HK1059671B - Improved luminescence characteristics detector and method for detecting - Google Patents

Description

Detector and detection method for improving luminescence characteristics

Technical Field

The present invention is in the field of security documents and articles. It relates to a method of determining the authenticity of such documents or articles. In particular, it relates to security documents or articles bearing luminescent properties and a device for quantitative measurement of the intensity of luminescent radiation and said luminescent properties.

Background

Luminescent compounds are well known as security elements for the protection of banknotes, value documents and other security articles. The compound may be placed in the substrate of the security article, printed onto the security article by an ink, or applied to the security article in the form of a security thread, foil or carrying label.

The detection of luminescent security elements is a well-known technique and is described in a number of patents. US5918960 describes a counterfeit banknote detection device that measures the intensity of emitted light compared to the background emission intensity from a UV lamp and two photocells that are energized to emit light. A particular problem in luminescence detection is the identification of weak luminescence signals from the usually strong background signal, which is caused by ambient light. The use of modulated excitation and synchronous detection makes it possible to overcome the above-mentioned difficulties.

US5608225 describes an improved fluorescence detection apparatus and method using a modulated excitation source, a photocell and a phase detector to suppress background signals. US4257299, US5548106, US5418855 and US5574790 further describe detection devices based on modulation excitation. US3656835 teaches the combined use of a constant UV excitation source and a modulated magnetic field to generate and detect modulated emissions from the magnetic triplet state of luminescence. US5315993 and US5331140 propose multiplexed luminescence delay detection using more than one modulation frequency of the excitation source, e.g. reading a bar code of invisible fluorescence. US5548124 and US 57013 propose to measure the number of light emission delays by a modulation product generating an excitation signal and a back side received light emission response signal.

The prior art modulation-based luminescence detection systems are mixed with the influence of ambient light that does not have the same modulation frequency and phase as the light source of the detector itself. On the other hand, they are very sensitive to their own modulation frequency. Some of the modulated excitation light has significant backscatter at the sample surface and leaks through the optical filter system into the detector photocell. No optical filter system has 100% rejection of out-of-band light components. The remaining excitation light having the same frequency as the luminescence response is added to the detected signal intensity. In the case of weak luminescence signals, the above-mentioned background signal prevents a correct recognition of the luminescence signal intensity.

This will be more disturbing, since the background signal depends on the reflectivity of the substrate, which varies independently of the intensity of the luminescence signal. In the case of banknotes identifying authenticity, the substrate reflectivity is significantly dependent on external factors such as soiling and wear, making it difficult to verify the authenticity of the banknote if there is no difference between the reflected background signal and the true luminescence signal.

The present invention discloses a method and apparatus for overcoming the disadvantages of the prior art.

In particular, it discloses a method and apparatus for discriminating between reflected excitation and luminescence signals and selectively determining the luminescence intensity.

The present invention further provides for the quantitative determination of the independence of luminescence intensity and background reflectance.

It further allows to derive absolute or relative luminescence intensities and to exploit them for coding and identification purposes.

Disclosure of Invention

A method of authenticating a security marking according to the first aspect of the invention, wherein said marking comprises a luminescent compound which can be excited by an exciting light source, said method comprising the steps of exposing said marking to excitation and determining the luminescence intensity, wherein the light intensity value is measured during or after exposure to the exciting light source at least during a first and a second two consecutive time intervals, after subtracting the intensity value collected during said first time interval from the intensity value collected during said second time interval, the result of the subtraction representing the light emitted from the luminescent substance, which is compared with a reference value as authentication criterion.

A method of marking and authenticating a security document or article according to a second aspect of the present invention, said method relying on the use of at least one luminescent compound which can be excited by an excitation light source and exhibits a temporal increase in luminous intensity after switching on the excitation light source and a temporal decay in luminous intensity after switching off the excitation light source, said method being characterized in that: the luminescent compound is part of a security document or article, and the excitation light source is switched on for a first time interval, switched off for a second time interval, and at least two luminescence intensity values of at least one luminescence wavelength are measured during at least two consecutive time intervals within the first time interval or the second time interval or both, and the at least two measured luminescence intensity values are subtracted from each other to obtain a net luminescence intensity value, which is compared to a reference value as a verification criterion.

In accordance with this, a device for authenticating a security document or article according to a second aspect of the invention, said document or article carrying at least one luminescent compound which can be excited by an excitation light source and exhibits a temporal increase in the luminescence intensity after switching on the excitation light source and a temporal decay in the luminescence intensity after switching off the excitation light source, said device comprising at least one excitation light source, at least one photodetector channel, and at least one microprocessor, said device being characterized in that: the excitation light source is capable of being turned on during a first time interval and turned off during a second time interval under control of the microprocessor, and the photodetector channel includes at least one photodetector that produces an analog output signal when illuminated by the light source, and at least one signal sampling unit capable of sampling and integrating, under control of the microprocessor, forward and reverse portions of the photodetector output signal during a third time interval and a fourth time interval, respectively, to produce at least one net output signal, and the microprocessor is capable of digitizing and storing the at least one net output signal.

Furthermore, a security system according to the invention comprises a plurality of luminescent marks having time-delayed emission characteristics and preferably emitting at different wavelengths, said luminescent marks being incorporated in different proportions in an ink or plastic material to produce a security document or article, and means for authenticating the security document or article preferably having a corresponding number of detection channels to determine the authenticity of said security document or article.

A verification device according to the invention comprises means for emitting radiation onto a security marking, means for measuring the intensity of light during at least two time intervals, means for subtracting the light intensity values obtained by said measurements during at least two time intervals and for providing an output signal for performing the method of verifying a security marking.

A system according to the invention comprises a verification means and a composition for producing a security marking comprising a luminescent substance which is detectable by said verification means.

The invention discloses a method for measuring luminescence intensity, which is used for getting rid of the influence of ambient light and backscattering excitation radiation. It relies on the use of at least one light-emitting compound that exhibits a time-delayed emission characteristic, i.e. the time-dependent emission of light increases after switching on the excitation light source and still emits a delayed light signal after switching off the excitation light source. A typical luminescence response of such luminescence, such as a function of time, is shown in fig. 1: a) showing the intensity and time of the pulsed excitation signal at wavelength λ 1; b) the intensity and time of the luminescence detection response are shown. The detection response at least comprises three parts: (1) backscattered radiation of wavelength λ 1 penetrating the optical filter system, (2) luminescent radiation of wavelength λ 2 emitted during excitation, and (3) luminescent radiation of wavelength λ 2 emitted after excitation.

The presence of the back radiation (1) of the detector makes it difficult to obtain an accurate absolute measurement of the actual emitted light intensity, such as by reflection of its "rising part" (2) and its "delaying part" (3). Especially in the case of weak luminescence and high excitation intensities, for example in the case of phosphors which have to be detected for up-conversion.

The method according to the invention overcomes this problem and is illustrated in connection with figure 2. As shown in fig. 1, the excitation light source is periodically turned on and off. The net luminous intensity measurements can be obtained by using the following method to obtain the rising and delayed portions.

The rising interval (a) between the switching on and switching off of the excitation light source can be subdivided into preferably equal double intervals. The detector signals are accumulated over the time intervals to obtain a value for each interval. The difference between the first and second signals is then calculated. Since the time intervals are equal, the leakage influence (1) of the backscattered excitation radiation can be removed together with the further present background radiation (ambient light). The remaining signal intensity is unique because of the luminescence.

In the embodiment of fig. 2, for example, the rising portion (a) may be completely subdivided into two equal time intervals (t1, t 2). The accumulated signal strength during time interval t2 is subtracted by the accumulated signal strength of time interval t 1. The effect of backscattering, background radiation and other light effects causing errors is collectively referred to as backscattering effect 1. By subtracting the intensity values, a net signal value representing only the luminescence intensity can be obtained.

Alternatively, the rising interval (a) portion may be subdivided into two equal time intervals (t5, t6) shorter than the preceding time interval (t1, t2) and located near the beginning and end of the rising interval (a). The accumulated signal strength during time interval t5 is subtracted from the accumulated signal strength of time interval t 6. The effect of back-scattering, background radiation, is cancelled, leaving a net signal value that represents only the luminescence intensity. This alternative solution is particularly suitable if several luminescent substances with different characteristic rise time constants have to be analyzed with one and the same detection device.

Likewise, the delay interval (D) after switching off of the excitation light source can be subdivided into at least two, preferably equal, time intervals. The detector signals are accumulated during said time interval and at least one difference signal between later and earlier forms an equal time interval. Since the time intervals are equal, additional present background radiation (ambient light) will be subtracted. The remaining signal is unique because of the presence of fluorescent emission.

In the embodiment of fig. 2, the delay interval (D) may be entirely subdivided into two equal time intervals (t3, t 4). The accumulated signal strength during time interval t4 is subtracted by the accumulated signal strength of time interval t 3. The effect of background radiation is cancelled out leaving a net signal value that is representative of only the luminescence intensity.

Alternatively, the delay interval (D) portion may be subdivided into two equal time intervals (t7, t8) shorter than the preceding time interval (t3, t4) and located near the beginning and end of the delay interval (D). The accumulated signal strength during time interval t7 is subtracted from the accumulated signal strength of time interval t 8. The effect of background radiation is cancelled, leaving a net signal value that represents only the luminescence intensity. This alternative solution is particularly suitable if several luminescent substances with different characteristic delay time constants have to be analyzed with one and the same apparatus.

The method of the invention thus relies on the use of luminescence that exhibits time-delayed radiation characteristics and provides internal compensation of the backscattered excitation radiation in the ambient light radiation and the detector itself, by suitable subdivision of the rise and delay signal observation intervals and the formation of the corresponding different values of the accumulated signal. This allows quantitative evaluation of weak luminescence intensities.

Other variations of the disclosed method can be readily derived and implemented by those skilled in the art in light of the present teachings, and in particular, depend on the extraction of more than twice the interval of the luminescent features, and also on the observation of different sized time intervals.

Detection devices suitable for determining luminescence intensity and other luminescence characteristics are disclosed to be immune to ambient light and backscattered excitation radiation. Such a device as described above, together with at least one luminescent compound exhibiting a luminescent characteristic of a time delay characteristic, is dependent on the implementation of the method of the invention.

FIG. 2b illustrates in more detail how two time intervals such as t5 and t6 are subtracted from each other: during t5 and t6, intensity values 1a and 1b that cause backscatter and other errors are determined. When times t5 and t6 are equal, the values of 1a and 1b are equal.

the total intensity value during t5 includes values 1a and 2 a. the total intensity value during t6 includes values 1b and 2 b. However, when the intensity value 2a caused by the luminescent material at the initial stage of luminescence is relatively low, and the value of 2b at the end of the luminescence period is relatively high, the derivation of (2b-1b) minus (1a +2a) is very close to the value of 2 b. By taking a small sample t5 at the beginning of the irradiation period and another sample t6 at the end of the irradiation period, it is possible to obtain a composite signal which corresponds to a high value of the luminous intensity. Of course, it may be decided to increase the length of one sampling period. For example, if t6 should be twice as long as t5, an accurate value is obtained by dividing the intensity value measured during t6 by 2 to compensate for the longer time period.

Figure 3 shows a schematic diagram of a functional block diagram of the above-described detection apparatus and the implementation of the above-described method of the present invention. The above-mentioned detection means comprise at least one laser diode or light emitting diode as light source (LD/LED) for exciting the luminescent marking (M) of the sample under test (S). The above-mentioned detection device further comprises at least one microprocessor (μ P) with a memory (Mem) and at least one analog-to-digital converter (A/D), and at least one detection channel. The detection path comprises a Photodiode (PD) followed by a transimpedance amplifier (T), a high-pass electronic filter (HP), a low-pass electronic filter (LP) and a first signal amplifier (a 1). The output of the signal amplifier a1 is connected to a switching unit comprising a positive branch consisting of a non-inverting amplifier of unity gain (+1) and switching unit (S +) and a negative branch consisting of an inverting amplifier of unity gain (-1) and switching unit (S-). The combined signal of the switching units (S +, S-) is input to an integrator (I) followed by a second signal amplifier (A2). The output of the signal amplifier a2 will eventually be input to the microprocessor (μ P) analog-to-digital converter a/D.

The detection means comprises at least one, but preferably two or more detection channels to provide a relative comparison of the intensity of the luminescence signal resulting from the deliberate mixing of different luminescence in the labels. Further optical or electronic components may be present in the detection device or in its respective detection channel, such as focusing or collecting lenses, optical filters, electronic filters, etc. Some of the functional blocks in fig. 3 are also incorporated together with the same electronic circuit unit.

The excitation light source (LD/LED) and the switching unit (S +, S-) are controlled by the microprocessor (μ P) such that the detection unit executes any and specific application sampling period by a corresponding program of the microprocessor.

The microprocessor (μ P) is obviously programmed to perform the following operations:

1. the excitation light source (LD/LED) is repeatedly switched on and off to determine time intervals,

2. according to a pre-established sampling plan to open and close the positive and negative switch units (S +, S-),

3. reading the detected signal values in digital form for at least some of the existing channels using the microprocessor's A/D converter,

4. mathematical processing is performed and compared with reference values of the signal values read in step 3 either absolutely or relatively,

5. the result of step 4 is made based on an indication of authenticity or non-authenticity of the sample being tested.

The above described detection apparatus may further be used as a stand-alone unit operating in a stand-alone manner using pre-stored reference values to determine the authenticity of a sample under test, or, alternatively, connected to a central, secure data server via an information transfer chain. The central server contains the real reference values and can perform some operations of the microprocessor (μ P), in particular as indicated in steps 4 and 5 above.

Also disclosed is a security system comprising a mixture of luminescent compounds, which can be identified using the above detection devices and methods. Mixtures of the above-mentioned luminescent compounds can be incorporated into inks and printed on security documents or articles, or molded into plastics or laminated between paper, for use in the production of foils, security threads, credit cards, identification or access cards, and the like. The security system described above can obviously be used to protect banknotes, valuable documents, official documents, cards, transportation tickets and all types of branded goods.

It has to be noted that the method and the device according to the invention need to take into account the need for a significantly reduced optical filtering. If the detection of the luminescence response is performed during the delay interval, where no excitation signal is present, it is not necessary to protect the photodiode especially from the excitation light. A simple corrugated 45 ° beam splitter may be sufficient to separate the wavelengths of the emitted light. Such filters are advantageous when they are mass produced by Lippmann holography and related techniques.

In special cases, it is conceivable to work without any optical filter and with the method and device of the invention, together with the analysis of the luminescence delay characteristics, to rely exclusively on the wavelength identification that has been achieved by selecting a suitable excitation source and a suitable photodiode. In this context, it is interesting to note that most LEDs can also be utilized as the selectable wavelength, albeit with somewhat lower efficiency, and photodiodes. This is particularly useful when working with up-converting phosphors in order to reduce the sensitivity of the photodetector to the longer wavelength light of the excitation source. Because there are a large number of different color LEDs on the market, covering the entire spectral range from near UV, from visible down to IR, there are many potential photodiodes available to choose for the person who needs them.

The invention is further described by means of the figures and examples.

Drawings

FIG. 1 shows a typical time evolution of the excitation signal and the detected luminescence response of the luminescent compounds used in the present invention: a) the intensity and time of the excitation signal at wavelength λ 1; b) the strength and time of the response signal are detected. Detecting the response signal includes: (1) backscattered radiation of wavelength λ 1 that permeates the optical filter system, (2) luminescent radiation of wavelength λ 2 that is emitted during excitation, and (3) luminescent radiation of wavelength λ 2 that is emitted after excitation.

Figure 2 shows a schematic diagram of a detection method according to the invention,

figure 3 shows a block diagram of a detection device according to the invention and a circuit implementing the method of the invention,

FIG. 4 shows a schematic diagram of the optics of an embodiment of the invention, including an excitation IR-LED and two detection channels: a) schemes using non-imaging optics; b) with the use of the scheme of the imaging optics,

FIG. 5 shows a circuit diagram of an electronic embodiment of a detection channel according to the present invention.

Fig. 6 shows timing charts of the activation signal (E) and the control signals (P1, P2) of the switching unit.

Detailed Description

A safety system implementing the method of the invention and a corresponding detection device can be implemented as follows. Up-conversion of a selected light-emitting compound to Y₂O₂S: er, Yb and Y₂O₂S: tm, Yb phosphorus. These substances are excited by intense infrared radiation in the wavelength range from 900 to 980 nm. After a two-photon excitation process, they emit light of shorter wavelength, erbium species in the green, 550 nm region and thulium species in the near infrared, 800 nm region. The time constant characteristic of the respective luminous intensity integration and delay has a sequence of 50 to 500 microseconds; they obviously depend on the precise nature of the luminescent substances.

The detection device was constructed according to figures 3, 4 and 5. The excitation source was a commercially available GaALAs IR-LED for remote control applications. The selected device, OPE5594S, emitted 120mW of optical power per 10 ° of spherical power at half angle. The peak emits a wavelength of 940 nm and a half-width spectrum of 45 nm.

Fig. 4a shows a schematic view of the optical system of the detection device. The light of the above IR-LED was fed through a 45 ° dielectric beam splitter (BS1) into a conical nozzle (N) of polymethylmethacrylate-acrylate (PMMA) and focused on the luminescent marker (M) of the sample to be tested (S). The above-described conical nozzle (N) apparently acts as a non-imaging optical concentrator (accepting angle changes), accepting low intensity, nearly parallel light at its wide end and delivering high intensity light, but delivering discrete light at its narrow end. Instead, it collects a large concentration of discrete light at the end and delivers a diluted near-parallel beam at the wide end. The beam splitter BS1 is of the high-pass type, with a 45 ° cutoff wavelength of 900 nm.

The marker (M) comprises the two up-converting phosphors described above at a preset ratio and emits the two above-mentioned shorter wavelength luminescent radiations of 550 nm and 800 nm when excited at a high intensity with the light of the above-mentioned 900 to 980 nm emitting IR-LED. The radiation is collected through a cone-shaped nozzle (N) at a wide acceptance angle, "collimated" and deflected at the first 45 ° beam splitter BS 1. A second 45 ° dielectric beam splitter (BS2), of the long pass type with a 45 ° cutoff wavelength of 700 nm, splits off the 550 nm and 800 nm components of the emitted luminescence response. Packing the 800 nm component into a silicon photodiode (PD1) through an optical 800 nm bandpass filter (F1); the 550 nm fraction was packed into a GaAsP photodiode (PD2) through an optical 550 nm bandpass filter (F2).

An alternative optical system layout is shown in figure 4 b. The essentially parallel-beam light of the narrow-angle emitting IR-LED is transmitted through two dichroic 45 ° beam splitters (BS1, BS2) and is focused by focusing a focusing lens (L) on the luminescent marker (M) of the sample under test. The mark M is thus set on the focal plane of the lens L. Light is emitted through the marker M in response to the 900 to 980 nm excitation light collected by the lens L and transmitted back as parallel rays onto the first 45 ° beam splitter (BS 1). The beam splitter is of the 45 ° pleated filter type and reflects a first narrow wavelength band of about 800 nm towards a first photodiode (PD 1). The remaining beam falls on the second 45 beam splitter (BS 2). The beam splitter is also of the 45 ° pleated filter type and reflects a second narrow wavelength band of about 550 nm towards a second photodiode (PD 2). An optical filter (F1, F2) that reduces the intensity of the back reflection of the IR light excitation source may optionally be inserted in front of the photodiode (PD1, PD 2).

FIG. 5 shows an embodiment of the electronic portion of one of the detection channels of the detection device. It relies on a microprocessor of the PIC 16F877 type. The microprocessor is common to all of the detection channels of the detection device. Detector electronics rely on inexpensive electronic components; i.e. the low noise operational amplifier may be of the NE 5532 type (two units per set) and the switching units may be of the 4066 type (4 units per set).

Which may be silicon, GaAsP or any other type of photodiode, is utilized in the photo mode and passes its signal to a symmetrical transimpedance amplifier stage (IC 1: a). The transimpedance amplifier stage is followed by a second amplifier stage (IC 1: B) which delivers its output to the positive and negative switching units (IC 3: a, IC 3: B) by capacitive coupling. For a positive switching cell (IC 3: A), IC 1: b, an output signal; for the negative switch cell (IC 3: B), it will first be fed into the IC1 through an analog conversion stage (IC 2: B): b, output signal. The combined output of the switching units (IC 3: A, IC 3: B) is input to an integrator terminal (IC 2: A) and the integrated signal enters the analog-to-digital converter (A/D) of the PIC processor. The control signals (P1, P2) of the switching units (IC 3: A, IC 3: B) are generated by a PIC processor.

Further embodiments of the detection device are easily conceivable to the person skilled in the art on the basis of the above teachings, in particular with more than one excitation light source or more than two detection channels.

The operating frequency of our example device was chosen to be 1kHz, with the same length as the time interval between the start of excitation and the end of excitation. However, this is not a requirement; other on/off frequencies may be equally selected.

Fig. 6 shows an example of a useful timing diagram of the activation signals (E) and the control signals (P1, P2) of the switch cells. Fig. 6a shows a square wave excitation signal (E) and a luminescence response (R). Fig. 6b shows an example of sampling the "up" part of the luminescence response (R) with the control signal (P1, P2) of the "switching cell". Fig. 6c shows an example of sampling the "delayed" part of the luminescence response (R). Fig. 6d shows an alternative example of sampling the "rising" part of the luminescence response (R).

By combining suitable different sampling schemes, the method and apparatus of the present invention allow for the extraction of information for both the luminous intensity and the characteristic time constant of the "up" and "delay" portions of the luminous response (R).

Claims (27)

1. A method of authenticating a security marking, wherein the marking comprises a luminescent compound which is excitable by an excitation light source, the method comprising the steps of exposing the marking to excitation and measuring the intensity of the luminescence,

wherein the light intensity values are measured during at least a first and a second consecutive time interval during or after exposure to the excitation light source, the result of the subtraction, after subtracting the intensity values collected during said first time interval from the intensity values collected during said second time interval, representing the light emitted from the luminescent substance, is compared with a reference value as a verification criterion.

2. A method according to claim 1, wherein the first time interval is selected during the initial phase of excitation of the luminescent substance to reduce the proportion of the intensity of light caused by the emission of the luminescent substance compared to the measured intensity of light caused by backscattered, diffuse or other light not caused by the emission.

3. A method according to claim 1, wherein the second time interval is selected at a phase where the light intensity caused by the emission of the luminescent substance rises to its maximum.

4. A method according to claim 2 or 3, wherein the duration of one time interval is shorter than 25% of the transmission period (a).

5. The method according to claim 1, wherein the label comprises one or more luminescent compounds which emit light at two different frequencies, and the light intensity of the light emitted at said frequencies is sampled.

6. The method of claim 5, wherein the intensity values of the different frequencies are compared.

7. The method of claim 1, wherein the light intensity is sampled during the exposing of the indicia to radiation.

8. The method of claim 1, wherein the light intensity is sampled after exposing the indicia to radiation.

9. The method of claim 1, wherein the light intensity values during said first and second two consecutive time intervals are integrated.

10. A method of marking and authenticating a security document or article, said method relying on the use of at least one luminescent compound which can be excited by an excitation light source and shows an increase in luminescence intensity over time after switching on the excitation light source and a decay in luminescence intensity over time after switching off the excitation light source, said method being characterized in that:

the luminescent compound is part of a security document or article, and

the excitation light source is turned on for a first time interval (T1), turned off for a second time interval (T2), and

measuring at least two luminescence intensity values of at least one luminescence wavelength at least during two consecutive time intervals within the first time interval (T1) or the second time interval (T2) or both, and

the at least two measured luminescence intensity values are subtracted from each other to obtain a net luminescence intensity value, which is compared with a reference value as a verification criterion.

11. The method according to claim 10, wherein the third time interval (T3) and the fourth time interval are the same (T4) and are comprised within the first time interval (T1).

12. The method according to claim 11, wherein the third time interval (T3) and the fourth time interval (T4) are half of the first time interval (T1).

13. The method of claim 12, wherein the third time interval (T3) and the fourth time interval (T4) are the same and are included in the second time interval (T2).

14. The method according to claim 13, wherein the third time interval (T3) and the fourth time interval (T4) are half of the second time interval (T2).

15. A method according to claim 10, wherein said excitation light source is repeatedly switched on and off and wherein said luminescence intensity values are repeatedly measured and subtracted to obtain an integrated net intensity value, which is compared to a reference value as a validation criterion.

16. An apparatus for authenticating a security document or article carrying at least one luminescent compound which can be excited by an excitation light source and exhibits an increase in luminescence intensity over time upon turning on the excitation light source and a decay in luminescence intensity over time upon turning off the excitation light source, said apparatus comprising at least one excitation light source, at least one photodetector channel, and at least one microprocessor, said apparatus being characterized in that:

the excitation light source being capable of being turned on during a first time interval (T1) and turned off during a second time interval (T2) under control of the microprocessor, and

the photodetector channel comprises at least one photodetector generating an analog output signal when illuminated by a light source, and at least one signal sampling unit capable of sampling and integrating, under control of the microprocessor, the forward (P1) and reverse (P2) portions of the photodetector output signal during a third time interval (T3) and a fourth time interval (T4), respectively, to generate at least one net output signal, and

the microprocessor is capable of digitizing and storing the at least one net output signal.

17. The apparatus of claim 16, wherein the third time interval (T3) and the fourth time interval (T4) are the same and are included in the first time interval (T1).

18. The apparatus of claim 17, wherein the third time interval (T3) and the fourth time interval (T4) are half of the first time interval (T1).

19. The apparatus of claim 18, wherein the third time interval (T3) and the fourth time interval (T4) are the same and are included in the second time interval (T2).

20. The apparatus of claim 19, wherein the third time interval (T3) and the fourth time interval (T4) are half of the second time interval (T2).

21. The apparatus of claim 13, wherein the excitation light source is repeatedly switched on and off, and wherein the signal sampling unit repeatedly samples and integrates the photodetector output signal to obtain at least one integrated net output signal.

22. The apparatus of claim 20, wherein the at least one net output signal or the at least one integrated net output signal is compared, locally by the microprocessor, to at least one internally stored reference value to derive the validation signal.

23. The apparatus of claim 20, wherein the at least one net output signal or the at least one integrated net output signal is transmitted over a communication link to a remote server for comparison with at least one stored reference value to obtain and return an authentication signal.

24. A security system comprising a plurality of luminescent indicia having time delayed emission characteristics and preferably emitting at different wavelengths, said luminescent indicia being incorporated in different proportions in an ink or plastics material to produce a security document or article, and a device according to any of claims 20 to 23 preferably having a corresponding number of detection channels to determine the authenticity of said security document or article.

25. A verification device comprising means for emitting radiation onto a mark, means for measuring the intensity of light during at least two time intervals, means for subtracting light intensity values obtained by said measurements during at least two time intervals and for providing an output signal for performing a method according to any one of claims 1 to 13.

26. The apparatus of claim 25, comprising means for measuring light intensity at two or more ranges of light frequencies.

27. A system comprising a verification device according to claim 25 or 26 and a composition for producing a security marking comprising a luminescent substance detectable by said verification device.