# Patent

## Title
Device and method for detecting infrared radiation through a resistive bolometer matrix

## Abstract

A device for detecting infrared radiation including a matrix of resistive imaging bolometers above a substrate, a read circuit forming an analog electrical signal constituting an image of the infrared radiation impinging on the matrix, a probe for measuring the temperature of the substrate, an ADC for converting the analogue electrical signals from the read circuit and probe, and a data processing unit for correcting the signal formed by the read circuit according digital values NC(i,j) corresponding to electrical signals from the bolometers exposed to a uniform scene at the measured temperature of the substrate. The data processing unit including a unit for computing the digital values NC(i,j) based on a single set of parameters of a predetermined physical model of the read circuit electrical signals and corresponding to the exposure of the matrix of imaging bolometers to a temperature substantially equal to the measured substrate temperature.

## Background
FIELD OF THE INVENTION
The invention relates to the field of infrared imaging and bolometric pyrometry.
To be more specific, the invention relates to the field of the correction of a spatial non-uniformity in the response from a resistive bolometer detection matrix.
BACKGROUND OF THE INVENTION
In the field of infrared detectors, there is a known technique of using devices arranged in a matrix form that are able to operate at ambient temperature, in other words they do not need to be cooled to very low temperatures, unlike the detection devices known as “quantum detectors”, which need to operate at a very low temperature, typically that of liquid nitrogen.
These non-cooled detectors conventionally use the variation of a physical variable of an appropriate material, as a function of the temperature, in the vicinity of 300K. In the case of bolometric detectors, this physical variable is the electrical resistivity of the material.
A non-cooled detector of this type generally associates:

    
    
        means for absorbing the infrared radiation and converting the same into heat;
        means for thermally insulating the detector, so as to allow it to warm up under the action of the infrared radiation;
        thermometry means which, in the context of a bolometric detector, employ a resistive element;
        and means for reading the electric variables supplied by the thermometry means.
    
    


Detectors intended for infrared imaging are produced conventionally in the form of a matrix of elementary bolometric detectors, or bolometers, arranged according to one or two dimensions, the matrix being suspended above a substrate, generally made out of silicon, via support arms for each elementary detector.
Provision is commonly made in the substrate for means for the sequential addressing of the elementary detectors and means for the electrical excitation and pre-processing of the electrical signals generated by these elementary detectors. These sequential addressing, electrical excitation and pre-processing means are therefore formed in the substrate and constitute a read circuit.
To obtain the image of a scene through the medium of this detector, the scene is projected through an appropriate optic onto the matrix of elementary detectors, and timed electrical stimuli are applied through the medium of the read circuit to each of the elementary detectors, or to each row of the detectors, in order to obtain an electrical signal constituting the image of the temperature reached by each of the elementary detectors. This signal is processed in a more or less sophisticated way by the read circuit, and then possibly by an electronic device external to the enclosure in order to generate the thermal image of the scene observed.
Such a detector has many advantages in terms of manufacturing cost and use but also drawbacks which restrict its performance.
To be more specific, a problem is posed of the uniformity of the signal formation by the bolometer matrix. Indeed, owing to a dispersion in the behavior of the bolometers, which do not all produce the same output level when they are brought to one and the same operating temperature, and which do not all respond exactly in the same way in the face of one and the same infrared radiation coming from the scene, the image of a uniform scene formed by the detector has a fixed pattern noise.
The dispersion can have a great many causes. We may cite the main cause as being technological dispersion of the bolometers which translates into resistance dispersion, entailing for the detectors a variation in their output levels, even when the matrix is fed by a uniform scene. Another cause of signal dispersion is thermal drift in the substrate and in its spatial temperature distribution, given that bolometers keep to the temperature of the substrate on which they are supported.
Commonly, the shift in the output level of one specific bolometer relative to the average output level of the bolometers in the matrix when it is fed by a uniforms scene, is denoted by the term “offset”. The term “Continuous Level (Niveau Continu)”, or NC in the interests of simplification, is used hereinafter to denote all the output levels in these particular uniform feed conditions.
To compensate for bolometer offset dispersion, which is the main cause of limitation in the quality of the signals supplied by the detector, many correction techniques have been perfected.
One first type of offset correction, as described for example in document US 2002/022938, comprises acquiring a reference image, namely that of a uniform scene. This reference image is then stored in the system (a term used here to mean all the electromechanical and software functions implementing the detector or sensor), and then subtracted, digitally or analogically, from each image acquired thereby. The reference image is commonly farmed by means of an essentially isothermic shutter, which is closed in order to obtain the uniform scene.
This first technique has the drawback of rendering the detector inoperative throughout the entire reference image acquisition time. Furthermore, installing a shutter involves a not inconsiderable additional cost and increases the sources of mechanical failure in the system and the energy consumption thereof.
The second type of offset dispersion correction is based on the fact that the offset of a resistive bolometer depends on the temperature thereof. In this second type of bolometer correction, as described for example in the document U.S. Pat. No. 5 811 808, offset tables in respect of different predetermined temperatures of the sensor are stored permanently in the system. The system is provided with a thermometric sensor that measures the temperature of the substrate, and a data processing unit then selects one of the stored offset tables as a function of the measured temperature, or creates a new offset table by interpolating stored tables as a function of the measured temperature. The offset table selected or created is then subtracted from the current image detected.
This type of correction does not then need a shutter but does however prove to be less effective than the first correction type. Furthermore, the higher the number of reference points and the degree of the interpolating polynomial the greater the accuracy of the correction by interpolation. In fact, quality interpolation requires significant quantities of calculation resources, and storage of a sufficient number of tables. Moreover, the offset table acquisition time is significant. Lastly and above all, by virtue of its principle, an interpolation is only valid with accuracy in the vicinity of the reference points. The manufacturing cost of a detector implementing the second technique thus becomes prohibitive as soon as accurate sampling is required of the operating temperature range of the detector.
The purpose of the invention is to resolve the aforementioned problems, by proposing an effective and accurate correction technique, which does not require a shutter, while using a limited quantity of tables.
SUMMARY OF THE INVENTION
To this end, the object of the invention is an infrared radiation detection device that comprises a substrate, a matrix of at least one line of elements for detecting the radiation, each comprising a resistive imaging bolometer, the matrix being formed above the substrate, the device comprising means for measuring the temperature in at least one point of the substrate (known as the focal plane temperature, or FPT), and means for correcting the signal formed from each bolometer as a function of the temperature measured in at least one point of the substrate.
According to the invention, the correcting means are capable of correcting the signal formed from the bolometer, by means of a predetermined physical model of the temperature behavior of the signal, and particularly by means of a predetermined physical model of the temperature behavior of the bolometer.
To advantage, the model is formulated in a compact way, in other words suitable for fast calculation without numerical resolution of differential equations.
The temperature of the resistive bolometer, and consequently the electrical resistance thereof, depend on the temperature of the scene observed whereof the infrared power is projected onto the bolometer matrix via the optic and is picked up by each bolometer, but above all on the focal plane temperature. Since bolometers are thermal detection elements, they are suspended above the substrate containing the read circuit by thermal insulation arms. Through thermal conduction, the focal plane temperature has a direct effect upon the temperature of the bolometer. It may be assumed as a general idea that the contribution of the focal plane to the temperature of the bolometer is about 99% as against about 1% for the scene temperature. Bolometer output level behavior, whatever the scene temperature, therefore mainly depends on the focal plane temperature. The focal plane temperature therefore sets the temperature operating point of the bolometers. The resistance dispersions of the bolometers also vary as a function of the focal plane temperature. The change in resistance of a specific bolometer may be estimated as a function of the focal plane temperature, and consequently also the change in the dispersion (since each can be known). Faithfully modelling the temperature behavior of the signal formed from the bolometer, and in particular modelling the temperature behavior of the bolometer, therefore ensures correction validity over time when changes occur in the focal plane temperature.
Bolometer signal “behavior” is the term used here to define the repeatable correspondence between the focal plane temperature, which constitutes the thermal quiescent operating point of the bolometer, and the signal formation process outcome, which depends on the various physical parameters of the elements involved in developing the signal. The most important of these parameters is the electrical resistance of the bolometer formed on this substrate, for given conditions of IR brightness, at any focal plane temperature but other parameters may also play a part, such as for example the electrical resistance of a compensation structure.
Indeed, a so-called compensation structure is very widely used during signal formation, with the aim of producing a so-called common mode current that represents the imaging bolometer current part independent of the scene. This current is subtracted from the imaging bolometer current prior to signal formation, as will be explained in more detail subsequently. Typically, this structure is itself bolometric, and formed with the same material as the sensitive imaging structure. In this way, the current used to form the signal is already largely compensated by the variations induced by the fluctuations in the focal plane temperature, and essentially represents the thermal scene. The previous considerations with regard to the effect of the focal plane temperature on the signal and its dispersions remain very widely applicable all the same.
This correspondence between the focal plane temperature and the formed signal is preferably established by means of a “physical model” that expresses the behavior law of the signal formed as a function of the focal plane temperature in these reference optical conditions.
It will be noted that a physical model here denotes one or more mathematical relationships, such as a function or a system of differential equations for example, whereof the outputs are calculable whatever the value of its inputs from a single set of identified behavioral parameters. Preferentially, the aim of the invention is signal modelling in a “compact” analytical form which allows direct and fast calculation free from sophisticated resolution loops.
In other words, a physical model of the temperature behavior of the signal formed from a bolometer allows the signal offset to be determined as a function of the measured focal plane temperature, with no need to use a shutter or an interpolation of a plurality of tables.
Moreover, physical models use a limited number of parameters so that parameter identification is less costly than identifying the plurality of tables of the prior art.
According to the invention, the temperature measurement means comprise at least one probe for measuring the temperature of the substrate.
According to one particular embodiment of the invention, the temperature measurement means include a plurality of temperature measurement probes capable of measuring the temperature of the substrate in a plurality of points thereof and means for modelling the spatial distribution of the temperature of the substrate as a function of the measured temperature in the plurality of points of the substrate, and in that the correcting means are capable of correcting the signal formed from the bolometer as a function of the modelled temperature of the substrate in the vicinity thereof.
In other words, the substrate may be spatially non-uniform in tennis of its temperature. By modelling the spatial distribution of the substrate temperature, it is possible to determine the temperature thereof underneath each bolometer and therefore to increase correction accuracy.
To advantage, the correcting means are capable of estimating the continuous level (NC) corresponding to the exposure of the matrix to a uniform scene of temperature substantially equal to the measured focal plane temperature.
The correcting means are capable on the one hand of estimating the resistance of the bolometer according to the relation:

Rest(i,j)=Rabs(i,j)·C(Tmes)

where:

    
    
        Rest(i, j) is the estimated resistance of the bolometer,
        Rabs(i, j) is a predetermined reference resistance of the bolometer,
        C(Tmes) is a parameter dependent on the measured focal plane temperature Tmes expressed in degrees Kelvin.
    
    


Preferably, the parameter C(Tmes) is calculated according to the relation:




  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  




where:

    
    
        EA is a predetermined bolometer conductivity activation energy,
        k is the Bolzmann constant.
    
    


According to one particular inventive embodiment, the device comprises a common mode compensation structure that includes a compensating bolometer associated with each column of the matrix, the read means being capable of forming the signal from a difference between a current flowing in the imaging bolometer and a current flowing in the compensating bolometer associated therewith, and the correcting means are capable of estimating a resistance of the compensating bolometer, corresponding to the exposure of the matrix to a uniform scene of temperature substantially equal to the measured temperature, in order to correct the signal.
In particular, the correcting means are capable of estimating the resistance of the compensating bolometer according to the relation:

Rcomp-est(j)=Rcomp-abs(j)·C(Tmes)

where Rcomp-est(j) is the estimated resistance of the compensating bolometer, Rcomp-abs is a predetermined reference resistance of the compensating bolometer, and C(Tmes) is a parameter dependent on the measured temperature Tmes of the substrate. To be more specific, the parameter C(Tmes) is calculated according to the relation:





  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  




where EA is a predetermined imaging bolometer activation energy, k is the Bolzmann constant, and Tmes is the measured temperature expressed in degrees Kelvin.

According to one particular embodiment of the invention, the correcting means are capable of:

    
    
        estimating a continuous level of the signal formed from the imaging bolometer, corresponding to the exposure of the matrix to a uniform scene of temperature substantially equal to the measured temperature, according to the relation:

NC(i,j)=α−β×(Icomp(j)−Iac(i,j))

        where α and β are predetermined parameters dependent on the read circuit, Icomp(j) is the current flowing in the compensating bolometer calculated from the estimated resistance thereof, and Iac(i, j) is the current flowing in the imaging bolometer calculated from the estimated resistance thereof; and
        correcting the signal Nbrut(i, j) formed from the imaging bolometer according to the relation:

Ncorr(i,j)=Nbrut(i,j)−NC(i,j)+ NC

        where Ncorr(i, j) is the corrected output signal and NC is the mean value of the estimated continuous levels.
    
    


The correcting means are on the other hand capable of estimating the continuous signal level (NC) from this resistance estimation and from the configuration of the circuit responsible for establishing the signal. In particular, the correcting means are capable of estimating the resistance of the common mode compensation structures according to a formulation directly inspired from the previous relations, where typically in particular the same conductivity activation energy will be used. The modelled signal is established by expressing the difference in the currents produced by the imaging bolometer and by the compensating bolometer, as will be discussed below.
The correcting means are thus capable of correcting output levels, temperature behavior dispersions of the bolometers of the matrix.
To advantage, the correcting means are at least partially implemented in the substrate.
To advantage, the reference resistances of the bolometers of the matrix can be tabulated in a permanent memory built into the substrate.
According to one inventive alternative, the correcting means are implemented in the form of an electronic circuit built into the substrate.
A further object of the invention is a method for detecting infrared radiation by means of a matrix of at least one line of elements for detecting the radiation each comprising a resistive imaging bolometer, the matrix being formed above a substrate, the method comprising a step of reading the bolometers of the matrix, a step of measuring the temperature in at least one point of the substrate, and a step of correcting the signal formed from each bolometer as a function of the temperature measured in at least one point of the substrate.
According to the invention, the correction step comprises correcting the signal formed from the bolometer based on a predetermined physical model of the behavior of the signal according to the focal plane temperature.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from reading the following description, given solely by way of example and provided in relation to the appended drawings, wherein identical reference numbers are used to denote identical or similar elements, and wherein:
FIG. 1 is a diagrammatic view of an inventive bolometric detection device;
FIG. 2 is a diagrammatic view of a layout showing an imaging bolometer, a compensation circuit and a measurement circuit forming constituent parts of the device in FIG. 1;
FIG. 3 is a flow chart of the operation of the system in FIG. 1 according to a first embodiment; and
FIG. 4 is a flow chart of the operation of the system in FIG. 1 according to a second embodiment.



DETAILED DESCRIPTION OF THE INVENTION
A diagrammatic view has been shown in FIG. 1, using the general reference number 10, of a bolometric detector for acquiring a scene in thermal infrared radiation, the detector being incorporated into the focal plane of an optic of an infrared camera. Such a detector comprises:

    
    
        a two-dimensional imaging matrix 12 that includes n lines and m columns of resistive imaging bolometers 14 with suspended membrane, where n and m are integers greater than or equal to 1. The imaging matrix 12 is arranged in the focal plane of an infrared transparent optic of the camera and suspended above a reflective substrate;
        a read circuit 16 formed in the substrate. The read circuit 16 includes a circuit for the line-by-line addressing of the imaging matrix 12 and, for each column thereof, a measurement circuit capable of being connected to each imaging bolometer 14 of the addressed column; these measurement circuits are represented collectively by the block 18, which further includes means for multiplexing the signals towards the analog output or outputs;
        the read circuit 16 further comprises a line 20 of compensation circuits including one or more compensating bolometers in relation to each column of the imaging matrix 12;
        at least one temperature probe 22 formed in the substrate and measuring a temperature thereof; and
        a conversion circuit 24 connected to the read circuit 16 and to the temperature probe 22 in order to convert analog signals delivered thereby into digital signals.
    
    


The bolometric detector structure is conventional and will not be explained in further detail below. For further information, reference may usefully be made to the document “Uncooled amorphous silicon enhancement for 25 μm pixel pitch achievement” by E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820.
The bolometric detector 10 is connected furthermore to a data processing unit 26 that implements an algorithm for processing the signals delivered thereby so as to determine an infrared image of the scene projected on the matrix 12, and a compensation of a dispersion of the behavior of the imaging bolometers 14, as will be explained in more detail below.
The data processing unit 26 is used for example in a personal computer that comprises, in a known way, a central processing unit, or CPU 28, associated with a random-access memory 30, and with a bulk store 32, such as a hard disk for example. The unit 26 is furthermore connected to a display screen 34 for viewing the infrared images captured by the camera. Clearly much better integrated material embodiments, based on appropriate inter-communicating circuits as known in the prior art in the field, can be employed in exactly the same way.
In FIG. 2, a preferred elementary layout has been shown:

    
    
        of an imaging bolometer 14 of the matrix 12;
        of a measurement circuit 40 of the read circuit 16 for measuring the imaging bolometer 14; and
        of a compensation circuit 42 of the line 20 for the compensation of a common mode current flowing in the imaging bolometer 14 during the reading thereof.
    
    


The bolometer 14 is subjected to infrared radiation IR coming from a scene and is connected to a constant voltage VDET via a first terminal A.
The measurement circuit 40 includes:

    
    
        an operational amplifier 44 whereof one non-inverting terminal (+) is set at a predetermined constant voltage VBUS;
        a capacitor 46, of predetermined capacity Cint, connected between an inverting terminal (−) of the amplifier 44 and an output thereof;
        a reset switch 48 connected in parallel to the capacitor 46 and controllable by means of a “Reset” signal;
        a read switch 50, controllable by means of a Select signal, and connected to the inverting terminal (−) of the operational amplifier; and
        a first MOS injection transistor 52, whereof the gate is set at a predetermined constant voltage GFID, whereof the source is connected to a second terminal B of the bolometer 14, and whereof the drain is connected to the other terminal of the read switch 50.
    
    


The compensation circuit 42 of the common mode current passing through the imaging bolometer 14 includes a resistive compensating bolometer 54 made out of the same material as the imaging bolometer 14 with low thermal resistance towards the substrate and provided, optionally, with a screen 56 against the radiation coming from the scene.
It will be noted that the resistance of the compensating bolometer 54 varies insignificantly relative to that of the imaging bolometer 14, when it is subjected to an incident infrared flux, since the compensating bolometer 54 is insensitive to the infrared radiation from the scene. It may thus be considered that the resistance of the compensating bolometer is substantially constant at the given focal plane temperature.
The compensating bolometer 56 is connected at one of its terminals to a predetermined voltage VSK and at the other of its terminals to the source of a second MOS injection transistor 58 of the compensation circuit 42. The drain of the transistor 58 is connected to the inverting input of the operational amplifier 44 and the gate is connected to a predetermined voltage GSK.
At the start of a read cycle of the bolometer 14, the reset switch 48, which is in an on-state subsequent to a discharge cycle of the capacitor 46, is switched into an off-state by adjusting the “Reset” signal to an appropriate value. The read switch 50, which is an off-state, is switched into an on-state by adjusting the “Select” control. The electronic circuit performs an operation of integrating the difference between the current Iac=Vac/Rac passing through the bolometer 14 and the current Icomp=Vcomp/Rcomp passing through the compensating bolometer 54, whereof the outcome is expressed in the form of voltage at the terminals of the capacitor 46. The voltages Vac and Vcomp seen at the terminals of the active and compensating bolometers respectively are the result of the various supply and control voltages of the transistors 52 and 58, so as to define an appropriate operating point for the active bolometer on the one hand, and an effective common mode compensation of the compensating bolometer on the other hand, and there is no need to go any further into the detailed relationships that can be defined by those skilled in the art from the diagram in FIG. 2. When a predetermined integration time ΔTint has elapsed from the start of the read cycle, the read switch 50 is switched into its off-state.
The voltage or output level of the amplifier 44, which will be termed Nbrut, which contains the signal from the imaging bolometer 14 of resistance Rac, is expressed, assuming, in the interests of simplification, that Rac varies little during the integration time ΔTint, according to the relation:




  
    
      
        
          N
          brut
        
        =
        
          VBUS
          -
          
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  T
                  int
                
              
              
                C
                int
              
            
            ×
            
              (
              
                
                  
                    V
                    comp
                  
                  
                    R
                    comp
                  
                
                -
                
                  
                    V
                    ac
                  
                  
                    R
                    ac
                  
                
              
              )
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




where:

    
    
        Vac is the voltage at the terminals of the imaging bolometer 14;
        Rac is the resistance of the imaging bolometer 14;
        Vcomp is the voltage at the terminals of the compensating bolometer 54; and
        Rcomp is the resistance of the compensating bolometer 54.
    
    


The conversion circuit 24, which is connected to the amplifier 44, converts the analog voltage Nbrut into a digital value. The circuit 24 delivers this digital value to the data processing unit 26 which determines as a function thereof a corresponding image pixel value, as is well known.
The arrangement and operation of the elements that have just been described are conventional and will not be explained in further detail in the interests of conciseness. For further detail, reference may be made to the following documents:

    
    
        “Uncooled amorphous silicon enhancement for 25 μm pixel pitch achievement” by E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820;
        “320×240 uncooled microbolometer 2D array for radiometric and process control applications” by B. Fieque et al., Optical System Design Conference, SPIE, vol. 5251; et
        “Low cost amorphous silicon based 160×120 uncooled microbolometer 2D array for high volume applications” by C Trouilleau et al., Optical System Design Conference, SPIE, vol. 5251.
    
    


A description will now be given, in relation to the flow chart in FIG. 3, of a method for compensating the offsets of the imaging bolometers 14 of the matrix 12, implemented by means of the device that has just been described.
In a first initialization phase 60, implemented before the first use of the inventive detector, for example in the factory, a reference resistance is identified for each of the imaging bolometers 14 of the matrix 12.
This initialization phase 60 includes a step 62 during which the detector 10 is exposed to a uniform scene of constant temperature, while ensuring that the focal plane temperature of the detector, and therefore of the imaging bolometers 14 and of the substrate, is substantially equal to the scene temperature. At the same time, the temperature of the substrate is measured by the temperature probe 22 and the measurement delivered after digital conversion to the data processing unit 26, and stored under the parameter Tref.
In a following step 64, a specific matrix read cycle is implemented, wherein the common mode current compensation implemented by the compensation circuits 42 is deactivated.
For a specific imaging bolometer 14, only the current passing through it is therefore integrated by the associated measurement circuit 40. At the end of the read-out of this bolometer, the output voltage Vout of the amplifier 44 is therefore expressed according to the relation:




  
    
      
        
          V
          out
        
        =
        
          VBUS
          +
          
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  T
                  int
                
              
              
                C
                int
              
            
            ×
            
              
                V
                ac
              
              
                R
                ac
              
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




In the particular case of step 64, the voltage VFID is commonly brought to VDD. The series resistance of the transistor 52 is negligible and the following simplified expression may be supposed:

Vac≅VBUS−VDET

Preferably, the voltages VBUS and VDET are selected to be close to each other so as to prevent too significant a current from passing through the imaging bolometer and causing a thermal load that may be detrimental thereto by Joule effect.
It will be noted that in order to implement the read step 64, a shorter integration time ΔTint and/or a more significant capacity Cint, which will in this case have to be provided in the read circuit, may be selected so as to prevent saturation of the capacitor 46 or of the amplifier 44.
Once the read-out of the matrix 12 is complete, a reference resistance Rabs is calculated, in a following step 66, for each imaging bolometer 14 by the data processing unit 26 from a physical model of the temperature behavior of its resistance.
To be more specific, in 66, the data processing unit 26 calculates the reference resistance according to the relations:




  
    
      
        
          
            R
            abs
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              R
              ac
            
            ⁡
            
              (
              
                i
                ,
                j
              
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                -
                
                  
                    E
                    A
                  
                  
                    k
                    ·
                    
                      T
                      ref
                    
                  
                
              
              )
            
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          
            R
            ac
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              VBUS
              -
              VDET
            
            
              (
              
                
                  
                    V
                    out
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
                -
                VBUS
              
              )
            
          
          ×
          
            
              Δ
              ⁢
              
                
              
              ⁢
              
                T
                int
              
            
            
              C
              int
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




where:

    
    
        exp is the exponential function,
        EA is a predetermined bolometer activation energy,
        k is the Bolzmann constant,
        et Tref is the substrate temperature measured at step 60.
    
    


Once the initialization step 60 is complete, a two-dimensional table Rabs is then obtained of reference resistances for the imaging bolometers of the matrix 12. This table is then stored in the bulk memory 32 of the data processing unit 26.
Once the detector has left the factory and been commissioned by the user, the method continues in respect of the real-time correction of the offsets of the imaging matrix 12.
At system power-up, the reference resistance table is loaded, at 68, into the random-access memory 30 of the data processing unit 26. Still at 68, two two-dimensional tables of dimensions corresponding to those of the imaging matrix 12, namely a table Rest of estimated resistances and a table NC of output levels, are reset to zero and stored in the random-access memory 30.
In a following step 70, the matrix 12 is exposed to a scene to be observed. Then, or at the same time as step 70, the temperature of the substrate is measured at 72 by means of a probe 22 and the temperature value is stored as a parameter Tmes.
The method then continues with a conventional read step 74 (with “ébasage” i.e. common mode compensation) of the imaging bolometers 14 of the matrix 12, as has been previously described. The different measured voltages (or output levels) Nbrut associated with these bolometers are digitized and stored in the random-access memory 30.
A test is then conducted at 76 by the unit 26 to ascertain whether the absolute value of the difference between the values Tref and Tmes is greater than a predetermined threshold value, for example equal to 0.2 degrees Kelvin.
If the result of this test is positive, an estimated resistance Rest(i, j) is calculated at 78 for each bolometer of the matrix 12 by the unit 26. This estimated resistance of the bolometer is calculated as a function of the corresponding reference resistance Rabs(i, j) of the table Rabs, according to the relation:




  
    
      
        
          
            R
            est
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              R
              abs
            
            ⁡
            
              (
              
                i
                ,
                j
              
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                
                  E
                  A
                
                
                  k
                  ·
                  
                    T
                    mes
                  
                
              
              )
            
          
        
      
    
    
      
        (
        5
        )
      
    
  




The value Rest(i, j) then replaces the corresponding value of the table Rest.
Once step 78 is complete, an estimated resistance has therefore been calculated for each bolometer of the imaging matrix 12 and stored in the table Rest.
The method then continues with a step 80 for the calculation, in respect of each imaging bolometer of the table 12, of an output level in front of the uniform scene NC(i, j) by the unit 26 according to the relations:




  
    
      
        
          NC
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          VBUS
          -
          
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  T
                  int
                
              
              
                C
                int
              
            
            ⁢
            
              (
              
                
                  I
                  comp
                
                -
                
                  
                    I
                    ac
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
              
              )
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




Those skilled in the art are able to express Icomp when the MOS 58 is in saturation mode, from the technological parameters and the circuit configuration, according to the relation:




  
    
      
        
          I
          comp
        
        =
        
          
            
              
                
                  
                    -
                    
                      (
                      
                        1
                        +
                        
                          2
                          ·
                          
                            R
                            comp
                          
                          ·
                          
                            
                              A
                              p
                            
                            ⁡
                            
                              (
                              
                                VSK
                                -
                                GSK
                                -
                                
                                  V
                                  thp
                                
                              
                              )
                            
                          
                        
                      
                      )
                    
                  
                  -
                
              
            
            
              
                
                  
                    1
                    +
                    
                      4
                      ·
                      
                        R
                        comp
                      
                      ·
                      
                        
                          A
                          p
                        
                        ⁡
                        
                          (
                          
                            VSK
                            -
                            GSK
                            -
                            
                              V
                              thp
                            
                          
                          )
                        
                      
                    
                  
                
              
            
          
          
            
              -
              2
            
            ·
            
              A
              p
            
            ·
            
              R
              comp
              2
            
          
        
      
    
    
      
        (
        7
        )
      
    
  




On the other hand Iac is expressed, when the MOS 52 is in saturation mode, by the relation:




  
    
      
        
          
            I
            ac
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              
                
                  
                    -
                    
                      (
                      
                        1
                        +
                        
                          2
                          ·
                          
                            
                              R
                              est
                            
                            ⁡
                            
                              (
                              
                                i
                                ,
                                j
                              
                              )
                            
                          
                          ·
                          
                            
                              A
                              n
                            
                            ⁡
                            
                              (
                              
                                GFID
                                -
                                
                                  V
                                  thn
                                
                              
                              )
                            
                          
                        
                      
                      )
                    
                  
                  -
                
              
            
            
              
                
                  
                    1
                    +
                    
                      4
                      ·
                      
                        
                          R
                          est
                        
                        ⁡
                        
                          (
                          
                            i
                            ,
                            j
                          
                          )
                        
                      
                      ·
                      
                        
                          A
                          n
                        
                        ⁡
                        
                          (
                          
                            GFID
                            -
                            
                              V
                              thn
                            
                          
                          )
                        
                      
                    
                  
                
              
            
          
          
            
              -
              2
            
            ·
            
              A
              n
            
            ·
            
              
                R
                est
                2
              
              ⁡
              
                (
                
                  i
                  ,
                  j
                
                )
              
            
          
        
      
    
    
      
        (
        8
        )
      
    
  




where

    
    
        Ap and An are known technological factors of the MOS 58 and 52 respectively;
        Vthp and Vthn are the known threshold values of the MOS 58 and 52 respectively; and
        VFID and GSK being the voltages applied to the gates of the transistors 52 and 58 respectively, also known.
    
    


The newly calculated continuous levels NC(i, j) then replace the values of the table NC corresponding to the imaging bolometers. Once the step 80 is complete, a so-called continuous, uniform scene level has therefore been calculated for each bolometer of the imaging matrix 12 and stored in the table NC.
The step 80 is followed by a step 82 of correction by the unit 26 of the digitized output voltage Nbrut(i, j) of each imaging bolometer according to the relation:

Ncorr(i,j)=Nb(i,j)−NC(i,j)+ NC  (9)

where Ncorr(i, j) is the corrected digitized output voltage and NC is the mean value of the output voltages in front of a uniform scene calculated at step 80.

If the test applied at step 76 is negative, in other words if the substrate temperature has not varied significantly between two read cycles of the imaging matrix 12, step 76 loops to the correction step 82. The continuous level is then corrected on the basis of a matrix NC of continuous levels calculated during a previous cycle.
Step 82 then continues with the display, or more generally the use, at 84, of the corrected image of the scene captured by the matrix 12, and then at 86 of the adjustment of the temperature value Tref to the value Tmes. Step 86 then loops to the exposure step 70 in order to fulfil a new read cycle.
In the embodiment that has just been described, only the imaging bolometers 14 are taken into account as a source for the dispersion of the offsets of the output signals. Thus, as may be noted from reading the relation (6), the continuous levels NC(i, j) depend on a plurality of variables including in particular the current Iac(i,j) passing through the imaging bolometers 14. In so far as the main dispersion source comes from the non-uniform behavior of the imaging bolometers, the continuous levels NC(i,j) are therefore calculated as a function of the currents Iac(i, j).
However, as may be noted from reading the relation (6), the continuous levels NC(i,j) also depend on other variables, and in particular the current Icomp passing through the compensating bolometers 54. In the first embodiment described above, this current is considered to be identical for each of the compensating bolometers 54, which means that they offer uniform temperature behavior, or at the very least negligible dispersions relative to the dispersions of the imaging bolometers 14.
In fact, this is not necessarily the case. Like the imaging bolometers 14, the compensating bolometers 54 respond differently when they are subjected to identical environmental conditions, thereby inducing a dispersion in the currents Icomp passing through them, and thereby a dispersion in the continuous levels NC(i,j).
A description is now given in relation to FIG. 4 of an embodiment that corrects the offset dispersion in the output signals of the detector which also takes account of the compensating bolometers 54 as the source of the dispersion. The structure of the detector remains unchanged, with only the offset compensation method being modified.
The offset compensation method according to the second embodiment comprises steps 62 and 64 identical to those described previously.
At a following step 65, a second specific read cycle is implemented, wherein the current from the active bolometers is deactivated while leaving the selection switch 50 in the off state, while the gate of the transistor 58 is earthed so that its resistance is rendered negligible.
For a specific compensating bolometer 54, only the current passing through it is therefore integrated by the associated measurement circuit 40. At the end of the read-out of this bolometer, the output voltage Vout of the amplifier 44 is therefore expressed according to the relation:




  
    
      
        
          V
          out
        
        =
        
          VBUS
          -
          
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  T
                  int
                
              
              
                C
                int
              
            
            ×
            
              
                V
                comp
              
              
                R
                comp
              
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




With substantially:

Vcomp≅VSK−VBUS  (11)

It will be noted as previously that to implement the read step 65, the difference between the voltages VSK and VBUS at step 65, and/or the integration time ΔTint and/or a larger capacitor Cint, may be selected to avoid saturation of the amplifier 44.
Once the read-out of the matrix 12 and of the line 20 are complete, a reference resistance Rabs is calculated, at a following step 66, for each imaging 14 and compensating bolometer 54 by the data processing unit 26 from a physical model of the temperature behavior of its resistance.
To be more specific, at 66, the data processing unit 26 calculates the reference resistance according to the relations:




  
    
      
        
          
            R
            abs
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              R
              ac
            
            ⁡
            
              (
              
                i
                ,
                j
              
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                -
                
                  
                    E
                    A
                  
                  
                    k
                    ·
                    
                      T
                      ref
                    
                  
                
              
              )
            
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          
            R
            
              comp
              -
              abs
            
          
          ⁡
          
            (
            j
            )
          
        
        =
        
          
            
              R
              comp
            
            ⁡
            
              (
              j
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                -
                
                  
                    E
                    A
                  
                  
                    k
                    ·
                    
                      T
                      ref
                    
                  
                
              
              )
            
          
        
      
    
    
      
        (
        12
        )
      
    
  
  
    
      
        
          
            R
            ac
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              VBUS
              -
              VDET
            
            
              (
              
                
                  
                    V
                    out
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
                -
                VBUS
              
              )
            
          
          ×
          
            
              Δ
              ⁢
              
                
              
              ⁢
              
                T
                int
              
            
            
              C
              int
            
          
        
      
    
    
      
        (
        4
        )
      
    
  
  
    
      
        
          
            R
            comp
          
          ⁡
          
            (
            j
            )
          
        
        =
        
          
            
              VSK
              -
              VBUS
            
            
              (
              
                VBUS
                -
                
                  
                    V
                    out
                  
                  ⁡
                  
                    (
                    j
                    )
                  
                
              
              )
            
          
          ×
          
            
              Δ
              ⁢
              
                
              
              ⁢
              
                T
                int
              
            
            
              C
              int
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




where:

    
    
        exp is the exponential function,
        EA is a predetermined bolometer activation energy,
        k is the Bolzmann constant,
        et Tref is the substrate temperature measured at step 60.
    
    


Once the initialization step 60 is complete, a two-dimensional table Rabs of reference resistances is thus obtained for the imaging bolometers of the matrix 12 and for compensation of the line 20. This table is then stored in the bulk memory 32 of the data processing unit 26.
Once the detector has left the factory and been commissioned by the user, the method continues in respect of the real-time correction of the offsets of the imaging matrix 12.
At system power-up, the reference resistance table is loaded, at 68, into the random-access memory 30 of the data processing unit 26. Still at 68, two two-dimensional tables of dimensions corresponding to those of the imaging matrix 12, namely a table Rest of estimated resistances and a table NC of output levels, are reset to zero and stored in the random-access memory 30.
At a following step 70, the matrix 12 is exposed to a scene to be observed. Next, or at the same time as step 70, the substrate temperature is measured at 72 by means of a probe 22 and the temperature value is stored as a parameter Tmes.
The method then continues with a step 74 for the conventional read-out (with common mode compensation) of the imaging bolometers 14 of the matrix 12, as has been described previously. The different measured voltages (or output levels) Nbrut associated with these bolometers are digitized and stored in the random-access memory 30.
A test is then applied at 76 by the unit 26 to ascertain whether the absolute value of the difference between the values Tref and Tmes is greater than a predetermined threshold value, for example equal to 0.2 degrees Kelvin.
If the result of this test is positive, an estimated resistance Rest(i, j) is calculated at 78 for each bolometer of the matrix 12 and of the compensation line 20 by the unit 26. This estimated resistance of the bolometer is calculated as a function of the corresponding reference resistance Rabs(i, j) and Rcomp-abs of the table Rabs, according to the relations:




  
    
      
        
          
            R
            est
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              R
              abs
            
            ⁡
            
              (
              
                i
                ,
                j
              
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                
                  E
                  A
                
                
                  k
                  ·
                  
                    T
                    mes
                  
                
              
              )
            
          
        
      
    
    
      
        (
        5
        )
      
    
  
  
    
      
        
          
            R
            
              comp
              -
              est
            
          
          ⁡
          
            (
            j
            )
          
        
        =
        
          
            
              R
              
                comp
                -
                abs
              
            
            ⁡
            
              (
              j
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                
                  E
                  A
                
                
                  k
                  ·
                  
                    T
                    mes
                  
                
              
              )
            
          
        
      
    
    
      
        (
        14
        )
      
    
  




The value Rest(i, j) or Rcomp-est(j) then replaces the corresponding value of the table Rest.
Once step 78 is complete, an estimated resistance has therefore been calculated for each bolometer of the imaging matrix 12 and of the line of compensation circuits 20 and stored in the table Rest.
The method then continues with a step 80 for the calculation, for each imaging bolometer of the table 12, of an output level in front of the uniform scene NC(i, j) by the unit 26 according to the relations:




  
    
      
        
          NC
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          VBUS
          -
          
            
              
                Δ
                ⁢
                
                  
                
                ⁢
                
                  T
                  int
                
              
              
                C
                int
              
            
            ⁢
            
              (
              
                
                  
                    I
                    comp
                  
                  ⁡
                  
                    (
                    j
                    )
                  
                
                -
                
                  
                    I
                    ac
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
              
              )
            
          
        
      
    
    
      
        (
        15
        )
      
    
  




Those skilled in the art are able to express Icomp when the MOS 58 is in saturated mode, from the technological parameters and from the circuit configuration, according to the relation:




  
    
      
        
          
            I
            comp
          
          ⁡
          
            (
            j
            )
          
        
        =
        
          
            
              
                
                  
                    -
                    
                      (
                      
                        1
                        +
                        
                          2
                          ·
                          
                            
                              R
                              
                                comp
                                -
                                est
                              
                            
                            ⁡
                            
                              (
                              j
                              )
                            
                          
                          ·
                          
                            
                              A
                              p
                            
                            ⁡
                            
                              (
                              
                                VSK
                                -
                                GSK
                                -
                                
                                  V
                                  thp
                                
                              
                              )
                            
                          
                        
                      
                      )
                    
                  
                  -
                
              
            
            
              
                
                  
                    1
                    +
                    
                      4
                      ·
                      
                        
                          R
                          
                            comp
                            -
                            est
                          
                        
                        ⁡
                        
                          (
                          j
                          )
                        
                      
                      ·
                      
                        
                          A
                          p
                        
                        ⁡
                        
                          (
                          
                            VSK
                            -
                            GSK
                            -
                            
                              V
                              thp
                            
                          
                          )
                        
                      
                    
                  
                
              
            
          
          
            
              -
              2
            
            ·
            
              A
              p
            
            ·
            
              
                R
                
                  comp
                  -
                  est
                
                2
              
              ⁡
              
                (
                j
                )
              
            
          
        
      
    
    
      
        (
        16
        )
      
    
  




On the other hand Iac is expressed, when the MOS 52 is in saturated mode, by the relation:




  
    
      
        
          
            I
            ac
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              
                
                  
                    -
                    
                      (
                      
                        1
                        +
                        
                          2
                          ·
                          
                            
                              R
                              est
                            
                            ⁡
                            
                              (
                              
                                i
                                ,
                                j
                              
                              )
                            
                          
                          ·
                          
                            
                              A
                              n
                            
                            ⁡
                            
                              (
                              
                                GFID
                                -
                                
                                  V
                                  thn
                                
                              
                              )
                            
                          
                        
                      
                      )
                    
                  
                  -
                
              
            
            
              
                
                  
                    1
                    +
                    
                      4
                      ·
                      
                        
                          R
                          est
                        
                        ⁡
                        
                          (
                          
                            i
                            ,
                            j
                          
                          )
                        
                      
                      ·
                      
                        
                          A
                          n
                        
                        ⁡
                        
                          (
                          
                            GFID
                            -
                            
                              V
                              thn
                            
                          
                          )
                        
                      
                    
                  
                
              
            
          
          
            
              -
              2
            
            ·
            
              A
              n
            
            ·
            
              
                R
                est
                2
              
              ⁡
              
                (
                
                  i
                  ,
                  j
                
                )
              
            
          
        
      
    
    
      
        (
        8
        )
      
    
  




where

    
    
        Ap and An are known technological factors of the MOS 58 and 52 respectively;
        Vthp and Vthn are the known threshold voltages of the MOS 58 and 52 respectively; and
        VFID and GSK being the voltages applied to the gates of the transistors 52 and 58 respectively, also known.
    
    


The newly calculated continuous levels NC(i, j) then replace the values of the table NC corresponding to the imaging bolometers. Once step 80 is complete, a so-called continuous uniform scene level has therefore been calculated for each bolometer of the imaging matrix 12 and stored in the table NC.
Step 80 is followed by a step 82 for the correction by the unit 26 of the digitized output voltage Nbrut(i, j) of each imaging bolometer according to the relation:

Ncorr(i,j)=Nbrut(i,j)−NC(i,j)+ NC  (9)

where Ncorr(i, j) is the corrected digitized output voltage and NC is the mean value of the output voltages in front of a uniform scene calculated at step 80.

If the test applied at step 76 is negative, in other words if the substrate temperature has not varied significantly between two read cycles of the imaging matrix 12, step 76 loops to the correction step 82. The continuous level is then corrected on the basis of a matrix of continuous levels NC calculated during a previous cycle.
Step 82 then continues with the display, or more generally the use, at 84, of the corrected image of the scene captured by the matrix 12, and then at 86 of the adjustment of the value of the temperature Tref to the value Tmes. Step 86 then loops to the exposure step 70 to fulfil a new read cycle.
As it is possible to note, the inventive detection system uses the following concepts.
For continuous level correction or imaging bolometer offset, a physical model thereof according to the relation (5) is used. This model requires only a reference resistance table, and possibly additionally an activation energy table (which is very easy to acquire from the initial recording of the NC matrices at two sufficiently different focal plane temperatures, in spatially isothermic conditions), in order to know the behavior of the imaging matrix over the entire operating range in focal plane temperatures thereof.
Furthermore, from a physical model of the formation of the signal from the imaging bolometer, it is possible to predict the bolometer offset by simulating an illumination of the imaging matrix by a uniform scene. Indeed, in the case of such an illumination, commonly implemented by means of a mechanical shutter, the temperatures of each element of the matrix 12, of the compensation line 20, of the substrate and of the scene are substantially equal.
Thus, at each matrix read cycle, using a physical model of the signal formed from an imaging bolometer, such as the one in relation (6) detailed by relations (7) and (8), and associated with the bolometer-specific relation (5) (first embodiment), or such as the one in relation (15) detailed by relations (8) and (16), and associated with relations (5) and (14) (second embodiment), allows simulation of output continuous level behavior consequential upon an illumination by a uniform scene of temperature equal to the measured temperature of the substrate.
The signal formation model can be refined on the basis of relation (5) or relations (5) and (14), which specifically model bolometric resistance behavior, in order to take account of the well-known self-heating effect related to the polarization current of the bolometers 14 during the read-out, or even the residual self-heating of the compensating bolometers 54.
The simulated behavior is shown in the tables Rest an and NC. The corresponding signal continuous levels NC(i,j) output from the read circuits 40 if they were placed in front of a uniform scene at the focal plane temperature are given by the relation (6) (first embodiment) or (14) (second embodiment).
The corrected output level in front of any scene Ncorr(i, j) of a bolometer according to the relation (9) does therefore correspond to an output voltage thereof, wherein an offset Nbrut−NC(i, j) relative to a mean output voltage Nbrut of the matrix is cancelled.
Clearly other embodiments are possible.
Therefore to further refine the accuracy of the compensation of the offsets appearing in the output signals of the matrix 12, it may also be advantageous to model the dispersions occurring in the voltage VBUS delivered to the amplifiers 44, or again the dispersion of the capacities Cint thereof.
In the embodiments that have just been described, the activation energy EA, provided for example by the manufacturer of the imaging bolometers, is presumed to be identical for all imaging bolometers. This is justified in particular in the situation where the bolometric material used for the imaging bolometers is amorphous silicon. It is observed indeed in this event that dispersion among the activation energies of the bolometers is absolutely negligible and that there is therefore no need to take account thereof.
However, in specific uses that require increased accuracy or in the event of a bolometric material being used that demonstrates not insignificant activation energy dispersion from one bolometer to the next, an activation energy is ascertained at the initialization step 60 in respect of each imaging bolometer, if not also of each compensating bolometer, since the individual acquisition protocol previously mentioned permits it in the same way. These energies are then stored in the bulk memory 32 in the form of a table of dimensions corresponding to those of the imaging matrix (plus the compensation line 20) and is used during offset correction.
For example, the activation energy EA(i,j) of an imaging bolometer is determined at the initialization step 60 by twice implementing steps 62 and 64 with different focal plane temperatures, but in front of a uniform scene of identical temperature.
The data processing unit 26 then calculates the activation energy of the bolometer 14 according to the relations:




  
    
      
        
          
            E
            A
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          k
          ·
          
            
              (
              
                
                  T
                  
                    ref
                    ⁢
                    
                      
                    
                    ⁢
                    2
                  
                
                ·
                
                  T
                  
                    ref
                    ⁢
                    
                      
                    
                    ⁢
                    1
                  
                
              
              )
            
            
              (
              
                
                  T
                  
                    ref
                    ⁢
                    
                      
                    
                    ⁢
                    2
                  
                
                -
                
                  T
                  
                    ref
                    ⁢
                    
                      
                    
                    ⁢
                    1
                  
                
              
              )
            
          
          ·
          
            ln
            ⁡
            
              (
              
                
                  
                    R
                    1
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
                
                  
                    R
                    2
                  
                  ⁡
                  
                    (
                    
                      i
                      ,
                      j
                    
                    )
                  
                
              
              )
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




where Tref1 and Tref2 are the two predetermined focal plane temperatures, and R1(i,j) and R2(i,j) are the measured resistances of the bolometer (i,j) for these two temperatures respectively. The same type of application gives access to the activation energies of the common mode compensating bolometers, if necessary.

A device has been described wherein the temperature is measured by means of a single probe. As an alternative, a plurality of temperature probes are implanted into the detector substrate, and a temperature table, denoted Tmes(i, j) is established, from the signals provided by the plurality of temperature probes.
Relation (5) is then replaced by the relation:




  
    
      
        
          
            R
            est
          
          ⁡
          
            (
            
              i
              ,
              j
            
            )
          
        
        =
        
          
            
              R
              abs
            
            ⁡
            
              (
              
                i
                ,
                j
              
              )
            
          
          ·
          
            exp
            ⁡
            
              (
              
                
                  E
                  A
                
                
                  k
                  ·
                  
                    
                      T
                      mes
                    
                    ⁡
                    
                      (
                      
                        i
                        ,
                        j
                      
                      )
                    
                  
                
              
              )
            
          
        
      
    
    
      
        (
        
          5
          ⁢
          b
        
        )
      
    
  




It will be noted that unless a temperature probe is installed in proximity to each bolometer, which is a direct possibility in order to obtain the table Tmes(i, j), a highly acceptable result is obtained by means of a reduced number of probes, arranged for example on the periphery of the matrix. The table Tmes(i, j) is then established for example by means of a spatial thermal model which approximates the focal plane temperature distribution in the immediate vicinity of each bolometer, as a function of the spatial sampling provided by the limited plurality of temperature probes.
In such a case, the offset correction of an imaging bolometer takes the focal plane temperature spatial dispersion into account, including in the vicinity of the compensating bolometers, dispersions whereof the effect is eliminated by the digital correction established by means of the table Tmes(i, j). Correction accuracy is thus increased.
It will be observed in this respect that the imaging bolometers can therefore be considered individually in respect of correcting their offset in so far as the relation (9) remains unchanged for the reasons mentioned previously in respect of the value Nbrut
A detection device has been described wherein the data processing unit 26 and the parameter table or tables used for the physical model of the imaging bolometers are external to the camera which is for example infrared.
As an alternative, the data processing unit 26 is built into an enclosure of the camera. In this alternative, the bulk memory is replaced by a read-only memory for the storage of the parameter tables. The data processing unit 26 is then for example implemented in the form of an electronic card.
In another alternative, the parameter tables are recorded in a read-only or flash memory, directly formed in the substrate wherein the read circuit is defined furthermore.
In another alternative wherein the unit 26 is built into the camera, the conversion circuit 24 and the data processing unit 26 are implemented in the same substrate as the one carrying the read circuit furthermore.
Clearly other physical models than that in relation (5) or relations (5) and (14) can be used depending on the degree of accuracy required and on the processing power of the data processing unit 26, although the one in relation (5) is preferred in respect of its accuracy, its straightforwardness, and its relevance in the specific event of amorphous silicon being used. Likewise, other physical signal formation modellings, exemplified here by the elements 40 and 42, described through relations (6) and sub-relations of currents (7) and (8) (first embodiment) or through relations (15) and sub-relations (8) and (16) (second embodiment) can be used depending on the specific configuration and details of the signal formation circuitry.
The device that has just been described therefore has the following advantages:

    
    
        use of a physical model allows more accurate correction than that achieved from multiple table interpolation. Indeed, an interpolation, which conceals no physical reality, remains less accurate in modelling bolometer behavior than a physical model thereof, which reflects the reality of phenomena with a high degree of accuracy;
        use of a physical model makes it possible to take accurate account of the scene temperatures outside the range of temperatures of the tables for interpolation, in other words allows extrapolation;
        use of a physical model only requires a limited number of tables, or even requires only one table in the case of the model in the relation (5). Memory savings are thus made, which is advantageous in the event of the tables being stored in read-only or flash memories; and
        offset correction according to the invention requires no mechanical element such as a shutter for example. However, correction according to the invention simulates the use of a shutter which is commonly the most effective means of obtaining a uniform scene and therefore the most accurate means to determine the bolometer offsets;
        the volume of calculations to be processed in order to implement the invention is sufficiently small to allow a real-time implementation thereof on a present-day processor. The advantage of the invention is therefore that it offers an advantageous compromise between correction accuracy and algorithmic implementation compactness.
    
    


The invention can be used in the field of image or thermometric sensors with bolometric detection, whatsoever the detection frequency band or the type of bolometric material used to manufacture the imaging and compensating bolometers, such as for example amorphous silicon (a-Si) or similar materials alloyed with carbon or germanium, and doped in various ways, vanadium oxides (VOx), a metal (for example titanium).

## Claims

1. A device for detecting infrared radiation comprising:
a substrate;
a matrix of at least one line of elements for detecting said infrared radiation, each of said elements comprising a resistive imaging bolometer, said matrix being formed above the substrate;
a read circuit for applying electrical stimuli to each imaging bolometer of said matrix to form an analog electrical signal constituting an image of said infrared radiation impinging on said matrix,
a probe for measuring the temperature in one point of the substrate;
an analog-to-digital converter for converting the analog electrical signals formed by the read circuit and the probe into digital values;
a data processing unit comprising:
a first memory space for storing digital values NC(i,j) corresponding to electrical signals from the matrix of imaging bolometers exposed to a uniform scene at the measured temperature of the substrate;
a second memory space for storing a single set of parameters of a predetermined physical model of the electrical signals formed by the read circuit and corresponding to an exposure of the matrix of imaging bolometers to a uniform scene of temperature substantially equal to the substrate temperature; and
a central processing unit for:
correcting the digital electrical signal formed from each imaging bolometer according to the relation:


Ncorr(i,j) =Nbrut(i,j) −NC(i,j) + NC


where: Nbrut(i,j) is the digital electrical signal from said bolometer, Ncorr,(i,j) is the corrected digital electrical signal of said bolometer, and NC is the mean value of the digital values NC(i,j) stored in said first memory space;
computing digital values NC(i,j) corresponding to the exposure of the matrix of imaging bolometers to a uniform scene of temperature substantially equal to the measured substrate temperature based on said single set of parameters; and
overwriting said computed digital values NC(i,j) in said first memory space.

2. The infrared radiation detection device as claimed in claim 1, further comprising a predetermined physical model of the electrical signals formed by the read circuit comprising:
a predetermined physical model of a resistance of the imaging bolometers as a function of the substrate temperature; and
a predetermined physical model of the electrical signals formed by the read circuit as a function of the resistance of the imaging bolometers.

3. The infrared radiation detection device as claimed in claim 2, wherein the central processing unit estimates the resistance of each imaging bolometer according to the relation:


Rest(i,j)=Rabs(i,j)·C(Tmes)

where Rest(i,j) is the estimated resistance of the imaging bolometer, Rabs(i,j ) is a predetermined reference resistance of the imaging bolometer stored in the second memory space for storing a single set of parameters, and C(Tmes) is a parameter dependent on the measured temperature Tmes of the substrate.

4. The infrared radiation detection device as claimed in claim 3, wherein the parameter C(Tmes) is calculated according to the relation:




  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  



where EA is a predetermined imaging bolometer activation energy, k is the Bolzmann constant stored in the second memory space for storing a single set of parameters, and Tmes is the measured temperature expressed in degrees Kelvin.

5. The infrared radiation detection device as claimed in claim 3, wherein the reference resistances Rabs of the imaging bolometers of the matrix are tabulated in a permanent memory built into the substrate.

6. The infrared radiation detection device as claimed in claim 2, wherein the central processing unit calculates a digital value NC(i,j) of each imaging bolometer according to the relation:


NC(i,j)=α−β×(Icomp(j)−Iac(i,j))

where α and β are predetermined parameters dependent on the read circuit stored in the second memory space for storing a single set of parameters, Icomp(j) is the current flowing in a jth compensating bolometer associated to the imaging bolometer calculated from the estimated resistance thereof, and Iac(i,j) is the current flowing in the imaging bolometer calculated from the estimated resistance thereof.

7. The infrared radiation detection device as claimed in claim 1, further comprising a common mode compensation structure that includes a compensating bolometer associated with each column of the matrix of imaging bolometers, the read circuit forming the electrical signal constituting the image of the infrared radiation impinging on said matrix from a difference between a current flowing in each imaging bolometer and a current flowing in the compensating bolometer associated therewith, wherein a predetermined physical model of the electrical signals formed by the read circuit comprises a predetermined physical model of a resistance of said compensating bolometer.

8. The infrared radiation detection device as claimed in claim 7, wherein the central processing unit estimates the resistance of each compensating bolometer according to the relation:


Rcomp-est(j) =Rcomp-abs(j)·C(Tmes) 

where Rcomp-abs(j) is the estimated resistance of the compensating bolometer, Rcomp-abs(j) is a predetermined reference resistance of the compensating bolometer of the jth column stored in the second memory space for storing a single set of parameters, and C(Tmes) is a parameter dependent on the measured temperature Tmesof the substrate.

9. The infrared radiation detection device as claimed in claim 8, wherein the parameter C(Tmes) is calculated according to the relation:




  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  



where EA is a predetermined imaging bolometer activation energy, k is the Bolzmann constant stored in the second memory space for storing a single set of parameters, and Tmes is the measured temperature of the substrate expressed in degrees Kelvin.

10. The infrared radiation detection device as claimed in claim 7, wherein the central processing unit calculates a digital value NC(i,j) of each imaging bolometer according to the relation:


NC(i,j)=α−β×(Icomp(j)−Iac(i,j))

where: α and β are predetermined parameters dependent on the read circuit stored in the second memory space for storing a single set of parameters, Icomp(j) is the current flowing in the jth compensating bolometer associated to the imaging bolometer calculated from the estimated resistance thereof, and Iac(i,j) is the current flowing in the imaging bolometer calculated from the estimated resistance thereof.

11. The infrared radiation detection device as claimed in claim 1, wherein the data processing unit is implemented at least partially in the substrate.

12. The infrared radiation detection device as claimed in claim 1, wherein the second memory space is a permanent memory built into the substrate.

13. The infrared radiation detection device as claimed in claim 1, wherein the central processing unit ascertains whether an absolute difference between the last measured temperature of the substrate and the temperature associated with the digital values NC(i,j) stored in the first memory space is greater than a predetermined threshold value, and for activating the means for computing digital values NC(i,j) if said absolute difference is greater than the threshold value.

14. A method for detecting infrared radiation by means of a matrix of at least one line of elements for detecting said radiation, each of said elements comprising a resistive imaging bolometer, the matrix being formed above a substrate, said method comprising:
applying electrical stimuli to each imaging bolometer of the matrix to form an analog electrical signal constituting the image of the infrared radiation impinging on said matrix;
measuring the temperature in one point of the substrate;
converting the analog electrical signals formed by a read circuit and a probe into digital values;
computing the digital values NC(i,j) corresponding to an exposure of the matrix of imaging bolometers to a uniform scene of temperature substantially equal to the measured substrate temperature based on a single set of parameters of a predetermined physical model of the electrical signals formed by the read circuit; and
correcting the digital electrical signal formed from each imaging bolometer according to the relation:


Ncorr(i,j)=Nbrut(i,j)−NC(i,j) + NC


where: Nbrut(i,j) is the digital electrical signal from said bolometer, Ncorr(i,j) is the corrected digital electrical signal of said bolometer, and NC is the mean value of the digital values NC(i,j).

15. The method for detecting infrared radiation as claimed in claim 14, wherein said predetermined physical model of the electrical signals formed by the read circuit comprises:
a predetermined physical model of resistance of the imaging bolometer as a function of the substrate temperature; and
a predetermined physical model of the electrical signals formed by the read circuit as a function of the resistance of the imaging bolometers.

16. The method for detecting infrared radiation as claimed in claim 15, wherein said predetermined physical model of the resistance of the imaging bolometer is a model according to the relation:


Rest(i,j)=Rabs(i,j)·C(Tmes)

where: Rest(i,j) is the estimated resistance of the imaging bolometer, Rabs(i,j) is a predetermined reference resistance of the imaging bolometer, and C(Tmes) is a parameter dependent on the measured temperature Tmes of the substrate.

17. The method for detecting infrared radiation as claimed in claim 16, wherein the parameter C(Tmes) is calculated according to the relation:




  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  




where: EA/k is a predetermined imaging bolometer activation energy divided by the Bolzmann constant, Tmes is the measured temperature of the substrate expressed in degrees of Kelvin.

18. The method for detecting infrared radiation as claimed in claim 15, wherein the digital value NC(i,j) of each imaging bolometer is calculated according to the relation:


NC(i,j)=α−β×(Icomp(j)−Iac(i,j))

where: α and β are predetermined parameters dependent on the read circuit, Icomp(j) is the current flowing in a jth compensating bolometer associated to the imaging bolometer calculated from estimated resistance thereof, and Iac(i,j) is the current flowing in the imaging bolometer calculated from the estimated resistance thereof.

19. The method for detecting infrared radiation as claimed in claim 14, wherein said method further comprises compensating a common mode by means of a compensating bolometer associated with each column of the matrix of imaging bolometers, wherein the generation of the electrical signal constituting the image of the infrared radiation impinging said matrix is based on a difference between a current flowing in each said bolometer and a current flowing in the compensating bolometer associated therewith, and wherein said predetermined physical model of the electrical signals formed by the read circuit comprises a predetermined physical model of a resistance of said compensating bolometer.

20. The method for detecting infrared radiation as claimed in claim 19, wherein said predetermined physical model of the resistance of the compensating bolometer is a model according to the relation:


Rcomp-est(j)=Rcomp-abs(j)·C(Tmes)

where: Rcomp-est(j) is the estimated resistance of the compensating bolometer of the jth column, Rcomp-abs(j) is a predetermined reference resistance of the compensating bolometer, and C(Tmes) is a parameter dependent on the measured temperature Tmes of the substrate.

21. The method for detecting infrared radiation as claimed in claim 19, wherein a parameter C(Tmes) is calculated according to the relation:




  
    C
    ⁡
    
      (
      
        T
        mes
      
      )
    
  
  =
  
    exp
    ⁡
    
      (
      
        
          E
          A
        
        
          k
          ·
          
            T
            mes
          
        
      
      )
    
  




where: EA/k is a predetermined imaging bolometer activation energy divided by the Bolzmann constant, and Tmes is the measured temperature of the substrate expressed in degrees of Kelvin.

22. The method for detecting infrared radiation as claimed in claim 19, wherein the digital value NC(i,j) of each imaging bolometer is calculated according to the relation:


NC(i,j)=α−β×(Icomp(j)−Iac(i,j))

where: α and β are predetermined parameters dependent on the read circuit, Icomp(j) is the current flowing in the jthcompensating bolometer associated to the imaging bolometer calculated from estimated resistance thereof, and Iac(i,j) is the current flowing in the imaging bolometer calculated from the estimated resistance thereof.

23. The method for detecting infrared radiation as claimed in claim 14, further comprising computing new digital values NC(i,j) if an absolute difference between the last measure of temperature of the substrate and the temperature associated with current digital values NC(i,j) is greater than a predetermined threshold value.