Photothermographic development system

Abstract

 An apparatus and a method for registering information on photosensitive and thermally developable image material (10) by means of imagewise exposure of the image material and thermal development of the image material around a rotating body of revolution (20) provided with specific heating means (60).
In this connection, a temperature variation is measured (68) on the body of revolution in the axial direction (a) and in the tangential direction (t) and the measured temperatures are converted into corresponding control signals (78) for the heating means.

Description

1. FIELD OF APPLICATION OF THE INVENTION

[0001] This invention relates to an apparatus and a method of developing a photothermographic material. More particularly, the present invention comprises the dry processing of a photothermographic material, also referred to as "image-forming element". A very specific application is in the market for dry-processable medical film.

2. PRIOR ART

[0002] Thermally developable silver-containing materials for making images by means of exposure and then heating are referred to as photothermographic materials and are generally known. For example: "Dry Silver®" materials from Minnesota Mining and Manufacturing Company. A typical composition of such thermographically image-forming elements contains photosensitive silver halides combined with an oxidation-reduction combination of, for example, an organic silver salt and a reducing agent for it. These combinations are described, for example, in US Patent No. 3,457,075 (Morgan) and in "Handbook of Imaging Science" by D. A. Morgan, ed. A. R. Diamond, published by Marcel Dekker, 1991, page 43.

[0003] A review of thermographic systems is given in the book entitled "Imaging systems" by Kurt I. Jacobson and Ralph E. Jacobson, The Focal Press, London and New York, 1976, in Chapter V under the title "Systems based on unconventional processing" and in Chapter VII under the title "7.2 Photothermography".

[0004] Photothermographic image-forming elements are typically processed by an imagewise exposure, for example, in contact with an original or after electronic image processing with the aid of a laser, as a result of which a latent image is formed on the silver halide. Further information about such imagewise exposures can be found in Agfa-Gevaert Patent Application EPA-96.201.530.1

[0005] In a heating step which then follows, the latent image formed exerts a catalytic influence on the oxidation-reduction reaction between the reducing agent and the nonphotosensitive organic silver salt, usually silver behenate, as a result of which a visible density is formed at the exposed points. The development conditions are determined by the choice of the nonphotosensitive organic silver salt and its reducing agent. For example, the development temperature is in the vicinity of 120°C and this is for about 5 seconds.

[0006] Further information about said thermographic materials can be found, for example, in said Patent Application EP-A-96.201.530.1.

[0007] Practical problems in the development of photothermographic image-forming elements often result from the fact that the density formed is dependent on the amount of heat supplied. In order to obtain a uniform density, a uniform heat transfer is therefore necessary.

[0008] In addition, a large number of grey values are required for medical applications and these should be shown in a reproducible way.

[0009] In many photothermographic systems according to the prior art (for example based on Patent Application WO 95/30934 in the name of 3M), exposure and development take place in separate units.

[0010] In another photothermographic system, a so-called "combined system", exposure and development take place in one and the same unit. Figure 1 shows the whole of a photothermographic system according to Patent Application DE 196 36 235 in the name of Agfa-Gevaert A.G., in which exposure and development take place on one and the same medium.

[0011] In the specialist literature, various apparatuses have already been described for the development of these materials. Some thermal processors of this type have, however, one or more disadvantages, such as thermal inertia (as a result of which the processing time becomes prohibitive), excessively high pressures (resulting in possible disadvantages, such as scratches or creases), uneven pressures and/or uneven temperature distribution (resulting in undesirable density differences).

[0012] In particular, if certain photothermographic materials are used which are relatively lore thermosensitive than other photothermographic materials, nonuniformity may be produced (for example, troublesome patterns). In addition, some types of photothermographic materials are relatively more thermosensitive than other types, as a result of which still greater problems may arise.

[0013] The present application presents an alternative thermal processing with relatively short development times and without undesirable density differences.

3. OBJECT OF THE INVENTION

[0014] The object of this invention is to provide an apparatus or system and a method or process for uniformly developing a photothermographic material implying that substantially equal temperatures occur during the development at all points on said photothermographic material.

[0015] Another object of this invention is to provide a method and an apparatus for developing a photothermographic material at a temperature which remains constant with time.

[0016] Further objects and advantages will become clear from the description which follows below.

4. SUMMARY OF THE INVENTION

[0017] We have now discovered that these objectives can be achieved by constructing an apparatus and performing a method according to the attached claims.

5. CONCISE DESCRIPTION OF THE FIGURES

[0018] 

Figure 1 shows a photothermographic system according to Patent Application DE 196 36 235.0-51;

Figure 2.1 shows a cross-section of a thermal development apparatus according to Patent Application BE 09600583;

Figure 2.2 shows a partial longitudinal section of a thermal development apparatus according to Patent Application BE 09600583;

Figure 3 shows a cross-section of a thermal development apparatus according to the present patent application;

Figure 4 shows a local cross-section through a drum and a scraper;

Figure 5.1 shows a diagrammatic view of a photothermographic material (m) which can be used in the present invention;

Figure 5.2 shows a diagrammatic view of a drum-shaped heating body according to the prior art;

Figure 5.3 shows a temperature variation measured in the axial direction on a body of revolution according to the prior art;

Figure 5.4 shows a temperature variation measured in the tangential direction to a body of revolution according to the prior art;

Figures 6.1 and 12.2 show a diagrammatic view of a photothermographic material (m) which can be used in the present invention;

Figure 6.2 shows a diagrammatic view of a drum-shaped heating body according to the present invention;

Figure 6.3 shows a temperature variation in the axial direction on a photothermographic material developed according to the present invention;

Figures 6.4 and 6.5 show the temperature variation measured in the tangential direction on a photothermographic material developed according to the present invention;

Figure 7 shows an example of an axially compensated heating system which can be used according to the present invention;

Figure 8 shows an example of a belt run with proportional control which can be used according to the present invention;

Figure 9 shows a control circuit for keeping the temperature of the drum constant;

Figure 10 shows a time diagram for compensation of the tangential temperature profile;

Figure 11 shows a series of activation pulses having a relatively high duty cycle;

Figure 12.1 shows diagrammatically a so-called "segmented drum";

Figure 12.2 shows diagrammatically a photothermographic material (m) which can be used in the present invention;

Figure 12.3 compares axial density profiles measured on image materials developed according to the prior art, with HW compensation according to the present invention or with HW and SW compensation according to the present invention, respectively;

Figure 13 shows a tangential density profile measured on a photothermographic material developed according to the present invention;

Figure 14 shows an axial density profiled measured on a photothermographic material developed according to the present invention;

Figure 15 shows a drum-shaped heating body with three different powers installed in the axial direction;

Figure 16 shows an embodiment of a drum in which different powers can be switched on by the hardware both in the axial direction and in the tangential direction;

Figure 17 shows three pulse trains with different duty cycles.

6. DETAILED DESCRIPTION OF THE INVENTION

6.1. Introductory concepts and definitions

[0019] In the present application, the term "body of revolution" is understood as meaning a "(geometrical) body which is considered to have arisen by rotating a surface around an axis, the axis of revolution" (cf. "Groot woordenboek der Nederlandse taal" ("Large-dictionary of the Dutch language"), published by Van Dale Lexicografie, Utrecht-Antwerp, 1984 edition, part two J-R, page 1883). Any cross-section at right angles to said axis of revolution therefore forms a circle. A cylindrical drum or "drum" for short, is a special case of a body of revolution.

[0020] In the present application, the term "heating unit" comprises various heating possibilities (for example, resistive via an ohmic resistor or via a lamp; or inductively) and various practical application facilities (without relation to any photographic material and therefore not limited to photography) The related term "development apparatus" is more specific is explicitly intended to develop a photographic material and is primarily directed at photothermophotography and direct thermography.

[0021] The terms "laser printer", "printer", "laser recorder" and "recorder" are regarded as equivalents in this application. As a more general term, "registration system" is used, it being possible for desired information to be written in with a laser, but possibly also with other means (such as cathode ray tubes (CRTs), light-emitting diodes (LEDs), luminescence panels, phosphor screens, etc).

[0022] In the present application, equivalence also applies to the terms photothermographic material, image-forming element, image material, film sheet, film or sheet of film, it being possible for images in transparency or in reflection to be included.

6.2. Outline description of a photothermographic system

[0023] In a photothermographic system according to known prior art (for example, based on Patent Application WO 95/30934 in the name of 3M) exposure and development generally take place in separate units. The use of a thermal development according to the present invention in such a "separate system" offers a number of advantages which have already been mentioned above and which will be clarified further.

[0024] In another photothermographic system, a so-called "combined system", exposure and development take place in one and the same unit. Figure 1 shows the whole of a photothermographic system 1 according to Patent Application DE 196 36 235.0-51 in the name of Agfa-Gevaert, in which exposure and development take place on one and the same medium, in this case preferably on a so-called body of revolution. In such a combined system, too, often referred to as "single drum printer (SDP)", the use of a thermal development according to the present invention offers the same advantages already mentioned above, which will be clarified further.

[0025] Although it is therefore quite possible that exposure and development take place in separate units for the present invention, Figure 1 can nevertheless serve as a basis for the outline description of a photothermographic system.

[0026] In general, laser printer 1 will first expose the image material 10 in an exposure unit 3 and then develop it in a thermal development unit 4. In the specific case of a so-called SDP according to Figure 1, laser printer 1 will first expose the image material 10 and then develop it in a common exposure and development unit 5. After processing, information is observable on the image material because nonexposed, developed zones are transparent and exposed and developed zones have an observable optical density (preferably with a maximum density above 3.5 D).

[0027] At the beginning of a copying cycle, one film sheet 10 is picked up from feed unit 2 (by means of a feed device 15) and transported (in the direction of movement Y along a guide 16) to exposure and development unit 5.

[0028] In said exposure and development unit 5, the outer casing (of thermally conducting material) of drum 20 (having direction of rotation n) is brought by a heating system 60 to a desired process temperature (for example, 123°C or 396 K) and kept at said temperature by at least one temperature sensor (for example, a thermocouple or a thermistor) and a control circuit (shown in Figure 9 and to be discussed later.

[0029] Film 10 is pressed against rotating drum 20 by means of rollers 31-33 and transport means 40, preferably a conveyor belt 41 (generally made of thermally insulating material) so that film 10 achieves virtually the same temperature as the outside surface of drum 20.

[0030] While film 10 moves synchronously with drum 20, the imagewise exposure takes place between rollers 32 and 33. For this purpose, modulated laser beam 57 inscribes the image material 10 at a sensitometrically suitable wavelength and with an imagewise-modulated intensity.

[0031] After the exposure, the film 10 is developed thermally so that a permanent blackening is produced at the exposed points. For this purpose, film 10 is pressed by conveyor belt or pressure belt 41 against the heated drum 20 and moved synchronously in the direction of rotation n.

[0032] In this connection, the contact tension can be influenced by adjusting tension roller 36. The development time can also be influenced (for example, between 3 and 20 s) by adjusting the contact angle α (having numerical reference 42) between belt 41 and drum (here shown at approximately 180°C) and by adjusting the speed of rotation.

[0033] After the thermal development, film sheet 10 is brought out via cooling rollers 38-39, which may be cooled by a fan 17, and is made available in a receiving tray 29 of an unloading unit 6.

[0034] The laser recorder 1 also comprises an electronic control unit 7 and an optical unit 8, which itself comprises yet other components, such as a laser source 51, which makes available an unmodulated laser beam 52, a modulator 53, objectives 54 and 55 and a rotating polygonal mirror 56. Various parameters (such as temperature, speed, contact force, contact angle α ...) can be set by means of an operating panel 9.

[0035] In the preferred embodiments of the present application, provision is made for the film to be presented tangentially to the "processor" (to avoid malfunctions). Provision has also been made by means of guides, small rollers, belt and a suitable receiving system for the processing of each sheet of film to start at a fixed point and to stop at a (different) fixed point and for the process to proceed in a straight line at right angles to the drum axis; each part of the film must then always have the same development history, each part of the film receiving the same amount of heat.

[0036] A cleaning unit 80 provides for the drum to be kept permanently clean by regularly removing any soiling elements which may be present.

[0037] For further information about the above-mentioned "single drum printer", reference is made to DE 196 36 235.0-51.

[0038] Now that an outline description of a photothermographic system 1 has been given, we will clarify a few essential concepts below relating to "axial and tangential temperature profiles" (symbolically represented by Ta and Tt, respectively) of a heated body of revolution. In this connection we distinguish "uncontrolled or natural" temperature profiles from (hardware- and/or software- "controlled") temperature profiles. It is precisely in the control or management of said temperature profiles that the characteristic part of the present application is situated.

6.3. "Natural" temperature profiles of a heated body of revolution according to the prior art

[0039] Reference is made first of all to Figure 5.1 which shows a diagrammatic view of a photothermographic material (m) which can be used in the present invention, and to Figure 5.2, in which a cylindrical drum is heated internally by, for example, an electrically resistant medium (for example, a so-called ohmic resistor which generates a Joule effect).

[0040] More specifically, Figure 5 shows a temperature variation such as is measured in the axial direction (symbolically indicated by arrow a) on a body of revolution according to the prior art. Figure 5.4 shows the temperature variation as measured in the tangential direction (symbolically indicated by arrow t) on a body of revolution according to the prior art.

[0041] On curve ca0 of Figure 5.3, we note that, on rotation in a thermally stabilized, but unstressed standby state (that is to say after the initial heating and before the final cooling, and without film) a temperature variation is encountered on the outside surface of said drum in the axial direction, a higher temperature prevailing in the centre than at the edges (this mainly because of varying heat losses to the environment). The graphical representation of said temperature variation in the axial direction (a) on the outside surface of the body of revolution is also referred to as "first temperature profile Ta".

[0042] In the loaded processing state, as soon as a film (having film width B) comes into contact with the drum, the surface temperature of the drum may drop locally (see curve ca1) . After the film has left the drum, the surface temperature of the drum rises again to the standby value.

[0043] In the curve ct0 of Figure 5.4 we note that, on the outside surface of a cylindrical drum along the circumference, that is to say in the tangential direction, no temperature variation occurs on rotation in a thermally stabilized standby state (that is to say after the initial heating and before the final cooling, and without film).

[0044] As soon as a film (having film length L) does come into contact with the drum, the surface temperature of the drum drops (see curve ct1). After the film has left the drum, the surface temperature of the drum rises again to the standby value. The graphical representation of this temperature variation in the tangential direction (t) on the outside surface of the body of revolution is referred to below as "second temperature profile Tt".

[0045] The present invention discloses an apparatus and a method in which both temperature profiles (Ta, Tt) can be controlled independently of one another even during processing of said image materials. In addition, both temperature profiles can be substantially flattened. How this can take place precisely in principle is explained, in particular, in a later section.

[0046] The technical features of some preferred embodiments are now first discussed in detail.

6.4. "Controlled/or compensated" temperature profiles of a heated body of revolution according to the present invention

[0047] As already reported, the physical centrepiece of the present invention comprises a specific drum having a modified heating.

[0048] In the first instance, the present invention comprises a rotating body of revolution 20 having an outer casing 21 made of thermally conducting a material, axial or lateral end faces 22-23 made of thermally insulating material, and heating means 60, for example an electrically resistant heating element 61, said body of revolution having on the outside surface a temperature variation in the axial direction (a) according to a first temperature profile (Ta) and a temperature variation in the tangential direction (t) according to a second temperature profile (Tt), characterized in that both temperature profiles (Ta, Tt) can be controlled independently of one another and that each of the two temperature profiles has a compensation control.

[0049] Here reference is made to Figure 6.1, which shows a diagrammatic view of a photothermographic material (m) which can be used in the present invention; to Figure 6.2, which shows a diagrammatic view of a drum-shaped heating body according to the present invention (a so-called "segmented drum" comprising, for example, three sections 21', 22' and 23', further discussed by reference to Figure 12.1); to Figure 6.3 which shows a temperature variation in the axial direction on a heated drum according to the present invention; and to Figures 6.4 and 6.5 which show the temperature variation as measured in the tangential direction on a heated drum according to the present invention. In these figures, curves ca0, ct0 and ct1 have the same meaning as in Figures 5.2 and 5.3. The other curves show various temperature variations, namely ca2 and ca3 show temperature variations measured in the axial direction on a segmented drum (cf. curve ca2) and on a segmented drum having axial compensation (here still incomplete) (cf. curve ca2) according to the present invention. Curves ct2 and ct3 show temperature variations as measured in the tangential direction on a heated drum having tangential compensation (here complete) according to the present invention curve ct2 showing a situation without image material 10 and curve ct3 a situation with image material.

[0050] Various practical embodiments of such compensations (including software axial compensations, hardware and/or software tangential compensations) are described in later sections.

[0051] Without proceeding here to an in-depth analysis of the above figures, it can already be clearly deduced in every respect from a comparison of the various curves that a compensated drum provides a much more uniform density profile (resulting from a similar temperature profile) than an uncompensated drum.

[0052] After an outline description of a photothermographic system has first been given above (in section 6.2) and after some essential concepts relating to "axial and tangential temperature profiles (Ta, Tt)" have been put in place (in sections 6.3 and 6.4) all the essential components are now described consecutively in detail along with their technical features. The following are discussed consecutively: the physical centrepiece of the invention, being a specific drum with modified heating; then the thermal processing of a photothermographic material by means of a specific heating unit; then a more completely developed apparatus, and finally a complete recording system. The method of the present invention is also explained explicitly in the detailed description which now follows.

6.5. Basic arrangement of a heated body of revolution which can be used in the present invention

[0053] More practical explanations relating to such a control follow in later sections of this description, with a statement of static and dynamic influences on the temperature, open-loop or closed-loop control circuit, feedforward or a feedback control, hardware or software control.

[0054] In general terms, a basic arrangement of a heated body of revolution which can be used in the present invention comprises a heating unit having means for transporting and heating a photothermographic material, electrically resistant heating means 60 mounted in a body of revolution and means of determining or managing (being registering and controlling) a temperature variation on said body of revolution in the axial direction (a) according to a first temperature profile (Ta) and a temperature variation in the tangential direction (t) according to a second temperature profile (Tt), characterized in that both temperature profiles (Ta, Tt) can be controlled independently of one another. Of course, synchronization means are also provided in order to activate the abovementioned means at the correct instants in time.

[0055] In further preferred embodiments of a heating unit 4 according to the present invention, said heating means 60 may comprise an electrically resistant heating element 61 and/or an electrical heat radiator 69. In this connection, the heat transfer between heating means 60 and body of revolution 20 can take place in various ways, namely, in the case of heating element 61, primarily by conduction, and in the case of heat radiator 69 (for example, a ceramic element or a lamp preferably having an infrared spectrum, IR), primarily by radiation.

[0056] In order to explain such a basic arrangement more specifically, reference is made to Figures 2 and 3, which show an apparatus according to a drum concept. Figures 2.1 and 3 show a cross-section of the apparatus and Figure 2.2 shows a longitudinal section. Said figures are further discussed jointly.

[0057] The processor comprises a heated drum around which a belt 41 runs, for example, over a contact angle α of 180°. To cause said belt to run against the drum, a number of rollers are provided, two transport rollers 34-35 next to the drum, and at least one tension roller (36 or 37) which provides for tensioning of the belt and controls the belt (so-called tension roller or control roller). If necessary, said tension roller can be replaced by a plurality of rollers, for example by two rollers (reference 36 and 37) as in Figure 1.

[0058] The film transport between inlet 46 and outlet 47 of development apparatus 14 takes place by means of a concomitantly rotating conveyor belt 41 which presses the film against the hot drum or cylinder 20.

[0059] The belt 41 itself is pressed against the heated drum 20 by two rollers 34 and 35. The contact angle, α of the belt can be adjusted from a few degrees to 180°, even to approximately 280° by positioning said rollers 34 and 35, and their diameter. The roller 36 provides for the tensioning of the belt 41 against the drum 20. This can take place either as a result of the weight of the roller 36 itself (cf. gravity) or, for example, as a result of a force applied via the bearing of said roller 36. This latter has the advantage that the apparatus can be arranged at an angle or even vertically without the tensioning force of the belt 41 being influenced.

[0060] The present invention thus describes a heating unit (4) comprising a body of revolution (20) having an outer casing (21) made of thermally conducting material and axial end faces (22-23) made of thermally insulating material, rotation means for rotating said body of revolution, heating means (60) for heating said body of revolution, measuring means (68) for measuring on said body of revolution at least one temperature to determine a temperature variation in the axial direction (a) according to a first temperature profile (Ta) and at least one temperature to determine a temperature variation in the tangential direction (t) according to a second temperature profile (Tt), conversion means (76) for converting said temperatures into corresponding (measured and digitized temperature measurements) temperature signals (77), control means (72-74) for converting said temperature signals into control signals (78) for said heating means.

[0061] In the further description, we first clarify in addition the primarily mechanical influences and then the primarily electrical influences including compensation means for compensating for static and dynamic interferences in said temperature profiles) on the temperature behaviour.

6.6. Basic construction of a drum which can be used in the present invention (Figures 2 and 3)

[0062] Central in this arrangement is a body of revolution 20, preferably in the form of a cylinder or drum.

[0063] By way of practical information about a preferred embodiment, we cite the following technical specifications, which are not however intended to be restrictive:

• metal drum having outside dimensions of 198 × 492 mm;
• outer casing 21 made of AlMgSi1 (DIN No. 3.2315);
• the outer casing sits between to thermally insulating flanges 22-23 (made, for example, of "hard fabric");
• the drum is suspended between two metal frame plates (for example, made of "zincor") having a thermal insulation on the inside (zincor is a type of steel DIN 1.0330.03 provided with an additional layer of zinc 2.5 µm thick);
• rollers 34-37, drum 20 and belt 41 are sealed from the environment by a housing 45 (made, for example, of metal) which is thermally insulated (for example, with cork);
• the drum is internally heated by a flexible heating mat (61) which is bonded into the inside;
• the power for the heating and also the signal from the temperature measurement 68 (for example a drum temperature probe using a sensor in the drum wall) are transmitted via slip rings and slip contacts;
• the drum is driven by an electric motor 30 (for example, a stepping motor or a synchronous motor, preferably placed outside the frame plates).

[0064] To achieve a satisfactory heat distribution (and satisfactory temperature profiles), the drum is mounted in a heat-insulating material, for example, so-called hard fabric having adequate resistance to high process temperatures (up to 140°C). Hard fabric is a laminate material based on a carrier (usually wool) and a resin (for example, phenol, epoxy or polyimide), it has a thermal conductivity of approximately 0.2 W/m·K and is subject to international standards such as DIN 7735. Known trade names are Batext, Epratex and Ferrozell; suppliers are, inter alia, Eriks, B2660 Hoboken and Vink, B2220 Heist op den Berg.

[0065] In the matter of thermal conductivity of usable construction materials, we understand, in the present application, "good conductors" as meaning materials having a thermal conductivity λ greater the 10 W/m·K and "thermally insulating" as meaning a thermal conductivity λ of less than 3 W/m·K, preferably less than 0.5 W/m·K.

[0066] Some material examples for the purpose of illustration are: good conductors are, inter alia, aluminium, having λ = 229 W/m·K, brass, having λ = 105, (steel having λ = 45), various types of cast iron having λ = 17 to 42, various types of stainless steel, having λ = 13 to 15 (RVS material designations according to DIN 17007: 1.4301, 1.4401, 1.4404, 1.4539, 1.4541 and 1.4571); thermal insulators are, inter alia, various types of hard fabric (having a thermal conductivity in the same vicinity as wood, namely λ = 0.2 W m·K).

[0067] In conformance with "De grote Oosthoek encyclopedie" (published by Oosthoek, Utrecht, 1980, Part 17, page 134), we understand, in the present application, "stainless steel", often also referred to as rust-free steel, as meaning an "alloyed type of steel which is very resistant to corrosion (chromium steel, chromium nickel steel, etc.)". Further information (inter alia structural classifications, alloys, properties etc.) can be found in, inter alia, the "Grote Winkler Prins encyclopedie" (published by Elsevier, Amsterdam-Brussels, 1982, Part 19, page 309) and in the "Winkler Prins technische encyclopedie" (published by Elsevier, Amsterdam-Brussels, 1977, Part 5, pages 377-378).

[0068] Said heating means 60 can be bent during manufacture or bent when fitted (preferably according to the radius of curvature of the drum). Said heating element 61 may be an etched foil (for example, supplied by WATLOW, 12001 Lacland Road, St Louis, Missouri 63146, USA), a wire-wound foil (for example, supplied by ELMWOOD, Elm Road, North Shields, Tyne and Wear NE29 8Sa, GB), ...

6.7. Surface properties of a drum which can be used in the present invention

[0069] Certain properties (such as thermal conductivity, hardness, roughness ...) of the drum surface are very important, this being because of process quality (especially with regard to heat transfer) and because of mechanical resistance to damage.

[0070] The drum surface must be uniformly heated and must give off the heat uniformly to the film. This requires a surface having equal thermal capacity and thermal conduction coefficient.

[0071] The drum surface should be hard enough not to be damaged by contact with film and scraper. Damage due to occasional contact with foreign objects for example, during the opening of the processor, during the removal of dust, etc.) must also be avoided.

[0072] The surface finish is also important. Firstly, the surface should be finely finished enough for no imprints to be left behind in the film during the processing (as a result of the soft emulsion layer, the film is, of course, very susceptible to damage during processing). Secondly, a surface which is insufficiently smooth will be polished by contact with the scraper, as a result of which fine particles may break off the drum surface and remain stuck at the level of the scraping system. Because said particles are very hard, damage to the drum surface and the scraper(s) is real. In an embodiment according to the present invention, the best results are obtained with a re-ground drum surface, preferably with a finish to Ra ≤ 0.5 µm and Rz ≤ 4 µm (cf. standards DIN 4762 and DIN 4768).

[0073] In a particular embodiment according to the present invention, an aftertreatment of the drum surface with the aid of a normal anodization of the drum proved insufficiently hard to obtain and to maintain adequate image quality. In a further embodiment according to the present invention, the drum surface has therefore been hard-anodized (preferably, to a Vickers hardness of approximately 500 HV).

[0074] In another embodiment according to the present invention, a normal Teflonization of the drum proved insufficient to obtain and to maintain adequate image quality. Of course, since Teflon is a thermal insulator in comparison with aluminium any temperature irregularity results in unacceptable process results. (Teflon is a trade name for PolyTetraFluoroEthylene (PTFE) and is available, inter alia, from Du Pont, Hoechst, ICI, Montedison).

[0075] In a further embodiment according to the present invention, the drum has therefore been subjected to a "hard anodization followed by Teflonization". This technology is known under the name "TUFRAM coating" and comprises a combination of first hard-anodizing and then sealing by smearing the anodization pores with Teflon; in this way "PTFE-impregnated Al₂O₃" is obtained. This aftertreatment has an immediate advantage in the fact that a very hard non-stick surface is obtained.

6.8. Cleaning of a drum which can be used in the present invention

[0076] Two other important functions which must be fulfilled in the apparatus comprise (1) the reproducible removal of the developed film from the drum and (2) the permanent maintenance of the cleanliness of the drum by means of regularly removing soiling elements. The drum can, in fact, be soiled by various elements, especially dust (from the environment, due to the film manufacture, ...) and emulsion residues.

[0077] The most important limitations if a scraper is used on the drum are imposed by the high temperature (which imposes a limitation in the materials to be used) and by the surface of the drum (which must not be damaged and must be capable of being easily cleaned).

[0078] Particular embodiments according to the present invention use a glass-fibre scraper impregnated with a resin, or a scraper made of plastics, such as PEN or nylon. (PEN is an abbreviation for PolyEthylene Naphthalate and is produced, inter alia, by the companies Du Pont, ICI and Teijin. Nylon is a polyamide plastic and is produced, inter alia, by the companies BASF, Du Pont and Monsanto.)

[0079] Yet another embodiment uses a thin steel sheet (for example, 0.1 mm thick).

[0080] A brass scraper (having a thickness of approximately 0.1 mm) provisionally gives the best result. In this connection, it is observed that the brass scraper settles down well to the shape of the drum and that less damage occurs.

[0081] In a particular embodiment, the drum cleaning function was combined with the film removal function by using one scraper both to remove the sheet of film from the drum and to keep the drum free from dust and dirt.

[0082] In another embodiment, both functions were performed separately, for example by a separate set of individual removal scrapers (preferably made of plastic and arranged next to one another on one and the same axial line) and by a continuous cleaning scraper (preferably made of metal and extending over the entire drum width).

[0083] Figure 4 shows an example of a local cross-section through a drum 20 having drum outer casing 21 and a removal scraper 81 having several components 82-83 of a scraper holder.

6.9. Mounting of a drum which can be used in the present invention

[0084] In a preferred embodiment according to the present invention, the drum is supported at the side edges by two flanges made of hard fabric (for example, EPRATEX from Eriks), this being because of its heat-resistant and insulating properties. Said flanges are provided with shaft ends made of stainless steel or RVS (limitation: heat losses).

[0085] Said shaft ends are mounted in rolling bearings, one bearing being fixed (preferably at the motor side) and the other bearing being laterally displaceable in order to absorb thermal expansion of the drum.

[0086] Because of the fairly high temperature which the bearings can assume, a heat-resistant grease, such as, for example, Barrierta™ type 55/2, is used to lubricate said bearings in a preferred embodiment of the present invention. (Barrierta™ is a registered trademark of Klueber Lubrification, D 8000 Munich and comprises a temperature-resistant lubricant based on perfluoroalkyl ether).

6.10. Belt control according to the present invention

[0087] Just as the static properties (such as thermal characteristics, dimensions, cylindricity, surface hardness, roughness ...) and the dynamic properties (such as true running, bearing, friction, uniform speed, synchronization ...) are important for the qualitative working of a photothermographic system in the case of the drum, such things also apply to the conveyor belt.

[0088] Let it be clear that an inadequate or an inadequately uniform pressing of the film against the drum during the processing results in unequal densities over the sheet of film. This often manifests itself in the belt structure becoming visible or in lighter and darker strips on the finished film.

[0089] Insufficiently smooth finishing of the belt also gives rise to fine damage of the film, which manifests itself in a microstructure on the finished film.

[0090] Because of the fairly short bending-round of the belt just in front of the drum and the fairly small diameters of the feed and take-off rollers, a high tension prevails in film and belt, as a result of which all kinds of faults may be produced.

[0091] Because of the conveyor belt, lateral forces at right angles to the running direction (cf. "tracking") and forces in the running direction may deform the film, certainly if the film is fairly weak (for example, PET with a thickness of only 100 µm and heated above 100°C). If these forces are sufficient, faults are produced by mechanical deformation of the film and, possibly, darker and lighter strips are produced.

[0092] Figure 8 shows a preferred embodiment of a belt path having proportional control. In this connection, a tension roller or control roller 36 is arranged movably in a shackle bearing 26 and can be displaced by means of a threaded spindle 27 and a screw motor 28. This is controlled in turn by (for example, two) position sensors which detect the run of the belt 41.

[0093] Instead of a threaded spindle, other control systems, such as an eccentric, an electromagnet, a pneumatic or a hydraulic cylinder, which are in turn controlled by a two-point control or by a proportional control, are, of course, also conceivable.

[0094] In a further embodiment of the present invention, means are also provided for switching over the image side of the image material, that is to say thermal processing of said image material with the emulsion side in contact with the heated drum, instead of the rear side.

[0095] After an outline description of a photothermographic system according to the present invention has already been set out above, followed by a detailed description of a body of revolution and various mechanical functions relating to it, subsequent attention will now be directed more specifically at a modified heating system, then at the thermal processing of a photothermographic material by means of a specific heating unit, then at a more complete development apparatus, and finally, at a complete recording system. The method of the present invention is also explicitly explained in the detailed description which now follows.

6.11. First embodiment of a drum heating system according to the present invention ("segmented drum")

[0096] The section below describes three preferred embodiments for heating a drum to a certain, set temperature and with a certain accuracy. Common to each of said preferred embodiments is the fact that, during processing of the image material, both temperature profiles (Ta, Tt) can be controlled independently of one another and, in addition, are controlled (or managed) in such a way that both temperature profiles are substantially flattened.

[0097] In a first preferred embodiment (cf. Figures 2 and 3), the system comprises at least one body of revolution 20 having an outer casing 21 made of thermally conducting material (for example, a metal, for example anodized aluminium) having axial or lateral end faces 22-23 made of thermally insulating material, and having electrical heating means 60.

[0098] A special feature of this first embodiment is precisely that the axial end faces 22-23, also referred to as side faces or flanges, are thermally insulated from the remaining construction (including outer casing and shaft ends). In a preferred embodiment, said end faces are made of "hard fabric".

[0099] Figure 12.1 (cf. also Figure 6.2) shows a diagrammatic view of a so-called "segmented drum"; Figure 12.2 shows diagrammatically a photothermographic material (m) which can be used in the present invention. In this (non-restrictive) example, drum 20 actually comprises three sections, namely a centre section 21', a left-hand section 22' and a right-hand section 23'. In this connection, the extremities 22 and 23 of said side sections 22' and 23' are additionally well insulated in order to limit heat losses via the flanges or via the shaft ends.

[0100] Running over the drum 20 (for example, having a diameter of 198 mm, a wall thickness of 14 mm and a shaft length W = 490 mm) over a certain contact angle α (for example, 180°) is a conveyor belt 41 (for example, having a length of 890 mm and a width of 470 mm).

[0101] In this embodiment according to the present invention, provision has been made, inter alia, for good synchronization between incoming sheets of film and the position of the temperature sensor(s). Of course, if the circumference of the drum is different (in this case greater) from the length of the longest sheet of film (for example, 620 mm instead of 430 mm), the sheet of film to be processed does not always travel at the level of the sensor, which has an unfavourable influence on the control system.

[0102] In this embodiment according to the present invention, provision must also be made, inter alia, for the belt not to be too severely cooled during the processing by the sheet of film fed through.

[0103] Another version used an alternative belt having a smaller thermal capacity (the lower the thermal capacity is, the less influence the belt has on the process) and having a greater thermal resistance and contact resistance (surface finish). Such belts may be composed of metal (for example of a stainless steel), possibly covered by a fine layer of rubber, or of a plastic sheet based on a polyester film, such as Mylar (registered Du Pont trademark) or a polyimide film such as Kapton (registered Du Pont trademark).

[0104] An electrically resistant heating element is preferably used as heating means.

[0105] Mounted in the wall of the drum is at least one, but most preferably, a plurality of temperature sensors (for example, four thermocouples Pt100) (cf. Figure 9), most preferably at various tangential points (for example, displaced through 90° with respect to one another) and at various axial points, possibly also at various depths in the drum wall thickness.

[0106] Both the measurement signal for the drum temperature and the power for the heating element are transmitted via respective slip contacts. In general, slip contacts are part of the prior art, so that they are not elaborated on. However, attention is drawn to a preference for slip rings having a rhodium coating because of two special advantages: such slip contacts give (i) a reproducible signal and (ii) a maintenance-free service life of at least one million revolutions.

[0107] If a contact-free system which is not susceptible to interference is opted for, an optical interface may, if desired, be used.

[0108] As heating element, a so-called flexible or a curved heating element is preferably mounted in the inside of the drum, for example bonded with RTV adhesive ("room temperature vulcanization", obtainable, inter alia, from Dow Corning). Used as heating elements are, inter alia: (i) an "etched foil flexible heater" from WATLOW having a rated power of 1500 W at 240 V, or (ii) a "wire-wound flexible heater" from ELMWOOD having a rated power of 1300 W at 240 V.

[0109] A control circuit 70 having a PID (proportional + integral + derivative) controller 73, namely a configuration having an SW controller around a microprocessor 72 (for example, a processor 80186 of the Intel™ type) is chosen for controlling the temperature of the drunk. The fed back signal 75 from a calibrated temperature sensor 68 (for example a Pt100 having an accuracy of 0.1°C within the calibrated working range) is converted and an ADC analogue-to-digital converter 76 into a binary signal 77 (for example, a 10 bit signal.

[0110] In a differential amplifier 72, a comparison is made between the digital value of the entered (or desired) target temperature 71 and the digital value of the (actual or) measured temperature 77.

[0111] In a preferred embodiment, a PWM (pulse-width-modulation) 74 is created in the microprocessor in order to control the heating element(s) 60 with a control signal 78 from the mains with so-called solid state relays (SSRs). For example, a period of 1 sec and a duty cycle δ adjustable in one hundred steps from 0 to 100%.

[0112] Since a pulse-wise activation is regarded as disclosed, the explanation below is limited to only a few features. First of all, Figure 11 shows a series of activation pulses having a relatively high duty cycle δ. The period (t_s) comprises an active time (t_son), for example a time during which a heating element is activated and can therefore heat up, and a passive time (t_s - t_son), for example a time during which a heating element is not activated and can therefore cool. The duty cycle δ is the ratio of an active pulse width (t_{s on}) to the total period (t_s).

[0113] In a printer which can be used according to the present invention, the duty cycle δ can be varied while retaining a constant period (t_s) but with varying activation time (t_{s on}). In another embodiment, the duty cycle δ can be varied, with the period (t_s) being varied with constant (t_{s on}).

[0114] Preferably, a number of criteria (for example, control parameters such as gain, integration time, differentiation time constant, filtering input signal) can be adjusted on a video screen and a number of measurements (such as percentage duty cycle, PID variables, temperature, speed) can be read off.

[0115] Initial test results, in which a number of sheets of film have been developed at a set temperature of 120C and with a processing time of 10 seconds, yield:

- heat capacity of 1 sheet of film, 17" × 14": 55 J/C	(calculated)
- processor heating-up time: 20 min	(measured)
- heating-up slope: 0.1C/sec	(measured)
- cooling slope: 0.025C/sec	(measured)
- maximum heat consumption: 183 W	(calculated)
- heat losses: 300 W	(measured)
- temperature drop due to 1 sheet of film: 0.5C	(measured)
- stability of unloaded system: 0.2C	(measured)
- stability of measurement signal PT100: 0.1C	(measured)
- stability of IR-sensor measurement signal: 0.2C	(measured)

[0116] Another version is provided with PT1000 sensors (instead of PT100). Said PT1000 sensors have the advantage that the sensitivity to the transfer/transition resistance of the slip contacts drops by a factor of 10. In order to remove interferences in the signal, a filter may be used, for example a "software filter".

[0117] Another possibility of transmitting signals in a contact-free manner makes use of optoelectronics, for example using emitter-sensor systems.

[0118] A further version is provided with infrared sensors from EXERGEN Corporation (1 Bridge Street, Newton, MA 02158, USA). These are thermocouples which, on receiving infrared, generate a thermal voltage. They receive the heat by radiation, so that no slip contacts are necessary.

[0119] Since the development temperatures are situated around 120°C, sensor types are preferably used which are calibrated at 120°C, i.e. the linear portion of their characteristic is centred around 120°C or 393 K.

[0120] In this connection, the following sensor models have been used:

a) Sensor type IRt/c™. 01-K120 C is a low-cost sensor having a plastic casing (namely ABS, an abbreviation for an acrylonitrile-butadiene-styrene copolymer). As a result, the maximum ambient temperature for the sensor is limited to 70°C, so that measures have to be taken to limit the ambient temperature.

b) Sensor type IRt/c™.1X-K120 C has a stainless steel casing and can therefore resist higher ambient temperatures.

6.12. Second embodiment of a drum heating system according to the present invention ("segmented heating system")

[0121] In a second preferred embodiment, the system comprises a heating element having power compensation at the side edges (see Figure 7), which offers possibilities for an axial compensation.

[0122] As an extension to the previous embodiment of a so-called "segmented drum", in which drum 20 actually comprises, for example, three sections, in this second embodiment, said side sections 63 and 64 of heating element 61 have a different electrical power from the centre section 62, which is sometimes referred to as "segmented heating" system.

[0123] More specifically, in a particular embodiment, said side zones 63-64 are geometrically symmetrical with respect to the centre zone 62 and mutually identical in dimensions and in available power.

[0124] In a particular preferred embodiment, each zone 62-64 has a separate sensor 65-67 and a separate controller.

[0125] In another preferred embodiment, both side zones 63-64 are connected electrically in parallel and jointly controlled by one thermal probe in the centre of one of said side zones. Since the large centre zone in this example has a separate temperature probe in the drum, this actually results in two separate control circuits.

[0126] In another preferred embodiment, only one sensor is fitted and, specifically, in the axial centre of the centre zone 62 of the drum and the three zone heating systems are all three connected in parallel to one control circuit 70, but the installed powers differ for the centre zone with respect to the side zone.

[0127] In still other embodiments, the installed powers can, in addition, be activated by software at various levels. Thus, by way of an example: left-hand zone 63 having an installed power of 0.522 W/cm² and activated at 75%, centre zone 62 having an installed power of 0.492 W/cm² and activated at 90%, right-hand zone 64 having an installed power of 0.522 W/cm² and activated at 80%. In this way, both hardware compensation and software compensation is possible for the axial temperature profile Ta of the drum.

[0128] To illustrate a possibility of axial compensation, Figure 15 shows a hardware possibility comprising a drum-shaped heating body 20 having, in the axial direction, three different installed powers P1-P3 in heating elements 61 (with terminals Mi-Ni). Figure 17 shows a software possibility with three pulse trains having different duty cycles (for example δ1 = 75%, δ2 = 50%, δ3 = 65%).

[0129] Of course, if necessary, more than three heating zones can also be used, with or without symmetrical heating.

[0130] In a further preferred embodiment, the heating element has a continuously extending power profile to compensate for possible static and dynamic interferences in said temperature profiles.

[0131] Figures 12.1-12.3 illustrate some comparative tests. Figure 12.3 compares density profiles measured on image materials developed according to the state of the art, with hardware compensation (HW) according to the present invention or with hardware and software compensation (HW & SW) according to the present invention. Curve 91 shoes a so-called "natural profile", curve 92 shows an axial hardware-compensated profile obtained with a so-called "segmented drum and segmented heating system", while curve 93 shoes a further optimization using a differentiated software control of the side zones of the heating element. This reveals a clear quality advantage (with regard to density uniformity) of the present invention.

[0132] At the design level, a further additional advantage ensues from the present application. Of course, if the useful film width of the drum is 356 mm (14"), a certain additional width is necessary because of the heat distribution across the drum. Tests have revealed that, with a conventional drum, even an additional width of 40% (for example for a film width of 356 mm, the drum width is then approximately 490 mm), the axial temperature profile is insufficiently uniform, depending, of course, on the desired accuracies. After further optimization of embodiments according to the present invention with thermal compensation towards the side edges of the drum, it was possible for the addition to the drum width W to be gradually reduced to 30% (resulting in 470 mm), to 25% (resulting in 450 mm) and even less.

[0133] Owing to the embodiment described of the body of revolution (with a "segmented drum") and of the heating means (with a "segmented heating system") we can specify said heating unit more precisely as a heating unit in which said heating means have a first power profile (Pa) in the axial direction with a modification of said body of revolution according to a first temperature profile (Ta), or also as a heating unit 4 in which said heating means 60 have, in the axial direction, a first power profile (Pa) which is controlled by said control means 72-74.

6.13. Third embodiment of a drum heating system according to the present invention ("HW- or SW-feedforward")

[0134] In a third preferred embodiment, the system comprises a very specific control system in order to obtain a better response to interferences due to the sheets of film being passed through. For this purpose, two "feedforward (FF)" control systems have been tested: an FF in software (in which an additional amount of energy is supplied if a sheet of film is presented) as opposed to an FF in hardware (with an associated heating element being switched on every time a sheet of film is fed through).

[0135] In a first FF version, namely a feedforward in software (FF-SW), a system is incorporated in the software so that, during the processing of a sheet of film, the output of the controller can be blocked and can be kept in position for a certain time.

[0136] A specific example: to process a sheet of film, a well-defined amount of energy is necessary (5500 J is necessary for a sheet of film measuring 17" × 14" and a temperature increase of 20°C to 120°C), which can be supplied by the net power which is available (being the maximum power minus the losses) during a certain time. For a 1300 W resistance element and a loss of 300 W, there is a power excess of 1000 W. This means that the output of the controller has to be kept for 5.5 sec at 100% (resulting from 5500 J to be delivered to 1000 W) in order to replace the energy in the drum which has been drawn off by the film.

[0137] In this connection, Figure 10 shows a time diagram for a compensation of the tangential temperature profile. That the time patterns in Figure 10 (for example, more in the region of seconds or fractions of seconds) are in principle an order of magnitude longer than those in Figure 11 (for example, more in the region of milliseconds) is shown in the drawings by two different hatchings (vertical as against horizontal).

[0138] Suppose that, at instant t0, no image material is present in the processor, the drum is then kept by the controller at a standby temperature (see time interval SB from t0 to t1, with a duty cycle δsb).

[0139] As soon as (at instant t1) the start of the image material appears at the inlet of the processor during the transportation of the image material, signal "DET IN" assumes, for example, a logic 1 state (say, a higher voltage).

[0140] As soon as (at instant t2) the start of the image material appears after that at the outlet of the processor, signal "DET OUT" also assumes a logic 1 state.

[0141] During the residence time of the image material in the processor, also referred to as development time (here from t1 to t4), at least one of said detection signals DET IN or DET OUT remains high, the heating being activated with a duty cycle δdev and as a function of the measured temperature (cf. controller, Figure 9).

[0142] After removing a developed sheet and before feeding in a subsequent sheet (still to be developed), a specific preheating can already occur (see signal FF during t5 to t6 with a duty cycle δff of, for example, 100%). After the elapse of said FF time, the controller may still be blocked at a lower duty cycle δ1 to avoid any temperature shocks.

[0143] Several of said parameters are adjustable: for example FF time (in 1/10 sec, duty cycle δ (in %) ... In a test arrangement an FF 400 W halogen lamp has been employed and the lamp's power is adjustable owing to a variable duty cycle δ.

[0144] A second version is provided with a feedforward in hardware (FF-HW). In this case, use is made of an additional heating element (for example, of 400 W) which additionally heats only the drum segment which will be cooled by the film fed in. This takes place in such a way that the heat distribution over the entire drum circumference remains constant (in contrast to the main heating, which always heats the entire drum).

[0145] Said additional heating element may be a halogen lamp or another heat radiator (such as a ceramic element) which is arranged in a stationary manner outside the drum (see also ref 69 in Figure 3) or an additional heating resistor inside the drum (see also ref 69 in Figure 15); preferably said additional heating element has an adequately high response speed.

[0146] Because such a lamp inherently has one defined, fixed length, such hardware feedforward can only operate optimally for one film width. To remedy this shortcoming, use has been made in a further embodiment according to the present invention of different lamps which have been arranged next to each other and which were capable of being activated separately.

[0147] In an alternative embodiment, one lamp having different segments is used.

[0148] In a particular embodiment according to the present invention, the hardware feedforward lamp is closed off in a light-tight manner towards the interior of the processor by means of a VITON™ rubber seal (VITON™ is a registered trademark of the company Du Pont and comprises a heat and chemical-resistant fluorinated elastomer). In another embodiment said sealing is carried out by means of horse-hair brushes.

[0149] In another embodiment, the temperature of the drum is not measured so much in the rotating drum but at a fixed point at the inlet of the processor.

[0150] In yet another embodiment, the hardware FF can be implemented by pressing one or more small heated rollers against the drum.

[0151] According to the above, the present invention therefore comprises a heating unit comprising a body of revolution having an electrically resistant heating element, drive means for rotating said body of revolution in a controlled manner, transport means for transporting a photothermographic material in a controlled manner around said body of revolution, heating means for heating said body of revolution in a controlled manner by means of said heating element, and means for determining, on said body of revolution, a temperature variation in the axial direction (a) according to a first axial temperature profile (Ta) and a temperature variation in the tangential direction (t) according to a second or tangential temperature profile (Tt), characterized in that both temperature profiles (Ta, Tt) can be controlled independently of one another (in the meaning of measurable and influenceable or manageable).

[0152] In regard to the temperature profile, as a departure from known prior art, the present invention also provides dynamic corrections or compensations. For this purpose, at least two heating zones are provided, of which the power is individually modified, for example by means of modified software, every time a sheet of film arrives.

[0153] Each axial and/or tangential heating zone is activated and/or compensated in a differentiated manner. In the static state, initially different power may be provided (cf. Figure 7). In the dynamic state, there is, in addition, the possibility of different activation by means of a variable duty cycle δ (see Figure 17). In alternative embodiments, such compensation can also be implemented by means of variable pulse numbers or even by means of a variable phase (by gating of a sinusoidal alternating voltage by means of a thyristor).

[0154] Rather sluggish influences on the temperature behaviour of the system, also referred to as "static interferences", may originate from a varying ambient temperature, moisture content, etc.

[0155] Rather fast influences on the temperature behaviour of the system, also referred to as "dynamic interferences", may originate from a subsequent sheet, from another type and/or thickness and/or heat capacity and/or moisture content.

[0156] An off-line compensation will primarily compensate for systematic variations in the thermal properties of a drum.

[0157] An on-line (or "instant") compensation will primarily compensate for dynamic variations in the thermal properties of a drum.

[0158] Figure 16 shows an embodiment of a drum, in which various powers (see P1,1 - P1, 2 - P1,3 up to and including Pm,n) can be switched on by hardware both in the axial direction and in the tangential direction (cf. heating elements 61 having terminals Mij-Nij).

[0159] Owing to the feedforward facilities described, we can specify said heating unit more precisely as a heating unit in which said heating means have, in the tangential direction, a second power profile (Pt) with a modification according to said second temperature profile (Tt) of said body of revolution, or also as a heating unit 4 according to one of the preceding claims, said heating means 60 having in the tangential direction a second power profile (Pt) which is controlled by said control means 72-74.

[0160] According to all the above, the present invention furthermore comprises a development apparatus 1 for developing photothermographic material, at least one feed unit 2, a development unit 4 an an unloading unit 6, characterized in that said development unit 4 comprises a heating module in the form of a body of revolution or drum 20. With the objective of a good image quality, it almost goes without saying that, during the thermal development, said photothermographic material and said drum move at a synchronous speed.

[0161] In a more complete embodiment, the present invention also comprises a registration system 1 for registering photothermographic material, having at least one feed unit 2, an exposure unit 3, a development unit 4 and an unloading unit 6, characterized in that said development unit comprises a heating unit in the form of a body of revolution or drum.

6.14. Overall comparative tests

[0162] A number of comparative experiments were performed on the basis of photothermographic material as described in detail in EP-A-96.201.530.1 cited above.

[0163] In comparative experiments, a number of sheets of film were exposed to a diode laser having a rated power of 100 mW and a spot size on the film of approximately 140 µm and developed at a process temperature of 120°C for a process time of 15 sec.

[0164] The tests took place especially around an optical density equal to 1 on the image-forming element because the human eye is particularly sensitive in this region to small variations in density. The density measurements were carried out with a MACBETH™ TD904 densitometer having an ortho-filter.

[0165] Further common characteristics for all the tests relate to the image material, having dimensions 17" × 14" (or approximately 432 mm × 356 mm), a heat capacity of 55 J/°C and a heat consumption during processing of 183 W.

[0166] Important results from a drum optimization (inter alia, with a wall thickness of 14 mm versus 8 mm) are summarized in Table I below.

[0167] Important results from an optimization of the control system (inter alia, with SW feedforward versus HW feedforward) are summarized in Table II below. An important advantage of the HW feedforward is based, inter alia, on the relatively short recovery time (under otherwise identical test conditions, the drum temperature recovered after only 15 seconds after processing five sheets of film using an HW feedforward, whereas almost 400 seconds were necessary in the case of a heating system without feedforward).

Table I

	Segment drum + segment heating (= emb. 2)	The same with optimization (thinner drum) (= emb. 2)
Drum dimensions	490 × 198 × 14 mm × mm × mm	470 × 198 × 8 mm × mm × mm
Drum volume	1358 cm3	744 cm3
Rated power	1500 W	1300 W
Drum heating-up time (to 120°C)	20 min	13 min
Drum heating-up slope	0.08 °C/s	0.13 °C/s
Heat losses	300 W	250 W
Ta uniformity (unloaded drum, 120°C)	2.5 °C	0.5 °C
Tt stability (unloaded drum, 120°C)	0.2 °C	0.2 °C

Table II

	Segment drum + segment heating + optimization (= emb. 2)	The same + SW feedforward (= emb. 3)	The same + HW feedforward (= emb. 3)
Drum dimensions	470 × 198 × 8 mm × mm × mm	ditto	ditto
Drum volume	744 cm3	ditto	ditto
Rated power	1300 W	1300 W	1300 W
Tt stability (120°C and 2 sheets/min)	1.2 °C	0.8 °C	0.5 °C
Recovery time after 5 sheets	6.5 min	1 min	15 sec
Tt stability (unloaded)	0.15 °C	0.15 °C	0.10 °C

[0168] A good picture of the density profiles now reached is illustrated by Figures 13 and 14. In these figures, Figure 13 shows a tangential density profile measured on a thermographic material developed according to the second embodiment of the present invention.

[0169] Figure 14 shows an axial density profile measured on a thermographic material developed according to the third embodiment of the present invention.

6.15. Summary of the detailed description

[0170] The above description results in a heating unit having means for developing a photothermographic material at a qualitative high level by supplying a well-defined amount of heat for a well-defined time.

[0171] In this connection, the most important advantages of the present invention succeed in (i) achieving a very constant temperature across the entire surface of a sheet of film both across the surface of one complete sheet of film and across different sheets of film, (ii) the achievement of a very constant development time, which is constant for every point in one (or more) surface(s) of sheets of film and (iii) the achievement of very short recovery times.

[0172] On the one hand, we obtain as a result of this an advantageous combination of high stability and low recovery time to achieve adequate film throughput and, on the other hand, density variations are limited in an observation region around 1.0 D to values better than 0.1 D and in an observation region around 0.3 D to values better than 0.02 D.

[0173] A synoptic summary of the various compensation means for compensating for static and dynamic interferences in said temperature profiles is presented in Table III below.

Table III

Temperature profiles	Hardware compensation	Software compensaton
Ta	Switch on locally varying power (segmentation)	Activate locally differentiated power (e.g. duty cycle)
	See Fig. 7, refs. 62-64; Figs. 15 & 16, refs. P1-P3	See Fig. 17
Tt	Switch on additional heating means temporarily (e.g. lamp, roller)	Activate differentiated power temporarily
	See Fig. 3, ref. 69; Fig. 15, ref. 69; Fig. 16, refs P1,1 & Pm,1	See Fig. 10, ref. 79; Fig. 17

[0174] It will be clear to the person skilled in the art that, on the basis of the heating unit described above, an extension to a complete development apparatus and even to a complete registration system is quite possible.

[0175] Phrased in more detail, the present application therefore protects also a development apparatus 1 for developing photothermographic material at least comprising a feed unit 2, a development unit 4 and an unloading unit 6, characterized in that said development unit comprises a heating unit as described above.

[0176] A further aspect of the present invention protects a registration system 1 for registering photothermographic material, at least comprising a feed unit 2, an exposure unit 3, a development unit 4 and an unloading unit 6, characterized in that said development unit comprises a heating unit as described above.

[0177] In a particular preferred embodiment of such a registration system 1, said exposure unit 3 and said development unit form one common unit around one and the same body of revolution.

[0178] From another standpoint, the present invention protects a method for registering information on photosensitive and thermally developable image material, comprising the following steps:

imagewise exposure of said image material,

thermal development of said image material around a rotating body of revolution,

measurement, on said body of revolution, of a temperature variation in the axial direction (a) according to a first temperature profile (Ta) and a temperature variation in the tangential direction (t) according to a second temperature profile (Tt), and

compensation of static and dynamic interferences in said temperature profiles.

[0179] In an extreme situation, said temperature variations can also be determined on the basis of only one sensor in the axial direction and one sensor in the tangential direction, or even from one single sensor which is then used for both directions.

[0180] For this purpose, we first suppose that, in a preceding test, the corresponding temperature variations (Ta, Tt) are measured for each discrete target value of the drum temperature (for example, 115°C, 116, ..., 124, 125°C) in each case (with either a plurality of sensors or with displaceable sensors) and registered (for example, in a look-up-table memory (LUT)) as characteristic curves (as depicted in Figures 6.3-6.5 for one temperature setting).

[0181] If at least one temperature is then measured in an actual printing cycle, the corresponding temperature variations can be derived from the electronic memory. This principle is successful both in the axial and tangential direction, but in the tangential direction an alternative can also possibly be used by means of "time sampling".

[0182] For complete clarity it may expressly be stated yet again:

(i) that the measurerment of said temperature variations can, of course, be performed both off-line in a previous test cycle and on-line during an actual printing cycle;

(ii) that both temperature profiles are controlled in such a way that a uniform temperature is produced on the image material.

[0183] A further preferred embodiment therefore comprises a method which also comprises a step for the preceding determination of said temperature profiles.

[0184] Yet a further preferred embodiment relates to a method such as that just described, but in which exposure and development of said (image) material take place around one and the same body of revolution.

6.16. Applicability of the present invention

[0185] In further preferred embodiments according to the present invention, various features have also been improved which are now specified concisely.

[0186] In a particular version, control parameters may change as a consequence of the error found.

[0187] In another version, namely having a cascade controller in which the temperature of the roller surface forms the "master" and in which the temperature of the heating element forms the "slave", it is possible to respond more rapidly and reduce the control error.

[0188] In another version, the sheets of film to be processed are already heated before they are fed into the processor. As a result the thermal shock in the processor and the temperature drop across the drum are reduced.

[0189] In yet another version, the belt is heated.

[0190] In yet other versions, said image-forming element may not have a sheet form but a belt form.

[0191] The film is preferably transported by means of a concomitantly rotating conveyor belt 41 which presses the film against the hot drum 30. However, if film on a roller is employed, the conveyor belt can itself be omitted and, for example, the tension on the film can provide for a good contact and for a satisfactory development.

[0192] A particular property of the present application is based on the fact that the thermal processing can take place in one of the following two divergent manners, namely (i) with the emulsion side of the image material in contact with the heated drum or (ii) with the emulsion-free side of the image material in contact with the heated drum. The first method has the advantage that lower development temperatures and/or shorter development times are possible.

[0193] The second method has the advantage that any temperature differences are also averaged out by the base of the photosensitive layer. Depending on the method chosen, some practical interventions have, however, to be performed (for example adjustment of temperatures and/or times, reversal of film run ...).

[0194] After being acquainted with the present patent application, it is quite possible that a person skilled in the art proposes other embodiments and/or other applications which are based, however, completely on the principle of the present invention.

[0195] Thus, a system may also comprise more than one heating body, for example a linear iteration of one and the same basic concept (two drums arranged on one side and in series after one another along a path followed by the image-forming element) or an iteratively alternating arrangement with two drums opposite one another on either side of a path followed by an image-forming element (with possibly two sides to be developed or to be dried).

[0196] Double-sided arrangement of two drums may be attractive, inter alia, in systems having a photosensitive layer on both sides of a base or having a photosensitive layer on one side of a base and an auxiliary layer (for example, an anti-halo layer or an anti-stress layer) on the other side of said base.

[0197] In a method according to the present invention, said image-forming element comprises a photothermographic material. Let it be clear that in a system or apparatus or in a method or process according to the present invention, said photothermographic materials comprise a silver halide or a mixture of silver halides, one or more organic salts and one or more reducing agents. After exposure and development, visual densities greater than 1 are obtained. In a further preferred embodiment said silver organic salt is silver behenate and said reducing agent is a phenolic reducing agent. Preferably, said photothermographic materials comprise one or more toning agents which result in a neutral grey density on development. In addition, said photothermographic materials preferably comprise one or more stabilizers to maintain the quality of the image formed.

[0198] The present invention can be used to produce both images in reflection (based, for example, on paper, inter alia, used in the copying sector) and images in transparency (based, for example, on black-and-white or coloured film, inter alia, used in medical diagnoses).

[0199] Applications are encountered both in graphical applications (generally with high contrast) and in medical applications (generally with reproduction of a large number of continuous tones).

[0200] In addition, applications are also conceivable in other fields, such as general photography (in connection with the drying of wet-developed photographic materials), electro(photo)graphy and toner-jet (in connection with the thermal fixing of toners), ink-jet (in connection with the drying of the sprayed image) and lithographic printing procedures (cf. drying of one or more inks) or the "on-press" exposure and development of printing plates, more specifically of photothermographic printing plates, etc. If necessary, such a uniform heating can also be carried out here in accordance with the present invention.

Claims

1. Heating unit (4) comprising

a body of revolution (20) having an outer casing (21) made of thermally conducting material and having axial end faces (22-23) made of thermally insulating material,

rotation means (30) for rotating said body of revolution,

heating means (60) for heating said body of revolution,

measuring means (68) for measuring on said body of revolution at least one temperature to determine a temperature variation in the axial direction (a) and at least one temperature to determine a temperature variation in the tangential direction (t),

conversion means (76) for converting said temperatures into corresponding temperature signals (77),

control means (72-74) for converting said temperature signals into control signals (78) for said heating means.

2. Heating unit (4) according to the preceding claim, wherein said heating means (60) comprise an electrically resistant heating element (61) and/or an electrical heat radiator (69).

3. Heating unit (4) according to Claim 1 or 2, wherein said heating means (60) have, in the axial direction, a first power profile (Pa) which is controlled by said control means (72-74).

4. Heating unit (4) according to one of the preceding claims, wherein said heating means (60) have, in the tangential direction, a second power profile (Pt) which is controlled by said control means (72-74).

5. Development apparatus (14) for developing a photothermographic image material (10), at least comprising a heating unit (4) according to one of Claims 1 to 4, an inlet (46), an outlet (47) and transport means (40) for transporting said photothermographic image material.

6. Registration system (1) for registering photothermographic image material (10), at least comprising a development apparatus according to Claim 5, a feed unit (2), an exposure unit (3) and an unloading unit (6).

7. Registration system (1) according to Claim 6, wherein said exposure unit (3) and said development unit (4) form one common unit (5) around one and the same body of revolution (20).

8. Method for registering information on photosensitive and thermally developable image material (10), at least comprising the following steps:

imagewise exposure of said image material,

thermal development of said image material around a rotating body of revolution (20) provided with heating means (60),

measurement, on said body of revolution, of a temperature variation in the axial direction (a) and a temperature variation in the tangential direction (t), and

conversion of said temperatures into corresponding temperature signals (77),

conversion of said temperature signals into control signals (78) for said heating means.

9. Method according to Claim 8, wherein exposure and development of said image material (10) take place on one and the same body of revolution (20).

Drawing