RU2727349C1 - Method of thermography of a remote object - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: invention relates to measurement equipment and a method for thermography of a remote object. Method includes formation in a given spectral range of an image of a remote object at a receiving site of a matrix radiation detector, recording electric signals from sensitive elements of the receiver and forming an array of digital data of temperature of the object. At that, contact temperature is used to measure temperature of atmosphere at receiver location, to correct reflected radiation receiver temperature, taking radiation coefficient of atmosphere equal to one, and achieving equality of the measured temperature of the atmosphere to its actual temperature, measured by a contact method at the location of the receiver. After correcting, a new array of digital data of the temperature field of the object is formed, which is taken as an array of its actual temperatures.EFFECT: technical result consists in improvement of accuracy of measurements and expansion of the range of measured objects.1 cl, 1 dwg, 1 tbl

Description

The invention relates to measuring technology, in particular to temperature measurements carried out using infrared measuring instruments, can be used in thermal tests of various man-made objects and is intended for remote measurements of actual temperature fields of remote or weakly heated objects.

At the present level of development of technology for remote temperature measurement (thermography) of remote objects, the following methods are used or known.

The known method, which includes the formation at one wavelength of infrared (IR) radiation of two images on each of the two matrix image receivers. One of the two images on each receiver is an image of the object of investigation, and the other is an image of a temperature standard, the formation of which is carried out under the same imaging conditions as for the object of investigation. The image on one of the two receivers is formed using a mirror scanner. Based on the data obtained, an array of digital data is formed from all images, taking into account the relationship between the brightness temperature reproduced by the temperature standard and the corresponding digital value of the electrical signal from the elements of both image receivers (RF patent No. 2552599, IPC G01J 5/00; G01J 5/52 , publ. 10.06.2015, BI No. 16). The claimed technical result is an increase in the time resolution of measurements.

In this method, in order to ensure the specified accuracy of temperature measurements of a remote or weakly heated object, a procedure for calibrating the device against a temperature standard is required, which must be carried out under conditions identical to those of real measurements. These conditions include the distance from the object to the receiver and the transmission of the optical path between them. Without preliminary calibration of the device, in the absence of a priori data on the distance to the object and the value of the transmission of the optical path, it is impossible to establish an accurate relationship between the power of IR radiation incident on the receiver and the temperature of the object. In most cases, perform a remote calibration, i.e. it is impossible to preliminarily place a standard heated to a known specified temperature on the site of the object under study. In the absence of calibration, it is only possible to measure the distance to the object (for example, with a laser rangefinder), while the transmittance of infrared radiation of this route is not amenable to operational control or measurement. Thus, the problem of ensuring high accuracy of thermography of remote or weakly heated objects by the absolute method (in terms of the power of the radiation flux incident on the receiver) cannot be solved using this method. This is his disadvantage.

There is also known a method consisting in the fact that an image of an object is formed on the receiving area of a matrix receiver, the elements of which are capable of recording radiation in at least two different parts of the IR spectrum, after which the temperature of the object surface is determined as a result of analyzing the images obtained in two spectral ranges (US patent, No. 6758595, IPC G01K 3/00, publ. 06.07.2004). The disadvantage of this method is the insufficient accuracy of temperature measurement. This drawback is due to the uncontrollable difference in the transmission of optical filters that set one or another spectral measurement range. In addition, the method provides for the measurement of the luminance rather than the actual temperature of the object. In the absence of a priori data on the spectral emissivity of the object, obtaining the true values of the actual temperature of the object in this method becomes impossible.

The closest to the proposed method is a method (prototype), which consists in the fact that the image of the object is formed on the receiving area of the matrix electronic image receiver in the infrared range of the spectrum, additional spectral selection of radiation incident on the receiving area of the image receiver is carried out within the wavelength interval from 7 μm to 14 μm using a spectral-selective element having a monotonic dependence of the transmission on the wavelength, additional registration of electrical signals from the elements of the matrix electronic image receiver is carried out, analog-to-digital conversion of these signals is performed and an additional array of digital data of object images is formed, and the array digital data of the object temperature is formed by determining the values of the ratio of the corresponding elements of the main and additional arrays of digital data of the object image (RF patent №2324152, IPC G01J 5/00, publ. 10.05.2008, BI №13) ... The claimed technical result is an increase in the accuracy of determining the temperature of weakly heated objects.

In comparison with other methods, this method allows you to more accurately measure the temperature, however, it is characterized by an error that is associated with the use of a calibration dependence, built on the assumption of equality of the spectral energy brightness of the object and an ideal black body. In reality, this is not the case - the values of the indicated spectral radiance differ by a certain amount, which is determined by the spectral emissivity of the object. Therefore, the method provides measurement of the temperature of the object, which differs from its actual temperature by a certain amount. Moreover, this difference is the greater, the more the emissivity of the object differs from unity. Therefore, this method is mainly applicable for objects with a high emissivity, for example, greater than 0.9. This is the main disadvantage of the method - insufficient accuracy and limitation on the nomenclature of measured objects, imposed by the emissivity of the object.

The purpose of the invention is to improve the measurement accuracy and expand the range of measured objects.

This goal is achieved due to the fact that in the proposed method of thermography of a remote object in a given spectral range, an image of a distant object is formed on the receiving area of a matrix radiation receiver, electrical signals are recorded from the sensitive elements of the receiver, and an array of digital data of the object temperature is formed from them, and the contact method is measured the temperature of the atmosphere at the location of the receiver, the reflected temperature of the radiation receiver is corrected, while the emissivity of the atmosphere is taken to be unity, and the atmospheric temperature measured by the receiver is equal to its actual temperature, measured by the contact method at the location of the receiver, after performing the correction, a new array of digital temperature data is generated. fields of the object, which is taken as an array of its actual temperatures.

The essence of the invention is as follows.

The proposed method is based on the measurement equation, the output of which is presented below. In the output, a measurement circuit was used, the block diagram of which is shown in Fig. 1, where: 1 is the object of measurement, 2 is a matrix receiver of thermal radiation, 3 is the optical system of the measuring instrument, while the set of elements 2, 3 forms a temperature measuring instrument.

Let us take as the measured object 1 any distant object surrounded by a layer of the atmosphere and having a low thermal contrast with respect to the surrounding atmospheric background (the object is slightly heated). There is no information about the object, but at the same time the technical characteristics of the measuring instrument 2 - the matrix receiver of infrared radiation (hereinafter referred to as the receiver) are known and the value of the temperature of the surrounding atmosphere at its location is known. The task is to measure the actual temperature of the surface of an object or its separate area.

To derive the measurement equation, we use the Stefan-Boltzmann law, which is the theoretical basis for radiation thermometry. According to this law, in the full spectrum of thermal radiation, the flux density of intrinsic radiation from a unit surface of an ideal black body (black body) q _rad ⁰ in vacuum is:

Where

σ - Stefan-Boltzmann constant,

_Blackbody T - actual blackbody temperature.

Any real heated object 1 is surrounded by the atmosphere and has an emissivity ε _ob , which is less than that of a blackbody (ε _blackbody = 1). In this case, the object emits a heat flux, which consists of the flux of its own radiation and the flux re-radiated from the atmosphere. Therefore, in contrast to the blackbody, the flux density of the natural radiation of a real object into the surrounding atmosphere, taking into account the emissivity, is described by the equation

Where

T _ob is the actual temperature of the object,

ε _back is the emissivity of the atmosphere,

T _back is the actual temperature of the atmosphere.

With further transformations, we will take into account that near the horizon the emissivity of the atmosphere is close to unity, i.e. ε _back ≈1 (Hackward Henry L. Infrared radiation. Transl. from English, M.-L .: Energiya, 1964, 336 p.), therefore, the radiation of the atmosphere near the horizon is similar to the radiation of an absolutely black body located at an atmospheric temperature T _back ... Based on this, let us assume that ε _back = 1, taking into account its equation (2) takes the form:

In the case when the temperature difference between the object T _ob and the surrounding atmosphere T _back is small (the object is weakly heated), i.e. ΔT = T _ob -T _back << T _ob , equation (3) can be given a simpler form. To do this, we represent its right side as:

If the value of ΔT is small, the expression in parentheses will slightly differ from the value 4T _back ³ , so you can take:

Equation (4) describes the density of the intrinsic radiation flux from a unit surface of a real body.

With regard to remote measurements, the specified flux from object 1, having passed through the atmosphere layer and the optical system 3 of the measuring instrument, is attenuated to the value q _rad ⁽¹⁾ and enters the receiver 2. Taking into account (4), the value of the radiation flux density q _rad ⁽¹⁾ can be described by an equation of the form:

Where

ΔT ₁ = T _ob -T _matrix is the difference between the actual temperature of the object 1 and the actual temperature T _{matrix of the} receiver 2,

τ _opt , τ are the transmittance of the radiation by the optical system 3 of the receiver 2 and the atmosphere, respectively.

Receiver 2 absorbs part of the flux incident into it, and reflects some of it. The ratio of these parts is determined by the emissivity ε _{matrix of the} receiver 2. The density of the radiation flux absorbed by the receiver 2 can be represented as similar to (5):

Where

ΔT ₂ = (T _{ob, meas} -T _{back, meas} ) is the difference between the temperature of the object T _{ob, meas} and the atmosphere T _{back, meas} measured by the receiver.

In the steady-state (stationary) thermal regime, the radiation flux absorbed by the receiver 2 is less than the incident flux by the value Δq _rad ⁽²⁾ = (1-ε _matrix ) q _rad ⁽¹⁾ , or q _rad ⁽¹⁾ = q _rad ⁽²⁾ / ε _matrix , therefore from (5), (6) equality follows:

Let us express the atmospheric transmittance from (7), we obtain:

The receiving element also receives atmospheric radiation, the flux density of which, taking into account the fact that ε _back = 1, is equal to:

Where

ΔT ₃ = T _back -T _matrix is the difference between the actual temperatures of the atmosphere and the receiving element.

Let us find the value of the radiation flux density that falls on the receiving element from the object's own radiation, for this we subtract equation (9) from equation (5), we obtain:

Since we have assumed that ΔT ₁ = T _ob -T _matrix and ΔT ₃ = T _back -T _matrix , so we get:

On the other hand, for a virtual absolutely black body surrounded by an atmosphere with a temperature T _back and located at the same temperature of a real body in the absence of absorption in the atmosphere and optical system, the value of the flux density falling on the receiving element from the black body, taking into account equations (3 ) and (4) can be expressed by an equation of the form:

This value should be equal to the value of the resulting radiation flux density, calculated according to equation (11) and should cause the same responses (output signals) of the receiver. Based on this, we equate equations (11) and (12), we obtain:

We substitute in (13) the previously obtained equation (8) for the atmospheric transmittance τ, after transformations we obtain the equation for the actual temperature of the object:

Since in the initial formulation of the problem it was assumed that the object is weakly heated, this means that the heating of the receiver from the radiation of the object is extremely small, which, in turn, gives reason to accept with high accuracy that the receiver's own temperature T _matrix is equal to the temperature atmosphere T _back , i.e. T _matrix = T _back . Then equation (14) is simplified and takes the form:

The resulting equation is quite logical: in the case when the measuring instrument gives the value of the atmospheric temperature equal to its actual temperature, i.e. T _{back, meas} = T _back , then the object temperature measured by it is equal to the actual temperature. This confirms the correctness of the obtained equation. In practice, due to the absorption of radiation by the atmosphere, the measured temperature of the atmosphere at the location of the object is always less than its actual value, therefore the measured temperature of the object is always less than its actual temperature.

From equation (15) follows the basis of the proposed method, which is briefly formulated as follows:

if the hardware for adjusting the measuring instrument to achieve the equality of the measured atmospheric temperature T _{back, meas} and its actual temperature T _back , i.e. equality T _{back, meas} = T _back , then in this case the measured temperature of the object becomes equal to its actual, i.e. T _{ob, meas} = T _ob .

To make this correction, it is necessary to know the actual temperature of the atmosphere T _back surrounding the object. If we assume that the temperature of the atmosphere at the location of the object and at the location of the measuring instrument (receiver) is the same, then in this case the desired atmospheric temperature T _back can be measured at the location of the receiver by any contact measuring instrument, for example, a thermometer. In addition, since the derivation of (15) used the assumption that the emissivity of the atmosphere is equal to one (ε _back = 1), therefore, simultaneously with the temperature correction, an emissivity correction is required, namely, the emissivity value must be set on the measuring instrument ε _back = 1. If these requirements are met, the temperature of the object measured by the matrix receiver 2, according to equation (15), will be equal to its actual temperature.

Thus, the resulting equation (15) is the measurement equation for the actual temperature of the object T _ob . To obtain the desired value T _ob, it is necessary using a measuring instrument, for example, using a thermal imaging camera, to measure the temperature of the atmosphere in the area of the object T _{back, meas} and the temperature of the object itself T _{ob, meas} , and using another measuring instrument, such as a contact, to measure the actual temperature of the atmosphere at the location of the camera T _back , then make the appropriate adjustments, as a result of which a data array of the actual temperature field of the object is formed in the thermal imaging camera.

At the same time, for the implementation of the method, the following conditions and assumptions must be met:

- the object is weakly heated, - this means that within the specified accuracy, approximation (4) is valid for it, in addition, the radiation from the object does not cause any significant heating of the receiver, therefore, the assumption is made that the temperatures of the receiver and the atmosphere are equal, i.e. ... T _matrix = T _back ;

- the temperature of the atmosphere at the location of the receiver is equal to the temperature of the atmosphere at the location of the object (the temperature of the on-site atmosphere).

It should be borne in mind that although the derivation of the measurement equation is made on the assumption that the measuring instrument registers the radiation flux from the object in the entire thermal spectral range, it is also valid for any part of the thermal spectrum in which, for example, the selected measuring instrument operates. When considering a specific spectrum, the derivation of the measurement equation becomes very voluminous, but leads to an identical result. Therefore, the obtained measurement equation (15) is fully applicable for quasi and monospectral measuring instruments.

The proposed method is easily implemented using a thermal imager and any contact temperature measuring device.

The order of implementation of the method.

The thermal imager is focused on the measured object and thermography of the object is performed. As a result of thermography, a thermal portrait of the object and the near-object layer of the atmosphere is obtained on the thermal imager screen, as well as a per-pixel array of temperature data corresponding to this portrait. At the same time, any contact temperature measuring device, for example, a thermocouple, measures the temperature of the atmosphere T _{back, cont} at the location of the thermal imager. Using the "emissivity" option of the thermal imager, set the emissivity value equal to 1.0, and using the "average temperature in the selected area" option _{, select an} area of the on-site atmosphere in the thermal portrait and find its average temperature T _{back, meas} . It always differs (less) from the actual temperature of the atmosphere by some value. Then, by alternately using the options "average temperature in the selected area" and "reflected temperature T _back ", a set temperature value T _back is selected, at which the temperature of the atmosphere measured by the thermal imager and the contact measuring device will be equal, i.e. T _{back, meas} = T _{back, cont} . As a result of the performed correction of the installation data of the thermal imager, a new thermal portrait and a new array of temperature data are formed on its screen, which corresponds to the actual temperature field. From the received data array, the necessary information about the actual thermal field of the measured object is obtained.

An example of the implementation of the method. The proposed method was used to measure the thermal field of a sea bunkering tanker in the near-horizon layer of the atmosphere. A 96 m long tanker with a deadweight of 3000 tons was removed from the observer at a distance of ≈925 meters. An IR913 thermal imager and Chromel-Copel thermocouples with a known sensitivity were used for measurements. The IR913 thermal imager is based on an uncooled microbolometric matrix of 320 × 240 pixels with a single pixel of 50 × 50 μm, equipped with a standard lens providing an angular field of view FOV = 12 × 16 ° and an angular resolution of IFOV = 0.9 mrad. Thermal sensitivity of the IR913 thermal imager ΔT _NETD = 0.08 K.

To assess the accuracy of the method, several thermocouples were installed on the control surface of the tanker, and two other thermocouples were used to measure the temperature of the atmosphere: one at the location of the thermal imager, the other at the location of the tanker. As a result of measurements according to the proposed method, the results are obtained, which are presented in the table,

Where

T _back1 is the temperature of the atmosphere measured by a thermocouple at the location of the thermal imager,

T _back2 - temperature of the atmosphere measured by a thermocouple at the location of the tanker,

T _{back, meas} - measured temperature of the on-site atmosphere after correction,

T _ob1 - actual temperature of the control surface of the object, measured by the thermal imager after correction,

T _{ob1_} - average temperature of the control surface of the object, measured by thermocouples. The estimation of the measurement error was carried out in terms of the ratio of temperature differences (T _ob2 -T _ob1 ) to the absolute temperature contrast (T _ob2 -T _back ) and amounted to 1.7%. For other measurements carried out at other objects and under different conditions, the discrepancy in the measurement results did not exceed 2%.

The result obtained for this type of measurement can be considered an accurate result and surpasses the results currently achieved. Thus, in comparison with known methods, the proposed method provides a higher measurement accuracy and at the same time expands the range of measured objects, because its implementation does not require knowledge of the radiation coefficients of the object and the transmittance of the atmosphere.

Claims (1)

The method of thermography of a remote object, which consists in the fact that in a given spectral range an image of a remote object is formed on the receiving area of a matrix radiation receiver, electrical signals are recorded from the sensitive elements of the receiver and an array of digital data of the object temperature is formed, characterized in that the contact method is measured the temperature of the atmosphere at the location of the receiver, the reflected temperature of the radiation receiver is corrected, while the emissivity of the atmosphere is taken to be unity, and the atmospheric temperature measured by the receiver is equal to its actual temperature, measured by the contact method at the location of the receiver, after performing the correction, a new array of digital temperature data is generated. fields of the object, which is taken as an array of its actual temperatures.

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