Thermal transfer foil for producing a true color image, process for producing a true color image, and true color image

Abstract

A thermal transfer foil and also a process for producing a true color image, and a true color image, wherein the thermal transfer foil includes at least one effect pigment layer and a carrier foil which has first effect pigments in one or more first regions.

Claims

1. A process for producing a color image comprising: providing one or more thermal foils having an effect pigment layer and a carrier foil, the effect pigment layer comprising effect pigments; and applying subareas of the effect pigment layer of at least one of the one or more thermal transfer foils to a first surface of a substrate using one or more thermal transfer printheads, whereby effect pigments having different optical effects and/or orientations are transferred from the subareas to form two or more halftone dots on the substrate, said two or more halftone dots being formed in such a way that additive and/or subtractive color mixing of the two or more halftone dots form the color image, wherein the color image consists of a multiplicity of color domains which, when illuminated and viewed in reflected light or in transmitted light show a color, and wherein the at least two halftone dots are formed in at least 10% of the color domains.

2. The process according to claim 1, wherein, in each of the color domains, two or more of the halftone dots are applied alongside one another and/or over one another and/or overlapping one another on the first surface of the substrate.

3. The process according to claim 1, wherein the halftone dots have at least one lateral dimension in the range between 40 μm and 100 μm, wherein the lateral dimensions of the halftone dots amount to between two times and five times the lateral dimension of the effect pigments.

4. The process according to claim 1, wherein the substrate is selected from or components of the substrate are selected from the following: PET, PP, PE, PA, PEN.

5. The process according to claim 1, wherein the substrate is transparent, and the transparent substrate is applied, by a surface opposite to the first surface, to a colored background.

6. The process according to claim 1, wherein a protective layer is applied to the first surface and/or to the surface of the substrate that is opposite to the first surface, said protective layer being selected from the following: transparent overprint, laminate, plastic sheet, glass sheet.

7. The process according to claim 1, wherein the substrate is transparent, and the transparent substrate is applied, by the first surface, to a colored background.

8. The process according to claim 1, wherein the substrate is opaque and/or is applied to a black or dark surface.

9. The process according to claim 1, wherein the substrate has at least one colored varnish coat on a second surface that is opposite to the first surface.

10. The process according to claim 9, wherein a colorimetric value of the visible intrinsic color of the colored varnish coat in a color space defined by coordinate axes a* and b* specifying the complementary colors and by coordinate axis L* specifying the luminance of the hue in a CIELAB color space, is provided in a range of L* of greater than or equal to 0 and less than or equal to 90.

11. The process according to claim 9, wherein the colored varnish coat is provided with one or more dyes and/or one or more different-colored pigments.

12. The process according to claim 11, wherein one or more of the pigments are selected from the following: optically variable pigments, pigments containing thin-film layers and/or liquid-crystal layers which generate a color shift effect dependent on viewing angle or illumination angle, organic pigments, inorganic pigments, luminescent additives, UV-fluorescent additives, UV-phosphorescent additives, IR-phosphorescent additives, IR upconverters, thermochromic additives.

13. The process according to claim 11, wherein a pigmentation number of greater than or equal to 5 cm.sup.3/g and less than or equal to 120 cm.sup.3/g, is provided.

14. The process according to claim 1, wherein a register tolerance in an advancement direction and/or perpendicular to the advancement direction between at least two regions, each of which is transferred or printed onto the substrate by different thermal transfer foils, to one another is greater than or equal to −0.15 mm and less than or equal to +0.15 mm.

15. The process according to claim 1, wherein a first of the thermal transfer foils comprises a red effect pigment layer, and wherein a second of the thermal transfer foils comprises a green effect pigment layer, and wherein a third of the thermal transfer foils comprises a blue effect pigment layer.

16. The process according to claim 1, wherein the thermal transfer foil has two or more regions in which the effect pigment layer comprises effect pigments which differ in respect of their color effect, and/or orientation.

17. The process according to claim 1, wherein the process comprises the following further steps: providing a multicolored motif; determining two or more grayscale images assigned in each case to a color channel; converting a third grayscale image assigned to a blue color channel; converting the respective grayscale images into a respective halftone image consisting of a multiplicity of halftone dots, based on frequency-modulated rastering and/or on periodic rastering; and driving the thermal transfer printhead or of the thermal printheads in such a way that the subareas of the effect pigment layer or effect pigment layers, in halftone dot formation, are transferred to the first surface of the substrate in accordance with the size and disposition of the halftone dots of the halftone images.

18. The process according to claim 17, wherein the periodic rastering is provided with two or more different halftone angles and/or two or more different halftone dot shapes.

19. The process according to claim 1, wherein the halftone dot shapes are selected from the following: punctiform, rhomboidal, cruciform.

20. The process according to claim 17, wherein the multicolored motif is selected from the following: photos, images, alphanumeric symbols, logos, microtexts, portraits, pictograms.

21. The process according to claim 1, wherein the process comprises the following further steps: applying further layers or layer sequences before and/or after the application of the true color image by means of a process selected from the following: thermal transfer printing, gravure printing, flexographic printing, screen printing, pad printing, inkjet printing, hot stamping, cold stamping.

22. The process according to claim 21, wherein the layers or layer sequences are each or partially selected from the following: transparent, translucent and/or opaque color layer, transparent, translucent and/or opaque metallic layer, open or embedded replication layer comprising diffractive and/or refractive relief structures, transparent, translucent and/or opaque reflection layer, thin metal layer, HRI layer, LRI layer, volume hologram layer, transparent, translucent and/or opaque thin-film construction, Fabry-Perot layer with absorption layer, spacer layer and/or reflection layer.

23. A true color image produced according to claim 1, wherein the true color image comprises a multiplicity of halftone dots applied to a first surface of a substrate, wherein the halftone dots are formed by subareas of an effect pigment layer of a thermal transfer foil or by subareas of effect pigment layers of two or more different thermal transfer foils.

24. The true color image according to claim 23, wherein a halftone dot comprises 10 to 1000 effect pigments, which are disposed partly or completely above one another and/or alongside one another.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic representation of a thermal transfer foil

(2) FIG. 2 shows a schematic representation of thermal transfer foils

(3) FIG. 3 shows a schematic representation of a thermal transfer foil

(4) FIG. 4 shows a schematic representation of an effect pigment

(5) FIG. 5 shows a schematic representation of a color space

(6) FIG. 6 shows a schematic representation of a thermal transfer printing apparatus

(7) FIG. 7 shows a schematic representation of a rastering

(8) FIG. 8 shows a schematic representation of a rastering

(9) FIG. 9 shows a schematic representation of a rastering

(10) FIG. 1 shows the layer construction of a thermal transfer foil 1 in principle.

(11) The thermal transfer foil 1 comprises a carrier foil 12 and an effect pigment layer 11. This thermal transfer foil 1 is designed, in terms of its layer construction and the design of the individual layers, in such a way that the effect pigment layer 11 can be applied regionally to a surface of a substrate by means of a thermal transfer process and more particularly by means of a thermal transfer printhead. For this purpose it is necessary for regions of the effect pigment layer 11 to be detachable from the transfer foil 12 on local introduction of heat by means of a thermal transfer printhead and adhered on the surface of the substrate correspondingly as mediated by the heat.

(12) For this purpose, the thermal transfer foil 1 is formed preferably as described below:

(13) The thermal transfer foil 1, in addition to the carrier foil 12, preferably has a backside coating 14, a detachment layer 13 and an adhesive layer 15.

(14) The carrier foil 12 consists preferably of a polymeric foil in a layer thickness of between 3 μm and 30 μm. It has proven particularly appropriate to use a PET foil for the carrier foil 12, and more particularly to use a PET foil in a layer thickness of between 3 and 15 μm, 5.7 μm for example. This choice of the layer thickness of the carrier foil 12 ensures that sufficient heat can be transported from the printhead through the carrier foil 12 in order to allow the subsequent layer to be transferred to the surface of the substrate.

(15) Particularly advantageous here, moreover, is the use of the backside coating 14. This is the case because the surface of customary plastic carrier foils is frequently too rough or too dull to glide sufficiently well over the printhead of the thermal transfer printer. The backside coating 14 hence consists preferably of a lubricious coating material which is applied preferably with a layer thickness of between 0.05 μm and 3 μm, in particular approximately 0.3 μm, to the carrier foil 12. The backside coating 14 is here applied preferably by gravure printing. The backside coating 14 preferably comprises one or more polyester resins or consists of one or more polyester resins.

(16) The optionally provided detachment layer 13 improves the detachment property of the effect pigment layer 11 from the carrier foil 12 during thermal transfer printing. The detachment layer 13 preferably has a layer thickness of between 0.1 μm and 3 μm, more preferably between 0.25 μm and 0.75 μm. The detachment layer 13 here consists preferably of a resin, more particularly a silicone resin, with a binder, more particularly an acrylate. Further, the detachment layer 13 may also consist of a wax, or one or more waxes may have been added to the detachment layer 13. The detachment layer 13 in this case will be applied preferably by means of a printing process, more particularly by means of gravure, screen, flexographic, offset, inkjet or pad printing, to the carrier foil 12.

(17) The effect pigment layer 11 comprises effect pigments which are preferably embedded in a binder matrix. The effect pigment layer 11 preferably has a layer thickness of between 0.5 μm and 5 μm, more particularly between 1 μm and 3 μm, more particularly between 1.5 μm and 2.5 μm.

(18) As already observed above, the effect pigment layer comprises not only the effect pigments but also, preferably, one or more binders from the following classes of compound: polyacrylate, polyurethane, polyvinyl chloride, polyvinyl acetate, polyester, polystyrene, and copolymers of the aforesaid classes of compound. Moreover, the effect pigment layer 11 has preferably been admixed with adjuvants, especially rheological additives, more particularly a phyllosilicate, preferably one or more bentonites.

(19) The effect pigment layer 11 preferably has a high degree of filling with effect pigments, more particularly a degree of filling of more than 30 weight percent, preferably between 50 weight percent and 70 weight percent, as for example 60 weight percent, in the solids.

(20) The addition of the above-cited rheological additive is particularly important here for the formulation of the decorative varnish, by means of which the effect pigment layer 11 is formed by means of a coating process on the detachment layer 13. In addition to the components cited, this decorative varnish further comprises one or more solvents, examples being organic solvents, which evaporate after coating has taken place. It is also possible, moreover, for the decorative varnish to be a water-based decorative varnish. As a coating process, a printing process has been found particularly appropriate, especially gravure, screen, flexographic or offset printing.

(21) The addition of the rheological additive to the decorative varnish reduces sedimentation of the effect pigments in the decorative varnish. In contrast to absorption pigments commonly used in printing inks, such as white pigments, for example, which have an approximately spherical form with a diameter of less than 5 μm, more particularly less than 1 μm, effect pigments customarily comprise decidedly large, lumplike structures. As a result of the size and high density of the material, there is comparatively rapid settlement of the pigments and compacting of this precipitate. The settling speed is dependent firstly on the particle morphology but secondly, also, on the property of the medium in relation to the viscosity, density, polarity, etc., and may range from a few days down to a few hours. As long as the printing medium is kept in motion, by shaking or stirring, the dispersion is usually retained. In a coating material at rest, in contrast, settlement is usually unavoidable over the longer term. Where such precipitation occurs, a critical factor is whether it is a soft, bulky precipitate, which can be disrupted again by gentle stirring or shaking, for example, or whether the precipitate is in so compacted a form that the forces between the particles cannot readily be undone by stirring or shaking. A printing medium compacted in this way should be avoided at all costs, since in that case any further use is impossible or virtually impossible.

(22) In order to obtain the precipitate in a soft and bulky form, the addition of the above-recited rheological additives has proven to be advantageous. These additives are added to the decorative varnish preferably at a weight percentage of 1 to 10, more preferably of 2 to 8, more preferably still of 3 to 5. By the corresponding addition of this additive and also, moreover, where appropriate by corresponding accompanying measures in the feeding of the decorative varnish to the printing mechanism, it is possible to improve the settlement characteristics of the effect pigments and so also to tailor the particle area density within the effect pigment layer 11 and also the alignment of the effect pigments of the effect pigment layer 11 through corresponding application of the decorative layer.

(23) It is also possible, furthermore, for the effect pigment layer 11 to comprise not only the effect pigments but also, additionally, absorbing inorganic and/or organic dyes and/or pigments. These dyes and/or pigments preferably absorb a sub-spectrum of the incident visible light and so generate the color of the respective dye or pigment. Furthermore, phosphorescent or fluorescent pigments and/or dyes may be admixed additionally to the effect pigment layer 11.

(24) The fraction of the absorbing pigments among the total amount of the pigments is preferably below 20%, more particularly below 5%, with further preference below 1%.

(25) It has proven appropriate, moreover, if the composition of the effect pigment layer 11 is selected such that at the same time the effect pigment layer 11 provides the function of an adhesive layer. By this means it is possible to do without the adhesive layer 15. This may be brought about in particular by using as binder or binder constituent of the effect pigment layer 11 a binder which is activatable thermally, for example possessing thermoplastic properties or being crosslinkable by means of heat and/or UV radiation. It is possible for the activation in particular also to generate or initiate a crosslinking reaction in the binder of the effect pigment layer 11. Additional curing of the binder of the effect pigment layer 11 may take place by means of UV radiation in an operating step (post-curing) which takes place following the activation by means of heat, in terms of time.

(26) Effect pigments used in the effect pigment layer 11 are preferably transparent, platelet-shaped interference layer pigments. As already observed above, firstly a part of the incident light in the case of such transparent interference layer pigments is reflected preferably at two or more interfaces of the interference layer pigment, and another part of the light is transmitted through the pigment. The transmitted fraction of the light is preferably then absorbed and/or reflected by the ground. Transparent interference layer pigments of this kind preferably have a transparency of more than 30%, more preferably of more than 50%, in the visible spectral range.

(27) A schematic representation of an effect pigment of this kind is shown for example in FIG. 4:

(28) FIG. 4 shows an effect pigment 2 which has an interference layer 22, an auxiliary carrier 20, a first auxiliary layer 21 and a second auxiliary layer 23. Moreover, the effect pigment 2 has a platelet-shaped morphology, with the effect pigment 2 having a diameter c and a thickness or height d. The interference layer 22 also has a layer thickness a, and the auxiliary carrier 20 has a layer thickness b.

(29) The auxiliary carrier 20 serves essentially for increasing the mechanical robustness of the pigment. The auxiliary carrier 20 consists preferably of natural or synthetic mica, aluminum oxide, silicon dioxide, borosilicate glass, or nickel or cobalt. The layer thickness d of the auxiliary carrier 20 is preferably in a range between 100 nm and 1000 nm.

(30) The interference layer 22 consists preferably of iron oxide, zinc sulfide, silicon dioxide, titanium dioxide, not only in the rutile but also in the anatase and brookite modifications, or magnesium fluoride.

(31) The layer thickness a of the interference layer 22 is preferably selected such that interference effects occur in the visible wavelength range. The optical thickness of the interference layer 22 is for this purpose preferably selected such that it meets the λ/2 or λ/4 conditions for a wavelength λ in the region of visible light.

(32) Optical thickness refers to the product of physical thickness and the refractive index of the layer. This means that layers having a higher refractive index must correspondingly be less thick in order to generate the same optical thickness as a layer having a lower refractive index.

(33) By the λ/2 or λ/4 condition is meant the path difference between two or more coherent waves of the incident light. This path difference is critical to the occurrence of interference phenomena. If the path difference between two waves of equal wavelength λ with the same amplitude is exactly one half wavelength (plus an arbitrary integral multiple of the wavelength), the two component waves cancel one another out. This attenuation of intensity is called destructive interference. If the path difference is an integral multiple of the wavelength, the amplitudes of the two component waves are added to one another. This case is called constructive interference. At values in between there is a partial cancellation or extinction.

(34) Depending on the refractive index of the material used for the interference layer 22, therefore, the layer thickness a is situated preferably within a range between 50 nm and 500 nm. Through the corresponding layer thickness of the interference layer, the effect pigment acts as a color filter, which reflects or transmits a specified color spectrum in dependence in particular on the incident angle of the light. This also, furthermore, produces preferably a more or less strongly pronounced color change as a function of the incident angle of the light. This color change is particularly strongly pronounced when substances are selected that have a low refractive index for the interference layer 22, whereas it is only weakly pronounced for substances with a high refractive index.

(35) The optional first auxiliary layer 21 serves preferably as a crystallization aid in order to generate the metal oxide layer in a particularly advantageous crystal modification, and it may consist, for example, of tin dioxide.

(36) The optional second auxiliary layer 23 may be provided in order to protect the effect pigment 2 from environmental effects. More particularly this layer ensures that any chemical and/or physical interaction of the effect pigment with the surrounding binder matrix is prevented or minimized. It is also possible, moreover, for a colored metal oxide to be used as second auxiliary layer 23, in order to modify appropriately the color of the effect pigment.

(37) As already observed above, the effect pigment 2 is preferably platelet-shaped in form. “Platelet-shape” here means preferably that the top and bottom sides of the effect pigment 2 are aligned approximately in parallel with one another. Moreover, the height or thickness d of the effect pigment 2 is also much smaller than its diameter c. Thus the height d of the effect pigment 2 is preferably less than 1 μm, whereas the diameter c is between 2 μm and 200 μm, preferably between 5 μm and 35 μm. As well as a discoid embodiment of the platelet-shaped effect pigments, more particularly of the effect pigment 2, however, any desired alternative morphology is also possible, more particularly an irregular morphology, an angular morphology or ellipsoidal morphology of the platelet-shaped effect pigments.

(38) The color impression imparted by such effect pigments derives—in contrast to that of absorbing pigments—essentially from interference phenomena. These phenomena are brought about by multiple reflection at interfaces in the effect pigments—for example, the interface at the front side and the reverse side of the interference layer 22. In this context it is also possible for the effect pigment 2 to have not only one interference layer 22, but instead an even or uneven number of interference layers having different refractive indices, so allowing the filter effect of the effect pigment to be set to a correspondingly narrower band.

(39) Through the choice of the layer thickness for the interference layer 22, as observed above, a portion of the irradiated white light, which contains all wavelengths of the visible spectrum, is extinguished by destructive interference, and another part is amplified by constructive interference, so producing a corresponding color impression in reflection. In transmission, moreover, a corresponding color impression is produced which is complementary to the reflection color.

(40) Because the effect pigments of the effect pigment layer 11 take the form of transparent effect pigments, a large part of the irradiated spectrum can be transmitted through the respective effect pigments and can interact with the background or else with adjacent effect pigments of the effect pigment layer. Furthermore, this also ensures that even on overlap of the halftone dots on the substrate, there is optical superimposition of the optical effects provided by the effect pigments of different halftone dots.

(41) In order to ensure this effect, it is also advantageous, moreover, for the binder of the effect pigment layer 11 as well to be selected such that it is transparent or largely transparent in the visible wavelength range, and more particularly possesses a transmissivity in the visible wavelength range of more than 30%, more preferably of more than 50%, more preferably of more than 80%, relative to a formation in the layer thickness of the effect pigment layer 11.

(42) The size distribution of the effect pigments is preferably selected such that the effect pigments have a lateral extent of between about 1 μm to 35 μm based on the longest extent of the effect pigment. It has further emerged that, as already observed above, the D.sub.x value of the distributors is a further important variable, with x standing for the percentage fraction of the particles which are smaller than the specified value. The preferred range of the particles lies in particular at D.sub.90≤35 μm, D.sub.50<20 μm, D.sub.10<12 μm. This means that only a very small fraction of the effect pigments are larger than 35 μm, whereas 40% are located in the middle range between 12 μm and 20 μm. This allows a particularly effective compromise between gloss and hiding power of the effect pigment layer 11 and also sufficient applicability of the halftone dots by means of a thermal transfer printhead.

(43) Effect pigments which may be used include, for example, the effect pigments available under the brand name Iriodin, Spectraval or Pyrisma from Merck.

(44) For the production of the true color images it is possible to use a plurality of thermal transfer foils, or else just one specially designed thermal transfer foil.

(45) The thermal transfer foils employed may in this case in principle be formed on the one hand so that they have one or more first regions which comprise first effect pigments. The first region may comprise preferably at least 90% of the area of the effect pigment layer of the thermal transfer foils and/or of the area of the carrier foil, or else may comprise fully the entire area of the effect pigment layer of the carrier foil.

(46) An exemplary embodiment of this kind is shown in FIG. 2:

(47) FIG. 2 shows by way of example a plurality of thermal transfer foils, namely a first thermal transfer foil 1a, a second thermal transfer foil 1b and a third thermal transfer foil 1c. The thermal transfer foils 1a, 1b and 1c have a formation as set out in relation to the exemplary embodiment according to FIG. 1, and they each have an effect pigment layer 11 with first, second and third effect pigments 211, 212 and 213, respectively. The advancement direction 100 of the thermal transfer foils 1a, 1b, 1c is labelled with an arrow, which preferably also provides the direction of the longitudinal extent of the thermal transfer foils 1a, 1b, 1c.

(48) The effect pigment layer 11 of the thermal transfer foil 1a is here formed identically over the entire area or at least 90% of the area of the effect pigment layer 11 or of the carrier foil 12, and in this region, for example, forms a first region 111 which comprises the first effect pigments 211. The thermal transfer foils 1b and 1c are designed correspondingly, so that their effect pigment layer 11 forms a second region 112 and a third region 113, respectively, in which the second effect pigments 212 and third effect pigments 213, respectively, are provided.

(49) At its most simple, therefore, the effect pigment layer 11 of the thermal transfer foil 1a comprises only one kind of color pigments, namely the first effect pigments 211. The second thermal transfer foil 1b likewise only comprises a single kind of effect pigments, namely the second effect pigments 212. The thermal transfer foil 1c in the simplest case likewise exhibits only one kind of effect pigments, namely the effect pigments 213.

(50) The first effect pigments 211, second effect pigments 212 and third effect pigments 213 differ preferably in terms of their optical effect, more particularly in terms of their color effect and/or alignment. In one preferred embodiment, for example, the first effect pigments 211 are formed, then, by interference pigments with a reddish perceived color, the second effect pigments 212 by interference pigments with a greenish perceived color, and the third effect pigments 213 by interference pigments with a bluish perceived color.

(51) It is also possible, moreover, for the regions 111, 112 and 113 each to comprise not just one effect pigment, but instead to comprise a mixture of two or more different effect pigments, so that the effect pigment layers of the thermal transfer foil 1a, 1b and 1c each comprise a mixture of two or more effect pigments. The mixture of the corresponding effect pigments is here selected preferably such that the regions 111, 112 and 113 differ in relation to their optical effect, more particularly in relation to their color effect. Thus, for example, the respective mixture of the effect pigments in the regions 111, 112 and 113 can be selected such that the regions 111 generate a perceived red color, the regions 112 a perceived green color and the regions 113 a perceived blue color in a particular viewing/illumination scenario.

(52) It is also possible, moreover, for a thermal transfer foil to comprise not just one region but instead two or more of the regions set out above, and so to comprise a plurality of regions each having different optical effects.

(53) Thus, for example, the exemplary embodiment according to FIG. 3 shows a detail of a thermal transfer foil 1d which is constructed like the thermal transfer foil according to FIG. 1. This transfer foil in this case has a plurality of first regions 111, second regions 112, third regions 113, which in particular are provided in iterative disposition on the thermal transfer foil 1d. In each of the regions 111, 112 and 113, the effect pigment layer generates a correspondingly assigned optical effect, with the optical effect of the first regions 111 being different from that of the second regions 112 and of the third regions 113. Accordingly, the effect pigment layer 11 is of mutually different design in the regions 111, 112 and 113. The advancement direction 100 of the thermal transfer foil 1d is marked by an arrow, which preferably also provides the direction of the longitudinal extent of the thermal transfer foil 1d.

(54) This has preferably been achieved by providing different effect pigments and/or different mixtures of effect pigments in each of the regions 111, 112 and 113.

(55) As a result of the use of different effect pigments or different mixtures of effect pigments in the regions 111, 112 and 113, a different optical color effect of the effect pigment layer, in particular, is produced in these regions, as already explained above with reference to FIG. 2.

(56) It is also possible, moreover, for the particle area density of the effect pigments to differ in the regions 111, 112 and 113 and/or for the alignment of the effect pigments that is selected to be different in the regions 111, 112 and 113.

(57) In particular, through the different alignment of the effect pigments in the regions 111, 112 and 113, it is possible to achieve, moreover, interesting optical effects in the true color image produced with the thermal transfer foil 1d or with the thermal transfer foils 1a, 1b and 1c:

(58) Thus, for example, it is possible for the alignment of the effect pigments to differ in the regions 111, 112 and 113 by virtue of the fact that the alignment exhibits in each case a different angle to the plane defined by the thermal transfer foil, or for the average alignment of the effect pigments to exhibit a correspondingly different angle. This may result in the effect pigments possessing a correspondingly different variable appearance, and so, for example, rendering specific color effects and/or other optical effects visible to the viewer in different spatial regions.

(59) It is also possible, moreover, for the alignment of the effect pigments to exhibit a different statistical distribution about an average alignment in the regions 111, 112 and 113. The effect of this is that, for example, the solid angular range in which the respective color effects are visible is different. Moreover, through a correspondingly selected statistical distribution it is possible on the one hand to generate specific glitter effects and the like and, by virtue of a correspondingly parallel alignment, it is possible on the other hand to generate intensive color flop effects in the regions 111, 112 and 113.

(60) The difference in alignment of the effect pigments in the regions 111, 112 and 113 may be brought about here by corresponding application of these subregions using different printing mechanisms and, further, optionally, by exerting corresponding influence on the alignment of the effect pigments by means of mechanical tools, especially stamping tools, and/or by means of electrical and/or magnetic fields, which are applied correspondingly during the printing operation or during the curing of the decorative varnish on the carrier foil.

(61) FIG. 3a shows an effect pigment layer 11 having a layer thickness e, comprising an effect pigment 2 having an effect pigment size c or a largest diameter c. The effect pigment 2 is tilted by an angle α relative to the plane or surface defined by the effect pigment layer. In this case the effect pigment 2 bears in each case against the first surface of the effect pigment layer 11a and the second surface of the effect pigment layer 11b. The spacing between the first surface of the effect pigment layer 11a and the second surface of the effect pigment layer 11b corresponds preferably to the layer thickness e of the effect pigment layer 11. The angle α corresponds to the angle between the surface defined by the effect pigment layer 11, and the normal to the surface defined by the effect pigment layer 11.

(62) When using effect pigments having effect pigment sizes of 1 μm to 35 μm, the D.sub.90 (90% quantile) of the corresponding effect pigment size distribution is situated for example at between 26 μm and 32 μm, the D.sub.50 (50% quantile) is located between 14 μm and 19 μm, and the D.sub.10 (10% quantile) is located between 7 μm and 11 μm. Preferably the greatest part of the effect pigment sizes is located between 10 μm and 30 μm. The layer thickness of the varnish layer e, more particularly of the dry varnish layer, is situated for example between 2 μm and 5 μm. Preferably, depending on the effect pigment sizes, there is an orientation of the effect pigments parallel to the surface defined by the substrate, if the layer thickness of the varnish layer e is less than, or less than or equal to, the effect pigment sizes of the effect pigments.

(63) The angle α is a product of the sine rule with

(64) $\frac{e}{\sin \left(\alpha \right)}=\frac{c}{\sin \left(\gamma \right)}.$

(65) The angle α is for example at most 3.8°, if the angle γ=90°, the layer thickness e=2 μm and the effect pigment size c=30 μm. The angle α is for example at most 9.6°, if the angle γ=90°, the layer thickness e=5 μm and the effect pigment size c=30 μm. The angle α is for example at most 11.5°, if the angle γ=90°, the layer thickness e=2 μm and the effect pigment size c=10 μm. The angle α is for example at most 30°, if the angle γ=90°, the layer thickness e=5 μm and the effect pigment size c=10 μm.

(66) The maximum angle α may provide a measure of the tilting of one or more effect pigments 2 included in an effect pigment layer 11. The maximum possible tilting of the respective effect pigments 2 here is limited by the layer thickness e of the effect pigment layer 11 and/or the effect pigment size c.

(67) The alignment of the effect pigments 2 in the effect pigment layer 11 is statistical, and the maximum value of the angle α indicates preferably the maximum disorientation of an individual pigment along a three-dimensional axis. The influence of adjacent pigments may reduce this value further.

(68) A virtual plane-parallel alignment, more particularly a plane-parallel alignment, of the effect pigments 2 parallel to the surface defined by the effect pigment layer 11 is preferred. A virtually plane-parallel or plane-parallel alignment of the effect pigments 2 in the effect pigment layer 11 is advantageous for very highly photorealistic reproduction of images, with avoidance in particular of any viewing angle-dependent change in the perceived color for the viewer.

(69) The alignment of the effect pigments 2 in the effect pigment layer 11 may be dictated with particular advantage through the production operation with predetermined parameters, by the use of predetermined substrates in combination with an extremely thin effect pigment layer 11.

(70) With preference, 90% of the effect pigments 2 have an angle α of less than 10° and/or 50% of the effect pigments 2 have an angle α of less than 5°.

(71) It is also possible, moreover, for the thermal transfer foils used in the process for producing the true color image to comprise not only the thermal transfer foils shown in FIG. 2 but also thermal transfer foils according to FIG. 3, and, moreover, for the thermal transfer foils shown in FIG. 2 to have, in regions, a different alignment or particle area density as in the case of the thermal transfer foil described according to FIG. 3.

(72) It is particularly advantageous if the particle area density of the effect pigments in the respective range 111, 112, 113 is substantially constant as seen over the area of the region in question. In particular it is preferred for this purpose for the standard deviation of the particle area density over the area of these respective ranges to be less than 30%, preferably less than 20%, more preferably than less 10%. This also applies correspondingly, moreover, to the alignment of the effect pigments in the respective range 111, 112 and 113 and/or in relation to the distribution of the alignment of the effect pigments in the regions 111, 112 and 113. This ensures that in the respective regions 111, 112 and 113 an identical, constant optical impression is generated in each case and, as a result, the advantages already set out above are achieved in the process.

(73) The thermal transfer foils designed as above in particular in accordance with the figures of FIG. 1 to FIG. 4 are employed preferably for producing a true color image. In this case, using a thermal transfer printhead, subareas of the effect pigment layer of the thermal transfer foil in the form of halftone dots, or subareas of effect pigment layers of two or more different thermal transfer foils, designed as halftone dots, using one or more thermal transfer printheads, are applied to the surface of a substrate in order to form the true color image. Thus, for example, one or more of the transfer foils 1a, 1b, 1c and 1d are used to produce the true color image, using a thermal transfer printer which comprises one or more thermal transfer printheads.

(74) The basic construction of a thermal transfer printer which can be used for this purpose is shown by way of example in FIG. 6.

(75) FIG. 6 shows the thermal transfer printer 3 with the thermal transfer printhead 35, the heating element 35a, the counter-pressure roller 36, the thermal transfer foil winder 37, the deflection roller 34, and the thermal transfer foil unwinder 32. Also shown in FIG. 6 is the thermal transfer foil 1, which is unwound by the thermal transfer foil unwinder 32 and is supplied via the deflection roller 34 to the printhead 35, and then wound up again on the thermal transfer foil winder 37. FIG. 6 additionally shows a substrate 31. The substrate 31 is unwound by the substrate unwinder 30 and then supplied to the nip between counter-pressure roller 36 and printhead 35 or heating element 35a. The thermal transfer foil unwinder 32, the thermal transfer foil winder 37, the counter-pressure roller 36 and/or the substrate unwinder 30, and also the printhead 35, are driven by a control means, not shown in FIG. 6, in such a way that by means of the printhead 35 or the heating element 35a, subareas in the form of halftone dots in the effect pigment layer 11 of the thermal transfer foil 1 are transferred onto the substrate 31 surface facing the printhead 35.

(76) The printhead 35 is designed preferably as a “flat head” printhead. In this case the position of heating elements 35a (thermocouples) of the printhead 35, at which the subareas of the effect pigment layer are applied to the substrate 31, is located preferably between 5 mm to 10 mm distant from the edge of a support plate, more particularly a ceramic support plate. The heating elements 35a in this case are designed in particular as a heating strip, on which the heating elements 35a are disposed closely alongside one another in a line. The carrier foil 12 of the thermal transfer foil 1, with the remaining, unapplied effect pigment layer 11, is taken off upwards preferably via an additional diverting plate, not shown in FIG. 6, and/or via an additional roll, from the substrate 36. Separation between the carrier foil 12 and the substrate 31 is accomplished with a certain temporal and spatial retardation after heat has been given off via the printhead 35. The temporal and spatial retardation may be advantageous in order to allow the applied effect pigment layer 11 to develop a greater adhesion on the substrate 31 within this time, with the carrier foil 12 only thereafter being peeled off from the applied effect pigment layer 11.

(77) It is possible, moreover, for the thermal transfer printer to use a “near-edge” thermal transfer printing process. In the case of this printing process, the position of the heating elements 35a (thermocouples) of the printhead 35 is located very close to the edge of the support plate. Here as well, the heating elements 35a take the form in particular of a heating strip, on which the heating elements 35a are disposed alongside one another closely in a line. The carrier foil 12 of the thermal transfer foil 1 with the unapplied residual effect pigment layer 11 is taken off upwards from the substrate 31 without additional diversion, at a sharp angle, as shown in FIG. 6. The separation of the carrier foil 12 from the substrate 31 therefore takes place immediately after the transfer of the subareas of the effect pigment layer 11 from the carrier foil 12 onto the substrate 31, by means of the partial heating of the thermal transfer foil 1 by the printhead 35. An advantage in this variant is that as a result it is possible to achieve higher printing speeds.

(78) With regard to inter-layer adhesion and force of adhesion to the substrate 31, respectively, the layers of the thermal transfer foil 1, more particularly the effect pigment layer 11 and the optionally provided detachment layer 13, and adhesive layer 15, respectively, are preferably set as follows:

(79) The partial heating of the thermal transfer foil 1 by the heating elements 35a of the printhead 35 employed in the respective process produces a change in the behavior of this layer system: in the regions in which the transfer foil 1, which is in contact with the substrate 31, is not heated by the heating elements 35a of the printhead 35, the inter-layer adhesion between the effect pigment layer 11 and the carrier foil 12 is higher than the force of adhesion between the effect pigment layer 11 and the substrate 31. In the regions in which the thermal transfer foil 1 in contact with the substrate 31 is heated by the heating elements 35a of the printhead 35, corresponding activity of the thermoactivatable adhesive layer 15 and/or of the thermoactivatable effect pigment layer 11 produces an increase in the force of adhesion between the effect pigment layer 11 and the substrate 31, and possibly a reduction in the force of adhesion between the effect pigment layer 11 and the carrier foil 12, as a result of reduced force of adhesion of these two layers to one another—by melting of the detachment layer 13, for example.

(80) The increase in the force of adhesion between the effect pigment layer 11 and substrate 31 is formulated here in such a way that within these regions the force of adhesion between the effect pigment layer 11 and the substrate 31 is higher than between the effect pigment layer 11 and the carrier foil 12. In this way, the subareas of the effect pigment layer 11 that are acted on by heat, by means of the heating elements 35a of the printhead 35, are applied to the substrate 31. In this case it is also possible for the effect pigment layer and/or the adhesive layer 15 to be able briefly to melt and so to enter into a particularly intimate connection with the substrate 31.

(81) A further effect of this setting of the force of adhesion of the layers of the thermal transfer foil 1, as described above, is that when the thermal transfer foil 1 is peeled from the substrate 31, the subareas of the effect pigment layer 11 that have been heated by the heating elements 35a of the printhead 35 remain on the substrate 31, and the remaining subareas of the effect pigment layer 11 are detached with the carrier foil 12 from the substrate 31.

(82) As already observed above, the thermal transfer printer 3 may have not only one printhead 35, but also two or more printheads 35. In that case it is also possible for each of these two or more printheads 35 to be assigned one thermal transfer foil among a plurality of thermal transfer foils used, or else for the same thermal transfer foil to be supplied to two or more printheads 35.

(83) These one or more printheads 35 the supplying of the one or more thermal transfer foils 1 and also the supplying of the substrate 31 is controlled in this case, depending on the thermal transfer foils used and also on the true color image to be produced, preferably as described below:

(84) At the print preparation stage, the print original—which, as set out above, preferably is a single-color or multicolor motif to be represented as a true color image—first broken down into its color channels.

(85) As already set out above, the color channels are oriented on the one or more thermal transfer foils used for producing the true color image. Preferably, then, each of the available regions of the one or more transfer foils possessing a different optical effect is assigned a color channel.

(86) These color channels may therefore be color channels of a customary color model, for example RGB, hence a red color channel, a green color channel and a blue color channel. As a result, the respective color of the respective color channel is generated by the particular region of the thermal transfer foil, as a result of the effect pigments provided there.

(87) Moreover, however, it is also possible and advantageous to define and provide here corresponding color channels which take account of the optical color effect in a predefined viewing angle range, or of additional optical effects besides the color effect, such as glitter effects, etc., for example. Hence for example it is possible for one and the same color to be assigned a plurality of color channels—for example, a first color channel in respect of a corresponding color effect in a first viewing angle range; a second color channel in respect of the same color effect in a different viewing angle range; and a third color channel with a corresponding color effect, but superimposed, for example, by a glitter effect, likewise in a specific viewing angle range.

(88) The corresponding breakdown of the motif into the color channels may be based here on the basis also of further information concerning the desired optically variable effects of the motif, or else based optionally on a three-dimensional representation of the motif.

(89) For each of the color channels, an assigned grayscale image is determined in the digital original of the motif and in the information that may additionally be available. In one preferred case, therefore, there is a first grayscale image for a red color channel, a second grayscale image for a green color channel, and a third grayscale image for a blue color channel.

(90) The respective grayscale images are then converted via appropriate algorithms and calculation methods, as for example by means of an RIP (RIP=Raster Image Processor) specifically designed for the purpose, into a respective raster image consisting of a multiplicity of halftone dots. The size of these halftone dots corresponds preferably to the size of the individual pixels which can be resolved by the printhead used. A raster image of this kind may consist, for example, of a binary black-white bitmap.

(91) In the course of this conversion, the grayscale image is broken down preferably into raster cells. Each raster cell comprises a certain number of binary pixels, namely the halftone dots. The halftone dots provided in the particular raster cell simulate the grayscale or color scale of the particular color channel.

(92) The conversion of the grayscale image into the respective raster image may be realized in this case by means of various rastering methods.

(93) With amplitude-modulated rastering with raster cells, for example, rastering takes place in raster cells following one upon another in a stipulated size and with a stipulated raster width, i.e., period. The individual halftone dots therefore comprise one or more of the individual pixels which can be implemented by the printhead 35. Within the raster cell, the respective grayscale is simulated by means of a variable size of the individual halftone dots. The halftone dots are varied in their size and may also have different shapes (for example dot shape, rhomboidal shape, cross shape). Through the size of the rasters the areal occupancy by the halftone dots within the raster cells, and hence the color gradation or gray gradation of the rasters, is stipulated in this way.

(94) Another method is that of frequency-modulated rastering with fixedly predetermined halftone dot sizes but with a varying distance between the halftone dots in the x and y directions and/or in the advancement direction and normal to the advancement direction of the substrate. In this case, preferably, the size of the halftone dots corresponds to the size of the individual pixels which can be implemented by the printhead 35. Here there is preferably a virtually random distribution of the spacings between the halftone dots, and for this reason this rastering may also be referred to as stochastic rastering.

(95) In specifying the parameters of the rastering, one consideration which must be made is that of the fineness which the representation is to have, necessary in particular for fine image details, and another is the level of gradation the particular color is to represent. The finer the selected raster width, the better the representation of fine image details. The finer the selection of the raster width, however, the smaller too are the raster cells generated and the fewer pixels are available in the respective raster cell for variation of the halftone dots. Since the respective grayscale or color gradation of the color channel is to be simulated within the respective raster cell, it is advantageous for there to be a maximum number of pixels available for the simulation of a maximum number of fine gray gradations. The fewer pixels there are in the raster cell, the fewer the color gradations that can also be simulated in the raster cell. The fewer color gradations there are available, the less realistic or natural the effect of the true color image, particularly as a result of tone separation effects (known as posterizing or posteration).

(96) If the true color image, for example, is to be executed with a resolution of 300 dpi (dpi=dots per inch, pixels per inch), it has proven appropriate to carry out the rastering of the color channels in each case with a raster width of 35 lpi to 70 lpi (lpi=lines per inch), in particular with amplitude modulation. This results in raster cells having sizes of between 8×8 pixels (35 lpi) and about 4×4 pixels (70 lpi). With 8×8 pixels it is possible to represent 64 gray gradations per color channel. With 4×4 pixels per color channel it is possible to represent 16 color gradations per color channel.

(97) FIG. 8 now illustrates a detail from a raster pattern, determined by means of the method, set out above, of amplitude-modulated rastering from an area with 50% gray gradation or color gradation, for one of the color channels:

(98) Accordingly, a representation 5 shows one such area detail of the raster pattern; a representation 50 shows a detail of the representation 5, enlarged by 500%; and a representation 500 shows a detail of the representation 5, enlarged again by 500%, with the representation of an individual raster cell 502. This is based on the examples given above with a raster width of 70 lpi. The representation 500 here illustrates by way of example the raster cell 502, which comprises 4×4 pixels and has the halftone dot 501, which is formed by the area of the pixels designed in white.

(99) FIG. 9 illustrates the corresponding detail from the raster pattern for the above-expanded raster width of 35 lpi. The representation 5 shows the detail from the raster pattern. The representation 50 a detail enlarged by 500%, and the representation 500 a detail therefrom again enlarged by 500%, with the raster cell 502, which comprises 8×8 pixels and features the raster dot 501.

(100) It is also possible, moreover, to use other raster methods for determining the raster pattern. Hence it is possible, for example, to use a frequency-modulated rastering which has no fixed raster cells. In that case the rastering follows only on the basis of the print resolution of 300 dpi with correspondingly free positioning of the individual pixels or halftone dots.

(101) FIG. 7 shows, by way of example, a representation 4 of such a detail, rastered by means of frequency-modulated rastering, also called “diffusion dither”, for a raster pattern corresponding to a gray gradation or color gradation of 50%. The representation 40 a detail therefrom enlarged by 500%, with the individual halftone dots and individual pixels 501.

(102) A resolution of 600×600 dpi corresponds in particular to a pixel size of 42 μm×42 μm, and a resolution of 300×300 dpi corresponds in particular to a pixel size of 84 μm×84 μm. Where the average largest diameter of the effect pigments is between 1 μm to 35 μm, for example, it is then advantageous that within one pixel, a plurality of effect pigments may be disposed partially or completely and also above one another and/or alongside one another, in order to generate as bright as possible an optical effect per pixel (and hence per color channel). The smaller the effect pigments used, the greater the number of effect pigments that can be disposed in particular within a pixel and the smaller, preferably, is the typical pearl luster effect which can be generated. The larger the effect pigments, the greater, in particular, the pearl luster effect and the fewer the number of effect pigments which can be disposed preferably within a pixel. Within a pixel, for example, it is possible for about 1 to about 7000 effect pigments, preferably about 10 to about 1000 effect pigments, more preferably about 10 to about 500 effect pigments, to be disposed partly or completely and also above one another and/or alongside one another.

(103) For the generation of the true color image on the substrate from the raster pattern of the color channels, the color channels must be combined with one another, by corresponding application of the halftone dots on the substrate, in such a way that additive and/or subtractive color mixing of the halftone dots produces the true color image, and more particularly the selected true color image or motif. This is brought about by driving the printheads and/or advancement apparatus in such a way that the raster patterns and therefore halftone dots assigned to the color channels are applied to the substrate, accordingly, in a manner with precise register to one another. In such a way that a correspondingly local color mixing can take place.

(104) Register, or register accuracy or in-register status, refers to a positional accuracy of two or more elements and/or layers relative to one another. The register accuracy here is to range within a prescribed tolerance, and is to be as minimal as possible. At the same time, the register accuracy of two or more elements and/or layers to one another is an important feature for increasing operational reliability. Site-accurate positioning may be accomplished here in particular by means of sensory, preferably optically detectable, registration marks or register marks. These registration or register marks may represent specific separate elements and/or regions and/or layers, or may themselves be part of the elements and/or regions and/or layers to be positioned.

(105) The driving in question takes place in particular here in such a way that the true color image has a multiplicity of true color domains which, when illuminated and viewed under reflected light and/or transmitted light, convey an assigned true color to the human viewer. This true color is generated in each case in particular by additive and/or subtractive color mixing of the halftone dots applied in the respective true color domain, on illumination.

(106) With the raster cells described above, comprising 8×8 pixels per color channel and 64 color gradations per color channel, accordingly, the number of shades resulting in the case of three color channels, for example, is 64×64×64=262 144 shades. With the above-described raster cells of 4×4 pixels per color channel and 16 color gradations per color channel, the number of shades in the case of three color channels is 16×16×16=4096 shades, which are available for a particular true color image. With this large number of shades, true color images with a realistic and natural effect can be produced.

(107) It has proven to be advantageous, furthermore, not to select too fine a rastering, in order in particular to select the rastering within the above-described ranges between 35 lpi and 70 lpi. Hence it has emerged that in the case of pixels or halftone dots which are too fine, there is reduced reproduction of detail and inaccurate shaping of the individual pixels, so falsifying the reproduction of color.

(108) The processes described above are carried out preferably by means of corresponding image processing software, which may be implemented on the controller of the printer 3 or separately on an external computer.

(109) Based on the raster patterns determined as set out above for the individual color channels, the printer 3 may be driven accordingly as described below in order to implement the process:

(110) If the printer 3 has only one printhead 35, which is disposed transverse to the advancement direction, i.e., print line transverse to the advancement direction, an advisable procedure is as follows:

(111) In one case it is possible to use different thermal transfer foils, each coated over their full area with an effect pigment layer which has a uniform optical appearance. Each of these thermal transfer foils is assigned to one of the color channels. These thermal transfer foils may therefore, for example, be the thermal transfer foils 1a, 1b and 1c elucidated with reference to FIG. 2.

(112) In one preferred implementation, the effect pigment layer of a first foil, on illumination (and at a defined angle), conveys the red color impression, while a second of the thermal transfer foils conveys the green color impression and a third of the thermal transfer foils conveys the blue color impression.

(113) First of all, then, the raster pattern assigned to the color channel of the first thermal transfer foil, such as to the red color channel, for example, is sent to the controller of the printer. The printer controller drives the printhead 35 in such a way that by means of the printhead 35 the halftone dots assigned to this raster pattern, consisting of subareas of the effect pigment layer of the first thermal transfer foil (for red color channel), are applied to the substrate 31, more particularly to a black substrate 31. Following application, the first thermal transfer foil is switched for the second thermal transfer foil (for green color channel). The substrate 31 is again moved into the start position. The raster pattern which is assigned to the second color channel, as for example the green color channel, is subsequently sent to the controller of the printer. The assigned halftone dots are then applied in the same way by means of the printhead 35, through corresponding application of subareas of the effect pigment layer of the second thermal transfer foil. This is repeated in the same way in a third step with the third thermal transfer foil and the third color channel, for the blue color channel, for example.

(114) The positioning of the substrate 31 at the starting position is accomplished here preferably by means of a stepper motor which controls the advancement of the substrate. Here there are two variants which have proven useful:

(115) In the first variant, the substrate 31 has a perforation in at least one edge region, and the corresponding lugs engage in this perforation. The substrate 31 is then moved forwards and backwards via this mechanical interlocking.

(116) In the second variant, the substrate 31 has no perforation. Here it is clamped in mechanically between two rolls and is fixed forward and backward there throughout the period of advancement, so that the forward path is known and the substrate can be moved back again correspondingly.

(117) Register tolerance in the advancement direction and/or perpendicular to the advancement direction here is approximately ±0.15 mm, preferably in the ±0.05 mm to ±0.5 mm range.

(118) Further, it is also possible in the case of such a printer to use only a single thermal transfer foil, having a plurality of regions with different optical effects, especially color effects. This thermal transfer foil may be designed in the same way, for example, as the thermal transfer foil 1d according to FIG. 3. Thus, for example, this thermal transfer foil has an iterative arrangement of regions 111, 112 and 113, which are each assigned to a different color channel and which on illumination, for example, reproduce the colors red, green and blue, respectively. The size of the regions is in this case oriented preferably on the running length of the image or motif to be printed. This single thermal transfer foil may additionally comprise further regions—for example, for an additional white or black color or another chromatic color or an optically variable color or optically variable layer sequences, or for a protective varnish which is applied partially or over the full area of the true color image following application of the true color image.

(119) The individual color channels are printed in the same way as described above in succession by corresponding transmission of the respectively assigned raster pattern to the controller of the printer, and, after the respective printing of a color channel, the substrate 31 is moved back into the starting position. There is no need here to change the thermal transfer foil, owing to the specific design of the thermal transfer foil, as described above.

(120) It is advantageous, moreover, if the printer has a plurality of separate printheads 35 with a respectively assigned transfer foil. Preferably in this case there is a printhead 35 with assigned thermal transfer foil 1 provided for each of the color channels. The printheads 35 here are positioned in succession, so that the halftone dots of the individual color channels are applied successively to the substrate 35, without any need for the substrate 35 to be moved back to the starting position. Here, preferably, the distance between the printheads 35 in the printer is known and fixed and is observed accordingly during printing. The register tolerance in advancement direction and/or perpendicular to the advancement direction here is approximately ±0.1 mm, preferably in the range between ±0.05 mm to ±0.5 mm.

(121) It is also possible, moreover, for the printer to have a printhead 35 which is disposed longitudinally to the advancement direction, i.e. printing line longitudinally to the advancement direction. With an arrangement of this kind it is advantageous to use a plurality of different thermal transfer foils. Preferably an assigned thermal transfer foil is used for each of the color channels, each of said foils being designed, as already described above, over the full area with an effect pigment layer, which exhibits an optical effect assigned to the respective color channel. The printhead 35 prints a corresponding stripe of the substrate 35 in accordance with the width of the printhead, in this case preferably with all color channels. The substrate 31 remains in position until all of the color channels have been printed. Thereafter the substrate 31 is displaced by a predetermined value (printhead width). In this case the change of the thermal transfer foil takes place preferably automatically. The register tolerance in advancement direction and/or perpendicular to the advancement direction here is approximately ±0.1 mm, preferably in the range between ±0.05 mm to ±0.5 mm.

(122) As already set out above, the optical appearance of the true color image is also determined by the substrate 31. With regard to the substrate used, more particularly the substrate 31, the following advantageous design variants arise in particular:

(123) Thus it is possible for the substrate 31 to be black or dark and/or to be applied on a black or dark surface. In view of the black or dark ground thus formed by the substrate, the light that is not reflected by the effect pigments is absorbed or largely absorbed. In reflection, as a result, all that can be seen is essentially the part of the spectrum reflected by the effect pigments, so producing a very clean and intense color impression.

(124) It is also possible, moreover, for the substrate to possess a strongly reflecting quality—having, for example, a metal layer or having a white ink layer or white ink area. The effect of this is that part of the light transmitted by the effect pigments of the halftone dots is reflected at this ground. As a result, interesting color effects can be achieved. This is the case because when transparent effect pigments are used, as elucidated above, the color spectrum differs in transmission and reflection and so the color generated by the effect pigments in transmission or in reflection becomes visible in dependence on angle.

(125) It is also possible, furthermore, for the substrate to form a colored ground or to have colored regions which, for example, reflect only part of the irradiated spectrum. As a result it is possible, in combination with the overlying effect pigments provided in the halftone dots, to achieve a deliberate modification of the perceived color.

(126) Hence the substrate preferably has at least one colored varnish coat, which may be provided over the full area or in patterns on the substrate. The luminance L* of the at least one colored varnish coat is preferably in the range from 0 to 90. The luminance L* here is measured preferably according to the CIELAB form L* a* b*, under the following conditions:

(127) According to geometry: diffuse/8 degrees as per DIN 5033 and ISO 2496 diameter of the measuring aperture: 26 mm spectral range 360 nm to 700 nm as per DIN 6174, standard illuminant: D65.

(128) FIG. 5 shows, in the upper part of FIG. 5, a two-dimensional coordinate system defined by the coordinate axes a* and b*, the system being designated here as “a*, b* chromaticity diagram”. In this case the color values on the axis a* range from green in the negative region through to red in the positive region of the possible values of a*. Moreover, the color values on the axis b* range from blue in the negative region through to yellow in the positive region of the possible values of b*.

(129) Furthermore, FIG. 5, in the lower part of FIG. 5, shows a three-dimensional coordinate system which is defined by the coordinate axes L*, a* and b*, and which also comprises the two-dimensional coordinate system defined by the axes a* and b*. In this case the color values on the axis a* range from green in the negative region through to red in the positive region of the possible values of a*.

(130) Moreover, the color values on the axis b* range from blue in the negative region through to yellow in the positive region of the possible values of b*. Furthermore, the luminance values on the axis L* range from black in the negative region through to white in the positive region of the possible values of L*.

(131) The individual colored varnish coats here may be colored using dyes and/or pigments. Pigments are given preference here, in view of the customarily higher hiding power relative to dyes.

(132) For the coloring of the pigments it is advantageous if the pigmentation of the at least one colored varnish coat is selected such that a pigmentation number PN is in the range from 1.5 cm.sup.3/g to 120 cm.sup.3/g, more particularly from 5 cm.sup.3/g to 120 cm.sup.3/g. The pigmentation number PN here is calculated as already set out above.

(133) As already set out above, it is advantageous for the color effect of the true color image if the substrate is black or dark or has a correspondingly black or dark layer.

(134) It is, however, also possible to combine with one another the implementation alternatives of the substrates that are described above. Thus, for example, a substrate may be provided which in regions is black or dark, in regions is strongly reflecting or white, and in regions is provided with different-colored colored varnish coats. Through the corresponding design and/or the preprinting of the substrate, the optical appearance of the true color image may be further influenced and by this means further optically variable effects can be generated, which are difficult to imitate by other methods.

(135) It is in particular also possible, before and/or else after application of the true color image to the substrate, for further layers or layer sequences to be applied to the substrate 31 that represent an overall motif together with the motif of the true color image. The further layers or layer sequences may likewise be applied to the substrate 31 by means of thermal transfer foils or else by means of other processes such as, for example, gravure, flexographic, screen, pad or inkjet printing, hot stamping, cold stamping, or other known processes.

(136) The further layers or layer sequences may for example take the form of transparent and/or translucent and/or opaque color layers, transparent and/or translucent and/or opaque metallic layers (applied by vapor deposition and/or sputtering and/or printing), an open or embedded replication layer with diffractive and/or refractive relief structures, more particularly with a transparent and/or translucent and/or opaque reflection layer disposed thereon in the form of a thin metal layer and/or an HRI layer with high refractive index (HRI=High Refractive Index) and/or as an LRI layer with a low refractive index (LRI=Low Refractive Index), a volume hologram, a transparent and/or translucent and/or opaque thin-film construction, particularly according to Fabry-Perot with absorption layer, spacer layer and reflection layer, or other known layers or layer sequences.

(137) By means of such layers applied previously and/or subsequently it is possible, for example, for individual subregions of the true color image to be emphasized with accentuation or else attenuated. For example, contours or subareas of the true color image may be given correspondingly different designs in this way. The true color image, for example, may be embedded or inserted into an overall motif and/or into an overall pattern by means of such layers applied before and/or after, so that the true color image can be disposed adjacently to the layers applied before and/or after.

(138) By way of example it is possible, by means of such layers applied previously or subsequently, for functional layers as well to be applied retrospectively to the true color image, these layers being in the form, for example, of a transparent protective varnish for sealing the true color image, applied in particular by means of thermal transfer printing, hot stamping or cold stamping. Likewise possible is the application of an adhesion promoter layer or primer layer to the substrate before the application of the true color image.

(139) The registered tolerance in advancement direction and/or perpendicular to the advancement direction between the true color image and the further layers or layer sequences here is approximately ±0.15 mm, preferably in the ±0.05 mm to ±0.5 mm range.

(140) Furthermore, it is also possible and advantageous if in the process, as well as the above-described transfer foils with effect pigment layer, use is made of one or more thermal transfer foils which have a transfer color layer containing no effect pigments. Thus it is possible, for example, to use the printer additionally to apply halftone dots to the substrate that have dyes and/or pigments which are based on absorption of the incident light. Hence it is possible, for example, additionally to use a thermal transfer foil which has a transfer ply formed by a white varnish layer.

(141) It is also possible, moreover, for further processing steps for producing the true color image to be carried out after the printing of the substrate.

(142) Hence it is possible, for example, for the substrate to be a transparent substrate whose facing side is printed with the printer 3. The substrate is subsequently applied by the reverse face to a preferably black/dark background, and the reverse face is printed in a further operation in order to provide in particular a multicolored background, as set out above.

(143) It is also possible, moreover, for the print to take place using the printer 3 onto the transparent substrate with mirror inversion. This is followed by the application of a preferably black/dark background to the printed side of the transparent substrate. In this way the transparent substrate protects the imprint provided between the transparent substrate and the black background.

(144) To improve the stability, the substrate printed with the printer 3 may also be protected on one or both sides with additional transparent overprints, laminates, plastic or glass sheets.

LIST OF REFERENCE SYMBOLS

(145) 1 Thermal transfer foil 1a First thermal transfer foil 1b Second thermal transfer foil 1c Third thermal transfer foil 1d Fourth thermal transfer foil 11 Effect pigment layer 11a First surface of the effect pigment layer 11b Second surface of the effect pigment layer 12 Carrier foil 13 Detachment layer 14 Backside coating 15 Adhesive layer 100 Advancement direction 111 First region 112 Second region 113 Third region 114 Fourth region 2 Effect pigment 20 Auxiliary carrier 21 First auxiliary layer 22 Interference layer 23 Second auxiliary layer 211 First effect pigments 212 Second effect pigments 213 Third effect pigments 214 Fourth effect pigments 3 Thermal transfer printer 30 Substrate unwinder 31 Substrate 32 Thermal transfer foil unwinder 34 Deflection roller 35 Thermal transfer printhead 35a Heating element 36 Counter-pressure roller 37 Thermal transfer foil winder