DE102014108971B4 - Calibration procedures and correction procedures for a shutterless infrared camera system and the like - Google Patents

Abstract

Calibration method for a shutter-free infrared camera system (11) comprising a housing (4), an infrared-permeable optics (5), a sensor matrix (6) hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation entering the housing (4) and in the Responding to this, deliver a signal to a signal processing unit (7) and each have an offset and a sensitivity, with an offset correction being carried out for each sensor element as a function of an ambient temperature that can be regulated in a temperature cabinet (1) and of controllable radiation sources (2), wherein under defined conditions, a measurement uncertainty that represents an interference component of the supplied signal influenced by the ambient temperature is determined as a function of a sensor temperature and / or at least one camera temperature, the influences of the ambient temperature by means of correction functions depending on the sensor temperature and / or the camera temperature are estimated and their camera system-specific correction coefficients are stored in a memory unit of the signal processing unit (7) for later correction of an infrared image, characterized in that - that an operating point is set for the sensor matrix (6), with an object temperature range and an ambient temperature range - that a camera temperature-specific non-uniformity of all sensor elements is determined by means of a first characteristic curve comparison of degree n, where n ≥ 2 and correction coefficient matrices a with n = 0, 1, 2, ... n in the storage unit of the signal processing unit (7) are stored, the nonuniformity correction being carried out at n + 1 different ambient temperatures and a constant, homogeneous object temperature, - that a correction curve for taking into account a camera interior radiation is determined, the camera temperature at m position len is measured within the housing (4) of the infrared camera system (11), with m ≥ 1, and the dependence of the signal voltage of the sensor (6) on the camera temperature is determined by means of a compensation calculation, in particular a polynomial approximation of at least second order, the polynomial coefficients α , β, γ are stored in the storage unit of the signal processing unit (7), - that an object temperature-specific non-uniformity of the infrared camera system (11) is determined by means of a second characteristic curve comparison of degree n, where n ≥ 1 and correction coefficient matrices b with n = 0, 1, 2, ... n are stored in the memory unit of the signal processing unit (7), the nonuniformity correction being carried out at n + 1 different object temperatures and a constant ambient temperature.

Description

The invention relates to a calibration method for a shutter-less infrared camera system comprising a housing, an infrared-permeable lens, a sensor matrix hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation entering the housing and, in response, each deliver a signal to a signal processing unit and each have an offset and a sensitivity, an offset correction being carried out for each sensor element as a function of an ambient temperature that can be regulated in a temperature cabinet and of controllable radiation sources.

The invention further relates to a correction method for a shutterless infrared camera system comprising a housing, an infrared-permeable lens, a sensor matrix hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation entering the housing and, in response to this, each deliver a signal to a signal processing unit, an offset correction being carried out for each sensor element by means of a correction calculation with correction parameters specific to the camera system.

Surface temperatures and their distribution are determined using infrared cameras. For this purpose, mostly uncooled microbolometers in the form of sensor matrices are used. A sensor matrix consists of individual microbolometer sensors that are arranged in large numbers in rows i and columns j. The individual sensors of the sensor matrix are also called sensor elements or pixels, with each sensor element being uniquely assigned to a position ij of the sensor matrix. Incident radiation is absorbed and the resulting change in temperature causes a change in the electrical resistance within the sensor material. A constant electrical current is sent cyclically through the individual sensors, with the change in resistance resulting in a variable signal voltage. This is further processed by digital signal processing steps, with an infrared recording in the form of an infrared image (IR image) of the surface temperature of an object to be examined or a measurement scene being able to be displayed. A calibration of the infrared camera or infrared camera system (IR camera) is used to calculate a radiation-proportional output signal (IR image) from a measurement signal (raw image), taking into account all technical-physical properties of the IR camera.

The output signal voltage of a sensor element is proportional to the exchanged radiation flux. The radiation-voltage characteristics of the individual sensor elements of a sensor matrix vary within certain limits.

Infrared-permeable lenses focus areas of the object space on the sensor elements. Due to the large opening angle of the sensor elements, they detect both the radiation components from the observed scene and radiation components that originate from the interior of the infrared camera ( 1 ). The amount of radiated interference depends on various factors, including the ambient temperature and the heat losses within the infrared camera. This dependence on external circumstances has the effect that the amount of detected interference radiation changes unpredictably and influences the temperature measurement. For the lowest possible measurement uncertainty, the influence of the interference radiation on the output signal of each sensor element must be corrected.

The main characteristic parameters of a microbolometer sensor element are the sensitivity (gain) and the operating point (offset). Due to the technology used, the individual sensor elements of a microbolometer array have individually different operating points (offsets) and sensitivities and thus differing characteristics with regard to the incident radiation flux. By correcting this non-uniformity (also called NUC - Non-Uniformity Correction), all sensor elements can be converted to a uniform characteristic, a so-called standard characteristic. So far, the non-uniformity of the sensor element radiation flux voltage characteristics is corrected by calculating a sensor element-dependent differential voltage from the image mean value with homogeneous object radiation and constant ambient temperature. The coefficients of the individual linear differential voltage curves are determined by comparing the characteristics. Two infrared images of a homogeneous radiation source at different temperatures (object temperatures), which cover the entire field of view of the infrared camera, are required for this.

Microbolometer sensors can be operated with and without temperature stabilization. Since the two main characteristic parameters of the sensor elements, sensitivity and offset, depend on the sensor temperature, they are almost constant with a temperature-stabilized microbolometer sensor. If the sensor is not temperature stabilized, the characteristic parameters vary depending on the sensor temperature. Assuming that all sensor elements have the same operating point temperature, the change in sensitivity and offset is the same for all sensor elements.

Due to thermal conduction through the camera housing, the sensor temperature is coupled to the ambient temperature. Changes in the ambient temperature thus affect both the amount of the absorbed radiation flux and the characteristic parameters, sensitivity and offset of the individual sensor elements. The sensor temperature is available during a measurement as an additional measured variable at the respective sensor element output. This makes it possible to correct the influence of the sensor temperature dependency on the signal voltage of each sensor element.

The proportion of the interference radiation in the respective signal voltage of a sensor element has so far been determined in most cases with the aid of an optical shutter in order to then be subtracted from the measurement signal and thus correct the output signal. For example, in Budzier, H., “Radiometric calibration of uncooled infrared cameras”, TUDpress, Dresden, 2014, ISBN 978-3-944331-45-4, the shutter is closed so that only the indoor radiation is detected by each sensor element of the sensor matrix . This offset of the output voltage is then weighted via the shutter characteristic and subtracted from the measurement signal when the shutter is open. For this purpose, the so-called shutter characteristic is recorded, which shows the ratio of shutter-to-signal and shutter-to-signal as a function of the temperature inside the infrared camera. If the temperature inside the infrared camera changes by a certain value or if a certain amount of time has passed, this process is repeated.

A second procedure is out Olbrycht, R. et al., "Thermal drift compensation method for microbolometer thermal cameras", Applied Optics, Vol. 51, No. 11, pp. 1788ff., 2012 known. Instead of a shutter, an infrared filter is used, which is swiveled cyclically into the beam path of the infrared camera. The further process steps correspond to the correction process with shutter.

The two methods described above rely on additional, mechanically moved aids, shutters or infrared filters, which limit the design options of infrared cameras and are mechanically susceptible.

An infrared camera system and an associated correction method that can do without the mechanical components would therefore be advantageous. In Bieszczad, G. et al., "Method of detectors offset correction in thermovision camera with uncooled microbolometer focal plane array", SPIE Proceedings, Vol. 7481, 748100-1-8, 2009 , a method is disclosed which estimates the amount of interference radiation for each sensor element based on the sensor temperature and subtracts it from the measured value, although only the temperature-dependent offset is considered. While the infrared camera is aimed at a homogeneous and constant source of radiation, the ambient temperature is varied at intervals over the entire permissible range. Before the measured values are recorded, it is waited until the temperature distribution in the infrared camera has assumed a steady state. For each sensor element, the compensation calculation to estimate the output voltage as a function of the sensor temperature is used as a polynomial 2 . Order carried out. During the infrared measurement, the sensor temperature is always available as a parameter. Although this method does not require any additional tools, it is limited to infrared cameras with sensors that are not temperature-stabilized. In addition, the compensation calculation of the correction line is carried out for each pixel and requires a correspondingly long time during the calibration of the infrared camera. With currently common sensor resolutions of 640x480 sensor elements or pixels, the time required is in the range of several hours. Another disadvantage is the very delayed reaction of the sensor temperature to a change in the ambient temperature. This means that the measurement uncertainty increases significantly after a sudden change in the ambient temperature and only slowly decreases again.

For a shutterless camera system, a calibration procedure is available in CAO, Yanpeng; TISSE, Christel-Loic: Solid-state temperatue-dependent NUC (Non-Uniformity Correction) in uncooled LWIR (Long-Wave Infra-Red) Imaging System. Proc. SPIE 8704 Infrared Technology and Applications XXXIX, 87042W (June 18, 2013) and SHINDE, Yogesh; BANERJEE, Arup: Design and calibration approach for shutter-less thermal imaging camera without thermal control. 2012 Sixth International Conference on Sensing Technology. Kolkata, India, Dec 18-21. 2012, ICST 2012. Conference Proceedings, ISBN 978-1-4673-2248-5, pp. 259-264. disclosed.

It is therefore the object of the invention to provide a method for correcting recorded infrared images (IR images) that does not require mechanical aids and can be used for infrared camera systems with temperature-stabilized as well as temperature-unstabilized sensors, and a method by means of which infrared camera systems can high sensor resolutions can be calibrated with significantly reduced expenditure of time.

In terms of the method, the object is achieved by a method according to the features of independent claim 1 and by a method according to the features of independent claim 3. In the proceedings under defined conditions, a measurement uncertainty that represents an interference component of the supplied signal influenced by the ambient temperature is determined as a function of a sensor temperature and / or at least one camera temperature, the influences of the ambient temperature being estimated by correction functions depending on the sensor temperature and / or the camera temperature and their camera system-specific correction coefficients be stored in a memory unit of the signal processing unit for later correction of an infrared image.

In one embodiment of the calibration method, the calibration method is used for an infrared camera system with a temperature-stabilized sensor. It is assumed that both the sensitivity of all sensor elements of the sensor matrix and the offset of all sensor elements of the sensor matrix are constant and if there are differences in the main characteristic parameters between the sensor elements, these vary only within certain limits.

In another embodiment of the invention, the calibration method is used for an infrared camera system with a temperature-unstabilized sensor. For this reason, both the sensor temperature-dependent change in sensitivity and the sensor temperature-dependent offset change of the sensor elements must be taken into account in a calibration process.

The following calibration method is proposed according to the invention for an infrared camera system with a temperature-stabilized sensor: An operating point is set for the sensor matrix, an object temperature range and an ambient temperature range being specified. A camera temperature-specific non-uniformity of all sensor elements is determined by means of a first characteristic curve comparison of degree n, where n ≥ 2 and correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n are stored in the memory unit, with the Correction of irregularity is carried out at n + 1 different ambient temperatures and a constant, homogeneous object temperature.

Furthermore, a correction curve is determined to take into account radiation inside the camera, the camera temperature being measured at m points within the housing of the infrared camera system, with m ≥ 1, and the dependence of the signal voltage of the sensor on the camera temperature being determined using a compensation calculation, in particular a polynomial approximation of at least second order the polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m} are stored in the storage unit. The number m and the position of the temperature measuring points within the housing of the infrared camera system are particularly important in order to determine the exact influence of the different camera temperatures within the housing on the signal voltage of the sensor.

Furthermore, an object temperature-specific irregularity of the infrared camera system is determined by means of a second characteristic curve comparison of degree n, where n ≥ 1 and correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n are stored in the memory unit, with the Correction of irregularity is carried out at n + 1 different object temperatures and a constant ambient temperature. The camera temperature is measured at the m locations within the housing of the infrared camera system, with m ≥ 1, and serves as the reference camera temperature for the subsequent correction process.

The first adjustment of the characteristic curve must be at least n = 2, i.e. based on three support points, and thus corresponds to a three-point correction. The second adjustment of the characteristic curve must be carried out at least with degree n = 1, i.e. based on two support points, and thus corresponds to a two-point correction. In order to reduce the measurement uncertainty of the corrected signal voltages of the sensor elements, both characteristic curve adjustments can be carried out to the same degree (but at least to degree n = 2) as a three-point correction.

After each calibration step, sensor elements should be determined whose behavior lies outside of defined limits. The output signals of these sensor elements are ignored for all further steps and replaced by averaged output signals from neighboring sensor elements. This leads to a lower measurement uncertainty of the corrected IR images.

Temperatures can be assigned to the corrected signal voltages of the individual sensor elements in a final signal processing step, a so-called radiometric comparison. In this radiometric comparison, a conversion rule is determined with the aid of calibrated radiation sources at different object temperatures in order to assign absolute object temperatures to the corrected sensor signal voltages. This conversion rule is stored in a memory unit of the signal processing unit.

The following calibration method according to the invention is proposed for an infrared camera system with a temperature-unstabilized sensor: An operating point is set for the sensor matrix, an object temperature range and an ambient temperature range being specified. It becomes a sensor temperature dependent The change in sensitivity of all sensor elements is approximated with a second-order polynomial and correction coefficients g ₂ , g ₁ , g ₀ are stored in the memory unit, a signal voltage difference change of at least two different constant object temperatures being measured as a function of the measured sensor temperature at different ambient temperatures.

Furthermore, a sensor temperature-dependent offset change of all sensor elements is approximated with a third order polynomial and correction coefficients o ₃ , o ₂ , o ₁ , o ₀ are stored in the memory unit, with a signal voltage change being measured at a constant object temperature depending on the sensor temperature at different ambient temperatures . After these calculation steps, the infrared camera system behaves as if the sensor would all be temperature-stabilized. The following calibration is similar to that of an infrared camera system with a temperature-stabilized sensor. A camera temperature-specific non-uniformity of all sensor elements is determined by means of a first characteristic curve comparison of degree n, where n ≥ 2 and correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n are stored in the memory unit, with the Correction of irregularity is carried out at n + 1 different ambient temperatures and a constant, homogeneous object temperature.

Furthermore, a correction curve is determined to take into account radiation inside the camera, the camera temperature being measured at m points within the housing of the infrared camera system, with m ≥ 1, and the dependence of the signal voltage of the sensor on the camera temperature being determined by means of a compensation calculation, in particular a polynomial approximation of at least second order the polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m} are stored in the storage unit.

Furthermore, an object temperature-specific irregularity of the infrared camera system is determined by means of a second characteristic curve comparison of degree n, where n ≥ 1 and correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n are stored in the memory unit, with the Correction of irregularity is carried out at n + 1 different object temperatures and a constant ambient temperature. The camera temperature is measured at the m points inside the housing of the infrared camera system, with m ≥ 1, and, together with the sensor temperature, which is also measured, serves as a reference temperature for the subsequent correction process.

The first adjustment of the characteristic curve must be at least n = 2, i.e. based on three support points, and thus corresponds to a three-point correction. The second adjustment of the characteristic curve must be carried out at least with degree n = 1, i.e. based on two support points, and thus corresponds to a two-point correction. In order to reduce the measurement uncertainty of the corrected signal voltages of the sensor elements, both characteristic curve adjustments can be carried out to the same degree (but at least to degree n = 2) as a three-point correction.

After each calibration step, sensor elements should be determined whose behavior lies outside of defined limits. The output signals of these sensor elements are ignored for all further steps and replaced by averaged output signals from neighboring sensor elements. This leads to a lower measurement uncertainty of the corrected IR images.

Temperatures can be assigned to the corrected signal voltages of the individual sensor elements in a final signal processing step, a so-called radiometric comparison. In this radiometric comparison, a conversion rule is determined with the aid of calibrated radiation sources at different object temperatures in order to assign absolute object temperatures to the corrected sensor signal voltages. This conversion rule is stored in a memory unit of the signal processing unit.

The raw data acquisition of the IR images and the acquisition of the respective associated sensor and camera temperatures for the individual steps of the calibration process can take place in any order and are not decisive for the final provision of the correction coefficients or polynomial coefficients. Rather, it is important that the determined correction coefficients or polynomial coefficients are stored in a memory unit for a later correction of an infrared image.

• - In einem ersten Korrekturschritt wird eine sensortemperaturabhängige Empfindlichkeitsänderung mit einem Polynom zweiten Grades korrigiert,
• - in einem zweiten Korrekturschritt wird eine sensortemperaturabhängige Offsetänderung mit einem Polynom dritten Grades korrigiert,
• - in einem dritten Korrekturschritt wird eine kameratemperaturspezifische Ungleichförmigkeit mittels eines Kurvenabgleichs vom Grad n, mit n ≥ 2 korrigiert,
• - in einem vierten Korrekturschritt wird der Einfluss einer Kamerainnenraumstrahlung entsprechend einer Korrekturkurve korrigiert,
• - in einem fünften Korrekturschritt wird eine objekttemperaturspezifische Ungleichförmigkeit des Infrarotkamerasystems mittels eines Kurvenvergleichs vom Grad n, mit n ≥ 1 korrigiert. Dabei werden die infrarotkamerasystemspezifische Korrekturkoeffizienten, welche in einem vorgeschalteten Kalibrierverfahren unter bekannten Bedingungen ermittelt wurden, für die Korrektur einer Infrarotaufnahme verwendet.

In terms of the method, the object is also achieved in that the correction coefficients or polynomial coefficients determined in the calibration method are subsequently used by a correction method according to the invention, the correction method having the following steps:

• - In a first correction step, a sensor temperature-dependent change in sensitivity is corrected with a polynomial of the second degree,
• - In a second correction step, a sensor temperature-dependent offset change is corrected with a third-degree polynomial,
• - In a third correction step, a camera temperature-specific non-uniformity becomes by means of a curve adjustment of degree n, corrected with n ≥ 2,
• - In a fourth correction step, the influence of radiation inside the camera is corrected according to a correction curve,
• In a fifth correction step, an object temperature-specific irregularity of the infrared camera system is corrected by means of a curve comparison of degree n, with n ≥ 1. The correction coefficients specific to the infrared camera system, which were determined in an upstream calibration process under known conditions, are used to correct an infrared image.

In one embodiment of the correction method, the first correction step uses correction coefficients g ₂ , g ₁ , g ₀ , which are derived from a measured signal voltage difference change between at least two different constant object temperatures as a function of a measured sensor temperature θ _s be determined.

In a further embodiment of the correction method, the second correction step uses correction coefficients o ₃ , o ₂ , o ₁ , o ₀ , which are derived from a signal voltage change at a constant object temperature as a function of the sensor temperature θ _s can be determined at different ambient temperatures.

In a further embodiment of the correction method, the third correction step uses correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n, which are determined at n + 1 different ambient temperatures and a constant, homogeneous object temperature.

In a further embodiment of the correction method, the fourth correction step uses polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m of} a correction curve and measured camera temperatures θ _{K, m} at m different locations within the housing of the infrared camera system, with m 1, where the polynomial coefficients are determined from the dependence of the signal voltage of the sensor on the camera temperature θ _{K, m} at different points m within the housing of the infrared camera system, with m ≥ 1.

In a further embodiment of the correction method, the fifth correction step uses correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n, which are determined at n + 1 different object temperatures and a constant ambient temperature.

The correction method can preferably be expanded by a radiometric comparison. The signal voltages are converted into absolute object temperatures using a conversion rule, this conversion rule being determined in a radiometric comparison as part of a calibration process of the infrared camera system with the help of calibrated radiation sources at different object temperatures.

In one embodiment of the correction method according to the invention, correction steps one to five are carried out for a temperature-unstabilized sensor, i.e. from the first correction step to the fifth correction step.

In another embodiment of the correction method according to the invention, only correction steps three to five are carried out for a temperature-stabilized sensor, i.e. from the third correction step to the fifth correction step. The first two correction steps are not necessary, since in an infrared camera system with a temperature-stabilized sensor it is assumed that the sensitivity and the offset of the sensor elements remain constant for all sensor elements when the ambient temperature changes.

On the arrangement side, the object is achieved by an infrared camera system, with at least two temperature measuring means measuring a camera temperature being arranged inside the housing of the infrared camera system.

In one embodiment of the infrared camera system, the temperature measuring means is designed as a thermocouple and / or a thermal resistor.

In the following, the invention will be explained in more detail with the aid of exemplary embodiments.

• 1 eine schematische Darstellung der von den Sensorelementen detektierten Strahlungsflüsse aus dem betrachteten Halbraum,
• 2 eine schematische Darstellung der Kalibrieranordnung eines Infrarotkamerasystems,
• 3 einen schematischen Ablauf eines Korrekturverfahrens für ein Infrarotkamerasystem mit einem temperaturstabilisierten Sensor, wobei drei Kamerainnenraumtemperaturen gemessen werden und beide Kennlinienabgleiche zum Grad n=2 erfolgen und die Matrixmultiplikationen elementweise durchgeführt werden,
• 4 einen schematischen Ablauf eines Korrekturverfahrens für ein Infrarotkamerasystem mit einem temperaturunstabilisierten Sensor, wobei drei Kamerainnenraumtemperaturen gemessen werden und beide Kennlinienabgleiche zum Grad n=2 erfolgen und die Matrixmultiplikationen elementweise durchgeführt werden,
• 5 eine Übersicht zur Bestimmung der Korrekturkoeffizienten-Matrizen für eine Drei-Punkt-Korrektur, wobei die Matrixmultiplikationen elementweise durchgeführt werden,
• 6 eine Übersicht zur Bestimmung der Korrekturkoeffizienten-Matrizen für eine Zwei-Punkt-Korrektur, wobei die Matrixmultiplikationen elementweise durchgeführt werden.

The accompanying drawings show

• 1 a schematic representation of the radiation fluxes detected by the sensor elements from the half-space considered,
• 2 a schematic representation of the calibration arrangement of an infrared camera system,
• 3 a schematic sequence of a correction method for an infrared camera system with a temperature-stabilized sensor, with three camera interior temperatures being measured and both characteristic curve comparisons for Degree n = 2 and the matrix multiplications are carried out element by element,
• 4th a schematic sequence of a correction process for an infrared camera system with a temperature-unstabilized sensor, whereby three camera interior temperatures are measured and both characteristic curve comparisons are made to the degree n = 2 and the matrix multiplications are carried out element by element,
• 5 an overview for determining the correction coefficient matrices for a three-point correction, whereby the matrix multiplications are carried out element by element,
• 6 an overview for the determination of the correction coefficient matrices for a two-point correction, the matrix multiplications being carried out element by element.

2 shows schematically an infrared camera system 11 in a temperature cabinet 1 , with the full field of view 10 of the infrared camera system 11 from a surface heater 2 is filled out. A movable black matt cover 3 can be swiveled into the field of vision of the infrared camera system so that it completely covers the field of vision of the infrared camera system. With this structure, the conditions for calibrating the infrared camera system 11 specifically adjusted and controlled.

In a first embodiment, the calibration method of an infrared camera system 11 with temperature stabilized sensor are explained in more detail. The sensor of the infrared camera system can 11 for example, be kept at a constant temperature by a Peltier element. Due to the constant sensor temperature, it is assumed that the sensitivity and the offset of the individual sensor elements are constant and independent of the ambient temperature. During the calibration, the infrared camera system is in a temperature cabinet 1 whose temperature can be regulated. The temperature in the temperature cabinet 1 sets the ambient temperature of the infrared camera system 11 and influences the temperature inside the camera of the infrared camera system 11 .

In a calibration step, the non-uniformity of the sensor elements with regard to the influence of different ambient temperatures, ie different components of the interfering radiation in the camera, is to be corrected so that all sensor elements react in the same way to changes in the components of the interfering radiation in the camera. The infrared camera system 11 is on a surface heater 2 with a homogeneous radiant surface with a constant temperature and thus a constant radiation ist. There are now at least three ambient temperatures T _U1 , T _U2 , T _U3 in the temperature cabinet 1 set, the camera temperature of the infrared camera system 11 is coupled to the ambient temperature. The temperatures in the temperature cabinet are preferably 1 set so that they match the values of the upper and lower limit of the permissible ambient temperature measuring range of the infrared camera system 11 correspond. The remaining number of required temperatures should be evenly distributed over the ambient temperature measuring range. In the steady state, the uncorrected sensor element voltages U _{p, ij are} measured at the at least three support points, ie at the set ambient temperatures T _U1 , T _U2 , T _U3 , the absolute values of the temperatures not being of interest here. From the measured signal voltages U _{p, ij of} all sensor elements at the respective support points, the element-wise squared values U _{p, ij} ² , the mean value U _p and the differences to the mean value ΔU _{p, ij are} calculated. Using the calculation rule 5 the correction coefficient matrices a _{n, ij are} calculated for a characteristic curve adjustment in the form of a three-point correction. Assuming that the deviations of the signal voltage curves of the sensor elements from a standard curve at different ambient temperatures and thus different camera interior radiation results as a quadratic relationship, a three-point correction is sufficient. In order to further reduce the measurement uncertainty, more than three support points can be used to calculate the correction coefficient matrices a _{n, ij} . For this purpose, several sets of auxiliary correction coefficient matrices a _{n, ij} ″ are calculated for different combinations of three support points each, the outer support points remaining the same and only the third required support point being varied. The calculated auxiliary correction coefficient matrices each have the same degree n (with n = 0, 1, 2) are then averaged and result in the final correction coefficient matrices a _{n, ij} . These correction coefficient matrices a _{n, ij} with n = 0, 1, 2 are subjected to an infrared image for a later correction process real measurement conditions in a storage unit of the signal processing unit 7th of the infrared camera system 11 , for example in an FPGA. After this first calibration step, the corrected infrared image is homogeneous for different ambient temperatures and the absolute value of the corrected signal voltages corresponds to the amount of the mean value of the uncorrected signal voltages of all sensor elements. In another calibration step, the influence of different ambient temperatures, ie different camera interior radiation components, should be corrected in terms of amount based on the measured camera interior temperature, hereinafter referred to as camera temperature, with the infrared camera system pointing to a homogeneously radiating surface radiator 2 is directed with a constant temperature T _obj and where the ambient temperature in the temperature _cabinet 1 is changed according to a predetermined, but arbitrary temperature regime. While the temperature is in the temperature cabinet 1 constantly changes, over time t at least one Set the camera temperature θ _{K, 1} and the mean value of all sensor elements within the infrared camera system U _p measured. The measurement takes place at different times i. There are preferably several temperature sensors within the housing 4th of the infrared camera system and the camera temperature can be measured at several points m = 1, 2, 3, ... simultaneously. It is assumed that a change in ambient temperature has different effects on the interior of the camera and that the total radiation component can be viewed as a linear superposition of various radiation components, which can be estimated by temperature measurements at different points m within the camera. The aim of this step is to correct the amount of the influence of different ambient temperatures and thus the temperature inside the camera so that the corrected absolute value of the signal voltage is independent of the ambient temperature. The signal voltage component U _{K, m} , caused by the radiation inside the camera, is determined indirectly by measuring the temperature inside the camera θ _{K, m} at points m and with a quadratic polynomial U _{K, m} (θ _{K, m} ) = α _{K, m} * θ _{K, m} ² + β _{K, m} * θ _{K, m} + γ _{K, m} approximated. This equation is overdetermined by the large number of recorded measured values and the associated camera temperatures at the respective measurement time i, so that the coefficients α _{K, m} , β _{K, m} , γ _{K, m} can be determined by a compensation calculation. These are also stored in a memory unit of the signal processing unit 7th of the infrared camera system, for example in an FPGA for a later correction process of an infrared recording under real measurement conditions.

In a further calibration step, the non-uniformity of the sensor elements with regard to the influence of different object radiation is to be corrected in such a way that all sensor elements react in the same way to changes in the object radiation. The infrared camera system is located in a temperature cabinet for this 1 . The ambient temperature is kept at a constant value Tu, the resulting camera temperatures θ _{K0, m being} measured and used as reference camera temperatures for the correction process. The temperature of the surface heater 2 at which the infrared camera system is aimed is varied. At least two different surface radiator temperatures, ie object temperatures T _{Obj, 1} , T _{Obj, 2 are} set. To reduce the measurement uncertainties, the number of object _temperatures T _{obj can correspond to} the number of set ambient temperatures Tu from the calibration _step for correcting the non-uniformity of the influence of different ambient temperatures, ie camera temperatures. In this case, the correction coefficients are determined analogously to the calculation rule shown in 5 . In the case of a two-point correction, the sensor element voltages U _{p, ij} are again measured at the at least two support points, ie at the set object temperatures, the absolute values of the temperatures not being of interest here either. The mean value is obtained from the measured signal voltages U _{p, ij of} all sensor elements at the respective support points U _p and the differences to the mean value ΔU _{p, ij are} calculated. Using the calculation rule shown in 6 the correction coefficient matrices b _{n, ij are} calculated for a characteristic curve adjustment in the form of a two-point correction. Assuming that the deviations of the signal voltage curves of the sensor elements from a standard curve at different object temperatures result as a linear relationship, a two-point correction is sufficient. These correction coefficient matrices b _{n, ij} with n = 0, 1, 2 are stored in the storage unit of the signal processing unit for a later correction method of an infrared recording under real measurement conditions 7th of the infrared camera system 11 , for example stored in an FPGA. After this step and the implementation of the calibration steps mentioned above, a homogeneous infrared image that is independent of ambient temperature and therefore also independent of the temperature of the camera interior is obtained with constant object radiation. Before the first and after each individual calibration step, those sensor elements can be determined whose behavior, ie for example their output signal voltages, are outside defined limits. The output signals of these sensor elements are ignored for all further steps and are replaced by averaged output signals from neighboring sensor elements.

The corrected signal voltages of the individual sensor elements are assigned temperatures by means of the radiometric comparison. For this purpose, the infrared camera system is aimed at calibrated radiation sources whose absolute object temperatures are known. The observed object temperatures should be evenly distributed over the permissible measuring range of the infrared camera system. The rule for converting sensor signal voltage into absolute object temperature can be described by a function with the aid of a compensation calculation or by an allocation table (look-up table) and in a memory unit of the signal processing unit 7th of the infrared camera system 11 get saved.

In a second exemplary embodiment, the calibration method of an infrared camera system with a temperature-unstabilized sensor is to be explained in more detail. In contrast to an infrared camera system with a temperature-stabilized sensor in which it is assumed that the sensor parameters (sensitivity and offset) do not depend on the ambient temperature are influenced, in an infrared camera system with a temperature-unstabilized sensor, the standard characteristic of the sensor parameters changes with the change in the ambient temperature. It is therefore imperative to first carry out a sensitivity and offset calibration in order to carry out the temperature-dependent correction of the standard characteristic.

The infrared camera system is located in a temperature cabinet for this purpose 1 whose temperature can be regulated. At least two homogeneous and constant surface emitters are required for sensitivity calibration 2 with different _heater temperatures (θ _obj1 , θ _obj2 with θ _obj1 ≠ θ _obj2 ) necessary. In the calibration setup ( 2 ) is located outside the temperature cabinet 1 and in the beam path 10 between the surface radiators 2 and the infrared camera system 11 a black matt cover 3 that the object radiation of the surface emitter 2 on the infrared camera system 11 can interrupt and for the sensor 6 of the infrared camera system 11 a homogeneous surface heater 2 represents. The infrared camera system 11 is on the two surface radiators 2 directed, the radiation from both surface emitters being seen by the infrared camera system with only a short delay time, ie at approximately the same time ti, so that the camera temperature 8a , 8b before and after swiveling the diaphragm in and out 3 can be considered the same.

In a first step, the ambient temperature is determined in the temperature cabinet 1 changed according to a given but arbitrary temperature regime and the aperture 3 repeatedly for a short time in the beam path 10 between the surface radiators 2 and the infrared camera system 11 swiveled in that the infrared camera system 11 alternately the homogeneous radiation surface of the screen 3 and that of the surface heater 2 sees. Raw data sets of sensor element voltages U _{p, ij} are repeatedly recorded, which can be distinguished with regard to their radiation source. The mean value is derived from this data U _{p of} the sensor element stresses is calculated. Then the differential voltage is the first calculated mean value when looking at the black matt screen 3 and the second calculated mean value when looking at the surface emitter 2 , calculated and stored over time t at different times ti, with the ambient temperature Tu in the temperature cabinet 1 is varied, whereby the sensor temperature changes. This differential voltage changes with increasing sensor temperature. This temperature dependency of the sensitivity can be described with a polynomial of the second degree: G _v (θ _S ) = g ₂ * θ _S ² + g ₁ * θ _S + g ₀ . A compensation calculation for determining the polynomial coefficients of the overdetermined equation G _V (θ _S ) = g ₂ * θ _S ² + g ₁ * θ _S + g _{0 is} carried out with the differential voltage values from the recorded series of measurements and an infrared recording is used for a later correction process using real measurement conditions in the storage unit of the signal processing unit 7th of the infrared camera system 11 , for example in an FPGA.

The sensor temperature dependency of the sensor element offset can be described with a third-degree polynomial: O _V (θ _S ) = o ₃ * θ _S ³ + o ₂ * θ _S ² + o ₁ * θ _S + o ₀ , although this change does not can be measured directly, since both the sensor temperature θ _S as well as the temperature inside the camera θ _K to change. The influence of the changing radiation inside the camera can depend on θ _K can be described with a polynomial of the second degree: O _K (θ _K ) = k ₂ * θ _K ² + k ₁ * θ _K + k ₀ . In order to determine the sensor temperature dependency of the offset, both influences on the sensor signal voltages, namely the influence of the sensor element offset that changes depending on the sensor temperature and the influence of the camera interior radiation that changes depending on the camera temperature, must be taken into account when the ambient temperature changes.

This is the infrared camera system 11 to a homogeneous radiation source 2 directed with constant object temperature. The ambient temperatures in the temperature cabinet 1 are changed according to a given but arbitrary temperature regime. The raw data sets for sensor element voltages U _{p, ij} and the sensor temperature θ _S and the camera temperature θ _{K, m} are at at least two different locations m inside the housing 4th of the infrared camera system 11 measured over time t. The coefficients of the linear superposition of all influences O ( θ _S , θ _{K, m} ) = O _V (θ _S ) + ΣO _{K, m} (θ _{K, m} ). The equation is over-determined due to the large number of nodes. The coefficients with which the influence of the sensor temperature can be calculated using the equation O _K (θ _K ) = k ₂ * θ _K ² + k ₁ * θ _K + k ₀ will be used for a later correction method of an infrared recording under real Measurement conditions in the storage unit of the signal processing unit 7th of the infrared camera system 11 , for example in an FPGA.

As a result of these calibration steps, it is possible to correct the influence of the sensor temperature on the sensitivity and the offset of the sensor elements of the infrared camera system, so that the corrected signal voltages behave as if the sensor were temperature-stabilized. The following calibration steps are the same as for an infrared camera system with a temperature-stabilized sensor, see the explanations relating to the first exemplary embodiment for a calibration method of an infrared camera system with a temperature-stabilized sensor.

In contrast to an infrared camera system with a temperature-stabilized sensor, in the second comparison of the characteristics of an infrared camera system with a temperature-unstabilized sensor, in addition to the camera temperatures θ _{K0, m} , the sensor temperature θ _{S0 is also} measured. Together, these temperatures are used as reference temperatures for the correction process.

An exemplary embodiment for the correction method using a temperature-stabilized sensor is to be specified below. 3 shows the schematic sequence of the correction method according to the invention. The infrared camera system records an object scene during a temperature measurement. The uncorrected sensor element voltages of all sensor elements of the sensor element array are sent to the signal processing unit as so-called raw data U _{p, ij} 7th of the infrared camera system 11 passed on. For each sensor element, using the coefficient matrices a _{2, ij,} a _{1, ij} , a _{0, ij} stored from the calibration process, a corrected sensor element voltage U ** _{p, korr, ij is used} to correct the irregularity with respect to a changing Camera temperature calculated. Since the correction coefficient matrices a _{n, ij} with n = 0,1,2 using at least three support points were calculated in the calibration process, the correction of the non-uniformity with respect to the radiation inside the camera is carried out as a characteristic curve adjustment of degree n = 2.

The influence of the temperature inside the camera, i.e. the radiation inside the camera, is calculated using the correction function U _K ( θ _K ) and the difference to the correction value U _K (θ _K0 ) at the reference camera temperature θ _K0 specified by the calibration is subtracted from the corrected sensor voltage U ** _{p, corr} , with U _K (θ _K ) = U _{K, 1} (θ _{K, 1} ) + U _{K, 2} (θ _{K, 2} ) + U _{K, 3} (θ _{K, 3} ) + ... + U _{K, m} (θ _{K, m} ), where for determining U _K ( θ _K ) the camera temperature at at least one point inside the housing 4th of the infrared camera system, preferably measured at two or more points with m = 1, 2, 3, ... and the coefficients α _{K, m} , β _{K, m} , γ _{K, m} stored from the calibration method are used in accordance with the second degree polynomial .

Then there is the nonuniformity correction with regard to different object temperatures, ie object radiation, a corrected sensor element voltage U _{p, korr, ij being} calculated for each sensor element with the correction coefficient matrices b _{n, ij} stored from the calibration process according to a two-point correction. The result is a homogeneous, ambient temperature-independent infrared image with constant object radiation.

The signal voltages of the sensor elements determined during the calibration, the output voltage signals of which are outside defined limits, are ignored and replaced by the corrected signal voltages of neighboring sensor elements.

The corrected signal voltages of the sensor elements are finally converted into temperatures, the conversion rule of the adjustment carried out during the calibration being used.

4th shows a second embodiment for an infrared camera system 11 with temperature unstabilized sensor. The infrared camera system takes 11 an object scene during a temperature measurement. The uncorrected sensor element voltages of all sensor elements of the sensor element array are sent to the signal processing unit as so-called raw data U _{p, ij} 7th of the infrared camera system 11 passed on. In the first correction step, the sensor temperature-dependent change in sensitivity of the sensor elements is based on the currently measured sensor temperature θ _S to U _{g, korr, ij} with a polynomial of the second degree using the coefficients g ₂ , g ₁ , g ₀ determined in the calibration method, which is normalized by the correction value at the reference sensor temperature θ _S0 measured in the calibration. The sensor temperature-dependent offset change is then corrected by U _{o, korr, ij} = U _{g, korr, ij} + o ₃ * θ _S ³ + o ₂ * θ _S ² + o ₁ * θ _S + o ₀ - O _V (θ _S0 ), whereby the correction coefficients o ₃ , o ₂ , o ₁ , o ₀ and the reference sensor temperature θ _S0 determined in the calibration process are used. After this correction step, the sensor elements of the sensor matrix of the infrared camera system behave as if there were an infrared camera system with a temperature-stabilized sensor. The other correction steps take place analogously to the first exemplary embodiment for the correction method using a temperature-stabilized sensor.

List of reference symbols

1

Temperature cabinet

2

Surface heater with object temperature

3

cover

4th

casing

5

infrared permeable lens system

6

Sensor matrix with sensor elements

7th

Signal processing unit

8a, b

Temperature measuring means

10

Field of view of the infrared camera system

11

Infrared camera system

θ _S

Sensor temperature

θ _K

Indoor temperature of the camera

Claims (13)

Calibration method for a shutterless infrared camera system (11) comprising a housing (4), an infrared-permeable optics (5), a sensor matrix (6) hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation incident in the housing (4) and Responding to this, deliver a signal to a signal processing unit (7) and each have an offset and a sensitivity, an offset correction being carried out for each sensor element as a function of an ambient temperature that can be regulated in a temperature cabinet (1) and of controllable radiation sources (2), wherein under defined conditions, a measurement uncertainty that represents an interference component of the supplied signal influenced by the ambient temperature is determined as a function of a sensor temperature and / or at least one camera temperature, the influences of the ambient temperature by means of correction functions depending on the sensor temperature and / or the camera temperature are estimated and their camera system-specific correction coefficients are stored in a storage unit of the signal processing unit (7) for later correction of an infrared image, characterized in that - that an operating point is set for the sensor matrix (6), with an object temperature range and an ambient temperature range - that a camera temperature-specific non-uniformity of all sensor elements is determined by means of a first characteristic curve comparison of degree n, where n ≥ 2 and correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n in the memory unit of Signal processing unit (7) are stored, the non-uniformity correction being carried out at n + 1 different ambient temperatures and a constant, homogeneous object temperature, - that a correction curve for taking into account a camera interior radiation is determined, the camera temperature is measured at m locations within the housing (4) of the infrared camera system (11), with m ≥ 1, and the dependence of the signal voltage of the sensor (6) on the camera temperature is determined by means of a compensation calculation, in particular a polynomial approximation of at least second order, the Polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m} are stored in the memory unit of the signal processing unit (7), - that an object temperature-specific irregularity of the infrared camera system (11) is determined by means of a second characteristic curve comparison of degree n, where n ≥ 1 and correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n are stored in the memory unit of the signal processing unit (7), the irregularity correction being carried out at n + 1 different object temperatures and a constant ambient temperature. Calibration procedure according to Claim 1 , characterized in that the calibration method is used for an infrared camera system with a temperature-stabilized sensor. Calibration method for a shutterless infrared camera system (11) comprising a housing (4), an infrared-permeable optics (5), a sensor matrix (6) hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation incident in the housing (4) and Responding to this, deliver a signal to a signal processing unit (7) and each have an offset and a sensitivity, with an offset correction being carried out for each sensor element as a function of an ambient temperature that can be regulated in a temperature cabinet (1) and of controllable radiation sources (2), wherein under defined conditions, a measurement uncertainty representing an interference component of the supplied signal influenced by the ambient temperature is determined as a function of a sensor temperature and / or at least one camera temperature, the influences of the ambient temperature by means of correction functions depending on the sensor temperature and / or the camera temperature are estimated and their camera system-specific correction coefficients are stored in a storage unit of the signal processing unit (7) for later correction of an infrared image, characterized in that - that an operating point is set for the sensor matrix (6), with an object temperature range and an ambient temperature range - that a sensor temperature-dependent change in sensitivity is approximated with a second order polynomial and correction coefficients g ₂ , g ₁ , g ₀ are stored in the memory unit of the signal processing unit (7), a signal voltage difference change of at least two different constant object temperatures depending on the measured sensor temperature is measured at different ambient temperatures, - that a sensor temperature-dependent offset change is approximated with a third order polynomial and correction coefficients o ₃ , o ₂ , o ₁ , o ₀ are stored in the memory unit of the signal processing unit (7), a signal voltage change being measured at a constant object temperature depending on the sensor temperature at different ambient temperatures, - that a camera temperature-specific non-uniformity of all sensor elements by means of a first characteristic curve comparison of degree n is determined, where n 2 and correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n stored in the memory unit of the signal processing unit (7) The non-uniformity correction is carried out at n + 1 different ambient temperatures and a constant, homogeneous object temperature, - that a correction curve is determined to take into account radiation inside the camera, the camera temperature at m locations within the housing (4) of the infrared camera system, with m ≥ 1 , is measured and the dependence of the signal voltage of the sensor on the camera temperature is determined by means of a compensation calculation, in particular a polynomial approximation of at least the second order, with certain polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m} in the memory unit of the signal processing unit (7 ) are stored, - that an object temperature-specific irregularity of the infrared camera system is determined by means of a second curve comparison of degree n, where n ≥ 1 and correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n in the storage unit of Signal processing unit (7) stored The non-uniformity correction is carried out at n + 1 different object temperatures and a constant ambient temperature. Calibration procedure according to Claim 3 , characterized in that the calibration method is used for an infrared camera system with a temperature-unstabilized sensor. Calibration method according to one of the preceding claims, characterized in that the first characteristic curve adjustment represents a three-point correction. Correction method for a shutterless infrared camera system (11) comprising a housing (4), an infrared-permeable optics (5), a sensor matrix (6) hereinafter referred to as a sensor, consisting of several sensor elements that detect radiation incident in the housing (4) and Responding to each supply a signal to a signal processing unit (7), an offset correction being carried out for each sensor element by a correction calculation with camera system-specific correction parameters, characterized in that - in a first correction step a sensor temperature-dependent change in sensitivity is corrected with a polynomial of the second degree, - in a second correction step a sensor temperature-dependent offset change is corrected with a third degree polynomial, - in a third correction step a camera temperature-specific irregularity by means of a curve adjustment of degree n, with n ≥ 2 is corrected, - in a vie rth correction step the influence of a camera interior radiation is corrected according to a correction curve, - in a fifth correction step an object temperature-specific irregularity of the infrared camera system is corrected by means of a curve comparison of degree n, with n ≥ 1, - infrared camera system-specific correction coefficients are used for the correction. Correction procedure according to Claim 6 , characterized in that the first correction step as a function of a measured sensor temperature θ _s uses a sensor temperature-dependent change in sensitivity correcting correction coefficients g ₂ , g ₁ , g ₀ which are derived from a measured signal voltage difference change between at least two different constant object temperatures as a function of a measured sensor temperature at different Ambient temperatures can be determined. Correction procedure according to Claim 6 , characterized in that the second correction step uses a sensor temperature-dependent offset change correcting correction coefficients o ₃ , o ₂ , o ₁ , o _{0 as a} function of a measured sensor temperature θ _s , which are derived from a signal voltage change at a constant object temperature as a function of the sensor temperature at different ambient temperatures be determined. Correction procedure according to Claim 6 , characterized in that the third correction step uses correction coefficient matrices a _{n, ij} with n = 0, 1, 2, ... n, which are determined at n + 1 different ambient temperatures and a constant, homogeneous object temperature. Correction procedure according to Claim 6 , characterized in that the fourth correction step polynomial coefficients α _{K, m} , β _{K, m} , γ _{K, m of} a correction curve and a measured camera temperature θ _{K, m} at m different locations within the housing (4) of the infrared camera system (11), wherein m ≥ 1, the polynomial coefficients of the correction curve being determined from the dependence of the signal voltage of the sensor on the camera temperature θ _{K, m} at various points m, with m ≥ 1. Correction procedure according to Claim 6 , characterized in that the fifth correction step uses correction coefficient matrices b _{n, ij} with n = 0, 1, 2, ... n, which are determined at n + 1 different object temperatures and a constant ambient temperature. Correction procedure according to Claim 6 , characterized in that the correction steps one to five are carried out for an infrared camera system (11) with a temperature-unstabilized sensor. Correction procedure according to Claim 6 , characterized in that only correction steps three to five are carried out for an infrared camera system (11) with a temperature-stabilized sensor.

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