RU2515086C1 - Pyrometric method of measuring temperature distribution on surface of object - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method involves forming a digital image of the object at a selected frequency using thermal radiation emitted by the object and obtaining an additional digital image of the same object illuminated with scattered radiation. The obtained two images and illumination of the surface of the object are used to calculate the reflection coefficient of scattered radiation in the direction of the video camera lens at points of measuring temperature at the selected frequency and temperature distribution is calculated from the obtained coefficients and the first image.

EFFECT: simple measurement method and enabling measurement of temperature without obtaining preliminary information on properties of the object.

3 cl, 1 dwg

Description

The invention relates to the field of measuring equipment, in particular to the field of optical pyrometry, and is intended to measure the temperature distribution on the surface of various objects.

Modern pyrometric methods for measuring temperature can be divided into three groups: brightness methods (temperature is determined by the spectral density of the thermal radiation of the object at a given frequency), radiation (temperature is determined by the energy luminosity of the object) and multi-frequency (temperature is determined by the spectral density of thermal radiation of the object at several frequencies ) All these methods have a sufficiently high spatial resolution and allow measuring the temperature distribution.

From the prior art, various variants of luminance methods are known. An example is the method implemented in the device described in [S.E. Aleksandrov, G.A. Gavrilov, A.A. Kapralov, G.Yu.Sotnikova, D.F. Chernykh, A.N. Alekseev , A.L.Dudin, I.V. Kogan, A.P. Shkurko, Device for pyrometric control of the temperature of a GaAs substrate for molecular beam epitaxy installation, Journal of Technical Physics, 2004, v. 74, issue 1, p.123- 127], which consists in receiving a pyrometric signal at a given frequency and calculating the temperature in accordance with the results of express calibration on one or more reference topics peraturam. A common drawback of all brightness methods is the need for information about the absorption capacity of an object. In this case, the absorption capacity is taken into account during calibration. The disadvantage of this method is the need for a reference temperature and the lack of full consideration of the temperature dependence of the absorption capacity of an object.

Various variants of multi-frequency methods are known. An example is the spectral ratio method, a description of which can be found in [M. Gumirov, Pyrometry of the processes of self-propagating high-temperature synthesis, Bulletin of the Tomsk Polytechnic University, Mathematics and mechanics. Physics, 2007, t.311, No. 2, p.135-138], which consists in receiving two pyrometric signals at different frequencies and calculating the temperature by their ratio. The main disadvantage of the method is that it is applicable only in those cases when the absorption capacity of an object in the selected spectral range is independent of frequency.

Known multi-frequency method described in the patent [RF No. 2365883, IPC G01J 5/60, publ. 08/27/2009, bull. No. 24], including the reception of at least three pyrometric signals at equidistant frequencies, the proximity of which provides a linear dependence of the logarithm of the product of the coefficients of spectral emissivity and transmission of the intermediate medium from the wavelength. The temperature in this method is calculated from the ratio of the first and third signals, then a correction is calculated, calculated from all at least three signals. The disadvantage of this method is that it is applicable only in cases when the aforementioned linearity condition is fulfilled, strictly speaking, the method requires a priori information about the properties of the object. The convergence of the frequencies of the pyrometric signals on the one hand expands the scope of the method, but at the same time requires an increase in spectral resolution and, therefore, leads to an increase in the cost of the method. An increase in the number of pyrometric signals received at different frequencies will also complicate the method.

Various multispectral methods are known, which include recording the spectrum of thermal radiation of an object (at several hundred or thousands of frequencies), selecting a portion of the spectrum similar to the spectrum of an absolutely black body, and determining the temperature from a selected area [A.N. Magunov, Spectral pyrometry (review), Instruments and experimental technique, 2009, No. 4, pp. 5-28]. These methods do not require any preliminary information about the absorption capacity of the object.

A common drawback of all multifrequency methods (and especially multispectral) is that they are technically more complicated and therefore more expensive than brightness methods, since they require measuring the pyrometric signal at several frequencies, which entails the use of appropriate spectral techniques. In order to measure the temperature distribution by the multifrequency method, it is actually necessary to obtain an image of an object formed by its thermal radiation at several frequencies.

The closest to the proposed technical solution in terms of essential features is the well-known method of relative brightness pyrometry, a description of which can be found in [F.F.Karimova, S.G. Orlovskaya, V.V. Kalinchak, M.S. Shkoropado, Research temperature fields of radiating objects, Physics of Aerodispersion Systems, 2006, issue 43, pp. 39-45] (prototype). This method is as follows. A digital image of the object is formed at the selected frequency due to the thermal radiation emitted by the object. Measure the average temperature in a certain area of the surface of the object using a pyrometer. The temperature distribution is calculated from the distribution of brightness of the pixels of the image of the object and the readings of the pyrometer.

The disadvantage of this method is the need for information about the dependence of the absorption capacity of an object on the coordinates on its surface and temperature, as well as the need to use an independent method of measuring temperature.

The technical result to which the invention is directed is a method for measuring the temperature distribution on the surface of an object that does not require any preliminary information about its properties, but at the same time preserves the simplicity and cheapness of the brightness methods (in particular, the simplicity and cheapness of the method of relative brightness pyrometry) .

The claimed technical result is achieved as follows. In the method of measuring the temperature distribution on the surface of an object, including the formation of a digital image of the object at a selected frequency due to thermal radiation emitted by the object, an additional digital image of the same object illuminated by the scattered radiation of the backlight is formed. The choice of the frequency at which the digital images of the object are formed is arbitrarily arbitrary.

Next, an array i ^{(heat) is formed} , each element of which represents the brightness of the corresponding pixel of the digital image of the object, and an array i ^(total) , each element of which is the brightness of the corresponding pixel of the additional digital image of the object.

Then, using the dependence I (i) (where i is the brightness of a given pixel, I is the spectral density of the radiation propagating from the corresponding part of the surface of the object towards the camera lens, at the selected frequency, per unit of visible area of this part of the surface of the object), an array i ^(warm) _:

I ^(heat) _{i, j} = I (i ^(heat) _{i, j} )

the indices i, j indicate the number of the array element. Similarly form an array I ^(total) :

I ^(total) _{i, j} = I (i ^(total) _{i, j} )

Next, an array I ^{(neg) is formed} , each element of which is the spectral density of the radiation of the backlight reflected from the corresponding portion of the surface of the object towards the camera lens, at the selected frequency, per unit of visible area of this portion of the surface of the object:

$$I_{i,j}^{\left(\mathrm{about}tR\right)}=I_{i,j}^{\left(\mathrm{about}bu\right)}-I_{i,j}^{\left(tePl\right)}$$

An array of I ^(neg) and the value of the illumination of the surface of the object at the selected frequency ω ₀ calculate the array of reflection coefficients of the scattered radiation towards the camera lens R:

$$R_{i,j}=\frac{I_{i,j}^{\left(\mathrm{about}tR\right)}}{I_0}$$

where I ₀ is the illumination of the surface of the object at the selected frequency.

Using the R array and the I ^(warm) array, in accordance with the Planck formula and Kirchhoff law recorded for the thermal radiation propagating towards the camera’s lens, the temperature array T is calculated:

$$T_{i,j}=\frac{\eta {\omega }_0}{k\cdot \ln \left[\frac{\eta {\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="00000005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0{}^3}{\mathrm{four}{\pi }^2{c}^2}\cdot \frac{\left(D_{i,j}-R_{i,j}\right)}{I_{i,j}^{\left(tePl\right)}}+\mathrm{one}\right]}$$

where ω ₀ is the frequency at which images of the object are formed,

D _{i, j} is the ratio of the radiation power emitted toward the camera lens by a flat Lambert source located perpendicular to the optical axis of the camera in place of the object’s surface area corresponding to this pixel, to the radiation power emitted by this source in all directions;

k is the Boltzmann constant;

η is the Planck constant;

c is the speed of light in vacuum.

If the illumination of the surface of the object at the selected frequency during the formation of an additional digital image of the object is not known (the backlight radiation source is not calibrated), a sample with a known (equal to a certain value of R ₀ ) reflection coefficient of the scattered radiation towards the camera lens at the selected frequency is placed next to the object under study. The illumination of the surface of the object at the selected frequency is determined by the value of R ₀ , the dependence I (i) and the pixel brightness of the digital image of the sample with a known reflection coefficient of scattered radiation towards the camera lens at the selected frequency.

If the backlight radiation source is calibrated, a sample with a known reflection coefficient of the scattered radiation towards the camera lens at the selected frequency can be used to correct (express calibration) the dependence I (i), which makes it possible to track the change in the transmission of the intermediate medium with time at the frequency ω ₀ . If the object is in a vacuum chamber, a change in the transmission of the intermediate medium may be associated with contamination of the window of the vacuum chamber.

If it is necessary to measure the dependence of the temperature distribution on the surface of the object on time and it is known in advance that the absorption capacity of the object at the selected frequency does not depend on temperature, an additional digital image of the object, necessary to determine the elements of the array R, can be formed only once. This approach increases the resolution in time.

The main difference from the prototype is that the temperature distribution is measured not by one digital image, but by two images, one of which was obtained under the conditions of illumination of the object by scattered radiation. The method for calculating the temperature distribution by the brightness of the pixels has corresponding differences. The temperature is calculated not by the absorption capacity, but by the reflection coefficient of the scattered radiation in a given direction to a given solid angle. Measurement of the average temperature in a certain area of the surface of the object is not required, respectively, does not require the use of an independent method of measuring temperature.

An example of the application of the described method for measuring the temperature distribution on the surface of an object

This method was used to measure the temperature distribution on the surface of samples of natural minerals during laser stepwise argon-argon dating. The experiments were performed on a standard Argus laser complex (Analytical Center IGM SORAN).

A sample of a natural mineral (object) was placed in a vacuum chamber of a mass spectrometer and heated using laser radiation to extract argon from it, the isotopic composition of which determines the age of samples of natural minerals during argon-argon dating. An image of a heated sample of a natural mineral formed on a semiconductor matrix of a video camera due to visible radiation was displayed on a computer monitor; the video camera was equipped with a red narrow-band optical filter (samples of natural minerals were used, which were thin, opaque plates in the visible spectrum; the heating temperature of samples of natural minerals made it possible to record their thermal radiation in the visible spectrum). The transmission of a red narrow-band optical filter at the frequency of maximum transparency ensured the complete suppression of the background illumination of the semiconductor matrix of the camera due to daylight. Illumination of a sample of a natural mineral by scattered radiation during the formation of an additional digital image of the object was carried out using the built-in illumination of the Argus laser complex. To calculate the illumination of the surface of a sample of a natural mineral, a sample was placed next to it with a known reflection coefficient of scattered radiation towards the camera lens at the maximum transparency frequency of a red narrow-band optical filter. Dependence I (i) was determined using the well-known model of an absolutely black body — a resistive stove with a small hole for the output of thermal radiation. Digital images were processed using a program written on the Framework 4.0 platform.

The figure (figure 1) shows the temperature distribution on the surface of the mica sample (biotite) at one of the heating stages. The coordinates on the sample surface (x, y) are indicated in pixels, the spatial resolution was 120 μm, the temperature measurement error was 10 ° C, and the temperature measurement error was mainly determined by the quality of the video camera. From this graph it is seen that the temperature distribution is heterogeneous, the temperature spread reaches 60 ° C.

The described method for measuring the temperature distribution on the surface of the object made it possible to control the temperature distribution on the surface of samples of natural minerals (the absorption capacity of which is not known and substantially depends both on the coordinates on their surface and on temperature) during laser heating during dating and, therefore, allowed exclude the previously unaccounted for error in the age of samples of natural minerals associated with the heterogeneity of their heating (as the experiment showed, the value of this ogreshnosti is 15 times greater than the analytical error). The practical implementation of the method of measuring the temperature distribution on the surface of the object did not require significant additional technical costs, the additional costs were limited to the acquisition of one red narrow-band optical filter.

The given example does not exhaust all the possibilities of the method of measuring the temperature distribution on the surface of an object and serves only illustrative purposes. An object can be understood as a solid body that is opaque at the selected frequency (mineral, semiconductor element, etc.); a drop of liquid that is opaque at the selected frequency and does not change shape during the formation of a digital image of an object and an additional digital image of an object (for example, a drop of mercury), etc. In addition, the above example is not the only option for the technical implementation of the described method for measuring the temperature distribution on the surface of an object. Specific implementations may differ by the choice of frequency ω ₀ , the method of forming digital images, etc.

Thus, in contrast to the known luminance (as well as two- or three-frequency) methods, the described method for measuring the temperature distribution on the surface of an object does not require any preliminary information about the properties of the object, all the necessary parameters are determined experimentally. Due to the fact that the reflection coefficients of the scattered radiation towards the camera lens at the selected frequency are determined directly under the conditions of the measured temperature distribution, the described method for measuring the temperature distribution on the surface of the object makes it possible to experimentally take into account the dependence of the absorption capacity of the object both on coordinates on its surface and on temperature. The described method for measuring the temperature distribution on the surface of an object suppresses the error in measuring temperature associated with irregularities on the surface of the object, since effective reflection coefficients are actually measured. Compared with multifrequency methods (and in particular multispectral), the described method is easier to implement and less expensive, since it involves the formation of digital images at a single frequency. Thus, the described method for measuring the temperature distribution on the surface of the object provides opportunities similar to the capabilities of multispectral methods, while maintaining the simplicity and low cost of brightness.

Claims (3)

1. A pyrometric method for measuring the temperature distribution on the surface of an object, comprising generating a digital image of the object at a selected frequency due to thermal radiation emitted by the object, characterized in that an additional digital image of the same object illuminated by scattered radiation is formed at the selected frequency; by the illuminance of the surface of the object at the selected frequency, the brightness of the pixels of the digital image of the object, the brightness of the pixels of the additional digital image of the object and the dependence of the brightness of the pixel on the ratio of the spectral density of radiation propagating from the corresponding pixel of the surface area of the object towards the camera lens, at the selected frequency, to the unit visible the area of this surface area of the object calculate the reflection coefficients of the scattered radiation towards the lens of the video camera s at the selected frequency into the temperature measuring points; according to the obtained reflection coefficients of the scattered radiation, the pixel brightness of the digital image of the object and the dependence of the pixel brightness on the ratio of the spectral density of the radiation propagating from the corresponding portion of the object’s surface to the camera’s lens, at the selected frequency, to the visible area of this object’s surface, in accordance with the Planck formula and the Kirchhoff law written for the thermal radiation propagating in the direction of the camera lens calculate temperature distribution:

where i, j are the coordinates of the point of the digital image of the object in pixels;
T _{i, j} is the temperature;
D _{i, j} is the ratio of the radiation power emitted toward the camera lens by a flat Lambert source located perpendicular to the optical axis of the camera in place of the object’s surface area corresponding to this pixel, to the radiation power emitted by this source in all directions;
ω ₀ is the frequency at which images of the object are formed;
R _{i, j} is the reflection coefficient of the scattered radiation towards the camera lens at a frequency ω ₀ ;

- the spectral density of thermal radiation emitted by the corresponding pixel of the surface area of the object towards the camera lens, at a frequency ω ₀ per unit of visible area of this surface area of the object;
k is the Boltzmann constant;
η is the Planck constant;
c is the speed of light in vacuum. 2. The method according to claim 1, characterized in that when determining the illumination of the surface of the object using a sample with a known reflection coefficient of the scattered radiation towards the camera lens at a selected frequency. 3. The method according to claim 1, characterized in that the dependence of the pixel brightness on the ratio of the spectral density of radiation propagating from the corresponding portion of the object’s surface to the camera’s lens, at a selected frequency, to a unit of the visible area of this object’s surface is adjusted according to changes in the transmission of the intermediate medium over time using a sample with a known reflection coefficient of scattered radiation towards the camera lens at a selected frequency.

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