Light sensor and decay-time scanner

Abstract

The disclosed scanner for detecting a decay time of light emitted by a luminescent material has a control unit operable to adapt the drive current, or the value of the drive voltage, powering its light source to accordingly adapt the intensity of excitation light delivered to the luminescent material so that its high sensitivity light sensor can reliably measure the luminescence light emitted in response to the excitation light, and thus accurately determine a corresponding decay time value.

Claims

1. A light sensor for detecting luminescence light received from a luminescent material, comprising: a bias regulator operable to deliver a bias voltage V.sub.b; a photodiode having a cathode connected to the bias regulator so that the photodiode is reversely biased by the delivered bias voltage V.sub.b, the photodiode being operable to deliver, in a photoconductive mode, a photocurrent intensity I.sub.p in response to received luminescence light in a given photodiode spectral range; an inverting transimpedance amplifier including an operational amplifier with a feedback resistor R.sub.f and a feedback capacitor C.sub.f mounted in parallel with the feedback resistor R.sub.f between an inverting input terminal and an output voltage terminal of the operational amplifier, the inverting input terminal of the operational amplifier being connected to an anode of the photodiode and operable to convert the delivered photocurrent intensity I.sub.p into an output voltage signal V.sub.out at the output voltage terminal; further comprising a PNP bipolar junction transistor of which emitter E and base B are connected in parallel with said feedback resistor R.sub.f and feedback capacitor C.sub.f, with its base B connected to said output voltage terminal and its collector C grounded.

2. The light sensor according to claim 1, wherein the bias regulator is a low noise, fast transient response bias regulator.

3. The light sensor according to claim 1, further comprising capacitors C.sub.1 and C.sub.2 in series connected to the cathode of the photodiode and grounded, a grounded resistor R.sub.g being connected to a non-inverting input terminal of the operational amplifier and a terminal between the capacitors C.sub.1 and C.sub.2, adapted to eliminate voltage variations caused by the bias regulator.

4. The light sensor according to claim 1, further comprising a bias current sensor connected between an output voltage terminal of the bias regulator and the cathode of the photodiode operable to measure an intensity of the bias current I.sub.bias delivered to the photodiode.

5. A scanner for detecting luminescence light from a luminescent material upon illumination with an excitation light within an excitation wavelength range, said luminescent material emitting said luminescence light within an emission wavelength range, comprising: a power source operable to deliver variable drive current or drive voltage; and a light source connected to said power source and operable to illuminate said luminescent material with said excitation light within said excitation wavelength range when powered with the drive current or the drive voltage delivered by the power source, during an excitation time interval Δt.sub.ex, said light source being operable to produce said excitation light with an excitation light intensity varying according to the delivered drive current or drive voltage, further comprising a light sensor according to claim 4 operable to deliver the output voltage signal V.sub.out to an input terminal of an analog-to-digital signal converter connected to the output voltage terminal upon illumination of said luminescent material with said light source and detection of corresponding emitted luminescence light, the analog-to-digital signal converter being operable to convert the output voltage signal V.sub.out into a digitalized luminescence light intensity signal over a measuring time interval Δt.sub.meas; and a control unit connected to the bias current sensor to receive a measured value of the intensity of the bias current I.sub.bias and further connected to a control bus, the control unit being operable to control the power source via a first digital-to-analog signal converter connected between the power source and the control bus by setting a value of the drive current or drive voltage and a value of the excitation time interval Δt.sub.ex, and the light sensor via both the analog-to-digital signal converter, further connected to the control bus, and a second digital-to-analog signal converter, connected to an offset resistor R.sub.o connected to the anode of the photodiode and further connected to the control bus to convert an offset current intensity I.sub.o into a digitalized offset current intensity, to set a value of the measuring time interval Δt.sub.meas and acquire the luminescence light intensity signal over the value of the measuring time interval Δt.sub.meas to form a digitalized luminescence light intensity signal profile I(t), wherein said control unit is further operable to receive said luminescence light intensity signal and control said power source to adapt the value of the drive current, or the value of the drive voltage, delivered to the light source so that a luminescence light intensity value corresponding to a delivered luminescence light intensity signal is below a maximum intensity value I.sub.max corresponding to a saturation threshold value of the photodiode.

6. The scanner according to claim 5, wherein the control unit, based on the measured value of intensity of the bias current I.sub.bias, is further operable to adapt the value of the drive current, or the value of the drive voltage, delivered to the light source so that a level of the corresponding current intensity in the photodiode is below a photodiode current intensity threshold value and the level of the corresponding current intensity through the PNP bipolar junction transistor is below a transistor current intensity threshold value.

7. The scanner according to claim 5, wherein the control unit, based on the measured value of intensity of the bias current I.sub.bias, a received value of the digitalized offset current intensity I.sub.o, and a received value of the digitalized luminescence light intensity signal, is further operable to set a value of the delivered offset current intensity I.sub.o via the second digital-to-analog signal converter.

8. The scanner according to claim 7, wherein the control unit is operable to switch off the light source and then acquire digitalized luminescence light intensity signal and set a value of the offset current so as to make the acquired digitalized luminescence light intensity signal close to zero, thereby compensating a current intensity due to stray light.

9. The scanner according to claim 8, wherein the control unit is further operable to power the light source and then form a digitalized luminescence light intensity signal profile I(t), check if a value of a digitalized luminescence light intensity signal acquired after the measuring time interval Δt.sub.meas is close to zero, and, in case said checked value is not close to zero, further set a value of the offset current to make a value of a digitalized luminescence light intensity signal further acquired after the measuring time interval Δt.sub.meas close to zero, and then control the scanner to illuminate the luminescent material during the excitation time interval Δt.sub.ex, acquire at least one corresponding digitalized luminescence light intensity signal profile I(t) over the measuring time interval Δt.sub.meas and store in a memory each acquired digitalized luminescence light intensity signal profile.

10. The scanner according to claim 9, wherein the control unit is further operable to determine a value of a decay time of the luminescent material from a stored digitalized luminescence light intensity signal profile.

11. The scanner according to claim 10, wherein the control unit is further operable to decide that the luminescent material is genuine in case the determined decay time value matches a reference value of decay time.

12. The scanner according to claim 5, wherein the illumination light source comprises a flat LED, the photodiode is a flat photodiode and said flat LED and flat photodiode are mounted adjacent and wired on a flat support member of a nose piece of the scanner for illuminating the luminescent material and collecting corresponding luminescence light, thereby allowing the nose piece to be disposed close to the luminescent material to improve illumination and luminescence light collection efficiency without necessitating a light guide.

13. The scanner according to claim 12, wherein the illumination light source comprises a plurality of flat LEDs wired in series on the support member, and a plurality of flat photodiodes wired in parallel on the support member.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustration of a typical shape of a luminescence light intensity signal received from a luminescent material in response to excitation illumination.

(2) FIG. 2 is a schematic illustration of an electrical circuitry of a light sensor for detecting luminescence light from a luminescent material according to an embodiment of the invention.

(3) FIG. 3 is a schematic illustration of the electrical circuit of the light sensor of FIG. 2, with further connection to a bias current sensor, according to the invention.

(4) FIG. 4 illustrates an electrical circuit scheme of a scanner according to the invention, incorporating the light sensor of FIG. 3.

(5) FIG. 5 illustrates a compact scanner nose piece integrating a light source with flat LEDs and a light sensor with flat photodiodes, according to an embodiment of the invention.

(6) FIG. 6 illustrates the electrical circuit scheme of the light source of FIG. 5.

(7) FIG. 7 illustrates the electrical circuit scheme of the light sensor of FIG. 5.

DETAILED DESCRIPTION

(8) In order to obtain a high sensitivity light sensor capable to detect weak luminescence light intensity signals from a luminescent material of a marking (to allow acquiring luminescence intensity profile I(t) and accurately calculating a decay time characteristic of the luminescent material from the profile), a specific electronic circuitry has been developed that makes possible a fast recovering (i.e. desaturation) of a high sensitive photodiode of the light sensor shortly after a powerful illumination with excitation light has been delivered to the marking so that accurate detection of luminescence light intensity signal emitted by the luminescent material of the marking in response to this excitation can start very soon after the end of illumination, while the (weak and decreasing) luminescence intensity signal is still close to the maximum of emission intensity (i.e. immediately after the end of illumination pulse), even in case the photodiode is disposed very close to an emitting surface of the luminescent material.

(9) FIG. 1 shows a typical shape of a luminescence light intensity signal from a luminescent material in response to excitation illumination. A powerful pulse of excitation light (with wavelength spectrum within an excitation wavelength range, for example by means of a FLED, “Flood LED”) first illuminates a marking comprising a luminescent material during an excitation time interval Δt.sub.ex and then, in response, the luminescent material emits luminescence light (within an emission wavelength range) with an intensity reaching a maximum value I.sub.em at the end of illumination at instant t.sub.0. Due to the powerful illumination, this value I.sub.em is generally above a maximum intensity value I.sub.max corresponding to a saturation threshold value of a photodiode of a light sensor used to detect the luminescence light emission. Typically, the emitted luminescence light intensity signal I.sub.L over time can be fitted by a decreasing exponential curve I.sub.L=I.sub.em exp(−(t−t.sub.0)/τ), with τ being a decay time value that is characteristic of the specific luminescent material considered. Generally, the photodiode starts accurately detecting the luminescence intensity signal only from an instant t.sub.1 after the end of illumination at t.sub.0, once being in an unsaturated state, to detect a corresponding value I.sub.1 of the emitted luminescence light intensity and continues detecting luminescence light intensities I.sub.2(t.sub.2), . . . , I.sub.N(t.sub.N) at respective subsequent instants t.sub.2, . . . , t.sub.N during a measuring time interval Δt.sub.meas before the emitted decreasing luminescence light intensity falls below a minimum intensity value I.sub.min (close to zero) corresponding to a noise threshold value of the photodiode (below which measured intensity values are not accurate enough). For example, to illustrate the meaning of the expression “close to zero”, the light sensor with the five photodiodes D5-D9 shown on FIGS. 5 and 7, a typical value of the minimum intensity I.sub.min is about five times the dark current intensity, i.e. about 5×5 nA=25 nA for a reverse (bias) voltage of about 20 V. Thus, the measured luminescence light intensity values I.sub.1(t.sub.1), . . . , I.sub.N(t.sub.N) can be used to determine (via curve fitting or interpolation methods) a luminescence light intensity profile I(t) from which a value of the characteristic decay time parameter τ can be determined (as well known in the art). In practice, in order to obtain a more reliable statistical (average) value for τ, the illumination-detection cycle is repeated a certain number of times to acquire a plurality of luminescence light intensity profiles. As an example, we mention a luminescent material that can be excited with illumination light in the infrared (IR) wavelength range (i.e. comprized between about 700 nm and 1 mm, for example around 900 nm), with illumination pulses corresponding to an excitation time interval Δt.sub.ex of about 100 μs, and emitting luminescence light in the infrared (IR) range (for example around 900 nm) with a measuring time interval Δt.sub.meas of several milliseconds (for example, about 4 ms). The emitted IR luminescence light, depending on the luminescent material, has a decay time characteristic τ (decay time constant) which can range between about few μs and few ms (for example, between 15 μs and 10 ms).

(10) To obtain the above mentioned high sensitivity fast recovering light sensor, illustrated on FIG. 2 with only one photodiode (1), the photodiode module comprises the photodiode (1) mounted to operate in the photoconductive mode (i.e. a reversely biased photodiode) with a positive high voltage V.sub.b applied at its cathode by means of a low noise, fast transient response bias regulator (2), and an inverting transimpedance amplifier including an operational amplifier (3) of which inverting input terminal is connected to the anode of the photodiode (1) and non-inverting input terminal is grounded. The inverting transimpedance amplifier also comprises a feedback resistor R.sub.f and a feedback capacitor C.sub.f (here, R.sub.f and C.sub.f respectively designate both the resistor element and its resistance value, and the capacitor element and its capacitance value), in parallel with the feedback resistor R.sub.f, connected between the inverting input terminal and the output voltage terminal (4) of the operational amplifier (3). The reverse bias increases the width of the depletion layer of the p-n junction of the photodiode, with the consequence that the capacitance of the junction is lowered and the response time is reduced (high-frequency performance is thus improved).

(11) The feedback resistor R.sub.f is for setting the (high) gain of the inverting transimpedance amplifier, and the (small value) feedback capacitor C.sub.f is for improving stability. When illuminated with luminescence light, the photodiode (1) delivers a photocurrent intensity I.sub.p to the operational amplifier (3) which delivers a corresponding output voltage V.sub.out at the output terminal (4). According to the invention, in order to drastically reduce the overall recovering time of the photodiode (1), even in case the photodiode is disposed close to the surface of the luminescent material (the operational amplifier being then not capable to evacuate all the current), a PNP bipolar junction transistor (5) is further connected in parallel between the feedback resistor and feedback capacitor, with its collector C connected to the ground to evacuate the surge current which appears when the photodiode delivers a strong current and saturates as the output photo-voltage approaches the reverse bias voltage V.sub.b, and thus allows shortening the desaturation time of the light sensor (the PNP “assists” the operational amplifier in evacuating the current). Connecting the collector C to the ground also reduces the ringing effect caused by disturbance of the power supply of the operational amplifier (not shown) due to the photocurrent variation when the illumination pulse stops. For example, a voltage regulator (not shown) supplying the operational amplifier (3) and the transistor (5) typically generates a huge current of about 500 mA, and causes ringing. The emitter E and base B of the transistor (5) are connected in parallel with the feedback resistor R.sub.f and the feedback capacitor C.sub.f, the base B being connected to the output terminal (4). As indicated, the gain of the operational amplifier is set by the value of the feedback resistor R.sub.f, but it is also the dominant source of noise (ringing effect). By connecting the collector C to the ground, the bias regulator is not perturbed and the ringing effect is strongly reduced. This configuration is particularly convenient for photodiodes illuminated with low light intensity levels requiring a high gain (large value of R.sub.f). As a result of this circuit configuration, fast switching speed compatible with the illumination-measurement cycles is allowed, shorter decay time characteristics can be obtained from the measured luminescence light intensity (as intensity is detected earlier), weak intensity signal due to a lower quantity of luminescent material in the marking can be detected (as the signal has not decayed too much when the photodiode starts detecting) and sensitivity of the light sensor is accordingly increased. Moreover, increasing the excitation pulse intensity delivered to the luminescent material (for detecting luminescence emission level due to reduced amount of luminescence material) generates a strong photocurrent delivered by the photodiode (1). However, the photodiode saturates when the output photovoltage approaches the reverse bias voltage V.sub.b, and the saturation of the operational amplifier can be avoided by evacuating the surge current in the photodiode through the ground (thanks to the PNP transistor which drains the surge current).

(12) Preferably, in order to eliminate voltage variations due to the bias regulator (2) (trying to maintaining the bias voltage at the value V.sub.b over the illumination-measurement cycles), two capacitors C.sub.1 and C.sub.2 are mounted in series and connected between the anode of the photodiode (1) and the ground (to block the AC variations of bias current I.sub.bias resulting from bias voltage variations due to the pulse) and a grounded resistor R.sub.g is connected between the non-inverting input terminal of the operational amplifier (3) and a terminal between the two capacitors C.sub.1 and C.sub.2. Moreover, a resistor R.sub.o may further be connected to the anode of the photodiode (1) and set to absorb offset current and shift down the measured offset current intensity I.sub.o to some reliable range.

(13) As shown on FIG. 3, the light sensor may also comprise a bias current sensor (6), connected between the output voltage terminal V.sub.b of the bias regulator (2) and the anode of the photodiode, for measuring an intensity of the bias current I.sub.bias delivered to the photodiode (1). Such a bias current sensor is necessary for controlling the level of current intensity in the photodiode (1) and avoid damaging it (this current intensity can reach high values, for example 300 mA or even higher), by limiting the level of excitation illumination.

(14) FIG. 4 shows an electrical circuit scheme of a scanner according to the invention, incorporating the light sensor shown on FIG. 3. The scanner comprises a power source (7) for delivering variable drive current or drive voltage to a light source (8). This light source (8) can illuminate a luminescent material (not shown) with an excitation light within an excitation wavelength range (adapted to the luminescent material) when powered by the power source (7), during an excitation time interval Δt.sub.ex, the light source (8) being operable to illuminate the luminescent material with said excitation light having an excitation light intensity varying according to the drive current or drive voltage delivered by the power source (7).

(15) The scanner further incorporates the light sensor of FIG. 3, which, upon illumination of the luminescent material with the light source (8) and detection of corresponding emitted luminescence light by the photodiode (1), is operable to deliver the output voltage signal V.sub.out to an input terminal of an ADC (9) (“analog-to-digital signal converter”) connected to the output voltage terminal (4). This ADC (9) converting the output voltage signal V.sub.out, received over a measuring time interval Δt.sub.meas into a corresponding digitalized luminescence light intensity signal.

(16) The scanner further comprises a control unit (10) connected to the bias current sensor (6) to receive a measured value of the intensity of the bias current I.sub.bias and is also connected to a control bus (11).

(17) The control unit (10) controls the power source (7) by means of a first DAC (12) (“digital-to-analog signal converter”), connected between the power source (7) and the control bus (11), by setting a value of the drive current or drive voltage and a value of the excitation time interval Δt.sub.ex. The control unit (10) also controls the light sensor by means of both the ADC (9), further connected to the control bus (11), and a second DAC (13), connected to the offset resistor R.sub.o connected to the anode of the photodiode (1) and further connected to the control bus (11), to convert the offset current intensity I.sub.o into a digitalized offset current intensity, to set a value of the measuring time interval Δt.sub.meas and acquire the luminescence light intensity signal over the value of the measuring time interval Δt.sub.meas, and to form a digitalized luminescence light intensity signal profile I(t).

(18) The control unit (10) is further operable to control the power source (7) to adapt the value of the drive current, or the value of the drive voltage, delivered to the light source (8), based on the received luminescence light intensity signal, so that the luminescence light intensity value corresponding to the received luminescence light intensity signal is below a maximum intensity value I.sub.max corresponding to a saturation threshold value of the photodiode (1) and of the operational amplifier (3).

(19) FIG. 5 shows a view of compact scanner nose piece, having a square shape of dimensions 8×8 mm, wherein a light source with four flat LEDs D1 to D4 wired in series, and a light sensor with five flat photodiodes D5 to D9 (here, fast response photodiodes) wired in parallel are integrated on a flat support (they are glued to the support), according to an embodiment of the invention. This wiring in parallel allows reaching a high sensitivity level for light detection. Here, the photodiodes have a spectral range of sensitivity of 750 nm to 1100 nm, a radiant sensitive area of 1 mm.sup.2, a rise and fall time of 5 ns, a capacitance of 11 pF, the spectral sensitivity of the chip being 0.65 A/W at wavelength λ=870 nm, with dark current of 1 nA and reverse voltage of 20 V. Such a compact nose allows positioning the LEDs and the photodiodes very close to the surface of a marking including the luminescent material, thus avoiding any light guide and corresponding losses while minimizing stray light. For example, this nose illumination-detection head allows measuring luminescence light intensity signals every 200 ns and typically acquire 30 to 40 values over a measuring time interval Δt.sub.meas.

(20) As an example, a measurement cycle comprises the following steps: a preliminary step for setting the offset (via the DAC (13)), wherein the light sensor of the scanner detects light for acquiring an intensity profile I(t) without excitation illumination by the light source (8). This allows eliminating the component of the photocurrent intensity signal due to stray light. illumination light pulses are delivered by the light source (8) which serve to set the amplitude of the pulses so that the detected maximum photocurrent intensity (I.sub.em at start of decay) is below the maximum intensity value I.sub.max corresponding to the saturation threshold value of the light sensor. Then, the offset current intensity I.sub.o is further checked, and possibly set (via the DAC (13)), so that the minimal value of the measured photocurrent intensity is very close to zero. in a subsequent step, a cycle of illumination with excitation light over excitation time Δt.sub.ex and acquisition of luminescence light intensity signal over measuring time interval Δt.sub.meas is performed to obtain (and store) digitalized luminescence light intensity profiles I(t) (typically, about one hundred), and a mean is calculated over said profiles which is in turn used for calculating a corresponding (test) decay time characteristic τ. a final step of authentication is then performed by comparing the calculated value τ to a reference value τ.sub.ref for the luminescent material: in case of (reasonable) matching, the marking including the luminescent material is considered as genuine, if not, the marking is considered as a fake. In this example, decay time values of about 100 to 120 μs can be measured. Other examples have been tested with successful measurement of decay time values of about 30 μs.

(21) The above disclosed subject matter is to be considered illustrative, and not restrictive, and serves to provide a better understanding of the invention defined by the independent claims.