EP1409263A1

EP1409263A1 - Continuous-tone image produced by printing

Info

Publication number: EP1409263A1
Application number: EP02750818A
Authority: EP
Inventors: Ludwig Brehm; Hannelore Erbar
Original assignee: Leonhard Kurz GmbH and Co KG
Current assignee: Leonhard Kurz Stiftung and Co KG
Priority date: 2001-07-25
Filing date: 2002-07-03
Publication date: 2004-04-21
Anticipated expiration: 2022-07-03
Also published as: WO2003011606A1; CA2452209C; ES2230508T3; CN1274516C; HUP0401205A2; PT1409263E; DE10136252A1; EP1409263B1; JP4390136B2; ATE283173T1; DK1409263T3; JP2004535963A; RU2004105277A; TWI237599B; DE50201638D1; DE20121871U1; US20040209096A1; RU2264296C2; HU225882B1; CN1533333A

Abstract

The invention relates to a method for producing colored continuous-tone images by using printing inks that contain fluorescent pigments so that the individual colors are obtained by additive color mixing.

Description

Halftone image created by printing

The invention is concerned with halftone images produced by printing on a substrate, consisting of at least two types of pixels of different colors arranged in a grid-like manner, the desired color being generated by color mixing the colors of the pixels.

In the case of colored halftone prints produced in conventional printing processes, for example offset printing, gravure printing or thermal transfer printing, the color impression is created by subtractive color mixing of four basic colors (generally cyan, yellow, magenta and black). In halftone prints of this type, the pigments of the printing inks absorb the respective complementary component from the incident white light. The non-absorbed corresponding color component of the white light is reflected, reaches the eye of the beholder and creates a corresponding color impression there. Each of the primary colors thus reflects only a portion of the striking light. The brightness of halftone prints produced in this way depends on the background on which the primary colors are printed. The lighter the background, the lighter areas can be achieved with the halftone image.

On the other hand, pictures are created on a television tube or a cinema screen using additive color mixing. Virtually every point on the screen or television tube represents a small light source that shines in a special color. As is the case with the TV tube, for example, three very specific areas of the visible spectrum are selected as colored light sources, for example red, green and blue-violet, which are distributed over the entire area of the visible spectrum and the appropriate Color receptors in the eye are able to stimulate, so it is possible to create color images with realistic color by additive color mixing.

Since the prerequisite for additive color mixing is the presence of corresponding luminous pixels, it has hitherto been necessary to dispense with the use of additive color mixing in printed halftone images.

The invention is based on the object of proposing a way in which halftone images can be produced on a substrate by a printing method, which are distinguished by high brilliance and the possibility of realistic color generation compared to the previous halftone images.

To achieve this object, it is proposed according to the invention to design a halftone image produced by printing in such a way that the pixels are formed by printing inks which contain fluorescent pigments in a specific color when excited by means of electromagnetic radiation. Halftone images according to the invention are preferably produced in such a way that pixels from three different printing inks are provided, the pigments of the different printing inks each fluorescing in one of three primary colors (for example red, green and blue-violet) for additive color mixing, because then by appropriate combination almost all colors of the visible spectrum can be generated from pixels of the individual pigments, each fluorescing differently.

Printed halftone images in accordance with the invention thus differ from the previously used, printed colored halftone images in that the respective colors can only be recognized if the pigments contained in the individual printing inks are excited by electromagnetic radiation of a suitable wavelength and then fluoresce. As soon as the pigments are excited, however, a very bright, color-rich halftone image is obtained. In this context, it should be pointed out that the term “printing inks” is of course to be understood in the broadest sense and includes all types of inks or varnishes that are suitable for producing a printing or raster image on a background. In particular, “printing inks "in the sense of the invention for example, the lacquer or sublimation layers of thermal transfer or thermal printing foils.

A very special feature of the halftone images according to the invention is that the desired color or colouration can only be observed if the halftone image is irradiated with electromagnetic radiation of the appropriate wavelength. This has the consequence that the color or color position of the halftone image changes when irradiated with light of different wavelengths, for example on the one hand with visible light and on the other hand with ultraviolet light. This effect can be used, for example, to display different structures on a substrate, which are alternately visible depending on the wavelength or frequency of the electromagnetic radiation used for the illumination.

A wide variety of types of electromagnetic radiation can be provided to excite the fluorescent pigments. In practice, however, it will generally be expedient to use pigments which fluoresce under the action of UV radiation.

It is further provided according to the invention that the pixels are arranged on a black background. The black background can either be formed directly by the substrate. However, it would also be conceivable to form the black background by means of an appropriate printing ink, in which case the printing ink forming the background can then be arranged over the entire surface or even only in spaces between the colored fluorescent pixels.

According to the invention, special effects can be achieved if at least one of the pigments used in a printing ink is selected such that it fluoresces in different colors when exposed to radiation of different frequencies. Depending on the radiation used to illuminate the halftone image, a different result is obtained, depending on the color in which the corresponding pigment is fluorescing, both Color changes, as well as changes in the subject, depending on the frequency used for the irradiation, can be achieved.

It is particularly advantageous if, according to the invention, the dimensions of the pixels - which generate the halftone image - are selected such that they cannot be resolved by the unarmed eye, which can be achieved in any case if the dimensions of the pixels are less than 0.3 mm according to the invention are selected. In this case, the colored light rays coming from the individual pixels mix and the impression of a continuous, appropriately colored surface is created.

Special effects, which are very valuable for security purposes, for example, can be achieved if - in a further development of the inventive concept - on the substrate on the one hand fluorescent pixels of printing inks containing pigments fluorescent when excited with a certain electromagnetic radiation and on the other hand non-fluorescent pixels of colored ones when excited with the certain printing inks containing non-fluorescent pigments are present. In this context, “non-fluorescent pixels” should not generally be understood to mean that the printing inks used to produce these pixels do not fluoresce at all. In the context of the present invention, these may also be pixels that consist of printing inks, the pigments of which, when excited, are present certain electromagnetic radiation fluoresce, but not when excited with the specific electromagnetic radiation that causes the fluorescent pixels to fluoresce.When a halftone image is composed of fluorescent and non-fluorescent pixels, there is a different effect depending on the radiation, since with radiation with which the electromagnetic radiation causing excitation of the fluorescent pixels illuminate the fluorescent pixels accordingly and produce a halftone color image, while the so-called non-fluorescent when irradiated with different radiation pigments produce the halftone color image. It can be achieved, for example, that when irradiated with UV light, the effect of the fluorescent pigments gives a first color impression, while at Illumination with daylight and a correspondingly low UV component results in a different color impression.

In principle, it is possible to provide the fluorescent pixels on the one hand and the non-fluorescent pixels on the substrate in their own areas. However, it is generally more expedient if the fluorescent pixels on the one hand and the non-fluorescent pixels on the other are provided nested on the substrate, because then on the same surface area of the substrate, which has both fluorescent and non-fluorescent pixels, the areas that are dependent on the illumination are present , set different effects.

It is further provided that the fluorescent pixels represent a first image and the non-fluorescent pixels represent a second image. For example, it would be possible in this way to provide a personal document with the portrait of the document holder in two ways, the first document being represented as a normal halftone image by mixing the colors cyan, magenta and yellow (and possibly black), while the second portrait through additive color mixing, e.g. is produced under UV light, fluorescent pigmented printing inks. In this way, the security of a personal document can be considerably increased and at the same time a simple method for checking the authenticity is obtained, namely by only having to check whether the portrait of the document holder created by subtractive color mixing with the one that appears when illuminated with very specific electromagnetic radiation, portrait created by positive color mixture of fluorescent colors. The generation of such images from normal pigments causing a subtractive color mixture and fluorescent pigments causing an additive color mixture can easily be done, for example, by means of thermal transfer printers, which must then be able to provide a corresponding number of colors for the printing dots.

Halftone images according to the invention can be used for a wide variety of purposes. It is particularly advantageous and is the subject of the invention however, the use of a corresponding halftone image as a security or guarantee element for valuables, documents, in particular securities, banknotes and ID cards, or for correspondingly valuable objects. For example, it is conceivable to provide a banknote, check, or other security with a corresponding halftone print, in which case the desired color effect only occurs when illuminated with appropriate radiation. For example, it could be achieved that a security element present on a bank note or the like shows a special color effect only when the bank note is irradiated with UV light of a certain frequency, whereas when irradiated with normal light there is only a slightly gray shimmer without the outlines or the like of the actually printed halftone image would be recognizable with this type of lighting. If the fluorescent pigments are selected accordingly and possibly non-fluorescent pigments are additionally added to the printing inks, it is also possible, for example, to generate a halftone image that appears white or gray when illuminated with normal light and that is illuminated when illuminated with light of a special wavelength, in particular UV light. then shows strong colors due to the fluorescence occurring. This effect (alternation between black and white representation and colored representation) is ideally suited as an easily recognizable security element.

Special security effects can be achieved if, as explained above, two images are combined on the document or the like, the first image being a normal halftone color image, while the second image is more specific only as a result of fluorescence when illuminated with light or electromagnetic radiation Wavelength is perfectly recognizable, whereby the special security effect can be seen in the possibility of providing two fundamentally matching images, which can then be compared accordingly.

In order to facilitate the application of appropriate security elements to valuables, it is advantageous if the halftone image is formed by the decorative layer of a transfer film, in particular hot stamping or thermal transfer film, which is transferred to the object to be secured. Halftone images can easily be created as part of transfer foils in conventional printing processes and then in form transfer of label-like stains, stripes, etc. to the objects to be secured in a simple manner. This has the advantage that the user of corresponding security elements can obtain them more or less completely and then only needs a relatively simple device for transferring the security element from the transfer film to the object to be secured.

Finally, it is within the scope of the invention that when using appropriate halftone images as a security element for objects, the halftone image is combined with an optically effective element, for example a grid structure, a hologram, a high-gloss reflective surface, a deliberately matted area or a color change or different Transparency-producing thin-film arrangement.

The halftone images according to the invention alone are difficult to imitate, because it is difficult to find out the exact combination between pigments, lacquer carrier substance and essential wavelength of the electromagnetic radiation. However, counterfeiting is made even more difficult if the optically active elements which are already known to be difficult to counterfeit are additionally present. This is especially true when a colored halftone image according to the invention and an optically effective structure in a single security element are immediately adjacent or even nested. Manufacturing processes are required here that make imitation practically impossible. In addition, the backup and verification options will be further improved. It would be e.g. it is conceivable to use the halftone image on the one hand and the optically effective element on the other hand to represent matching or complementary patterns, which opens up additional verification options, be it under normal lighting or be it when illuminating with light of a special wavelength, whereby these verification options can be designed in such a way that they are easy to grasp even by an inexperienced observer.

As can be seen from the above explanations, halftone images according to the invention are extremely versatile. For example, it would also be conceivable to print relatively large halftone prints on large-format printers in one To produce the design according to the invention in order to produce large-area, for example UV-fluorescent, prints which can be used for special effects, for example in the advertising industry. It would, for example, be conceivable to hang up advertising posters or the like produced in discotheques according to the invention, the content of which can then only be recognized when irradiated with adapted light, for example UV irradiation, advertising posters of this type being known from previously known fluorescent under UV radiation Distinguish elements in that you actually get halftone color images and thus there are extremely versatile design options. Despite the possibilities created, the production costs for such advertising materials are comparatively low.

Some principles and examples of halftone prints according to the invention are explained in more detail below.

If a halftone print is printed on a dark, preferably black, surface using printing inks with fluorescent pigments and the pigments are selected so that they shine in red, green and blue when irradiated accordingly, a halftone image can be produced which In terms of its properties, it basically corresponds to the image produced by a TV picture tube, with the individual pixels of the halftone image also being so small that the eye can no longer resolve them individually. This condition is met when viewing the halftone image from a normal reading distance if the pixels have a diameter of less than 0.3 mm, preferably even less than 0.1 mm. If, in addition, the pixels of the halftone image are printed so densely that the dark, preferably black, background no longer shines through, halftone images with different properties can be printed. If one assumes that UV-fluorescent pigments are used which do not shine in a special color when irradiated with normal daylight, the halftone images appear as pale black and white images when illuminated with normal daylight (because of the Self-coloring of the fluorescent pigments). On the other hand, if you illuminate the halftone image with appropriate UV radiation, the pigments fluoresce in the respective colors, the pigments should preferably be selected appropriately in the sense of the additive color mixture so that they shine in red, green and blue. Depending on which pixels are assigned to the individual colors at a specific point in the halftone image and in what density, a corresponding color image can then be generated, similar to a television picture tube, the dark or black background ensuring that that dark areas of images can also be produced, since it is possible to produce the color white by means of additive color mixing, but not the color black.

As already mentioned, special effects can be achieved if, at least for one printing ink, pigments are used which not only fluoresce at one wavelength in the color characteristic of them, but can also be excited at a second wavelength, fluorescence then occurring in a second color , For example, pigments that are UV fluorescent could be used, on the one hand at the wavelength 365 nm and on the other hand at the wavelength 254 nm.

The halftone images can be produced using appropriate printing inks in the customary printing processes, offset printing (digital offset) or thermal transfer printing expediently being used. The use of these printing methods has the advantage that the image information associated with each image (generally as a red, green and blue color separation) can be used directly in these methods.

Examples of different pigment combinations in printing inks for producing corresponding halftone images are explained below, for example following pigments used:

BF11 (red): bifluorescent pigment (red at 254 nm, blue-white at 365 nm) Manufacturer: Specimen Document Security Division, Budapest CD 120 (red): monofluorescent pigment (orange-red at 254 nm, red at 365 nm)

Manufacturer: Allied Signal Special Chemicals Riedel De Haen

CD 130 (orange-yellow): monofluorescent pigment (orange at 254 nm and

365 nm)

Manufacturer: Allied Signal Special Chemicals Riedel De Haen

CD 397 (yellow-green): monofluorescent pigment (yellow-green at 254 nm and 365 nm)

Manufacturer: Allied Signal Special Chemicals Riedel De Haen

MF 1 (green): monofluorescent pigment (green at 254 nm and

365 nm)

Manufacturer: Specimen Document Security Division, Budapest

MF 40 (blue): monofluorescent pigment (blue at 254 nm and

365 nm) Manufacturer: Specimen Document Security Division, Budapest

MF 50 (blue): monofluorescent pigment (pale blue at 254 nm, no fluorescence at 365 nm) Manufacturer: Specimen Document Security Division, Budapest Offset printing inks were produced using appropriate pigments, 10 to 40% by weight of the UV-fluorescent pigment being rubbed with an oxidatively drying offset film and used immediately in a manner known per se.

If a corresponding thermal transfer film with color layers with corresponding fluorescent pigments is to be produced, a thin PET carrier is coated in a manner known per se with a lacquer layer into which the respectively desired fluorescent pigments are incorporated.

The following halftone prints were produced using the pigments mentioned above:

Example 1 :

Halftone printing in red-green-blue technology on black, excitable at 365 nm

CD 120 (red)

MF 1 (green)

MF 40 (blue)

With an even distribution or intensity of the three pigments used, white light is obtained by additive color mixing when irradiated with UV light with a wavelength of 365 nm. When irradiated with UV light with a wavelength of 254 nm, on the other hand, a pale orange results because the pigment CD 120 fluoresces not red but orange at this wavelength.

Example 2:

Halftone printing in red-green-blue technology on black at 254 nm can be excited

BF 11 (red)

CD 397 (yellow-green)

MF 50 (blue) With a uniform distribution, the color white results when irradiated with UV light of 254 nm wavelength due to additive color mixing, whereas when irradiated with UV light with a wavelength of 365 nm, the color is green-white, because the pigment BF 11 only fluorescent red at 254 nm, blue-white at 365 nm. However, this means that when the halftone image is designed for irradiation with UV light with a wavelength of 254 nm, a customary three-color halftone print can be carried out, while with 365 nm irradiation the combination according to Example 2 only for a type of black and white Pressure is suitable.

Example 3:

Halftone printing with black and white on black, excitable at 365 nm

BF 11 (blue-white)

CD 130 (orange-yellow)

With a correspondingly uniform distribution and intensity of the printing inks containing the individual pigments, the color white is obtained when irradiated with UV light of a wavelength of 365 nm, while the color is red when irradiated with UV light of the wavelength 254 nm. This is due to the fact that the bifluorescent pigment BF 11 is used. Halftone prints according to Example 3 therefore appear when irradiated with UV light of the wavelength 365 nm as black and white prints (the two color pigments complement each other to white), while when irradiated with UV light of the wavelength 254 nm a red image on black Reason observed.

As can be seen from the above examples, using the basic idea of the invention, namely the additive color mixing via fluorescent pigments, it is possible to achieve a multitude of color effects, color changes in particular being particularly conspicuous when irradiated with light of different wavelengths and therefore particularly good as easy detectable security features are suitable. According to the invention, machine-readable security elements can also advantageously be produced, which can then only be properly evaluated by means of devices which generate the special electromagnetic radiation required for the excitation of the pigments, which can differ considerably from the radiation from normal daylight.

Example 4:

Using the pigments BF 11 (red, fluorescent at 254 nm), MF 1 (green, fluorescent at 254 nm) and MF 40 (blue, fluorescent at 254 nm), a halftone image of a portrait is created on a substrate, the distance between the individual pixels is chosen to be sufficiently large that further pixels can be inserted into the spaces. However, the size and spacing of the pixels must be such that the individual pixels cannot be resolved individually with the unarmed eye at a normal viewing distance of approx. 30 cm.

In the spaces between the fluorescent pixels made of pigments BF 11, MF 1 and MF 40, colored printing dots of a correspondingly small size are printed, these printing dots being printing dots in the four primary colors for subtractive color mixing (generally cyan, yellow, magenta and Black).

If six (or seven when using black) colors are used in accordance with the above proposal, precautions may have to be taken to ensure that there is no overlaying or moiree formation in the print. This can be achieved, for example, by printing the different representations of fluorescent pixels on the one hand and normal-colored print dots on the other in different screenings (eg 48 and 60 screen). Another possibility would be to use frequency-modulated rasters instead of amplitude-modulated rasters, as is already common today with a large number of digital printers. Both the pressure dots with the fluorescent pigments and the pressure dots from normal printing inks are each arranged in such a way that a halftone image is obtained, for example the portrait of a person can be shown in both images. The selection of the colors for the generation of the halftone image is then made in such a way that, under normal lighting, for example with daylight or artificial light, the printing dots which produce a subtractive color mixture represent the first halftone color image of the person, while the fluorescent printing dots when illuminated with suitable radiation, eg UV radiation, reproduce an essentially matching image. Checking the conformity of the two images is a suitable means of checking the authenticity.

Claims

Expectations

1. Halftone image produced by printing on a substrate, consisting of at least two types of pixels of different colors arranged in a grid-like manner, the desired color being produced by color mixing of the colors of the pixels, characterized in that the pixels are formed by printing colors which, when excited contain fluorescent pigments in a specific color by means of electromagnetic radiation.

2. Halftone image according to claim 1, characterized in that pixels from three different printing inks are provided, the pigments of the different printing inks each fluorescing in one of three primary colors (for example red, green and blue-violet) for additive color mixing.

3. Halftone image according to claim 1 or 2, characterized in that the pigments are UV-fluorescent pigments.

4. Halftone image according to one of the preceding claims, characterized in that the pixels are arranged on a black background.

5. Halftone image according to one of the preceding claims, characterized in that at least one of the pigments fluoresces in different colors when exposed to radiation of different frequencies.

6. Halftone image according to one of the preceding claims, characterized in that the dimensions of the pixels are chosen so that they cannot be resolved with the unarmed eye.

7. Halftone image according to claim 6, characterized in that the dimensions of the pixels are chosen to be less than 0.3 mm.

8. Halftone according to one of the preceding claims, characterized in that on the substrate on the one hand fluorescent pixels from printing inks containing fluorescence when excited with a certain electromagnetic radiation and on the other hand non-fluorescent pixels of colored pigments containing non-fluorescent when excited with the particular electromagnetic radiation Printing inks are available.

9. halftone according to claim 8, characterized in that the fluorescent pixels on the one hand and the non-fluorescent pixels on the other hand are provided nested on the substrate.

10. Halftone according to claim 8 or 9, characterized in that the fluorescent pixels represent a first image and the non-fluorescent pixels represent a second image.

11. Use of a halftone image according to one of claims 1 to 10 as a security or guarantee element for valuables, documents, in particular securities, banknotes, ID cards, or objects.

12. Use according to claim 11, characterized in that the halftone image is formed by the decorative layer of a transfer film, in particular hot stamping or thermal transfer film, which is transferred to the object to be secured.

13. Use according to claim 11 or 12, characterized in that the halftone image is combined with an optically active element.