) , gave incomplete fixing, but incorporated the dye into the receiver without sublimation. The second flash (through only the KB4 filter) then completed the fixing process. The results were compared with oven fixing under standard conditions: It can be seen that under these conditions the OD from flash fixing was slightly higher than for oven fixing, and that there was considerably less loss of the image when stripped with tape. Example 4 In this example, the duration of the flash was lengthened to 500 μs. The dye sheet was as used in Example 1, and the receiver sheet had the following composition: The results were as follows: Under these conditions, there was better control of the fixing process, and there was a monotonic increase in fixing, as the flash energy was increased, until a plateau was reached. In the more densely printed areas, the OD obtained by flash fixing exceeded that of oven fixing. In other experiments, the formulation using Pro-Jet 830NP gave similar results to those obtained using SC101743. The main difference being a small increase in efficiency.

Claims: Claims 1. A method of dye sublimation transfer printing, in which an infrared absorber is included in the receiver medium, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of infrared light to fix the transferred dye therein. 2. The method of claim 1, wherein the infrared flash is provided by a flash source which emits light having an infrared peak, and wherein the absorber absorbs at the peak wavelength 3. The method of claim 2, wherein the peak occurs at about 800 nm. 4. The method of claim 1, 2 or 3, wherein the flash of infrared light is produced by one or more discharge tubes. 5. The method of claim 4, wherein six to ten discharge tubes are used. 6. The method of claim 4, wherein eight discharge tubes are used. 7. The method of claim 4, 5 or 6, wherein the discharge tubes are longer than the length of the portion of the receiver medium which is to be flashed for fixing. 8. The method of any of claims 4 to 7, wherein the energy output of the discharge tubes are controlled independently. 9. The method of any of claims 4 to 8, wherein infrared reflecting means are provided behind the discharge tubes . 10. The method of any of claims 4 to 9, wherein reflecting means are provided at the ends of the discharge tubes . 11. The method of any of claims 4 to 10, wherein reflecting means are provides about the sides of the discharge tubes . 12. The method of claim 11, wherein the side reflecting means are angled outwardly to concentrate light along the edges of the area of the receiver medium to be fixed. 13. The method of any of claims 9 to 12, wherein the reflecting means comprises a blank folded into a five-sided box, having openings in two opposed sides to accommodate the discharge tubes. 14. The method of any of claims 4 to 13, wherein the reflecting means is comprised of aluminium, an aluminised reflective sheet or a white diffusive reflector. 15. The method of any preceding claim, wherein the flash has a duration of from about 0.1 μs to about 500 ms . 16. The method of any preceding claim, wherein the flash has a duration of between about 50 μs and about 5 ms . 17. The method of any preceding claim, wherein the flash has a duration of between about 0.15 and about 1 ms . 18. The method of any preceding claim, wherein the flash has a duration of about 0.5 ms . 19. The method of any preceding claim, wherein the energy delivered by the flash at the surface of the receiver medium is between about 0.5 and about 10 Jem "2 . 20. The method of any preceding claim, wherein the energy delivered by the flash at the surface of the receiver medium is between about 1 and 4 Jem "2 . 21. The method of any preceding claim, wherein the receiver medium has a substrate designed to reflect back infrared light into the receiver medium. 22. The method of any preceding claim, wherein reflecting means are provided on the opposite side of the receiver medium to the source of infrared light. 23. The method of any preceding claim, wherein a filter is used to modify the characteristics of the light from the infrared source. 24. The method of any preceding claim, wherein a filter is used to reduce the proportion of visible and/or ultra-violet radiation in the light. 25. The method of any preceding claim, wherein means are provided for scattering or dispersing light from the tubes in order to produce a more uniform intensity distribution. 26. The method of any preceding claim, wherein the receiver medium is substantially transparent to visible light. 27. The method of any preceding claim, wherein the average optical density produced by the absorber in the receiver medium is less than about 0.3 in the visible region. 28. The method of any of claims 1 to 26, wherein the average optical density produced by the absorber in the receiver medium is about 0.15 or less in the visible region. 29. The method of any preceding claim, wherein the absorber is distributed throughout the receiver medium at a set concentration. 30. The method of any of claims 1 to 28, wherein the absorber is distributed throughout the receiver medium at a concentration which is greater towards the print receiving surface of the receiver medium. 31. The method of any of claims 1 to 28, wherein the absorber is provided in a layer close to or on the surface of the receiver medium. 32. The method of any preceding claim, wherein the receiver medium includes dyes therein to neutralise any colour that the absorber gives to the receiver medium in the visible region of the spectrum. 33. The method of any preceding claim, wherein a lower energy pre-flash is applied before the main fixing flash. 34. The method of any preceding claim, wherein a number of differently coloured dyes are successively printed onto the receiver medium, and wherein intermediate fixing flashes are provided between each print run. 35. The method of claim 33, wherein the intermediate fixing flashes are carried out at lower energies than the energy of a final fixing flash provided when all of the dyes have been transferred onto the receiver medium. 36. The method of claim 33, 34 or 35, wherein the energy delivered by the intermediate fixing flashes at the surface of the receiver medium is between about 1.2 and 1.8 Jem "2 . 37. The method of any preceding claim, wherein a number of fixing flashes are provided to progressively fix the transferred dye into the receiver medium. 38. The method of any preceding claim, wherein the receiver medium comprises a 35 mm slide. 39. The method of any preceding claim, wherein dye transfer is by laser dye transfer printing. 40. A method of fixing a dye or dyes into a receiver medium, in which an infrared absorber is included in the receiver medium, and in which the receiver medium with the dye thereon is exposed to a flash or burst of infrared light . 41. A dye sublimation transfer receiver medium incorporating an infrared absorber therein in amounts and arrangements suitable for fixing dye transferred to the receiver medium, when irradiated by a flash or burst of infrared radiation. 42. Apparatus for dye sublimation transfer printing including an infrared light source which provides a flash or burst of infrared radiation for fixing an image on a dye receiver medium after the receiver medium has been printed to. 43. Apparatus for fixing a dye or dyes into a receiver medium, after it has been printed to, which includes means for exposing the receiver medium to a burst or flash of infrared light. 44. A method of dye sublimation transfer printing in which an image is fixed in a receiver medium by exposing the medium to a flash of infrared light. 45. Dye sublimation transfer apparatus incorporating a source of infrared light for providing a flash of light to fix an image in a receiver medium. 46. A method of fixing a dye or dyes into a receiver medium, in which the receiver medium with the dye thereon is exposed to a flash or burst of infrared radiation. 47. A method of dye sublimation thermal transfer printing, in which an absorber of electromagnetic radiation is included in the receiver medium, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of said electromagnetic radiation to fix the transferred dye therein. 48. A method of dye sublimation thermal transfer printing, in which an absorber of ultraviolet radiation is included in the receiver medium, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of ultraviolet radiation to fix the transferred dye therein. 49. A method of dye sublimation thermal transfer printing, in which an absorber of infrared radiation and an absorber of ultraviolet radiation is included in the receiver medium, and in which, after dye transfer, the receiver medium is exposed to a flash or burst of infrared and ultraviolet raditation to fix the transferred dye therein. 50. A dye sublimation transfer receiver medium incorporating an absorber of electromagnetic radiation therein in amounts and arrangements suitable for fixing dye transferred to the receiver medium, when irradiated by a flash or burst of said radiation. 51. Apparatus for dye sublimation transfer printing including a source of electromagnetic radiation which provides a flash or burst of said radiation for fixing an image on a dye receiver medium after the receiver medium has been printed to.

Inventors: HANN RICHARD ANTHONY, HUTT KENNETH WEST, MAZAR SCOTT THOMAS, NOURBAKHSH FARHAD, SU DAONING, WHALEY GREGORY JOSEPH, HANN, RICHARD, ANTHONY, HUTT, KENNETH, WEST, MAZAR, SCOTT, THOMAS, NOURBAKHSH, FARHAD, SU, DAONING, WHALEY, GREGORY, JOSEPH
Applicants: HANN RICHARD ANTHONY, HUTT KENNETH WEST, ICI PLC, MAZAR SCOTT THOMAS, NOURBAKHSH FARHAD, SU DAONING, WHALEY GREGORY JOSEPH, HANN, RICHARD, ANTHONY, HUTT, KENNETH, WEST, IMPERIAL CHEMICAL INDUSTRIES PLC, MAZAR, SCOTT, THOMAS, NOURBAKHSH, FARHAD, SU, DAONING, WHALEY, GREGORY, JOSEPH
Application year: 1997
Main classification: B41M5
IPC codes: B41J2, B41M5, B41M7