ES2389278T3 - Safety elements readable by machine and products containing them - Google Patents

Abstract

A machine-readable safety element comprising a mixture of at least two pigments, in which a first pigment comprises a first inorganic host network or matrix having a first dopanteluminescent ion that emits electromagnetic radiation in a first emission wavelength band , and a second pigment comprises a second inorganic host network having at least two dopants in which a first dopant of the second inorganic host network is the same as the first luminescent dopant ion and a second dopant is a rare earth junction (i) capable of be excited through non-radioactive energy transfer from the first dopant of the second inorganic host network and (ii) emit electromagnetic radiation in a second emission wavelength band, in which the second emission wavelength band has an overlap negligible with the first band of emission wavelength and the second dopant e reduces or turns off the emission widely from the first dopant of the second inorganic host network in one or more emission peaks

Description

Safety elements readable by machine and products containing them

Background of the invention

This invention relates to machine-readable safety elements and products that contain them. Products that require machine-readable security features include, for example, valuable documents (for example paper and plastic substrate materials such as banknotes, passports, visas, and the like), labels, tickets and identity cards .

In order to prevent impersonations and counterfeiting, in the industry attempts have been made to build products, which should be protected, with safety elements readable by machines in such a way that unauthorized persons cannot change or reproduce them without being detected.

The use of a host network or matrix doped with rare earth ions is generally known within the art. These compounds can absorb radiation at a frequency, and emit radiation at a different frequency, in which the radiation refers to UV radiation, visible, near infrared and infrared. When irradiated with a given wavelength of radiation, these compounds glow at a second wavelength and such luminescence can be detected by a detector. Such luminescent compounds are called "luminophores," which can be ions or compounds.

Host networks doped with luminophores compounds have absorption and emission profiles that are generally characteristic of the ion, but modified by the effects of being incorporated into a host network of luminescent substance. It is possible to measure both absorption and emission with standard laboratory equipment.

It is preferred to distinguish the spectra produced by the luminophore to make it more difficult for potential counterfeiters to reverse engineering safety features. The broadcast can be distinguished using more than one network with rare and / or host metals.

WO2006 / 024530 A1 describes the use of at least two luminescent materials with spectral emission bands that overlap as a luminescent safety feature system. Preferably, the luminescent materials have the same host network but a different luminophore or alternatively a different host network with the same luminophore, in which the luminophore can be a rare metal.

WO2005 / 035271 A2 describes a machine-readable coding system for security documents comprising a first and second luminescent substance which are both luminescent in a common emission range outside the visible spectral range. The emission spectra of the first and second luminescent substances overlap in at least a partial area of the known emission range, such that the emission spectrum of the first luminescent substance is characteristically supplemented by the emission spectrum of the second luminescent substance, in which the luminescent substances can be host networks doped with rare metals.

The emission spectra of luminophores can be disguised by the use of absorbent materials. The absorption spectrum of an absorbent material overlaps or covers the absorption spectrum of the luminophore and changes characteristically. These changes can, for example, take place by "restricting" the spectral range or "deforming" the excitation and / or emission spectra. In the simplest example, such restriction occurs by separating edge regions from the spectra, while such deformation can occur by designed damping of narrow spectral regions of broadband spectra or eliminating given spectral areas.

US 4451530 describes the alteration of known emission spectra and the use of the altered portion in a characteristic way for authentication of the security document. The security element consists of an inorganic host network doped with a luminophore, such as a rare earth and one or more absorbent materials such as dyes that alter the emission spectrum of the luminophore by restricting the spectral range or deforming the excitation spectra and / or broadcast.

US 6506476 B1 describes the use of an inorganic host network doped with at least one rare metal, including tulium as an authentication feature for a printed value document. The authentication feature can be printed on the value document printed or added to the paper pulp. The doped host network absorbs widely and is excitable in the visible region of the spectrum and is transparent in at least part of the infrared spectrum. With high efficiency, the host network transfers the absorbed energy to the tulium. Preferably, the quantum yield of the luminescent substances is in the range of 50 to 90%. By absorption, the host network suppresses the emission lines of the tulium that occur in the visible regions and possibly close to the

infrared.

US 6344261 B1 describes a value document printed with at least one authentication feature in the form of a luminescent substance based on a host network doped with at least one rare metal. The host network absorbs widely in the entire visible region of the spectrum, and contains chromium as an absorbent substance. This very broadband absorption by the host network causes the doped rare metal lines found in this region to be suppressed. At the same time, a transfer of energy takes place from the host network to the doped rare metal, by means of which the emissions by the luminescent substance are induced. A much more effective stimulation of rare metals takes place, also leading to higher emission intensities.

US 6479133 B1 describes the use of an inorganic host network, doped with two rare metals, tulium and holmium, as an authentication feature for a printed value document. The authentication feature can be integrated into a paper pulp or added to the printing ink. The host network contains broadband absorbent components and with high efficiency, transfers the absorbed energy to the rare doping metal. Preferably, the quantum yield of the luminescent substances is in the range of 50 and 90%. The emission spectrum of the rare metal is influenced in a characteristic way by the absorbent components.

US 4463970 describes the use of camouflage materials, which may be luminophores, to prevent the detection of the design of indirectly excitable marking materials by means of chemical laboratory analyzes. Excitable marking materials are used to record information in a code. For example, the signal used in the coding to be hidden may involve the emission spectra of camouflage materials including luminophores. Camouflage materials can be broadband emitters, for example organic luminophores. The characteristic signal of the excitable marking material can only then be observed as a small tip added to the emission spectra. Alternatively, one or more narrowband luminophores emitting at wavelengths other than the signal to be measured can be added and therefore the emission spectrum is complex.

It is generally known within the art that two luminescent substances can be used as a security feature for a printed value document. A luminescent substance acts as the absorbent and the other luminescent substance acts as the emitter. There may be an efficient transfer of energy from the absorbent to the emitter.

WO2006 / 099642 A1 describes the use of two inorganic luminescent substances in a security element for a security document. The first luminescent substance is excited by the incident radiation. The energy is transferred between the first and second luminescent substances, whereby the frequency range of the excitation of one of the luminescent substances corresponds to the frequency range of excitation of the other luminescent substance.

JP2002212552 describes an infrared luminescent fluorescent substance comprising an inorganic material coactivated with tulium, which has an absorption in the infrared range, and holmium, which corresponds to a level of tulium energy to efficiently transmit absorbed light energy and have luminescence in the infrared range as optionally active elements.

EP1241242 A2 describes an anti Stokes phosphor that is a luminophore for application in security documents, in which the emission waveband has a shorter wavelength than the absorption waveband. The Stokes match is a doped host network with two rare earth ions. One of the dopants, for example yterbium, forms an absorbent and the other dopant, for example thallium, forms an emitter. The intensity levels between individual groups of emission lines are variable and depend on the concentration of the absorbent and / or the emitter in the phosphorus.

Summary of the invention

In accordance with the first aspect of the invention, a machine-readable safety element has been provided comprising a mixture of at least two pigments, in which a first pigment comprises a first inorganic host network having a first luminescent doping ion emitting electromagnetic radiation in a first emission wavelength band, and a second pigment comprises a second inorganic host network having at least two dopants in which a first dopant of the second inorganic host network is the same as the first luminescent dopant ion and a second dopant is a rare earth ion (i) capable of being excited through non-radioactive energy transfer from the first dopant of the second inorganic host network and (ii) emitting electromagnetic radiation to a second wavelength band of emission, in which the second band of emission wavelength has a negligible overlap with the prim It was an emission wavelength band and the second dopant reduces or cuts the emission widely from the first dopant of the second inorganic host network

in one or more emission peaks.

In accordance with the second aspect of the invention, an article comprising a substrate bearing a security element according to the first aspect of this invention has been provided, in which the article is an article of value. The substrate may be a paper or plastic product comprising paper pulp or polymer material and a mixture of at least two pigments, in which when the pigments are added to the pulp during papermaking or to the polymer during extrusion for forming a security element within a paper or plastic product, the mixture is as defined in the first aspect of the invention.

In accordance with the third aspect of the invention, a collection of articles according to the second aspect of this invention has been provided.

In accordance with the fourth aspect of the invention, an ink comprising a vehicle, and a mixture of at least two pigments, is provided in which when the ink is applied to a substrate to form a security element, the mixture is as defined in the first aspect of this invention.

In accordance with the fifth aspect of the invention, a printing process has been provided in which a substrate is printed with an ink in accordance with the fourth aspect of the invention.

In accordance with the sixth aspect of the invention, a method has been provided to detect the presence of a security element according to the first aspect of this invention, in which the element is irradiated with incident radiation at one or more wavelengths. which include in an absorption wavelength of a first dopant / first pigment and in an absorption wavelength of a first dopant / second pigment and the emission is detected in the first emission band and in the second emission band by The use of detectors.

Detailed Description of the Invention

In accordance with the first aspect of the invention, a machine-readable safety element has been provided comprising a mixture of at least two pigments, in which a first pigment comprises a first inorganic host network having a first luminescent doping ion emitting Electromagnetic radiation in a first emission wavelength band and a second pigment comprises a second inorganic host network having at least two dopants in which a first dopant of the second inorganic host network is the same as the first luminescent dopant ion and a second dopant is a rare earth ion (i) capable of being excited through non-radioactive energy transfer from the first dopant of the second inorganic host network and (ii) emitting electromagnetic radiation in a second wavelength band of emission, in which the second emission wavelength band has negligible overlap with the prim It was an emission wavelength band and the second dopant reduces or cuts the emission widely from the first dopant of the second inorganic host network in one or more emission peaks.

"Reduces or extinguishes widely" means that the emission peak is reduced so that the maximum intensity of the emission peak is substantially reduced, preferably the emission peak is reduced to less than 20% of the maximum intensity of the peak in the first pigment , more preferably at less than 10% of the maximum peak intensity in the first pigment and more preferably less than 5% of the maximum peak intensity in the first pigment so that it can be easily distinguished from background noise in the spectrum .

In one embodiment, the first dopant absorbs electromagnetic radiation in the UV spectral region, visible or infrared in a first absorption wavelength, and preferably emits electromagnetic radiation in the UV spectral region, visible or infrared in said first wavelength band of emission that has a wavelength greater than the first doping absorption wavelength.

The first luminescent ion can be a transition metal such as chromium, vanadium, manganese, titanium, nickel, iron or cobalt. Preferably, the first luminescent ion is a rare earth ion, more preferably a lanthanide. Examples have been provided below.

The first and second broadcasts can both be in the IR interval.

The host network for the pigments can be an aluminate, bromide, chloride, fluoride, gallate, garnet, (including all mixed garnets), germanate, molybdate, niobate, oxide, oxyfluoride, oxisulfide, ytrium sodium fluoride, silicate, sulfate , sulfur, titanate, tungstate, and vanadate. The host network may be any of the known inorganic host networks for rare earth cations, for example any of those described in WO2006 / 024530 A1.

The host networks for the first and second pigments may be the same or different. When the host networks are different, in one embodiment, the first doping absorption is preferably in the same region of the spectrum, and may be within a few nanometers, both for the first and the second

pigments in order to allow the same source of incident radiation to be used for both pigments.

The incident radiation that excites the first dopant in the first pigment and the incident radiation that excites the first dopant in the second pigment can be from different sources. This could be required in the case where narrowband excitation sources have been used when the two host networks are substantially different, making the absorption band of the first dopant sufficiently different in the first and second pigments such that the Narrowband excitation cannot adequately excite the main ion in both host networks. The excitation of the first dopant can be performed at various wavelengths. In order to generate a higher intensity, more compact source of excitation, combining the wavelength can be beneficial, for example when two light sources are directed to the security element. An example of such a case would be when there are multiple absorption peaks that feed the desired higher state manifold.

The first and second emission bands are typically different by at least 50 nm. The desired result is to essentially have a single excitation of both pigments with the resulting spectral outputs of each separated pigment so that they can be widely separated from each other even if they contain at least one common emitting dopant. The emissions can be either narrowband or broadband, or combinations of any, and it is the wavelengths at maximum intensity that are preferably at least 50 nm apart.

The emission peak of the first dopant in the second pigment that comes from the same transition that occurs in the first emission band is greatly reduced by the second dopant.

The most efficient sources of incident radiation are ones in which light is more completely absorbed by the spectral absorption characteristic. The emission of light that can be part of the excitation source that does not overlap an absorption characteristic should be avoided if high efficiency is required. You can choose to use a relatively wide waveband source, for example an LED, because it is economically advantageous because it is commercially available easily, is robust in operation and is easy to maintain. A narrowband laser light source may be advantageous because it can be more efficient and LEDs of suitable wavelength may not be available for absorption spectra of any of the suitable pigment combinations, but it is also more expensive to operate and to maintain One skilled in the art must select a suitable source to meet the system requirements. There should be enough excitation, and the system's function should meet all other performance criteria. These criteria include life, environmental factors, eye safety and economic considerations.

The incident light may have a narrow waveband and may be at least 50 nm shorter in wavelength than both the first and second emission bands.

The incident light can have a very wide waveband such as a bulb or a flash lamp. It is possible to filter the incident light to prevent irradiation of the security element by light of the same wavelength as said first and second emissions, and subsequent detection by the detector. The incident source may be a narrower band than conventional bulbs, but wider than lasers, such as LEDs.

The incident radiation is preferably in the UV, visible and IR regions of the spectrum. The host networks of the first and second pigments in a preferred embodiment are both widely transparent in the visible range, that is, in the preferred embodiment the host networks are substantially colorless, but this is not a requirement. An advantage of pigments that are substantially colorless is that even if large amounts are applied to a substrate, for example, on a printed element, the color of the ink will not change substantially compared to the discharged state. The pigments should not be substantially colorless, they may be used in some embodiments at sufficiently low levels to avoid the changing color of the substrate, or in other embodiments at levels such that the color is visible.

The first and second dopants can be an M2 +, for example Eu, Co, V and Dy, M3 +, for example Cr, Ho, Pr and Nd, M4 +, for example Cr or M5 +, for example Mn ion, preferably M3 +, in where M is a rare earth, preferably a lanthanide or, for example, for the first dopant a transition metal and the number + represents the state of valence of the ion. The valence states impact on the emission properties of the element. The host network is selected to accommodate the valence state of the element.

The first dopant in the first and second pigments can also emit electromagnetic radiation in one or more bands of additional emission wavelength at a wavelength greater than the absorption wavelength of the first dopant. One or more such emissions may not be substantially reduced by the second dopant in the second pigment. It may be useful to detect such other emissions from the first dopant in both pigments.

In another embodiment, another emission wavelength is at a wavelength greater than the first doping absorption wavelength and the first emission wavelength, and in said second pigment, the second dopant, in

sufficient concentration can greatly reduce the other emission from said first dopant in the second pigment. The reduced emission may correspond to another emission that takes place from the first dopant in the first pigment.

The detectors are selected with respect to the wavelength or lengths of the emission or emissions that are detected, using, when necessary, filters to separate said first and second (and other) emissions. The detectors have design parameters based on a variety of requirements and conditions. They can be selected for the particular system requirement by those skilled in the art based, for example, on spectral sensitivity, noise characteristics, impedance, cost, speed, availability and the like. The detectors include, for example, photomultipliers, Si detectors, PbS, CdS, PbSe, InAs, InSb, Ge, HgCdTe, GaP, InGaAs detectors.

A silicon detector can detect photons up to 1,100 nm and the silicon-based detector exhibits extremely low noise. This feature is advantageous when the emission level of a phosphorus is very low requiring greater electronic amplification. Silicon detectors also have the advantage of being readily available and cheap.

An InGaAs detector can be used to detect longer wavelengths. It has a lower signal to noise ratio, is generally more expensive, and has fewer commercial providers. Such a detector is useful in the invention for detecting emissions greater than 900 nm with some detector models exhibiting a detection capacity of more than 2600 nm. Detector models that exhibit sensitivity at longer wavelengths, however, suffer from significantly higher noise and reduced shunt resistance.

The first and second emission bands can be measured at the same time, and the respective matches have similar fall time constants, but this is not a required condition.

One skilled in the art will be able to select appropriate amounts of the first and second pigments in a safety element that has respect to the detector used to detect the first and second emissions. For example, silicon detectors exhibit a very small NEP (equivalent noise power) so that the signal can be amplified to a higher degree than, for example, an InGaAs detector that has a considerably higher noise characteristic. One skilled in the art would make a decision on the amount of safety feature to be used based on the emission wavelength, emission intensity, emission grouping optics (if required), filter parameters, type of detector, response, area, environmental conditions and associated electronic amplification needs in order to achieve a satisfactory signal-to-noise ratio over all anticipated operating conditions of the safety feature and the detection system. It is desirable to have the minimum amount of pigments, and dopants in the security element to make it more difficult for a counterfeiter to reverse the engineering of the security element. One skilled in the art would evaluate dopants with respect to host network choices, ability to create the luminescent pigment, pigment size requirements, emission wavelengths, and the detection system with respect to the desired use. (See PHOSPHOR HANDBOOK, Edited by Shigeo Shionoya, William M. Yen, CRC Press 1999, ISBN 0-8493-7560-6).

It may be desired to have sufficient amounts of the second dopant in the second pigment to greatly reduce one

or more emission peaks by the first dopant in the second pigment. The relative levels of dopant can be selected by one skilled in the art according to the available information about luminescent efficiency and non-radioactive energy transfer efficiency, and / or empirically.

In accordance with the second aspect of this invention, an article comprising a substrate on which the security element is supported is provided.

The article may be valuable documents (for example, paper and plastic substrate materials such as banknotes, passports, visas, and the like), labels, tickets and identity cards.

The security element may be incorporated in the substrate body, preferably when the substrate is paper or plastic or may be applied to a substrate, preferably when the security element is in the ink. One skilled in the art will be able to select an appropriate particle size for each application. The mixture of pigments can be incorporated during the production of paper or plastic.

Document US4874188 describes the incorporation of particles such as grains, pellets or fibers into the paper pulp at the time of paper preparation. These are of the order of approximately 10 µm for the cross section of the grains or pellets and of the order of a few mm for the length of the fibers or the diameter of the pellets. The particle sizes to be used in this invention vary with the application, but may be of the order of 0.1 to 50 µm.

The first and second pigments each have a particle size distribution that can be characterized by a commercially available laboratory equipment such as a Microtrac laser diffraction particle size measurement system. The distributions are generally characterized by a value of D50 while

Half the volume is made of particles smaller than the specified diameter. The values for D90, D95 or D99 can also be referenced. There are situations in which the values D50 and D90, D95 or D99 can be very different. The particle size distributions are selected according to the desired method of application and eventual detection. The requirements are established in such a way that when the particles are used, the performance in terms of luminescence is within the specified tolerances. If the particle size is too large, standard deviations detected may be undesirably large. If the particle size is too small, the intensity of the emissions from the two pigments could be reduced, making detection more difficult.

The article may have the security element provided in a layer or on the substrate, preferably as a discontinuous layer, for example forming a predetermined design on the substrate. The mixture could be incorporated into fibers that have been woven into the substrate to form a design or be incorporated randomly. The polymer film strips containing the pigments can be incorporated into an article for example sandwiched between outer layers.

The article may have the security element provided in a discontinuous layer by printing and the layer may comprise other ink components.

According to a third aspect of this invention, a collection of articles has been provided in accordance with the second aspect of the invention in which the security element is identical in each article such as a collection of different denomination banknotes that all have The same security element. In addition, a collection of articles may be provided in which each security element differs from the predetermined design on the substrate such as a different printed pattern for each banknote denomination.

According to the fourth aspect of the invention, an ink comprises a vehicle and a mixture, in which when the ink has been applied to a substrate to form a security element, the mixture is as defined in the first aspect of this invention.

The ink can be used in a lithographic, offset, intaglio, flexographic, rotogravure, inkjet, typographic or screen printing process. Each process has requirements established by the printing process time used. Larger particles may clog certain print transfer surfaces, but they may be acceptable in others. Screen printing methods can accommodate fairly large particles.

A suitable particle size for each printing method can be selected by one skilled in the art, such as vehicles and additives, and suitable formulation methods. GB2258660 describes that particles suitable for use in a range of printing techniques have a maximum diameter of not more than 40 µm, preferably not more than 20 µm, and more preferably below 10 µm, for example 1 ! to 5! m or even 1! to 2! m. Such particle sizes can be used in this invention.

According to the fifth aspect of this invention, the ink of the fourth aspect of this invention can be used to print on a substrate, preferably by a lithographic, offset, intaglio, flexographic, rotogravure, inkjet, printing process. Typographic or serigraphic.

In accordance with the sixth aspect of this invention, a method has been provided to detect the presence of a security element in accordance with the first aspect of the invention. Typically, in the method the element is irradiated with incident radiation at one or more wavelengths that include a first dopant / first pigment in an absorption wavelength and a first dopant / second pigment absorption wavelength and the emission is detected in the first emission band and in the second emission band through the use of detectors.

In one embodiment, different detectors are used to detect the first and second emissions, in which the radiation optionally passes through a filter before reaching each detector, the filter being selected so as to allow the transmission of one of the first and second emissions and that prevents the transmission of the other of the first and second emissions. The detectors can be of the same type or alternatively of different types.

The detectors will be chosen by the requirements of the particular work. This will be based on performance, environment, availability and economy. This invention requires that each emission to be detected has its own detection system that can include appropriate detectors and filters.

Another embodiment of the sixth aspect of this invention includes moving a safety element by a mechanical conveyor to the detection site. This may occur during the manufacture of an article to ensure that the security element is being produced correctly. This can also occur at a later stage when the security elements need to be validated. The conveyor may or may not make the stationary safety element at the detection location. The conveyor is a component of the machine that comprises the detectors and allows machine readability of the safety element to be used. Detectors can also be produced for

static measurements such as quality control. Design considerations for each type of detector may vary according to the specified requirements.

There are several methods known in the art to produce pigment particles of appropriate size for use in printing and for incorporation into a substrate for a printed value document. One skilled in the art will be able to select an appropriate method for each of the two pigments used in this invention. The methods indicated below are a possible way for pigments to be produced; however, they are not intended to limit the scope of the claims in any way. It is to be understood that other appropriate methods can be used to produce the particles.

Document DE10056462 A1 describes a method for producing a luminescent material. This comprises charging an ion exchanger with rare metal cations and heat treating the charged ion exchanger. Examples of rare earth cations include a mixture of Yb and Er cations, or Nd and Cr cations or Yb and Y cations or Yb cations. Preferably, the charged ion exchanger is crushed before or after heat treatment to a pigment size, preferably by grinding. Preferably the ion exchanger is a silicate, more preferably a zeolite. The heat treatment is performed at 1,100 ° to 1200 ° C, preferably at 1,150 ° C for 2-5 hours.

EP1386708 A2 describes an aerosol method and an accompanying apparatus for preparing powdered products that involve the use of an ultrasonic aerosol generator, including a plurality of ultrasonic transducers that are underlying and ultrasonically exciting a reservoir of liquid fed which form droplets of the spray. A gas carrier is delivered to different portions of the tank by a plurality of delivery holes or openings that deliver gas from a gas delivery system. The aerosol is pyrolized to form particles that are then cooled and collected. Powdered products may include inorganic host networks doped with rare metal ions.

WO2006 / 078826 A2 describes a process for forming nanoparticles in a flame spray system, in which the described nanoparticles include inorganic host networks doped with rare metal ions. The process comprises the operations of (a) providing a precursor means comprising a liquid vehicle and a precursor to a component; and (b) flame spray the precursor medium under effective conditions to form a population of nanoparticles, in which the nanoparticles thus formed comprise less than about 5% by volume of particles having a particle size greater than 1 μm.

US 6344261 B1 describes crushing and grinding inorganic host networks endowed with rare earth ions to produce particles with an average grain size of less than 1 μm.

The following is a brief description of the drawings.

Fig. 1 shows the absorption spectrum of Y2O2S doped with 1% of Tm (atomic%), which is substantially the same as the absorption spectrum of Y2O2S doped with 1.0% of Tm (atomic%) and 2.0% of Ho (atomic%);

Fig. 2 shows the emission spectrum of Y2O2S doped with 1% of Tm (atomic%), and also the emission spectrum of Y2O2S doped with 1.0% of Tm (atomic%) and 2.0% of Ho (atomic%) .

The figures were generated using a high resolution optical spectrometer. The curve of fig. 1 was created by monitoring an emission peak and scanning through the desired excitation range (for example 500-1000 nm). In fig. 1, the excitation results were the contribution to the designated emission peak in the wavelength range indicated by the scan. Fig. 2 was generated by adjusting the excitation wavelength and scanning through the designated spectral emission range monitored by a detector.

The invention will be illustrated below by the following examples that are not intended to limit the scope of the claims in any way. The pigments in the examples were evaluated in powder form on a non-optical brightener containing paper back in a multi-graticle spectrometer arrangement consisting of tunable excitation detection and variable emission.

EXAMPLE 1a

The first pigment was Y2O2S doped with 1% Tm (atomic%). The particles of the first pigment containing the dopant Tm were produced using common phosphorus production methods. The first pigment can be excited using light of for example 700 nm, 805 nm or 910 nm corresponding to the absorption peaks shown in fig. 1. Suitable LEDs are available at Roithner-laser.com. The emission spectrum is shown in fig. 2. The emission of Tm centered at approximately 1800 nm was detectable by an extended InGaAs detector with a high resolution spectrometer. This emission may correspond to said first emission band in the invention.

EXAMPLE 1b

The second pigment was Y2O2S doped with 1% Tm (atomic%) and 2.0% Ho (atomic%). The particles of the second pigment contain both Tm and Ho dopants were manufactured using common phosphorus production methods. The second pigment was excited using the same light source as in Example 1a. The spectrum of

The absorption is substantially the same as in fig. 1. The emission of Tm (fig. 2, Trace A) that was present in Example 1a centered and at approximately 1800 nm was substantially reduced in the second pigment as seen in fig. 2, Trace B. The Ho emits over a band with major peaks at 1975 nm and 2050 nm. The emission of Ho in this spectral range can be detected using extended InGaAs detectors with a high resolution spectrometer. This emission may correspond to said second emission band in the invention.

10 EXAMPLE 1c

The first and second pigments were produced separately. The first pigment was manufactured as described in Example 1a and the second pigment was manufactured as described in Example 1b. The two pigments were then mixed together in a 1: 1 ratio and stirred to produce a homogeneous mixture. This mixture was used as a machine readable safety element. Said first and second mission bands would be

15 measurements as described in Examples 1a and 1b.

EXAMPLE 1d

The mixture as described in Example 1c can be mixed with vehicles to produce a printing ink. When this ink is used to print a substrate, assuming the ink is dry, the ink can be illuminated with the light source of Example 1a. The detection system described in examples 1a and 1b would identify the first and

20 second emission bands in the invention.

Claims (14)

one.

 A machine-readable safety element comprising a mixture of at least two pigments, in which a first pigment comprises a first inorganic host network or matrix having a first luminescent doping ion that emits electromagnetic radiation in a first wavelength band of emission, and a second pigment comprises a second inorganic host network having at least two dopants in which a first dopant of the second inorganic host network is the same as the first luminescent dopant ion and a second dopant is a rare earth ion (i) capable of being excited through non-radioactive energy transfer from the first dopant of the second inorganic host network and (ii) emitting electromagnetic radiation in a second emission wavelength band, in which the second length band The emission wave has a negligible overlap with the first emission wavelength band and the second or dopant reduces or extinguishes the emission widely from the first dopant of the second inorganic host network in one or more emission peaks.

2.

 A safety element according to claim 1, wherein the first dopant absorbs electromagnetic radiation in the UV spectral region, visible or infrared in a first absorption wavelength, and preferably emits electromagnetic radiation in the UV spectral region, visible or infrared in said first emission wavelength band that is in a wavelength greater than the absorption wavelength of the first dopant.

3.

A safety element according to any preceding claim, wherein said first luminescent is a rare earth ion or a transition metal.

Four.

A security element according to any preceding claim, wherein the wavelengths of the first and second emission bands are both in the IR range.

5.

 A security element according to any one of claims 1 to 4, wherein each host network is broadly transparent in the visible spectrum.

6.

A safety element according to any preceding claim, wherein the wavelengths of the first and second emission bands are different by at least 50 nm.

7.

 An article comprising a substrate that supports a security element according to any preceding claim.

8.

An article according to claim 7, wherein the security element is incorporated into the substrate body, preferably when the substrate is paper.

9.

 An article according to claim 7, wherein the security element is provided in a layer on the substrate, preferably as a discontinuous layer, for example forming a predetermined design on the substrate.

10.

 An article according to claim 9, wherein the layer has been applied by printing and in which the layer comprises ink components.

eleven.

 An ink comprising a vehicle, and a mixture of at least two pigments, wherein when the ink is applied to a substrate to form a security element, the mixture is as defined in claim 1.

12.

 A method for detecting the presence of a safety element according to claim 1, wherein the element is irradiated with incident radiation in one or more wavelengths that include an absorption wavelength of a first dopant / primer pigment and at an absorption wavelength of a first dopant / second pigment and the emission is detected in the first emission band and in the second emission band by the use of detectors.

13.

 A method according to claim 12, wherein different detectors have been used to detect the first and second emissions, wherein the radiation optionally passes through a filter before reaching each detector, each filter being selected so as to allow transmission of one or the other of the first and second emissions and that prevents the transmission of the other of the first and second emissions.

14.

 A method according to claim 12 or 13, wherein the incident radiation that excites the first dopant in the first pigment and the incident radiation that excites the first dopant in the second pigment are from different sources.