METHOD FOR CALIBRATING AN OPTO-ECTRONIC DEVICE

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of optical sensors, in particular optoelectronic sensors in the field of thermal infrared imaging. It finds particularly advantageous application in the field of calibration of this type of optoelectronic devices. STATE OF THE ART

[0002] Infrared imaging is experiencing significant growth thanks to advances in microelectronics. This growth has been driven by the ability to create more sensitive and smaller sensors.

[0003] In the field of thermal infrared imaging, it is crucial 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 with these devices is a thermal drift of optoelectronic sensors such as bolometers, for example, mainly with sensors designed based on amorphous silicon. This drift requires that the sensors be calibrated regularly.

[0004] This calibration is therefore generally done cyclically with what is called a reference element. For example, a temperature cover known as a "shutter" (for alternator in English) can be placed in front of the sensor while the latter can measure its infrared image and the electronics adjust the measurements with the temperature of the known cover.

[0005] This type of solution is very effective, but poses a major problem. Indeed, while some applications require uninterrupted imaging, this solution involves losing the desired image for a few moments while the sensor is recalibrated. Several tricks to speed up recalibration have been proposed, but all present problems for use in the field. For example, we know of document US8378290 B1 which describes a method for calibrating an optoelectronic device.

[0006] An object of the present invention is therefore to propose a solution to at least some of these problems.

[0007] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY OF THE INVENTION

[0008] The present invention relates to a method for calibrating an optoelectronic device according to claim 1.

[0009] The present invention makes it possible to calibrate, preferably only once, and advantageously in the factory, an optoelectronic device. It relates to a calibration method which does not need to be reproduced in the field during use of the optoelectronic device.

[0010] The present invention allows the determination of operating parameters and their testing in the same process comprising a step-like temperature increase and a linear temperature decrease. The step-like increase allows the necessary time to make the adjustments and data recordings necessary for several temperatures, then the linear decrease allows the calibration of the optoelectronic device to be tested in a dynamic thermal environment.

[0011] Advantageously, the method comprises a calibration phase by temperature steps, therefore a so-called static calibration phase, and a so-called dynamic verification phase by a change in temperature.

[0012] Advantageously, the method comprises a discrete calibration phase and a continuous verification phase.

[0013] Preferably, the present invention allows the optoelectronic device to be calibrated according to its future use by positioning the range relative to the dynamic range of the converter so as to prefer either applications in cold, hot or ambient temperatures.

[0014] The present invention makes it possible to create tables of operating and behavior parameters for each bolometer.

[0015] The present invention makes it possible to do without a cover or shutter and a calibration process that needs to be repeated regularly.

[0016] The optoelectronic device can therefore be used continuously in the field without interruption for recalibration, and without the optoelectronic device having to be kept at a specific temperature.

[0017] The present invention also relates to a computer program product comprising instructions, which when executed by at least one processor, executes at least the method according to the present invention. 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

[0018] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which: There figure 1schematically represents certain steps of the method according to an embodiment of the present invention. The figure 2 schematically represents a matrix of bolometers according to an embodiment of the present invention. The figure 3 schematically an optoelectronic device arranged in a climatic chamber according to an embodiment of the present invention. The figure 4 represents the evolution of the temperature of the bolometer matrix during the method according to an embodiment of the present invention. The Figure 5 illustrates the schematic positioning of an optoelectronic device in front of a black body.

[0019] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions are not representative of reality. DETAILED DESCRIPTION

[0020] Before commencing a detailed review of embodiments of the invention, optional features which may optionally be used in combination or alternatively are set out below.

[0021] In one example, the preselected range includes the middle of the dynamic range, preferably is centered on the middle of the dynamic range, advantageously has an extension of less than 20% on either side of the center of the dynamic range.

[0022] This allows the full dynamic range of each bolometer to be maintained in order to reduce or even avoid saturation, both at low and high temperatures.

[0023] In one example, the preselected range includes the 25% level of the dynamic range, preferably is centered on the 25% level of the dynamic range, advantageously has an extension of less than 20% on either side of the 25% level of the dynamic range.

[0024] This allows the full dynamic range of each bolometer to be maintained in the high temperature direction for applications where the observed temperatures are relatively high.

[0025] In one example, the preselected range includes the 75% level of the dynamic range, preferably is centered on 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.

[0026] This allows the full dynamic range of each bolometer to be maintained in the low temperature direction for applications where the observed temperatures are relatively low.

[0027] According to one example, if the measurement of the operating criterion indicates a disagreement greater than a threshold value between the temperature value evaluated by the bolometer array and the temperature of the observed scene, preferably of the bolometer array, the calibration phase is executed again.

[0028] This allows the calibration process to be completed only when the optoelectronic device is correctly calibrated.

[0029] According to one example, the step of recording the correction electrical voltage values ​​is carried out after a step of averaging a predetermined number of thermal images at the same temperature.

[0030] This improves the reliability of determining the correction electrical voltage values ​​for each bolometer.

[0031] According to one example, the evaluation of the operating criterion of the optoelectronic device comprises the evaluation of the fixed spatial noise and / or the average thermal value of at least one image.

[0032] This allows the level of success of the calibration phase to be quantified.

[0033] According to one example, the bolometer matrix has a row and column arrangement of the bolometers, and the acquisition by the readout circuit of the raw analog signals is carried out row by row.

[0034] According to one example, the ratio between the stabilization time Tps_stab and the temperature modification time is greater than 1, preferably 5 and advantageously 10. According to one example, the linear temperature reduction step from Tf to T0 has a direction coefficient less than or equal to 2°C / minute, preferably 1°C / minute and advantageously 0.5°C / minute.

[0035] 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 T0; b. Acquisition of a second plurality of images when the black body has a temperature T4 higher than T0. c. Determination of the bolometers exhibiting a measurement error.

[0036] The term "temperature of an element" means the temperature that the element releases in radiative form, that is to say in the form of an electromagnetic flux whose wavelength(s) are included in the infrared range.

[0037] “A stabilized temperature” means a temperature whose fluctuations are less than 1°C, preferably 0.01°C, and advantageously 0.001°C.

[0038] By "black body" we mean an element or device whose temperature is considered stable and whose surface emissivity (notation: ε) is close to ε=1 (or typically ε>0.97).

[0039] In the present description, a dynamic range of an analog signal converter to digital signals extends over several values, for example from 0 to 100.

[0040] The middle, center, is then understood to be the 50% point of the dynamic range, the value 50 for example. Similarly, the 75% of the dynamic range will be interpreted as the value 75, and the same is true for the 25% point of the dynamic range, which therefore corresponds to the value 25. All this in the case where the dynamic range extends, for example, from 0 to 100.

[0041] 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 an optoelectronic thermal vision device intended to visualize the infrared radiation of a scene, and preferably to be able to evaluate the temperature of the elements present in said scene.

[0042] According to one embodiment, the optoelectronic device comprises at least one matrix of infrared detectors, preferably bolometers and advantageously microbolometers. In the remainder of this description, the term bolometer designates both bolometers and microbolometers.

[0043] Preferably, but not limited to, the bolometers of the bolometer array are based on amorphous silicon. This allows the relative gain between each bolometer to remain identical, regardless of the temperature of the bolometer array as long as the gain-related parameters remain constant, i.e. as long as, for example, the integration time, or the electronic gain, etc. remain constant. This also allows the behavior of the bolometers as a function of temperature to be perfectly reproducible under identical measurement conditions.

[0044] Advantageously, these bolometers are arranged in rows and columns, thus forming a matrix of bolometers. This bolometer matrix is ​​electrically connected to a reading circuit. This reading circuit is configured to collect the raw analog signals from each bolometer. Indeed, each bolometer is configured to generate a raw analog signal based on the thermal radiation it receives.

[0045] According to a preferred embodiment, the readout circuit collects the analog signals from the bolometer array row by row, preferably from one end of the array to its opposite end.

[0046] Advantageously, the reading circuit comprises an analog output capable of providing a plurality of raw analog signals, i.e. the raw analog signals collected line by line.

[0047] Preferably, this analog output is electrically connected to an analog signal to digital signal converter. This converter includes at least one dynamic range, generally expressed in bits. This converter also allows the generation of a thermal image of the scene observed by the optoelectronic device.

[0048] As previously indicated, the prior art suffers from thermal drifts requiring a regular calibration phase to recalibrate the operating parameters of each bolometer, this may further include the bias voltages applied to each bolometer of the matrix, the electrical correction 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 bolometers to maintain a raw analog signal consistent with the intensity of the thermal radiation flux received.

[0049] The prior art thus proposes using a cache whose temperature is known in order to expose the matrix of bolometers to this cache, then to adjust the operating parameters until obtaining a thermal image consistent with the temperature of the cache placed in front of the matrix. This causes the use of the device to stop in its basic function, that is to say the observation of a scene.

[0050] The present invention proposes an innovative solution that does not require the use of a cover and therefore the cessation of the use of the optoelectronic device. A cessation that can be disastrous in the case of use of this technology in a military field for example.

[0051] The present invention proposes a method for calibrating, preferably in the factory, the optoelectronic device. With this calibration, the use of a cover, also called a shutter, is no longer necessary.

[0052] There figure 1 illustrates, according to one embodiment, the different stages of this process.

[0053] This method advantageously 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 a bolometer 12 is consistent with the temperature of the observed scene.

[0054] In particular, this calibration phase 100 allows the adjustment and recording of the bias voltage values ​​to be applied to each bolometer 12 so that each raw analog signal is in the middle range of the dynamic range of the converter.

[0055] Here, the term "midrange" means a range comprising the middle of the dynamic range over an extent of plus or minus 20%, and preferably being centered on the middle of the dynamic range.

[0056] This positioning, roughly in the middle of the dynamic range, helps to reduce, or even avoid, saturation problems at the edge of the range.

[0057] Likewise, advantageously, this calibration phase 100 makes it possible to determine and record the electrical correction voltage of each bolometer 12, this electrical correction voltage is a direct voltage, depending on the temperature of the observed scene; this electrical correction voltage, also called “Offset” in English, is distributed randomly on the matrix 11 of bolometers 12 and is found to be superimposed on each raw analog signal of each bolometer 12. This electrical correction voltage therefore applies a voltage offset to the raw analog signal which should be evaluated 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.

[0058] Advantageously, once the calibration phase 100 has been carried out, the verification phase 200 is carried out. The objective of this verification phase 200 is to control, to verify that the temperature evaluated by each bolometer 12 is in agreement with the temperature of the climatic chamber 20, and therefore with the temperature of the observed scene.

[0059] This method is advantageously carried out once the optoelectronic device 10 is placed facing a black body 40 in a climatic chamber 20 at a temperature T0, 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 and then recording the behavior of the bolometers 12 so as to be able to correlate their optoelectronic response with the temperature of the observed scene generated by the black body 40.

[0060] According to one embodiment, the calibration phase 100 comprises at least the following plurality of successive steps: a. Modify 101 the temperature inside the climatic chamber 20 to reach a first temperature T1 different from T0, preferably lower than T0, advantageously T1 = -30°C; b. Wait 102 for a stabilization time Tps_stab until the temperature of the matrix 11 of bolometers 12 is constant, preferably, but not limited to, equal to T1; it will be noted that, according to one embodiment, the reading of the matrix 11 of bolometers 12 can generate heat; the temperature of the matrix 11 then equilibrates at a temperature substantially higher than T1; advantageously, but not limited to, Tps_stab is less than 2 hours, preferably 1 hour and advantageously 30 minutes;it will be noted that Tps_stab is dependent on the size of the optoelectronic device and its mechanical construction. c. Adjusting 103 the bias voltage of each bolometer 12 so that the value of each raw analog signal is within a range of the predetermined dynamic range of the converter;According to one embodiment, the range is a median range, comprising the middle of the dynamic range, preferably centered on the middle of the dynamic range, advantageously with an extension of less than 20% on either side of the center of the dynamic range;This makes it possible to maintain the full dynamic range of each bolometer 12 in order to reduce, or even not suffer from, saturation, and this for both low and high temperatures;According to one embodiment, the range is a range comprising 25% of the dynamic range, preferably centered 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 makes it possible to maintain the full dynamic range of each bolometer 12 in the high temperature direction for applications in which the observed temperatures are relatively high; According to one embodiment, the range is a range comprising 75% of the dynamic range, preferably centered on the 75% of the dynamic range, advantageously with an extension of less than 20% on either side of the 75% of the dynamic range; This makes it possible to maintain the full dynamic range of each bolometer 12 in the low temperature direction for applications in which the observed temperatures are relatively low;This adjustment of the bias voltages makes it possible to configure the optoelectronic device for its future applications by favoring a homogeneous dynamic, or accentuated on high or low temperatures; d. Recording 104 the adjusted bias voltage of each bolometer 12; this makes it possible to store in memory the bias voltage necessary for the raw analog signals to be in the selected range of the dynamic range; e. Preferably, acquiring a plurality of thermal images so as to be able to carry out an averaging of these thermal images; f. Preferably, determining, for each bolometer 12, the so-called correction electrical voltage value; g. Recording 105, for each bolometer, the correction electrical voltage value corresponding to it;according to one embodiment, at least one of steps f and g can be carried out before or at the same time as at least one of steps c and dh Modifying the temperature inside the climatic chamber to reach a second temperature T2 different from T0 and T1, preferably higher than T1; i. Repeating steps b to h of the calibration phase until a final temperature Tf higher than the initial temperature T0. ;

[0061] Once this calibration phase is complete, the climatic chamber 20 is at a temperature Tf, for example 50°C. During this calibration phase 100, the polarization voltages and the correction electrical voltage values ​​for each temperature having had a thermal stage and for each bolometer were recorded.

[0062] Once this data has been acquired, the method continues with the verification phase 200. This verification phase 200 consists of lowering the temperature 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 decreasing 201 the temperature inside the climatic chamber 20 from Tf until the temperature of the matrix of bolometers is equal to T1; b. Measuring 202 at least one operating criterion of the optoelectronic device 10, during the step 201 of linearly decreasing the temperature from Tf to T1, using the bias voltage values ​​and the offset values ​​recorded as a function of the temperature of the matrix 11 of bolometers; c. If the verification phase meets the evaluation criteria, then the calibration method ends 203, otherwise the calibration phase 100 resumes 204.

[0063] This verification phase 200 thus makes it possible to check that the parameters recorded during the calibration phase 100 are correct and allow the optoelectronic device 10 to operate correctly, i.e. the correct evaluation of the temperature of the observed scene.

[0064] During the verification phase 200, and according to one embodiment, an evaluation of the fixed spatial noise is carried out. In particular, this step seeks to detect the presence or absence of fixed spatial noise. Preferably, minimal, or even non-existent, fixed spatial noise is desired. Fixed spatial noise or FPN in English for Fixed Pattern Noise implies the fact that all the components of the matrix are not exactly identical. Consequently, there may be differences between the bolometers 12, and therefore between the pixels, each bolometer 12 representing a pixel. This step is therefore intended to evaluate the level of this fixed spatial noise in order to estimate whether the adjustments and corrections made result in a reduction, or even cancellation, of the fixed spatial noise.

[0065] 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 and then the average temperature estimated by the optoelectronic device 10 is compared to the temperature of the climatic chamber 20. If there is a match, or if there is a difference less than a threshold value, the test is considered a success, otherwise, it may be necessary to carry out a new calibration phase 100.

[0066] In the event that the verification phase 200 is not conclusive or that it turns out that the adjustment and correction parameters are not correct, the calibration process continues by repeating the calibration phase 100 followed by another verification phase 200, until a calibration estimated to be correct is obtained.

[0067] 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 placed on a test bench facing a black body 40 whose temperature is controlled. This series of successive steps comprises: a. Acquisition of a first image when a black body 40 has a temperature T3 lower than T0; b. Acquisition of a second image when a black body 40 has a temperature T4 higher than T0. c. Detection of bolometers 12 having a measurement error.

[0068] Advantageously, the raw analog signals of these bolometers 12 will be corrected in real time when using the optoelectronic device 10 as a function of the surrounding bolometers 12.

[0069] Advantageously, the acquisition of the first image and the acquisition of the second image make it possible to calculate a relative gain between each bolometer 12 so that all, preferably almost all, of the bolometers 12 give the same response regardless of the temperature of the observed scene. Then, a detection of the bolometers 12, whose response is erroneous or unsatisfactory, is carried out on all of the stored tables to then be used in real time to be able to replace them with values ​​from the surrounding bolometers 12 that are not erroneous.

[0070] Thus, in a clever way, the signals acquired during the acquisition of the first image and the second image are used to calculate a relative gain between pixels, i.e. bolometers, so that they all 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 carried out on all the stored tables to be then used in real time to be able to replace them with values ​​of the surrounding pixels that are not erroneous.

[0071] We will now describe, according to one embodiment, the present invention through the figures 2 to 5 .

[0072] There figure 2 represents a matrix 11 of bolometers 12 comprising a plurality of bolometers 12 arranged in rows 15 and in columns 14. According to one embodiment, the matrix 11 of bolometers 12 can be carried by or can support the reading circuit 13.

[0073] There figure 3 represents a diagram of an installation configured to implement the method according to an embodiment of the present invention.

[0074] In this figure, we notice 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 radiation of the scene generated by the black body 40 can penetrate into the optoelectronic device 10 and be captured by the matrix 11 of bolometers 12 arranged in the optoelectronic device 10.

[0075] The climatic chamber 20 is temperature-controlled so as to impose a temperature on the entire climatic chamber 20 and on everything inside it.

[0076] Advantageously, the temperature inside the climatic chamber 20 can vary for example from -20°C to 60°C.

[0077] 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 one computer, and furthermore a non-transitory memory comprising a computer program product comprising instructions, which when carried out by at least the processor and / or the computer, executes at least the method according to the present invention.

[0078] There 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.

[0079] As previously indicated, the starting temperature T0 corresponds to the ambient temperature. During the calibration phase 100, the first step consists of decreasing the temperature from T0 to T1 which for example can be equal to -10°C. The governing coefficient of the temperature reduction curve 310 from T0 to T1 is less than 10°C / minute, preferably 5°C / minute and advantageously 2.5°C / minute. The governing coefficient of the temperature reduction curve 310 from T0 to T1 is configured to avoid a thermal shock which could lead to either breakage or condensation.

[0080] Once at T1, the temperature of the climatic chamber 20 tends to stabilize and the computer system 30 waits for a stabilization time Tps_stab before continuing the process. Preferably, Tps_stab is between 10 minutes and 180 minutes and advantageously between 30 minutes and 120 minutes. Preferably, this stabilization time is dependent on the camera 10.

[0081] Once the temperature of the optoelectronic device 10 and in particular of the matrix 11 of bolometers 12 is stabilized and equal to the temperature of the observed scene, that is to say to the temperature of the black body 402, the computer system 30 carries out a temperature plateau 320 and the method continues the following different steps: i. Adjusting 103 the bias voltage of each bolometer 12; ii. Recording 104 the adjusted bias voltage of each bolometer 12; iii. Preferably, acquiring a plurality of thermal images; iv. Preferably, determining, for each bolometer 12, a correction electrical voltage value: v. Recording 105, for each bolometer 12, the correction electrical voltage value corresponding to it.

[0082] Once this series of steps has been carried out, in various orders according to an embodiment of the present invention for example, the temperature of the climatic chamber 20 changes.

[0083] Preferably, the temperature of the climatic chamber varies from T1 to T2. The temperature T2 is preferably higher than T1. The coefficient of the temperature rise 330 from T1 to T2 is higher than 2.5°C / minute, preferably 5°C / minute and advantageously 10°C / minute. Advantageously, the different temperature stages are not large in temperature difference from each other, preferably approximately 5°C; thus, the coefficient of the temperature rise 330 from T1 to T2 is configured so that the temperature rise 330 from T1 to T2 is as rapid as possible in order to allow the optoelectronic device 10 the maximum time to reach the chosen temperature, i.e. to stabilize its temperature.

[0084] Again, the computer system 30 waits until the temperature of the matrix 11 of bolometers 12 is stabilized and substantially equal to that of the climatic chamber 20, that is to say at T2, T2 therefore being the temperature of the observed scene.

[0085] Once these conditions are met, the process repeats the previous steps, and so on until the temperature Tf, called the final temperature.

[0086] Thus, according to one embodiment, the calibration phase 100 comprises at least 15, preferably at least 20, and advantageously at least 25 temperature levels, i.e. measurement points of the calibration elements.

[0087] There figure 4 also illustrates the verification phase 200 with the linear temperature decrease 340 from Tf to T1. Advantageously, the direction coefficient of this linear temperature decrease 340 is less than 2°C / min, preferably 1°C / min and advantageously 0.5°C / min.

[0088] Once arrived at T1, if the verification phase 200 is conclusive, the temperature is brought back to 350 to T0 so that the optoelectronic device 10 can be removed from the climatic chamber 20; if the verification phase 200 is not conclusive, the calibration phase 100 starts again.

[0089] There Figure 5 illustrates the positioning of the optoelectronic device 10, for example on a test bench, opposite a black body 40. Thus, the observed scene includes said black body 40. Then, the additional phase described previously can be implemented.

[0090] The present invention proposes an innovative and effective solution to the problems of thermal drift of optoelectronic devices intended to evaluate the temperature of an observed scene. Instead of resorting to regular and very frequent calibration, or resorting to temperature control of the bolometer matrix for example, the present invention proposes a calibration method, preferably in the factory, and advantageously which no longer needs to be implemented outside the factory.

[0091] This method makes it possible to quantify, record and adjust the operating parameters of each bolometer in the bolometer array by subjecting them to a plurality of temperatures, then to carry out a check of this calibration by a linear and non-static evaluation. The present invention thus makes it possible to obtain an optoelectronic device calibrated for the remainder of its use and thus able to continuously observe a scene if necessary without having to interrupt itself to be recalibrated.

[0092] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the claims. List of references

[0093] 10Optoelectronic device 11Bolometer matrix 12Bolometers 13Reading circuit 14Bolometer column 15Bolometer row 16Optical input 20Climatic chamber 30Computer system 40Black body 100Calibration phase 101Modification of the chamber temperature from T0 to T1 102Waiting for temperature stabilization 103Adjustment of the bias voltage of each bolometer 104Recording of the adjusted bias voltage of each bolometer 105Recording of a correction electrical voltage for each bolometer 106Modification of the chamber temperature from T1 to T2 107Repetition of steps 112 to 116 until temperature Tf 200Verification phase 201Linear decrease of the temperature from Tf to T0 202Measurement of at least one operating criterion of the optoelectronic device during step 211 203Validation and end of the calibration process 204Return to the calibration phase 300Temperature curve 301Time scale302Temperature scale 310Temperature decline curve 320Temperature plateau 330Temperature rise 340Linear temperature decline 350Return to temperature T0

Claims

1. Method for calibrating an optoelectronic device (10) comprising an optical input (16), comprising a calibration during which said optoelectronic device (10) is placed in a climatic enclosure (20) at an ambient temperature T0, the optoelectronic device (10) comprising at least one matrix (11) of bolometers (12), preferably microbolometers, configured to measure at least one temperature and at least one reading circuit (13) comprising an analogue output capable of providing a plurality of raw analogue signals intended to form a thermal image, each raw analogue signal corresponding to a bolometer (12) and each raw analogue signal being a function of a scene observed by said optoelectronic device (10), the analogue output of the reading circuit (13) being connected to an analogue to digital signal converter having a predetermined dynamic range, the calibration of said method comprising at least the following two successive phases during which the observed scene is a black body having the temperature of the climatic enclosure (20),: (1) A calibration phase (100) comprising at least the following successive steps: a. Modifying (101) the temperature inside the climatic enclosure (20) to reach a first temperature T1 different from T0 and lower than T0, by temperature drop from T0 to T1; b. Waiting (102) for a stabilisation time Tps_stab for the temperature of the matrix (11) of bolometers (12) to be constant, preferably equal to T1; c. Adjusting (103) the bias voltage of each bolometer (12) such that the value of each raw analogue signal is within a preselected range of the dynamic range, preferably the preselected range corresponds to the midrange of the dynamic range; d. Recording (104) a bias voltage value adjusted of each bolometer (12); e. Recording (105), for each bolometer (12), a correction electric voltage value, this electric voltage being a direct voltage and a function of the temperature of the observed scene, this electric voltage being randomly distributed through the matrix (11) of bolometers (12), and being determined as a voltage offset of an identical voltage response for all bolometers (12) with respect to the raw analogue signal of each bolometer (12) and being superimposed on said raw analogue signal of each bolometer (12) to produce said voltage response; f. Modifying (106) the temperature inside the climatic enclosure (20) to reach a second temperature T2 different from T0 and T1, and greater than T1; g. Repeating 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 greater than the initial temperature T0; (2) A verification phase (200) comprising at least the following successive steps: a. Linearly dropping (201) the temperature inside the climatic enclosure (20) from Tf until the temperature of the matrix (11) of bolometers (12) is equal to T1. b. Measuring (202) at least one operating criterion of the optoelectronic device (10) observing said scene, during the step of linearly dropping temperature from Tf to T1, using the bias voltage values and the correction electric voltage values recorded as a function of the temperature of the matrix (11) of bolometers (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, advantageously has an extension of less than 20% on either side of 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 on 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 of the preceding claims, wherein, if the measurement of the operating criterion indicates a mismatch greater than a threshold value of the temperature value evaluated by the matrix (11) of bolometers (12) relative to the temperature of the observed scene, the calibration phase (100) is executed again.

6. The method according to any of the preceding claims, wherein the step of modifying (101) the temperature is carried out according to a drop curve whose slope is less than 10°C / minute.

7. The method according to any of the preceding claims, wherein the step of recording (105) the correction electric voltage values is performed after a step of averaging a predetermined number of thermal images at a same temperature.

8. The method according to any of the preceding claims, wherein the evaluation of the operating criterion of the optoelectronic device (10) comprises the evaluation of the fixed spatial noise and / or the average thermal value of at least one image.

9. The method according to any of the preceding claims, wherein the matrix (11) of bolometers (12) has an arrangement of the bolometers in rows (15) and in columns (14), and wherein the acquisition by the reading circuit (13) of the raw analogue signals is performed row (15) by row (15).

10. The method according to any of the preceding claims, wherein the ratio between the stabilisation time Tps_stab and the temperature change time is greater than 1, preferably than 5, and advantageously than 10.

11. The method according to any of the preceding claims, wherein the step of linearly dropping temperature from Tf to T1 has a slope less than or equal to 2°C / minute, preferably 1°C / minute.

12. The method according to the preceding claim, wherein the step of linearly dropping temperature from Tf to T1 has a slope less than or equal to 0.5°C / minute.

13. The method according to any of the preceding claims comprising, after calibration, an additional phase in which the optoelectronic device (10) is disposed outside the enclosure so that the optical input (16) is facing a black body (40), the additional phase comprising at least the following successive steps: (1) Acquiring a first plurality of images when the black body (40) has a temperature T3 lower than T0; (2) Acquiring a second plurality of images when the black body (40) has a temperature T4 greater than T0. (3) Determining bolometers (12) with measurement error.

14. A computer program product comprising instructions, which when performed by at least one processor, execute at least the method according to any of the preceding claims, said processor being configured to control a climatic enclosure (20) in which an optoelectronic device (10) is disposed and to control said optoelectronic device (10).