In dye thermal transfer printing, selected regions of a dye donor sheet are heated to cause dye to pass from the heated regions of the donor sheet to corresponding regions of a dye receiver sheet to form an image therein.

The heating may be carried out in a number of ways, for example by a thermal head comprising a number of heatable elements or by a scanning laser beam which is modulated as it is scanned.

Transfer of the dye may be by a diffusion process in which the donor and receiver sheets are held in intimate contact with one another. Alternatively, transfer may be by a sublimation process in which there is a gap of typically a few microns between the donor and receiver sheets. In sublimation transfer, the heated dye enters the vapour phase, crosses the gap between the sheets, and condenses onto the relatively cool surface of the receiver sheet. 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 transfer in a laser printing process is that it requires less energy for a given amount of dye transferred than does diffusion transfer. Further, because of the gap, the printing is less prone to image defects caused e. g. by dust particles between the donor and receiver sheets.

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 the dyes are 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 receivers 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.

An improved method of fixing has been proposed in which an infrared absorber material is included in the receiver sheet, and in which, after printing, the receiver sheet is exposed to a flash of light rich in the infrared, e. g. by using a flash from a xenon flash tube. The receiver is thus heated by the absorption of the infrared light by the infrared absorber, and this causes the dye to diffuse into the receiver. This fixing method is disclosed in International Patent Application No. PCT/GB97/01050 to Imperial Chemical Industries PLC et al., the contents of which are incorporated herein by reference. Instead of infrared radiation, other radiation, such as UV radiation, and a corresponding UV absorber may be used. Such methods of fixing will hereinafter be referred to as"flash fixing".

Flash fixing has a number of advantages, and provides a quick, simple and efficient method of fixing an image into a receiver sheet, for example into the dye receiving portion of a 35 mm slide.

The present invention relates to such flash fixing methods and apparatus, and, from a first aspect, provides a method of flash fixing in which a disposable

shield transparent to the fixing electromagnetic radiation is placed between the flash source and the receiver sheet. The invention also extends to an apparatus for flash fixing which includes a disposable shield transparent to the fixing radiation, the shield being positioned in use between the flash source and the receiver sheet.

It has been found that flash fixing can cause dye which has been transferred to the receiver sheet during printing to resublime. Only a small quantity of the dye will generally sublime, and so the resublimation does not present a problem with regard to the final fixed image on the slide. Over time and a number of flash fixings, however, the sublimed dye may build up on the flash source, e. g. on a reflector behind the flash tubes, and may also cause degradation of the tubes.

This may over time prevent energy from the flash from reaching the receiver sheet, and may lead to inefficient and incomplete fixing.

By incorporating a disposable shield between the flash source and the receiver, the source may be protected from the dye resublimed from the receiver by the fixing flash, and, instead of the dye building up on the flash source, it may condense onto the disposable shield. As the shield is disposable, however, it may be replaced before the build-up of dyes becomes large enough to influence the fixing process to any substantial degree.

The shield may take any suitable form, and may be fixed in place between the flash source and the receiver in any suitable manner. The shield may for example be mounted directly to the printer apparatus. In a particularly preferred embodiment, in which the dye sheet is mounted in a cassette, e. g. as a ribbon wound between two spools, the shield is preferably provided on the cassette rather than e. g. on the printer apparatus into which the cassette is placed. The shield may be integral with the cassette body or formed from a

separate element which engages with the cassette.

The provision of the shield on the cassette has the advantage that when placing the cassette in the printing apparatus, the shield is automatically positioned, without requiring any further action. Further, the user need not have to specifically replace the shield itself, and the fixing efficiency and the quality of the final print will not be degraded by a user forgetting to regularly check and replace the shield. Instead, the shield will automatically be replaced each time the dye sheet cassette is replaced.

As a cassette may typically house sufficient dye sheet material for about 100 prints, the cassette and so the shield will generally be replaced before the dye built up on the shield becomes significant enough to prevent good fixing.

The shield may for example comprise a flat, curved or U-shaped plate/screen that may be slotted or otherwise fixed into place on, for example, the printing apparatus or a cassette.

The shield may also comprise a substantially tubular element of any suitable cross-section, which may be four-sided, and may be mounted over the flash source to completely surround it. This form of shield may be particularly advantageous when the shield is provided on the cassette, as the rear wall portion of the shield adjacent the cassette may be used to protect the material of the cassette itself from being degraded by the heat from the tubes. When a reflector is built into the flash system, it will normally stop direct exposure of the cassette to the flash, but the shield may provide an additional thermal barrier.

Where the shield surrounds the flash source, e. g. flash tubes, it may also include reflective portions at the rear/side portions so as to reflect the light from the flash source towards the front of the shield and onto the receiver medium, and so do away with the need for a separate reflector mounted e. g. on the printer.

This arrangement would also allow the reflector to be replaced at the same time as the cassette in the case where the shield is mounted on the cassette. The reflective portions of the shield may be formed in any suitable manner, e. g. by making the shield from reflective material or by coating the shield inside and/or out with evaporated aluminium.

Besides the flash shield being disposable (and possibly also the reflector where e. g. the shield is the reflector), the flash source, e. g. the flash tubes, may also be disposable, and be e. g. mounted on the cassette within the shield. The flash source too may then be replaced with each change of cassette. In this case, the printer would have a power socket for receiving the flash tubes therein on insertion of the cassette into the printer. Although the present application relates mainly to a disposable shield for flash fixing, this feature of having the flash source/flash tubes on the cassette is in itself inventive, and, viewed from a further aspect, the invention provides a dye sheet cassette for use in flash fixing having the flash source, e. g. flash tubes, thereon, and further provides a flash fixing apparatus having a socket for receiving the flash source/tubes of such a cassette.

Turning back to the situation where the flash source is mounted on the printer assembly, the dye sheet cassette may be shaped with a contoured/recessed portion so that when in position on the printer apparatus, the flash source, e. g. an array of flash tubes, is accommodated in the contour/recess. The shield may be provided in or across the contour/recess.

In one preferred embodiment, the cassette and shield are connected together by longitudinal slots on one and corresponding ribs on the other.

In a preferred form of the dye sheet cassette, the cassette comprises a pair of tubular housings for the dye sheet unwind and take-up spools, the spools being inserted into the housing longitudinally and end covers

being provided over the housings after the insertion of the spools. In such an arrangement, the shield is preferably mounted on the wall of the take-up housing, preferably through guide slots mounted on the wall, and the take-up end cover preferably has pins thereon which fit into the guide slots.

The shield may be configured to curve about a wall of the spool housing.

Such a cassette is disclosed in a co-pending International patent application filed on the same day as the present application and based on GB 9722370.5 to Imperial Chemical industries PLC entitled"Dye sheet cassette and printing apparatus".

The mounting of the shield on a dye sheet cassette is a particularly advantageous arrangement, and, viewed from a further aspect, the present invention extends to a dye sheet cassette for dye thermal transfer printing, the cassette having a shield thereon which in use lies between a dye receiver to which dye from the dye sheet in the cassette is transferred and a flash source for the flash fixing of the receiver. The invention also extends to printing apparatus for receiving such a cassette, in which the print head for transferring the dye to the receiver and the flash fixing source are arranged in close proximity to one another.

The shield may be made from any suitable materials.

The shield, at least in the region opposed to the print area of the receiver medium in use, should be transparent to the wavelengths of the fixing electromagnetic radiation, and, for example, for infrared flash fixing, the material should preferably be transparent to radiation from about 400 nm to about 1000 nm. It should also be able to withstand repeated exposure to the fixing flashes without significant degradation.

The shield may for example be made from a polycarbonate, polysulphone or styrene copolymer, and may be poly (styreneacrylonitrile) or polystyrene.

The shield may be manufactured in any suitable manner, e. g. by being moulded.

The shield may be designed also to act as a protective barrier around the tubes, limiting the risk of electrical shock to users, or damage from an exploding flash tube.

The shield may further be formed to provide other advantages. For example, the shield may be designed to route cooling air over the flash tubes, thereby preventing excessive temperature rise in the tubes or shield. In one preferred embodiment, the shield has a chimney portion that helps to create an updraught through the shield to keep the tubes cool. The buoyancy of the warm air in the chimney may maintain the flow of air without the need for a fan or other mechanical extractor means.

The shield may be designed to selectively diffuse, refract or scatter the flash to make the illumination more uniform. For example, the shield may be frosted and/or may have 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. In one embodiment, the shield may include an arrangement of lenses for providing an even illumination front, and may for example be moulded with a series of cylindrical lenses arranged such that in use they are aligned over the individual tubes.

The shield may be designed to act as a filter to modify the characteristics of the light from the flash source. For example, the shield may contain dyes to alter the spectral distribution of the energy from the flash. In infrared flash fixing, the shield may also 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 dyes on the receiver sheet. This may help to mitigate against the loss of the dye from the image by resublimation, and so reduce the build-up of dye on the shield.

When the shield acts as a filter, the flash may need to be of a higher energy than when no filter is used, as some of the fixing, e. g. infrared, energy may also be filtered out.

The use of filtering material in the shield is particularly advantageous when short flash durations are used, as flash tubes generally emit a higher proportion of visible light at shorter flash durations.

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 flash fixing), the invention also extends to shields for flash fixing systems, in which a flash of any suitable wavelengths of electromagnetic radiation (preferably ones which are invisible to the eye) is used, preferably in combination with a suitable absorber in the receiver medium.

Although the above relates especially to disposable shields, it is also possible to provide a removable shield that may be removed for cleaning and then reinstalled in position, and the invention also extends to a reusable dye sheet cassette/shield arrangement, in which the shield may be reused or replaced.

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 apparatus for printing and fixing an image onto a 35 mm slide in accordance with a first embodiment of the present invention; Fig. 2 is a perspective view of a casing for the dye sheet cassette shown in Fig. 1; Fig. 3 is a perspective view of the flash fixing shield of Fig. 1; Fig. 4 shows the portion of the printing apparatus

of Fig. 1 on which the dye sheet cassette is mounted; Figs. 5 and 6 show respective ends of a spool which may be used as a take-up or unwind spool in the cassette of Fig. 1; Figs. 7 and 8 show respectively take-up and unwind end covers for the cassette casing of Fig. 1; Fig. 9 shows a ribbon insertion guide element for the cassette of Fig. 1; Fig. 10 shows a cassette/shield according to a second embodiment of the present invention; and Fig. 11 shows a shield according to a third embodiment of the present invention.

Referring to Fig. 1 printing apparatus is shown for printing and fixing an image onto a 35 mm slide 1 by laser sublimation transfer printing. Dye is transferred to the receiver portion of the slide 1 from a dye sheet ribbon 2 mounted in a dye sheet cassette 3. Fig. 1 shows the slide 1 in the printing position, with the fixing position shown in phantom.

The slide 1 is mounted on a suitable support 4 so as to contact the dye sheet ribbon 2 which is mounted between unwind and take-up spools 5,6 of the dye sheet cassette 3. The slide 1 contacts the ribbon 2 at the point where the ribbon passes about a rod lens 7 of the printing apparatus.

The slide 1 may take a form as disclose in PCT/GB98/01324, the contents of which are incorporated herein by reference. Such a slide has a sheet of unprinted receiver material mounted between a pair of frame halves, one of which has a larger aperture than the other for accommodating rod lens 7 and dye sheet 2.

Of course, similar apparatus to that shown in Fig. 1 could be used to print to an unmounted receiver sheet that is placed in a slide mount after printing and fixing.

In use, a modulated laser beam 8 is scanned along the length of the rod lens 7 (in a direction perpendicular to the plane of Fig. 1), and is focussed

by the lens 7 into the dye sheet 2. Dye from the heated portions of the dye sheet 2 transfers onto the slide 1 to form an image therein. During printing, the slide support 4 moves the slide 1 across the rod lens 7 in a direction perpendicular to the laser beam scan direction to produce a 2D scan over the slide.

The beam 8 is orientated with respect to the lens 7 so that the beam is focussed into the dye sheet 2 along a line which is offset slightly from the pinch of the lens 7 and slide 1, at a point where the dye sheet ribbon 2 and slide 1 do not touch. This produces a gap between the ribbon 2 and slide 1 that allows for dye sublimation transfer printing, heated dye from the ribbon 2 entering the vapour phase and crossing the gap to the dye receiving portion of the slide 1. Such a printing method is disclosed in USSN 08/682,905 filed on 16 July 1996 (corresponding to EP 96305216.2 and JP 8- 220270), the contents of which are incorporated herein by reference.

After printing, the slide 1 is moved by its support 4 to a position opposite to an array of, e. g. xenon, flash tubes 9. The tubes 9 are then flashed in order to fix the dye into the body of the dye receiving material of the slide 1. This method of fixing the dye is discussed in the above-mentioned International application PCT/GB97/01050. The flash tubes 9 are designed to produce a flash rich in infrared light, and an infrared absorber is included in the slide to absorb the infrared radiation. The absorber is heated by the flash and this causes the transferred dye on the surface of the slide to diffuse into the bulk of the receiver material.

A reflector 10 is mounted behind the flash tubes 9 in order to direct the light from the flash forward onto the slide 1.

Once the dye has been fixed into the slide 1, the slide may be removed and a new slide inserted. The dye ribbon 2 is wound on by the spools 5,6 to provide a

fresh area of the ribbon for printing to the new slide.

In colour printing, the dye ribbon 2 may have alternate areas of e. g. magenta, cyan and yellow dyes, and the slide 1 may be printed to three times, each time winding the ribbon 2 on to a different colour area. In this case, the slide may be moved opposite the flash tubes 9 for fixing between each individual colour dye transfer and/or after all three coloured dyes have been transferred.

During the flash fixing process, small amounts of transferred dye may resublime from the slide 1. This does not degrade the printed image on the slide, as the amounts of dye resublimed will generally be small, but over time could be problematic if the resublimed dye were to condense onto the flash source, e. g. onto the reflector 10 and build up on it, and perhaps also degrade the flash tubes 9.

In accordance with the present invention, however, the cassette 3 has a shield 11 thereon which extends over the flash tubes 9 in use. When the cassette 3 is placed on the printing apparatus, the shield 11 lies between the flash tubes 9 and the slide 1, so as to prevent dye which is resublimed from the slide 1 by the fixing flash from building up on the flash source. The resublimed dye instead condenses on the shield 11.

Eventually, this build up may prevent the energy from the flash tubes 9 from passing through the shield 11 and reaching the slide 1. Before this happens however, the cassette ribbon 2 will have been used up, and the cassette 3 will have been replaced by a new cassette with a clean shield 11.

A casing 12 for the cassette 3 of Fig. 1, and the shield 11 of Fig. 1, are shown separately in Figs. 2 and 3 respectively. The elements of the printing apparatus which cooperate with the cassette 3 are shown in Fig. 4.

The casing 12 comprises a base portion 13, an unwind spool housing portion 14, a take-up spool housing portion 15, biasing springs 16 for the take-up and

unwind spools 5 and 6, locating cylinders 17, a channel 18 and locking mechanism 19 for a ribbon insertion guide 20, and guide slots 21 for mounting the flash fixing shield 11.

The shield 11 comprises a four-sided tubular member 22 having a pair of ribs 23 that engage with the guide slots 21 on the cassette casing 12. The shield 11 is made from polycarbonate. It is transparent to the infrared radiation from the flash tubes 9 and is resistant to damage caused by the exposure to the flashes.

As can be seen from Fig. 1, the cassette 3 is configured so as to accommodate the flash tubes 9, the take-up side of the cassette to which the shield 11 is mounted being recessed somewhat. The printing apparatus has the flash tubes 9 and rod lens 7 closely adjacent one another to facilitate the mounting of the flash shield on the cassette.

The unwind and take-up spools 5,6 are configured as shown in Figs. 5 and 6, and have end caps 24 with central pips 25 that engage with the casing springs 16.

The spools 5,6 are inserted into the housing portions 14 and 15 of the cassette 3, with the end caps 24 providing a dust seal over the casing openings 26 at the springs 16. The ribbon insertion guide 20 is placed in the channel 18, and unwind and take-up end covers 27 and 28 shown in Figs. 7 and 8 respectively are mounted on the housing portions 14,15 via pins 29 that push into openings 30 (see Fig. 1) on the casing 12. The ribbon insertion guide 20 is held between a guide slot 31 on the unwind end cover 27 and a guide in the base 13 of the casing 12.

The end covers 27 and 28 have ribs 32 thereon against which ribs 33 of spool end caps 34 are biased by the springs 16 in order to keep tension in the ribbon 2 between the spools. The spools 5,6 are hollow for receiving drive shafts 35 (See Fig. 4) of the printing apparatus which connect with longitudinal splines 36 on

the inner surface of the spools.

The shield 11 is placed on the casing 12 before the end covers 27 and 28, and pins 37 and 38 of the take-up cover 28 engage with the ends of the shield guide slots 21.

The cassette 3 is mounted accurately in place on the printer apparatus through the placing of the locating cylinders 17 over locating rods 39 of the printer apparatus. The placing of the cassette 3 on the printer apparatus automatically positions the shield 11 over the fixing flash tubes 9.

The ribbon insertion guide 20 is used to hold the ribbon 2 out of contact with the rod lens 7 on placing the cassette 3 into the printer apparatus, and is normally held in place by the locking mechanism 19 in an the extended position. When the cassette 3 is mounted on the printer apparatus, the locking mechanism 19 is engaged by a release pin 40, and a hook 41 of the ribbon insertion guide 20 is engaged by an arm of the printer apparatus which moves the ribbon insertion guide into the retracted position shown in Fig. 1.

As can be seen from Fig. 4, the rod lens 7 is mounted in a cantilevered fashion on an optical plate 42 (which also mounts the locating rods 39 and the flash tubes 9). The cantilevered mounting allows the rod lens 7 to pass through a window 43 of the ribbon insertion guide 20 during retraction of the ribbon insertion guide, so as to contact with the dye sheet ribbon 2 in readiness for printing.

Also shown mounted on the optical plate 42 is a further lens 44 of the optical system for focussing the laser beam 8.

The laser beam 8 may be produced by a multimode laser diode, which is first collimated, then anamorphically expanded in its multimode mode axis, and then separately focused onto the transfer ribbon 2 in its single mode and multimode axes. In Fig. 4, the lens 44 is a part of a lens system for focussing the beam 8

in the single mode axis, whilst the rod lens 7 focuses the beam in the multimode axis.

To remove the cassette 3, the printer arm which engages the hook 41 of the ribbon insertion guide 20 is pivoted forward to move the ribbon insertion guide into its extended position. This moves the ribbon away from the rod lens 7, and allows the cassette 3 to be easily removed. Further, the movement of the ribbon insertion guide 20 to its extended position causes a cleaning pad 45 on the ribbon insertion guide 20 to wipe against the rod lens 7 and so clean it.

The shield 11 may include dyes therein to act as a filter to modify the characteristics of the flash reaching the slide 1. The filtering may be used to reduce the proportion of visible and/or ultra-violet radiation, to thereby ensure that only the receiver element of the slide 1 is significantly heated, and to prevent large amounts of heat from being absorbed directly by the transferred dye. This helps to reduce the loss of dye from the slide by resublimation, and so reduces the build up of dye on the shield.

The shield 11 may be used with other filters, and 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.

The shield 11 may be designed to disperse, e. g. scatter or refract, the light from the flash fixing tubes 9 to provide a more even light intensity distribution. For example, the shield 11 could be frosted or include shielding (e. g. frosted or filtering) striped areas which reduce the intensity of the light in the brightest regions.

Fig. 10 shows a second embodiment which is identical to the of Fig. 1, except that the shield 11 is only three-sided, and does not include the rear side adjacent the cassette 2. This drawing also shows the ribbon insertion guide 20 in phantom in the extended position, which it assumes when the cassette 3 is not in

the printer. In this position, the ribbon insertion guide 20 is able to seal the unwind spool housing 14 from dust, and so protects the unused dye sheet.

Fig. 11 shows a third embodiment of the present invention, in which the shield 11 includes a chimney element 46 that draws air through the shield to cool the tubes and the shield. The chimney is not mounted on a dye sheet cassette, but is integral to the printer assembly itself. It is arranged so that the hollow section extends vertically or generally vertically so as to facilitate the convective flow of the warm air.

The above described embodiments are examples only, and various alternatives and modifications are possible.

Any other suitable cassette designs could be used, as could any other suitable method of dye transfer, such as by a thermal head comprising a number of heatable elements, flash printing or ultrasonic means. Although the invention is well suited to the production of 35 mm slides, the invention may also be applied to any other suitable receiver mediums.

Also, the flash fixing may be carried out using UV light or other electromagnetic radiation, which is preferably invisible to the eye, the receiver sheet incorporating a corresponding absorber material therein.

In this case, the shield material should be transparent to the radiation wavelengths of relevance.

The shield 11 may include reflective rear and possibly side surfaces, so as to reflect the light from the flash source onto the receiver medium. The rear and sides of the shield may be made of a reflective material or may be coated in a reflective material, e. g. a coating of evaporated aluminium.

The flash tubes may also be mounted on the cassette, so that they too are disposable, in which case the printer will have a power socket for engagement by the flash tubes on insertion of the cassette into the printer.