AU2021219267A1

AU2021219267A1 - Method for calibrating an optoelectronic device

Info

Publication number: AU2021219267A1
Application number: AU2021219267A
Authority: AU
Inventors: Damien DIAZ; David ORIOT DE LA CHAPELLE; Emmanuel Soulie
Original assignee: Bertin Technologies SAS
Current assignee: Bertin Technologies SAS
Priority date: 2020-02-10
Filing date: 2021-01-27
Publication date: 2022-08-25
Also published as: FR3107116B1; FR3107116A1; CA3165712A1; WO2021160433A1; EP4103921A1

Abstract

The invention relates to a method for calibrating an optoelectronic device comprising an optical input and being placed in a climatic chamber at an ambient temperature T0, the optoelectronic device comprising at least one matrix of bolometers configured to measure at least one temperature, and at least one reading circuit comprising an analogue output capable of supplying a plurality of raw analogue signals intended to form a thermal image, said method comprising at least the following two successive phases: 1. a calibration phase (100); 2. a verification phase (200).

Description

METHOD FOR CALIBRATING AN OPTOELECTRONIC DEVICE

TECHNICAL FIELD OF THE INVENTION The present invention relates to the field of optical sensors, in particular optoelectronic sensors in the field of thermal infrared imaging. It finds as a particularly advantageous application the field of calibration of this type of optoelectronic devices. PRIORART Infrared imaging is increasingly growing thanks to advances in microelectronics. The ability to make more sensitive and smaller sensors has enabled this development. In the field of thermal infrared imaging, it is essential to have a calibrated correspondence table between the radiation intensity perceived by the sensor and the temperature of the imaged body. One of the main problems of these devices consists of a thermal drift of the optoelectronic sensors of the bolometre type for example, mainly with sensors designed based on amorphous silicon. This drift implies that the calibration of the sensors is done on a regular basis. Hence, this calibration is generally done cyclically with what is called a reference element. Thus, for example, a temperature mask known as a "shutter" (for alternator) may be placed in front of the sensor until the latter could measure its infrared image and the electronics adjust the measurements with the temperature of the known mask. This type of solution is very effective, but poses a major problem. Indeed, while some applications require uninterrupted imaging, this solution implies losing the desired image for a few moments to recalibrate the sensor. Several tips to accelerate the recalibration have been proposed, but all present problems of use in the field. Thus, for example, the document US8378290 B1 is known which describes a method for calibrating an optoelectronic device.

Hence, an object of the present invention is to propose a solution to at least some of these problems. The other objects, features and advantages of the present invention will become apparent upon examining the following description and the appended drawings. It should be understood that other advantages may be incorporated. SUMMARY OF THE INVENTION The present invention relates to a method for calibrating an optoelectronic device comprising an optical input and being placed in a climatic chamber at an ambient temperature TO, the optoelectronic device comprising at least one array of bolometres, preferably microbolometres, configured to measure at least one temperature and at least one reading circuit comprising an analog output adapted to supply a plurality of raw analog signals intended to form a thermal image, each raw analog signal corresponding to a bolometre and each raw analog signal being a function of a scene observed by said optoelectronic device, the analog output of the reading circuit being connected to an analog signal to digital signal converter having a predetermined dynamic range, said method comprising at least the following two successive phases: a. A calibration phase comprising at least the following successive steps: i. Modify the temperature inside the climatic chamber to reach a first temperature T1 different from TO and lower than TO; ii. Wait for a stabilisation time Tpsstab until the temperature of the array of bolometres is constant, preferably equal to T1; iii. Adjust the polarisation voltage of each bolometre so that the value of each raw analog signal is within a preselected range of the dynamic range, preferably the selected range corresponds to the middle interval of the dynamic range; iv. Record an adjusted polarisation voltage value of each bolometre; v. Record, for each bolometre, a corrective electrical voltage value, this electrical voltage being a direct voltage and depending on the temperature of an observed scene, this electrical voltage being randomly distributed throughout the array of bolometres, and being superimposed on said raw analog signal of each bolometre; vi. Modify the temperature inside the climatic chamber to reach a second temperature T2 different from TO and T1, and preferably higher than T1; vii. Repeat steps b, c, d, e and f of the calibration phase up to a final temperature Tf higher than the initial temperature TO; b. A verification phase comprising at least the following successive steps: i. Linearly lower the temperature inside the climatic chamber from Tf until the temperature of the array of bolometres is equal to T1. ii. Measure at least one operating criterion of the optoelectronic device, during the step of linear drop of the temperature from Tf to T1, using the polarisation voltage values and the corrective electrical voltage values recorded as a function of the temperature of the array of bolometres.

The present invention allows calibrating, preferably only once, and advantageously in the factory, an optoelectronic device. It relates to a calibration method that does not need to be repeated in the field during the use of the optoelectronic device. The present invention enables the determination of the operating parameters and testing thereof in the same process comprising a stepped temperature rise and a linear temperature drop. The stepped rise allows having the time needed to perform the necessary adjustments and data recordings for several temperatures, then the linear descent enables the calibration of the optoelectronic device to be tested in a dynamic thermal environment. Advantageously, the method comprises a phase of calibration by temperature steps, therefore a so-called static calibration phase, and a so-called dynamic verification phase through a change in temperature. Advantageously, the method comprises a discrete calibration phase and a continuous verification phase. Preferably, the present invention allows calibrating the optoelectronic device according to its future use by positioning the interval relative to the dynamic range of the converter so as to prefer applications either in cold, hot environments or at ambient temperatures. The present invention allows creating tables of operating and behaviour parameters of each bolometre. The present invention allows getting rid with a mask or shutter and a calibration process that must be repeated on a regular basis. Hence, the optoelectronic device can be used continuously in the field without recalibration interruption, and without the optoelectronic device having to be maintained at a determined temperature. The present invention also relates to a computer program product comprising instructions, which when performed by at least one processor, executes at least the method according to the present invention. Preferably, said processor is configured to control a climatic chamber in which an optoelectronic device is arranged and to control said optoelectronic device. DETAILED DESCRIPTION OF THE FIGURES The aims, objects, as well as the features and advantages of the invention will appear better from the detailed description of an embodiment of the latter which is illustrated by the following appended drawings in which: Figure 1 schematically represents some steps of the method according to an embodiment of the present invention. Figure 2 schematically represents an array of bolometres according to an embodiment of the present invention. Figure 3 schematically an optoelectronic device arranged in a climatic chamber according to an embodiment of the present invention. Figure 4 represents the evolution of the temperature of the array of bolometres during the method according to an embodiment of the present invention. Figure 5 illustrates the schematic positioning of an optoelectronic device in front of a black body.

The drawings are provided as examples and do not limit the invention. They consist of schematic principle representations intended to facilitate understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions are not necessarily representative of reality. DETAILED DESCRIPTION Before starting a detailed review of embodiments of the invention, the optional features which could be used in combination or alternatively are set out hereinafter. According to one example, the preselected range comprises the middle of the dynamic range, is preferably centred on the middle of the dynamic range, advantageously has an extension of less than 20% on either side of the centre of the dynamic range. This allows preserving the full dynamics of each bolometre in order to reduce, and possibly not to suffer from saturation, and that being so for both low and high temperatures. According to one example, the preselected interval comprises the level of 25% of the dynamic range, preferably is centred on the level of 25% of the dynamic range, advantageously has an extension of less than 20% on either side of the level of 25% of the dynamic range. This allows preserving the full dynamics of each bolometre in the direction of high temperatures for applications in which the observed temperatures are relatively high. According to one example, the preselected range comprises the level of 75% of the dynamic range, preferably is centred on the level of 75% of the dynamic range, advantageously has an extension of less than 20% on either side of the level of the 75% of dynamic range. This allows preserving the full dynamics of each bolometre in the direction of low temperatures for applications in which the observed temperatures are relatively low. According to one example, if the measurement of the operating criterion indicates a discrepancy larger than a threshold value between the value of the temperature assessed by the array of bolometres and the temperature of the observed scene, preferably of the array of bolometres, the calibration phase is executed again. This allows completing the calibration method only when the optoelectronic device is correctly calibrated. According to one example, the step of recording the corrective electrical voltage values is performed after a step of averaging a predetermined number of thermal images at the same temperature. This allows improving the reliability of the determination of the corrective electrical voltage values for each bolometre. According to one example, the assessment of the operating criterion of the optoelectronic device comprises the assessment of the fixed spatial noise and/or of the average thermal value of at least one image. This allows quantifying the level of success of the calibration phase. According to one example, the array of bolometres has an arrangement of bolometres in rows and in columns, and the acquisition by the reading circuit of the raw analog signals is carried out row-by-row.

According to one example, the ratio between the stabilisation time Tps stab and the temperature modification time is greater than 1, preferably greater than 5 and advantageously greater than 10. According to one example, the step of temperature linear drop from Tf to TO has a slope coefficient less than or equal to 2°C/minute, preferably less than or equal to 1°C/minute and advantageously less than or equal to 0.5°C/minute. According to one example, the method comprises an additional phase in which the optoelectronic device is arranged so that the optical input faces a black body, the additional phase comprising at least the following successive steps: a. Acquisition of a first plurality of images when the black body has a temperature T3 lower than TO; b. Acquisition of a second plurality of images when the black body has a temperature T4 higher than TO. c. Determination of the bolometres featuring a measurement error. By "temperature of an element", it should be understood the temperature that the element releases in a radiative form, i.e. in the form of an electromagnetic flux whose wavelength(s) is/are within the infrared range. By "a stabilised temperature", it should be understood a temperature whose fluctuations are less than 10C, preferably less than 0.01°C, and advantageously less than 0.001°C. By "black body", it should be understood an element or device whose temperature is deemed to be stable and whose surface emissivity (notation: E) is closeto0 =1 (or typically E>0.97). In the present description, a dynamic range of an analog signal to digital signal converter extends over several values, for example from 0 up to 100. By middle, centre, 50% point of the dynamic range, it should then be understood the value 50 for example. Similarly, the 75% of the dynamic range will be interpreted as the value 75, finally the same applies with the 25% point of the dynamic range which therefore corresponds to the value 25. All this being so in the case where the dynamic range extends for example from 0 up to 100. The present invention relates to a method for calibrating an optoelectronic device. This optoelectronic device comprises an optical input intended to receive an electromagnetic flux, preferably in the thermal infrared range. Indeed, preferably, the present invention relates to a method for calibrating a thermal vision optoelectronic device intended to visualise the infrared radiation of a scene, and preferably to be able to assess the temperature of the elements present in said scene. According to one embodiment, the optoelectronic device comprises at least one array of infrared detectors, preferably of bolometres and advantageously of microbolometres. In the remainder of this description, the term bolometre refers to both bolometres and microbolometres. Preferably, yet without limitation, the bolometres of the array of bolometres are based on amorphous silicon. This allows the relative gain between each bolometre to remain identical, regardless of the temperature of the array of bolometres as long as the parameters relating to the gain remain constant, i.e. for example as long as the integration time, or the electronic gain, etc...

remain constant. This also allows the behaviour of the bolometres as a function of temperature to be perfectly repeatable under identical measurement conditions. Advantageously, these bolometres are arranged in rows and in columns thus forming an array of bolometres. This array of bolometres is electrically connected to a reading circuit. This reading circuit is configured to collect the raw analog signals from each bolometre. Indeed, each bolometre is configured to generate a raw analog signal according to the thermal radiation it receives. According to a preferred embodiment, the reading circuit collects the analog signals from the array of bolometres row-by-row, preferably from one end of the array towards its opposite end. Advantageously, the reading circuit comprises an analog output adapted to supply a plurality of raw analog signals, i.e. the raw analog signals collected row-by-row. Preferably, this analog output is electrically connected to an analog signal to digital signal converter. This converter comprises at least one dynamic range, generally expressed in bits. This converter further allows the generation of a thermal image of the scene observed by the optoelectronic device. As indicated before, the prior art suffers from thermal drifts requiring a regular calibration phase to recalibrate the operating parameters of each bolometre, this may also include the polarisation voltages applied to each bolometre of the array, the corrective electrical voltages to be considered, etc... Indeed, depending on the more or less significant thermal drift, it is important to be able to adjust various operating parameters of the bolometres to maintain a raw analog signal consistent with the intensity of the received thermal radiation flux. Thus, the prior art suggests using a mask whose temperature is known in order to expose the array of bolometres to this mask, then to adjust the operating parameters until obtaining a thermal image consistent with the temperature of the mask arranged in front of the array. This causes stopping the use of the device in its basic function, i.e. the observation of a scene. The present invention proposes an innovative solution which does not require the use of a mask and therefore the stoppage of the use of the optoelectronic device. Which stoppage might be disastrous in the case of the use of this technology in a military field, for example. The present invention proposes a calibration method, preferably in the factory, of the optoelectronic device. With this calibration, the use of a mask, also called shutter, is no longer useful. Figure 1 illustrates, according to one embodiment, the different steps of this method. Advantageously, this method comprises at least two phases, a so-called calibration phase 100 and a so-called verification phase 200. The calibration phase 100 aims to adjust and determine operating parameters so that the temperature estimated by one bolometre 12 is consistent with the temperature of the observed scene. In particular, this calibration phase 100 enables the adjustment and recording of the polarisation voltage values to be applied to each bolometre 12 so that each raw analog signal is within the median interval of the dynamic range of the converter.

By median interval, it should be understood herein a range comprising the middle of the dynamic range over a range of more or less 20%, and preferably being centred on the middle of the dynamic range. This positioning substantially at the middle of the dynamic range allows reducing, and possibly avoiding, saturation problems at the edge of the range. Similarly, this calibration phase 100 advantageously allows determining and recording the corrective electrical voltage of each bolometre 12, this corrective electrical voltage is a direct voltage, depending on the temperature of the observed scene; this corrective electrical voltage, also called "Offset", is distributed randomly over the array 11 of bolometres 12 and happens to be superimposed on each raw analog signal of each bolometre 12. Hence, this corrective electrical voltage applies a voltage shift to the raw analog signal which should be assessed in order to take it into account to correct the raw analog signal. This correction is intended to consider this offset in the processing of the raw analog signals. Advantageously, once the calibration phase 100 is completed, the verification phase 200 is carried out. This verification phase 200 aims to check, verify that the temperature assessed by each bolometre 12 is in compliance with the temperature of the climatic chamber 20, and therefore with the temperature of the observed scene. Advantageously, this method is carried out once the optoelectronic device 10 is arranged opposite a black body 40 in a climatic chamber 20 at a temperature TO, preferably corresponding to the ambient temperature, for example 20°C. This climatic chamber 20 is configured so that the temperature in the climatic chamber 20 is homogeneous, stable and controllable. The principle is based on the principle of varying the temperature in the climatic chamber 20 and adjusting then recording the behaviour of the bolometres 12 so as to be able to correlate their optoelectronic response to the temperature of the observed scene generated by the black body 40. According to one embodiment, the calibration phase 100 comprises at least the plurality of the following successive steps: a. Modify 101 the temperature inside the climatic chamber 20 to reach a first temperature T1 different from TO, preferably lower than TO, advantageously T1=-30°C; b. Wait 102 for a stabilisation time Tps stab until the temperature of the array 11 of bolometres 12 is constant, preferably, yet without limitation, is equal to T1; it should be noted that, according to one embodiment, the reading of the array 11 of bolometres 12 could generate heat; the temperature of the array 11 then stabilises at a temperature substantially higher than T1; advantageously, yet without limitation, Tps stab is less than 2 hours, preferably less than 1 hour and advantageously less than 30 minutes; it should be noted that Tps stab depends on the size of the optoelectronic device and its mechanical construction. c. Adjust 103 the polarisation voltage of each bolometre 12 so that the value of each raw analog signal is within an interval of the predetermined dynamic range of the converter; According to one embodiment, the interval is a median interval, comprising the middle of the dynamic range, preferably centred on the middle of the dynamic range, advantageously with an extension of less than 20% on either side of the centre of the dynamic range; this allows preserving the full dynamics of each bolometre 12 in order to reduce and possibly not to suffer from saturation, and that being so for both low and high temperatures; According to one embodiment, the interval is an interval comprising 25% of the dynamic range, preferably centred on the 25% of the dynamic range, advantageously with an extension of less than 20% on either side of the 25% of the dynamic range; this allows preserving the full dynamics of each bolometre 12 in the direction of high temperatures for applications in which the observed temperatures are relatively high; According to one embodiment, the interval is an interval comprising 75% of the dynamic range, preferably centred on the 75% of the dynamic range, advantageously with an extension of less than 20% on either side of the 75% dynamic range; This allows preserving the full dynamics of each bolometre 12 in the direction of low temperatures for applications in which the observed temperatures are relatively low; The adjustment of the polarisation voltages allows configuring the optoelectronic device for its future applications while favouring dynamics that are homogeneous, or accentuated on high or low temperatures; d. Record 104 the adjusted polarisation voltage of each bolometre 12; this allows keeping in memory the polarisation voltage necessary for the raw analog signals to be within the selected interval of the dynamic range; e. Preferably, acquire a plurality of thermal images so as to be able to average these thermal images; f. Preferably, determine, for each bolometre 12, the so-called corrective electrical voltage value; g. Record 105, for each bolometre, the corresponding corrective electrical voltage value; according to one embodiment, at least one of steps f and g may be performed before or at the same time as at least one of steps c and d. h. Modify the temperature inside the climatic chamber to reach a second temperature T2 different from TO and T1, preferably higher than T1; i. Repeat steps b to h of the calibration phase until a final temperature Tf higher than the initial temperature TO. Once this calibration phase is complete, the climatic chamber 20 is at a temperature Tf, for example at 50°C. During this calibration phase 100, the polarisation voltages and the corrective electrical voltage values for each temperature having had a thermal step and for each bolometre have been recorded. Once these data have been acquired, the method continues with the verification phase 200. This verification phase 200 consists in lowering the temperature again from Tf to T1 and measuring, preferably continuously, one or more operating parameters of the optoelectronic device 10. In particular, the verification phase comprises at least the following successive steps: a. Linearly lower 201 the temperature inside the climatic chamber 20 from Tf until the temperature of the array of bolometres is equal to T1; b. Measure 202 at least one operating criterion of the optoelectronic device 10, during step 201 of temperature linear drop from Tf to T1, using the polarisation voltage values and the offset values recorded as a function of the temperature of the array 11 of bolometres; c. If the verification phase meets the evaluation criteria, then the calibration method ends 203, otherwise the calibration phase 100 resumes 204. Thus, this verification phase 200 allows checking that the parameters recorded during the calibration phase 100 are correct and allow the correct operation of the optoelectronic device 10, i.e. the correct assessment of the temperature of the observed scene. During the verification phase 200, and according to one embodiment, an evaluation of the fixed spatial noise is performed. In particular, this step seeks to detect the presence, or not, of a fixed spatial noise. Preferably, a minimum, and possibly non-existent, fixed spatial noise is desired. The Fixed Pattern Noise or FPN implies the fact that all of the components of the array are not exactly identical. Consequently, there may be differences between the bolometres 12, and therefore between the pixels, each bolometre 12 representing a pixel. Hence, this step is intended to evaluate the level of this fixed spatial noise in order to estimate whether the adjustments and corrections that are made lead to a reduction, and possibly a suppression, of the fixed spatial noise. During the verification phase 200, and according to one embodiment, an evaluation of the average value of the image is carried out. This step comprises the acquisition of a plurality of images, preferably a predetermined number of images, then an averaging of these images is carried out, then the average temperature estimated by the optoelectronic device 10 is compared with the temperature of the climatic chamber 20. If there is a correspondence, or if there is a discrepancy below a threshold value, the test is considered as a success, otherwise it might be necessary to proceed with a new calibration phase 100. In the case where the verification phase 200 is not conclusive or it turns out that the adjustment and correction parameters are not correct, the calibration method continues by restarting the calibration phase 100 followed by another verification phase 200, and that being so until obtaining a calibration deemed to be correct. According to one embodiment, this calibration method may also comprise an additional phase comprising a series of successive steps in which the optoelectronic device 10 is no longer in the climatic chamber 20, but is arranged on a test bench facing a black body 40 whose temperature is monitored. This series of successive steps comprises: a. Acquisition of a first image when a black body 40 has a temperature T3 lower than TO; b. Acquisition of a second image when a black body 40 has a temperature T4 higher than TO. c. Detection of the bolometres 12 featuring a measurement error. Advantageously, the raw analog signals of these bolometres 12 will be corrected in real-time during the use of the optoelectronic device 10 according to the surrounding bolometres 12. Advantageously, the acquisition of the first image and the acquisition of the second image allow calculating a relative gain between each bolometre 12 so that all, preferably almost all, of the bolometres 12 give the same response regardless of the temperature of the observed scene. Afterwards, a detection of the bolometres 12, whose response is erroneous or unsatisfactory, is performed on all of the memorised tables to be used afterwards in real-time in order to be able to replace them with values of the non-erroneous surrounding bolometres 12. Thus, in a cleverway, the signals acquired during the acquisition of the first image and the second image are used to calculate a relative gain between the pixels, i.e. the bolometres, so that all of them give the same response regardless of the temperature of the observed scene. Then, a detection of the various pixels, whose response is erroneous or unsatisfactory, is performed on all of the memorised tables to be used afterwards in real-time so as to be able to replace them with values of the non-erroneous surrounding pixels. We will now describe, according to one embodiment, the present invention through Figures 2 to 5. Figure 2 represents an array 11 of bolometres 12 comprising a plurality of bolometres 12 arranged in rows 15 and in columns 14. According to one embodiment, the array 11 of bolometres 12 may be carried by or may support the reading circuit 13. Figure 3 represents a diagram of an installation configured to implement the method according to an embodiment of the present invention. In this figure, one could notice that an optoelectronic device 10 arranged in a climatic chamber 20. The electronic device 10 comprises an optical input 16. This optical input 16 is configured so that the thermal radiations of the scene generated by the black body 40 could penetrate the optoelectronic device 10 and be captured by the array 11 of bolometres 12 arranged in the optoelectronic device 10. The climatic chamber 20 is driven in temperature so as to impose a temperature on the entirety of the climatic chamber 20 and on everything therein. Advantageously, the temperature inside the climatic chamber 20 could vary for example from 20°C to 60°C. According to one embodiment, the method according to the present invention is intended to be implemented by a computer system 30 comprising at least one processor and/or a computer, and furthermore a non-transitory memory comprising a computer program product comprising instructions, which when performed at least by the processor and/or the computer, executes at least the method according to the present invention. Figure 4 represents a curve of the temperature 300 of the optoelectronic device 10 during the implementation of the method according to an embodiment of the present invention. As indicated before, the starting temperature TO corresponds to the ambient temperature. During the calibration phase 100, the first step consists in reducing the temperature from TO to T1 which for example may be equal to -10°C. The slope coefficient of the temperature drop curve 310 from TO to T1 is less than 1 0 °C/minute, preferably less than 5°C/minute and advantageously less than 2.5°C. The slope coefficient of the temperature drop curve 310 from TO to T1 is configured to avoid a thermal shock which might cause either a breakage or a condensation.

Once at T1, the temperature of the climatic chamber 20 tends to stabilise and the computer system 30 waits for a stabilisation time Tps-stab before carrying on the method. Preferably, Tps stab is comprised between 10 minutes and 180 minutes and advantageously between 30 minutes and 120 minutes. Preferably, this stabilisation time depends on the camera 10. Once the temperature of the optoelectronic device 10 and in particular of the array 11 of bolometres 12 is stabilised and equal to the temperature of the observed scene, i.e. the temperature of the black body 402, the computer system 30 performs a temperature step 320 and the method continues with the following different steps: i. Adjust 103 the polarisation voltage of each bolometre 12; ii. Record 104 the adjusted polarisation voltage of each bolometre 12; iii. Preferably, acquire a plurality of thermal images; iv. Preferably, determine, for each bolometre 12, a corrective electrical voltage value: v. Record 105, for each bolometre 12, the corrective electrical voltage value corresponding thereto. Once this series of steps is completed, in various orders according to an embodiment of the present invention for example, the temperature of the climatic chamber 20 changes. Preferably, the temperature of the climatic chamber changes from T1 to T2. Preferably, the temperature T2 is higher than T1. The slope coefficient of the temperature rise 330 from T1 to T2 is greaterthan 2.5°C/minute, preferably greater than 5°C/minute and advantageously greaterthan 10°C/minute. Advantageously, the different temperature steps are not large in temperature discrepancy from each other, preferably about 5°C; thus, the slope coefficient of the temperature rise 330 from T1 to T2 is configured so that the temperature rise 330 from T1 to T2 is as quick as possible in order to let as much time as possible for the optoelectronic device 10 to set itself at the selected temperature, i.e. to stabilise its temperature. Again, the computer system 30 waits for the temperature of the array 11 of bolometres 12 to stabilise and to be substantially equal to that of the climatic chamber 20, i.e. to T2, T2 therefore being the temperature of the observed scene. Once these conditions are met, the method repeats the previous steps, and that being so on until the temperature Tf called the final temperature. Thus, according to one embodiment, the calibration phase 100 comprises at least 15, preferably at least 20, and advantageously at least 25 temperature steps, i.e. measurement points of the calibration elements. Figure 4 also illustrates the verification phase 200 with the temperature linear drop 340 from Tf to T1. Advantageously, the slope coefficient of this temperature linear drop 340 is less than 2°C/min, preferably less than 1C/min and advantageously less than 0.5°C/min. Once at T1, if the verification phase 200 is conclusive, the temperature is brought from 350 to TO so that the optoelectronic device 10 could be taken out of the climatic chamber 20; if the verification phase 200 is not conclusive, the calibration phase 100 starts again.

Figure 5 illustrates the positioning of the optoelectronic device 10, for example on a test bench, facing a black body 40. Thus, the observed scene comprises said black body 40. Afterwards, the previously-described additional phase could be implemented. The present invention proposes an innovative and effective solution to the problems of thermal drift of optoelectronic devices intended to assess the temperature of an observed scene. Instead of resorting to a regular and quite frequent calibration, or else of resorting to a check-up of the temperature of the array of bolometres for example, the present invention provides a calibration method, preferably in factory, and advantageously which no longer needs to be implemented outside the factory. This method allows quantifying, recording and adjusting the operating parameters of each bolometre of the array of bolometres by subjecting them to a plurality of temperatures, then performing a check-up of this calibration by a linear and non-static assessment. Thus, the present invention allows obtaining an optoelectronic device calibrated for the rest of its use and thus able to observe a scene continuously where necessary without having to be interrupted for recalibration. The invention is not limited to the previously-described embodiments and encompasses all of the embodiments covered by the claims. List of the references 10 Optoelectronic device 11 Array of bolometres 12 Bolometres 13 Reading circuit 14 Column of bolometres 15 Row of bolometres 16 Optical input 20 Climatic chamber 30 Computer system 40 Black body 100 Calibration phase 101 Modification of the temperature of the chamber from TO to T1 102 Waiting for the stabilisation of the temperature 103 Adjustment of the polarisation voltage of each bolometre 104 Recording of the adjusted polarisation voltage of each bolometre 105 Recording of a corrective electrical voltage for each bolometre 106 Modification of the temperature of the chamber from T1 to T2 107 Repetition of steps 112 to 116 up to the temperature Tf 200 Verification phase 201 Linear drop of the temperature from Tf to TO 202 Measurement of at least one operating criterion of the optoelectronic device during step

203 Validation and end of the calibration method 204 Return to the calibration phase 300 Curve of the temperature 301 Time scale 302 Temperature scale 310 Temperature drop curve 320 Temperature level 330 Temperature rise 340 Linear temperature drop 350 Return to the temperature TO

Claims

1. A method for calibrating an optoelectronic device (10) comprising an optical input (16) and being placed in a climatic chamber (20) at an ambient temperature TO, the optoelectronic device (10) comprising at least one array (11) of bolometres (12), preferably microbolometres, configured to measure at least one temperature and at least one reading circuit (13) comprising an analog output adapted to supply a plurality of raw analog signals intended to form a thermal image, each raw analog signal corresponding to a bolometre (12) and each raw analog signal being a function of a scene observed by said optoelectronic device (10), the analog output of the reading circuit (13) being connected to an analog signal to digital signal converter having a predetermined dynamic range, said method comprising at least the following two successive phases:

1. A calibration phase (100) comprising at least the following successive steps: a. Modify (101) the temperature inside the climatic chamber (20) to reach a first temperature TI different from TO and lower than TO; b. Wait (102) for a stabilisation time Tpsstab until the temperature of the array (11) of bolometres (12) is constant, preferably equal to TI; c. Adjust (103) the polarisation voltage of each bolometre (12) so that the value of each raw analog signal is within a preselected range of the dynamic range, preferably the selected range corresponds to the median interval of the dynamic range; d. Record (104) an adjusted polarisation voltage value of each bolometre (12); e. Record (105), for each bolometre (12), a corrective electrical voltage value, this electrical voltage being a direct voltage and depending on the temperature of an observed scene, this electrical voltage being randomly distributed throughout the array (11) of bolometres (12), and being determined as a voltage offset with respect to the raw analog signal of each bolometre (12) and being superimposed on said raw analog signal of each bolometre (12); f. Modify (106) the temperature inside the climatic chamber (20) to reach a second temperature T2 different from TO and T1, and higher than TI; g. Repeat steps b, c, d, e and f of the calibration phase (100) for different second temperatures T2 up to a second final temperature T2 equal to a final temperature Tf higher than the initial temperature TO; 2. A verification phase (200) comprising at least the following successive steps: a. Linearly lower (201) the temperature inside the climatic chamber (20) from Tf until the temperature of the array (11) of bolometres (12) is equal to T1. b. Measure (202) at least one operating criterion of the optoelectronic device (10) observing said scene, during the step of linear drop of the temperature from Tf to T1, using the polarisation voltage values and the corrective electrical voltage values recorded as a function of the temperature of the array (11) of bolometres (12).

2. The method according to the preceding claim, wherein the preselected range comprises the middle of the dynamic range, preferably is centred on the middle of the dynamic range, advantageously has an extension of less than 20% on either side of the centre of the dynamic range. 3. The method according to claim 1, wherein the preselected range comprises the 25% level of the dynamic range, preferably is centred on the 25% level of the dynamic range, preferably has an extension of less than 20% on either side the 25% level of the dynamic range. 4. The method according to claim 1, wherein the preselected range comprises the 75% level of the dynamic range, preferably is centred at the 75% level of the dynamic range, advantageously has an extension of less than 20% on either side of the 75% level of the dynamic range. 5. The method according to any one of the preceding claims, wherein, if the measurement of the operating criterion indicates a discrepancy larger than a threshold value between the value of the temperature assessed by the array (11) of bolometres (12) and the temperature of the observed scene, the calibration phase (100) is executed again. 6. The method according to the preceding claim, wherein the threshold value is located between the value of the temperature assessed by the array (11) of bolometres (12) and the temperature of the array (11) of bolometres (12). 7. The method according to any one of the preceding claims, wherein the step of recording (105) the corrective electrical voltage values is performed after a step of averaging a predetermined number of thermal images at the same temperature. 8. The method according to any one of the preceding claims, wherein the assessment of the operating criterion of the optoelectronic device (10) comprises the assessment of the fixed spatial noise and/or of the average thermal value of at least one image. 9. The method according to any one of the preceding claims, wherein the array (11) of bolometres (12) has an arrangement of the bolometres in rows (15) and in columns (14), and wherein the acquisition by the reading circuit (13) of the raw analog signals is carried out row (15) by row (15). 10. The method according to any one of the preceding claims, wherein the ratio between the stabilisation time Tpsstab and the modification time of the temperature is greater than 1, preferably greater than 5 and advantageously greater than 10. 11. The method according to any one of the preceding claims, wherein the step of temperature linear drop from Tf to TO has a slope coefficient less than or equal to 2°C/minute, preferably 1°C/minute. 12. The method according to the preceding claim, wherein the step of temperature linear drop from Tf to TO has a slope coefficient less than or equal to 0.5°C/minute. 13. The method according to any one of the preceding claims, comprising an additional phase in which the optoelectronic device (10) is arranged so that the optical input (16) faces a black body (40), the additional phase comprising at least the following successive steps: 1. Acquisition of a first plurality of images when the black body (40) has a temperature T3 lower than TO; 2. Acquisition of a second plurality of images when the black body (40) has a temperature T4 higher than TO.

3. Determination of the bolometres (12) having a measurement error. 14. A computer program product comprising instructions, which when performed by at least one processor, executes at least the method according to any one of the preceding claims, said processor being configured to control a climatic chamber (20) in which an optoelectronic device (10) is arranged and to control said optoelectronic device (10).