WO2011058496A2

WO2011058496A2 - Sensor device with light emitting diode

Info

Publication number: WO2011058496A2
Application number: PCT/IB2010/055074
Authority: WO
Inventors: Josephus A. H. M. Kahlman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-11-16
Filing date: 2010-11-09
Publication date: 2011-05-19
Also published as: WO2011058496A3

Abstract

The invention relates to a method and a sensor device (100) for executing optical examinations in the investigation region (3) of a carrier (5). The sensor device (100) comprises an LED (22) that is operated with a constant current (Ip) and that emits an input light beam (Ll) towards the investigation region (3). An output light beam (L2) coming from the investigation region (3) is detected with a light detector (30), typically a camera, to provide intensity related raw measurement signals (Spix). The forward voltage (VF) across the LED (22) is sensed and used in a correction unit (50) to generate corrected measurement signals (Spix nrm). Thus the measurements can be made robust with respect to variations of the input light source (22).

Description

SENSOR DEVICE WITH LIGHT EMITTING DIODE

FIELD OF THE INVENTION

The invention relates to a sensor device and a method for the execution of optical examinations in an investigation region of a carrier, said method comprising the emission of light from a light emitting diode into the investigation region and the detection of light coming from the investigation region. Moreover, it relates to the use of such a sensor device.

BACKGROUND OF THE INVENTION

The WO 2008/155723 Al discloses a sensor device in which a light emitting diode (abbreviated "LED" in the following) emits an input light beam into an investigation region, and in which a light detector senses an output light beam resulting after a frustrated total internal reflection of the input light beam. In order to make the measurements robust with respect to disturbances, the original emission of the LED is monitored and taken into account when a measurement of the light detector is evaluated. Moreover, the LED intensity is modulated with a particular frequency, and the measurement signal is demodulated accordingly.

SUMMARY OF THE INVENTION

Based on this situation it was an object of the present invention to provide an alternative robust measurement procedure that can be realized with reduced cost.

This object is achieved by a sensor device according to claim 1, a method according to claim 2, and a use according to claim 12. Preferred embodiments are disclosed in the dependent claims.

The sensor device according to the present invention is intended for making optical examinations in an investigation region of a carrier (which do not necessarily belong to the device). In this context, the term "examination" is to be understood in a broad sense, comprising any kind of manipulation and/or interaction of light with some entity in the investigation region, for example with biological molecules to be detected. The investigation region will typically be a small volume at the surface of the (preferably transparent) carrier in which material of a sample to be examined can be provided. The sensor device comprises the following components: a) A light emitting diode ("LED") for emitting a light beam, which is called "input light beam" in the following, towards the investigation region. The LED may optionally be provided with suitable optics for shaping and directing the input light beam.

b) A constant current source for providing a stabilized current to the aforementioned LED. A constant current source comprises means for keeping its output current within a given (small) interval, i.e. (approximately) constant, even it its external load varies largely. The value of the output current can typically be chosen from a given operating range, wherein this value is referred to as the "stabilized current" in the context of this application. Suitable circuits for realizing a constant current source are well known to a person skilled in the art and may be found in respective textbooks.

c) A voltage sensor for sensing the forward voltage across the above mentioned LED. This sensed forward voltage depends on the applied (stabilized) current and the internal resistance of the LED.

d) A light detector for providing a signal, which is called "raw measurement signal" in the following, that is related to the intensity of a light beam coming from the investigation region. The aforementioned light beam will be called

"output light beam" in the following, and it is usually directly or indirectly generated by the input light beam. The light detector may comprise any suitable sensor or plurality of sensors by which light of a given spectrum can be detected, for example a photodiode, a photo resistor, a photocell, or a photo multiplier tube. Moreover, the raw measurement signal may be a single value, or it may be a multidimensional vector. In the latter case, any statement made for "the raw measurement signal" applies to each of its components.

e) A "correction unit" for determining a value that is called

"corrected measurement signal" in the following and that is calculated based on the raw measurement signal and the sensed forward voltage. The correction unit is typically realized by dedicated electronic hardware, digital data processing hardware with associated software, or a mixture thereof. The invention further relates to a method for executing with a sensor device - particularly a sensor device of the kind described above - optical examinations in an investigation region of carrier, said method comprising the following steps:

a) Providing an LED with a stabilized current to make it emit an input light beam towards the investigation region.

b) Sensing the forward voltage across the LED.

c) Generating with a light detector a raw measurement signal that is related to the intensity of an output light beam coming from the investigation region.

d) Determining a corrected measurement signal from the raw measurement signal and the sensed forward voltage.

The described sensor device and the method have the advantage that they allow for robust optical examinations with high accuracy while requiring only comparatively simple means which can be realized very cost effectively. In particular, it is neither tried to keep the light output of the LED perfectly constant by a feedback loop (comprising e.g. photosensors or temperature sensors), nor is the light output modulated and the measurement signal demodulated. Instead, the proposed sensor device and method operate the LED with a stabilized current, which can readily be achieved by a constant current source. Fluctuations of the light output of the LED, which may particularly be generated by a variation of the operating temperature, are an inevitable consequence of this approach. However, these fluctuations are dealt with by sensing the forward voltage across the LED and using this value for a correction of the raw measurement signal. Furthermore, the proposed approach achieves a high accuracy due to the fact that the sensed forward voltage is usually indicative of the LED die temperature, which temperature has an important influence but cannot easily be measured using an external temperature sensor.

In the following, various preferred embodiments of the invention will be described that relate both to the sensor device and the method described above.

According to a first preferred embodiment of the invention, the corrected measurement signal may correspond to the raw measurement signal that was normalized with the sensed forward voltage or a value derived therefrom. As will be explained in more detail with respect to the Figures, the sensed forward voltage relates to the intensity of the input light beam if the LED is operated with a stabilized current.

Normalization with respect to the sensed forward voltage hence corresponds to a normalization of the raw measurement signal with respect the intensity of the input light beam, thus making the measurements independent of variations in the light source.

The determination of the corrected measurement signal from the raw measurement signal and the sensed forward voltage will usually be based on one or more given parameters. At least one such parameter is preferably determined or adjusted in a calibration procedure, said procedure comprising measurements under invariant (constant) conditions at the investigation region but under temperature fluctuations at the LED. Under such circumstances, the raw measurement signal will vary according to the temperature fluctuations, while the corrected measurement signal should ideally be constant. The parameter(s) of the correction procedure can hence be optimized in such a way that the corrected measurement signal best matches its ideal behavior.

The light detector may preferably comprise a plurality of sensor elements, wherein each sensor element provides a component of the (multidimensional) raw measurement signal. In this case a plurality of measurements can be made simultaneously.

In the aforementioned embodiment, the sensor elements are preferably arranged in a (one- or two-dimensional) array onto which the investigation region is imaged. With other words, the light detector comprises an image sensor in which each sensor element or "pixel" is associated to a part of the output light beam coming from a particular spatial position of the investigation region. The image sensor may preferably be realized by a CCD or a CMOS chip as it is for example known from digital cameras. Using an image sensor as a light detector has the advantage that an extended

investigation region can be monitored in a spatially resolved way, allowing for example to evaluate different spots in the investigation region in parallel.

According to a preferred embodiment of the invention, the sensor device comprises a control unit by which it is controlled such that a raw measurement signal is (only) generated during an exposure time, wherein such exposure times follow in an intermittent sequence. Such a discontinuous generation of raw measurement signals is typical for the above mentioned image sensors, which are preferably realized by a camera in which an integrated control unit controls the opening and closing of a shutter in front of the pixel array. In a further development of the aforementioned embodiment, the sensor device comprises a synchronization circuit by which the activity of the LED is synchronized with the activity of the light detector. In particular, the synchronization may be such that the stabilized current is provided to the LED only during the measuring ("shutter-open") times. This approach helps to avoid an unnecessary activity of the LED during times when no raw measurement signals are generated. This saves energy, which may be an important issue in portable sensor devices operated with a battery, and reduces thermal disturbances by limiting the power dissipation of the LED.

The sensor device may optionally comprise a memory for temporarily storing the sensed forward voltage and/or values derived therefrom. Such a memory may for example be useful in the aforementioned embodiment to bridge times of inactivity.

The sensor device may optionally comprise an analog-to-digital converter (ADC) for converting analog signals into digital signals for further processing. With other words, at least a part of the data processing is done digitally, which provides a high DC stability and avoids typical inaccuracies that can occur in analog processing hardware. Most preferably, the ADC is applied as a voltage sensor for sensing (and digitizing) the forward voltage across the LED.

The described sensor device can be applied in a variety of setups and apparatuses. In a particular example, the output light beam that is detected by the light detector comprises light of the input light beam that was totally internally reflected in the investigation region. To this end, the investigation region must comprise an interface between two media, e.g. glass and water, at which total internal reflection (TIR) can take place if the incident light beam hits the interface at an appropriate angle (larger than the associated critical angle of TIR). Such a setup is often used to examine small volumes of a sample at the TIR-interface which are reached by exponentially decaying evanescent waves of the totally internally reflected beam. Target components - e.g. atoms, ions, (bio-)molecules, cells, viruses, or fractions of cells or viruses, tissue extract, etc. - that are present in the investigation region can then scatter the light of the evanescent waves which will accordingly miss in the reflected light beam. In this scenario of a "frustrated total internal reflection", the output light beam of the sensor device will consist of the reflected light of the input light beam, wherein the small amount of light missing due to scattering of evanescent waves contains the desired information about the target components in the investigation region. Thus the signal one is interested in (missing light) is very small in comparison to a large base signal, making accurate measurements difficult. The proposed correction of raw measurement signals helps in this situation to make the results more accurate.

The invention further relates to the use of the sensor device described above for molecular diagnostics, biological sample analysis, or chemical sample analysis, food analysis, and/or forensic analysis. Molecular diagnostics may for example be accomplished with the help of magnetic beads or fluorescent particles that are directly or indirectly attached to target molecules.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. These

embodiments will be described by way of example with the help of the accompanying drawings in which:

Fig. 1 chematically illustrates an embodiment of a sensor device

according to the present invention;

Fig. 2 illustrates a modification of the sensor device of Figure 1 in which the activities of the LED and the light detector are synchronized;

Fig. 3 shows experimental data obtained with a sensor device

according to the present invention during the temperature step; Fig. 4 shows an exemplary layout of a constant current source.

Like reference numbers in the Figures refer to identical or similar components.

DESCRIPTION OF PREFERRED EMBODIMENTS

Though the present invention will in the following be described with respect to a particular setup (using magnetic particles and frustrated total internal reflection as measurement principle), it is not limited to such an approach and can favorably be used in many different applications and setups. The sensor device 100 shown in Figures 1 and 2 comprises a light source 20 for emitting an "input light beam" LI, a light detector 30 for detecting and measuring an "output light beam" L2, and an evaluation unit 60 to which said two components are coupled via correction unit 50. As is only schematically indicated in the Figures, the input light beam LI is emitted into a (disposable) carrier 5 that may for example be made from glass or transparent plastic like poly-styrene. The carrier 5 is located next to a sample chamber 2 in which a sample fluid with target components to be detected (e.g. drugs, antibodies, DNA, etc.) can be provided. The sample further comprises magnetic particles 1 , for example superparamagnetic beads, wherein these particles 1 are usually bound as labels to the aforementioned target components (for simplicity only the magnetic particles 1 are shown in the Figures). It should be noted that instead of magnetic particles other label particles, for example electrically charged of fluorescent particles, could be used as well.

The interface between the carrier 5 and the sample chamber 2 is formed by a surface called "binding surface" 4. This binding surface 4 may optionally be coated with capture elements, e.g. antibodies, which can specifically bind the target

components.

The sensor device 100 optionally comprises a magnetic field generator, for example an electromagnet 6 with a coil and a core, for controllably generating a magnetic field at the binding surface 4 and in the adjacent space of the sample chamber 2. With the help of this magnetic field, the magnetic particles 1 can be manipulated, i.e. be magnetized and particularly be moved (if magnetic fields with gradients are used). Thus it is for example possible to attract magnetic particles 1 to the binding surface 4 in order to accelerate the binding of the associated target component to said surface.

The light source 20 comprises an LED 22, e.g. a red 650 nm LED, that generates the input light beam LI which is transmitted into the carrier 5. The input light beam LI arrives at the binding surface 4 at an angle larger than the critical angle of total internal reflection (TIR) and is therefore totally internally reflected as the output light beam L2. The output light beam L2 leaves the carrier 5 through another surface and is detected by the light detector, e. g. by the light-sensitive pixels of a camera 30. The light detector 30 thus determines a "raw measurement signal" S_pi_x corresponding to a multidimensional vector whose components represent the amount of light of the output light beam L2 at a particular pixel position of the imaged binding surface (e.g.

expressed by the light intensity of this light in the whole spectrum or a certain part of the spectrum). The raw measurement signal is further processed in a correction unit 50 that is coupled to the output of the light detector 30 and that will be described in more detail below.

It is optionally possible to use the detector 30 (or a separate detector) for detecting fluorescence light emitted by fluorescent particles 1 which were stimulated by the evanescent wave of the input light beam LI .

The described sensor device 100 applies optical means for the detection of magnetic particles 1 and the target components one is actually interested in. For eliminating or at least minimizing the influence of background (e.g. of the sample fluid, such as saliva, blood, etc.), the detection technique should be surface-specific. As indicated above, this is achieved by using the principle of frustrated total internal reflection. This principle is based on the fact that an evanescent wave propagates (exponentially dropping) into the sample 2 when the incident light beam LI is totally internally reflected. If this evanescent wave then interacts with another medium having a different refractive index from water like the magnetic particles 1 , part of the input light will be coupled into the sample fluid (this is called "frustrated total internal reflection"), and the reflected intensity will be reduced (while the reflected intensity will be 100% for a clean interface and no interaction). Further details of this procedure may be found in the WO 2008/155723 Al, which is incorporated into the present text by reference. It should be noted that the scattering of evanescent light waves may also be used for other purposes, for example to estimate the distance of (e.g. single) magnetic particles from the refraction surface.

In order to make the measurements more robust against various disturbances, it is possible that one dedicated area on the carrier 5 is modified such that magnetic beads 1 cannot reach the evanescent field and the total internal reflection is not frustrated, thereby keeping the detected signal at its maximum, i.e. reference, level. This "True- White-Reference" (TWR) may be used to suppress (by normalization) common-mode intensity-variations in each of the binding spots induced by e.g. the light source or camera. This approach is based on the assumption that the TWR intensity correlates with the observed intensity in the binding spots. In the spatial frequency domain, said normalization suppresses low spatial frequencies (common mode disturbances) or in other words: it acts as a spatial High Pass Filter (HPF). The cut-off frequency is determined by the distance between the TWR and the spot.

However, when the described sensor device 100 is used to detect very low concentrations of a target substance, for example 1 pM, the detected signal is a sub- percent (0.1 %) darkening of the reflected light due to magnetic beads immobilized at the binding surface 4. In the low concentration regime (below 10 pM) there is hence the problem that the instability of the normalized background intensity amply exceeds the desired signal originating from the binding spot.

It was recognized by the inventor that output fluctuations of the LED 22 as a function of its temperature is a major problem, which is only for a small part solved by use of a TWR. As a result this appears as a strong drift when the LED 22 is switched on, and as random drifts/fluctuations due to environmental- and dissipation related temperature changes.

Possible approaches to reduce such drifts on beforehand comprise stabilizing the LED output based on temperature control, on generating 650 nm light by fluorescence from e.g. blue LEDs which are more stable, and on the use of a monitor photodiode. However, these solutions increase largely the complexity and the price- level of the system.

It is therefore desirable to solve the mentioned problems without adding optical and mechanical complexity to the setup. The generic idea that is proposed here to achieve this goal comprises:

using a constant current source during activation of the LED; measuring the forward voltage during activation of the LED; normalizing the measured intensity by said forward voltage; and optionally limiting the power dissipation by activating the LED only when the camera shutter is open.

In order to realize the aforementioned general concepts, the embodiment shown in Figure 1 comprises the following components:

A constant current source 21 which operates the LED 22 by providing it with a stabilized forward current Ip. Preferably, an extremely stable current source 21 is used for this purpose.

A voltage sensor for sensing the forward voltage VF across the LED 22. The voltage sensor is here realized by an analog-to-digital converter ADC 40. A correction unit 50 that receives the (digitized) measured forward voltage V_F from the ADC 40 and the raw measurement signals, i.e. the individual pixel values S_pi_x, from the camera 30. From these data, the correction unit 50 calculates normalized pixel values S_pi_{x nrm} as will be described in more detail below, and provides them to the evaluation unit 60 for further processing.

The calculations that are done in the correction unit 50 will in the following be explained with respect to the example of an LXHL-BD03 LED. From the specifications of this particular LED, it can be found that the forward voltage change AV_F at constant current I_F = 350mA as a function of the junction temperature fluctuation AT_S changes according to

AV_F

-2.0 mV/°C

AT_r

It can be derived that the relative power ΔΡ/Ρ at constant current I_F as a function of temperature fluctuates behaves in first order according to

API P

- = -1.15 %/°C

By combining the above two formulas, a factor a can be derived as

a = = 0.575 %/mV = 5.75 V^"1

AV_F

As a result of the power variation in the LED, the camera pixel signal S changes with a factor (1 + API P). This can be corrected by normalizing the camera signal S_pi_x according to This is illustrated in Figure 1 by a block 51 of the correction unit 50 that determines the factor 1/(1+CCAV_F) from the forward voltage V_F and a given value for a, and by a multiplication node 52.

The factor a may be fine tuned experimentally by minimizing the signal drift of the corrected light intensity as a function of an unknown temperature deviation with respect to the nominal condition. This may for example be done as a calibration step under random (e.g. overnight) or on purpose applied temperature fluctuations.

Experiments showed that in a FTIR setup where the LED 22 is excited at 80 mA instead of 350 niA, a = 1.46 V^"1 is a pretty accurate value. Due to the low thermal dissipation in the LED 22 at these low currents, the junction voltage AV_F can be used as a temperature sensor, as temperature is a critical parameter in low concentration measurements (e.g. cardiac assays).

In general, the high accuracy that can be achieved by the proposed method is largely due to the fact that the forward voltage V_F is indicative of the LED die temperature, which temperature cannot easily be measured using an external temperature sensor. Moreover, when using the LED as a temperature sensor, the outcome may be used to correct for other (non-LED related) temperature effects, like the biochemical process or drift in the camera.

The accuracy may further be improved by taking the factor a as a linear function of the temperature change ATj, assuming that

AV

= -2.0 mV/°C is a constant given by the physics of the semiconductor material. A linear fitting of calibration data shows that

a = ( _j +a₂AV_F provides good results if

cci = 1.3648 and a₂ = -2.8/V.

As a result, the formula of the normalized intensity changes to s 1 s 1

' Pixel , norm ' Pixel Pixel

1 + AV F, 1 + 1.3648ΔΓ, - 2.8ΔΓ F,

During operation of the sensor device 100, the forward voltage V_F(0) is measured at time t = 0 by the ADC 40. Next, deviations V_F = V_F (t) - V_F (0) are used to normalize the camera signal S_pi_x. When the LED 22 is continuously excited, a cold start typically induces large output fluctuations due to heating up the LED die, followed by more random fluctuations due to dissipation in camera and actuation coils and due to environmental changes.

Figure 2 illustrates a modification of the sensor device 100 of Figure 1, in which the activities of the LED 22 and the camera 30 are synchronized. To this end, a control unit 31 that controls the opening and closing of the shutter (not shown) in the camera 30 is coupled to a first switch SI between the constant current source 21 and the LED 22. This switch SI is closed (i.e. the LED 22 is excited) only during the exposure times during which the shutter of the camera 30 is open. This typically corresponds to only a few per cent of time. If the LED is for example operated at 80 mA during 1 ms shutter time and 1 frame per second, a 1000-fold reduction of the dissipation is achieved which consequentially shortens the warm-up time of the LED.

During the excitation time of the LED 22, the forward voltage V_F is measured. Preferably, a large number of samples are taken and averaged during this time in order to gain quantization ADC accuracy. When the shutter of the camera 30 is closed, the previous obtained normalization factor is "hold", which is conceptually illustrated in Figure 2 by a switch S2 and a memory M.

Figure 3 depicts measurement data that illustrate the operation of the described sensor device 100. The data refer to an experiment in which a temperature step of 20° C was applied to the LED of a sensor device by external heating.

The upper diagram shows the resulting temperature T of the LED over time t as calculated from the measured forward voltage variation AV_F and the specification of the LED. The lower diagram shows on a corresponding timescale the raw measurement signal S_pi_x for one monitored spot in the investigation region and the corrected measurement signal S_pi_{x nrm} obtained with the procedure described above. The experiments prove that suppression of temperature dependent illumination errors can be achieved by the described approach with little hardware and cost effort.

Figure 4 illustrates a possible circuit layout of a constant current source 21 for operating the LED 22. The shown example is particularly suited for a use in the sensor device of Figure 2, as it provides a terminal at which a shutter signal XI from the camera (not shown) can be received, said signal XI switching the current to the LED 22 on and off (off if a high voltage is on XI).

As described above, the forward voltage V_F across the LED 22 is sensed by a voltage sensor (e.g. the differential ADC 40 of Figures 1 or 2).

Finally it is pointed out that in the present application the term

"comprising" does not exclude other elements or steps, that "a" or "an" does not exclude a plurality, and that a single processor or other unit may fulfill the functions of several means. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Moreover, reference signs in the claims shall not be construed as limiting their scope.

Claims

CLAIMS:

1. A sensor device (100) for optical examinations in an investigation region (3) of a carrier (5), comprising

a) an LED (22) for emitting an input light beam (LI) towards the investigation region (3);

b) a constant current source (21) for providing a stabilized

current (I_F) to the LED (22);

c) a voltage sensor (40) for sensing the forward voltage (V_F) across the LED (22);

d) a light detector (30) for providing a raw measurement

signal (S_pi_x) that is related to the intensity of an output light beam (L2) coming from the investigation region (3);

e) a correction unit (50) for determining a corrected measurement signal (S_pi_{x nrm}) from the raw measurement signal (S_pi_x) and the sensed forward voltage (V_F).

2. A method for executing with a sensor device (100) optical examinations in an investigation region (3) of a carrier (5), comprising:

a) providing an LED (22) with a stabilized current (I_F) to make it emit an input light beam (LI) towards the investigation region (3);

b) sensing the forward voltage (V_F) across the LED (22);

c) generating with a light detector (30) a raw measurement

d) determining a corrected measurement signal (S_pi_{x nrm}) from the raw measurement signal (S_pi_x) and the sensed forward voltage (V_F).

3. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the corrected measurement signal (S_pi_{x nrm}) corresponds to the raw measurement signal (S_pi_x) normalized with the sensed forward voltage (V_F) or a value derived therefrom.

4. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that at least one parameter (a) that is used to determine the corrected measurement signal (S_pi_{x nrm}) is adjusted from measurements at an invariant investigation region (3) under temperature fluctuations at the LED (22).

5. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the light detector (30) comprises a plurality of sensor elements that provide the components of a multidimensional raw measurement signal (S_pix).

6. The sensor device (100) or the method according to claim 5, characterized in that light detector (30) comprises an image sensor, particularly as CCD or CMOS device.

7. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the sensor device (100) comprises a control unit (31) for controlling it such that raw measurement signals (S_pi_x) are generated during an intermittent sequence of exposure times.

8. The sensor device (100) or the method according to claim 7, characterized in that the sensor device (100) comprises a synchronization circuit (S 1 , S2) by which the activity of the LED (22) is synchronized with the activity of the light detector (30).

9. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the sensor device (100) comprises a memory (M) for storing the sensed forward voltage (V_F) and/or values derived therefrom.

10. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the sensor device (100) comprises an analog-to- digital converter (40) for processing the sensed forward voltage (V_F).

11. The sensor device (100) according to claim 1 or the method according to claim 2,

characterized in that the output light beam (L2) comprises light of the input light beam (LI) that was totally internally reflected in the investigation region (3).

12. Use of the sensor device (100) according to any of the claims 1 to 10 for molecular diagnostics, biological sample analysis, or chemical sample analysis.