CZ2016217A3 - Non-contact measurement of temperature fields using thermographic colours - Google Patents

Description

Non-contact measurement of temperature fields using thermographic paint

Field of technology

The invention relates to non-contact measurement of temperature fields, i.e. a method of determining the total emissivity of material surfaces as a function of temperature, using thermographic paint.

Current state of the art

First, the concept of emissivity itself will be clarified. Emissivity is defined as the ratio of the emission intensity of a real body He to the intensity of an absolute black body H _E o with the same temperature. Emissivity thus determines the body's ability to radiate heat. It is a dimensionless quantity.

The emissivity is generally not constant for a given surface, but is a function of a number of parameters. These parameters can be, for example: the angle of deviation from the surface normal, the temperature of the object, the wavelength, the state of the surface, the structure of the surface, and the like

Bodies for which, from a practical point of view, the emissivity can be considered independent of frequency, we call gray emitters. For so-called selective emitters, we consider that the emissivity is a function of frequency.

Absolute blackbody emissivity ε = 1

The emissivity of a real body ετ thus acquires values εγ<1, maximum ετ = 1

Calculation of emissivity ετ= He/Heo

He is the radiation intensity of a real body. It indicates the power radiated by the surface of a real body into the entire half-space.

Heo is the intensity of absolute black body radiation. It indicates the power radiated by the surface of the black body into the entire half-space.

Non-contact measurement of temperature fields using thermal imaging camera systems is based on the detection of infrared radiation (heat flow) falling on the camera's infrared detector. The temperature distribution (thermogram) is then evaluated using hardware and/or software means, while the user must enter the values of the physical parameters characterizing the processes of radiation, reflection and penetration of the measured radiation into the evaluation. These are typically the emissivity of the measured surface, the apparent reflected temperature of the radiation environment, the permeability and the temperature of the environment between the measured surface and the thermal imaging camera.

Since the actual value of the emissivity of the measured surface is mostly unknown, or changes in space, time or temperature, thermographic reference colors are used to facilitate quantitative thermographic measurements. The applied thermographic reference color on the measured surface creates an area with a known high and spatially homogeneous emissivity value. By locally applying the thermographic reference color and entering its properties into the thermographic quantitative evaluation, it is possible to achieve a correct determination of the surface temperature at the point of application of the thermographic reference color at a certain point of the thermogram. This temperature can then be used to determine the unknown emissivity in other areas of the measured surface.

In order to make these measurement methods more efficient, software has been developed that allows point, line, or area analyzes to be inserted into the thermogram, where the emissivity value can be set, that is, mark the area with a thermographic reference color and evaluate the surface temperature by entering the emissivity of the thermographic color. Current software also allow, after inserting an emissivity map, to display a thermogram with consideration of different emissivities on different areas of the measured surface. They also allow inverted evaluation of the unknown emissivity on embedded analyzes after entering a known surface temperature.

Brands used:

11-infrared intensity at location A1

12-infrared intensity at location A2

I3- intensity of infrared radiation at location A3 d-emissivity at location A1 s2-emissivity at location A2 c3-emissivity at location A3

T1 - temperature at location A1

T2-temperature at location A2

T3-temperature at location A3 (measured temperature)

The main disadvantage of the current methods of measurement using software is that they do not allow working with the temperature or, if necessary, time dependence of emissivity. This distorts the measurement results. The measurements are then inaccurate.

The essence of the technical solution

The mentioned shortcomings are largely eliminated by the non-contact measurement of temperature fields using the thermographic paint according to the invention, the essence of which is that a reference thermographic paint with a known emissivity εΐ is applied to the measured surface. Radiometrically, the intensities of infrared radiation II, 12 and 13 from areas Al, A2 and A3 are measured by a thermal imaging camera, and the temperature TI is determined using the function II = f(cl, TI) from the known intensity of infrared radiation II and the known emissivity of the reference thermographic color εΐ. This temperature, assuming that the temperature T2 = the temperature TI, using the function 12 = f(a2, T2) will enable the determination of the emissivity ε2. Using this emissivity under the condition that emissivity ε3= emissivity ε2, using the function 13 - f(c3. T3) will enable the determination of the measured temperature T3.

Clarification of drawings

The invention will be explained in more detail with the help of a drawing in which the non-contact measurement of temperature fields using the thermographic paint according to the invention is schematically indicated in the elevation and in the ground plan the field of view of the thermal imaging camera with defined places where the reference color is applied, where the emissivity of the measured surface is unknown, but the same surface temperature and where the unknown emissivity is the same but the temperature is generally different. The arrow with the description indicates the direction of the emitted radiation.

An example of an embodiment of the invention

A practical example of non-contact measurement of temperature fields using the thermographic paint according to the invention can be seen in the attached picture.

Fig. 1 shows a thermal imaging camera that detects infrared radiation from the measured surface. A reference thermographic color with known emissivity is applied to the measured surface with unknown emissivity at the AI location. Its temperature or time dependence is also known. The thermogram is defined by the geometric position of the AI analysis, which corresponds to the place of application of the reference thermographic color. The geometric position of the A2 analysis is defined in the thermogram. which corresponds to the place where the emissivity of the measured surface is unknown and where the same surface temperature is assumed as at the place of application of the AI reference thermographic color. The geometric position of the A3 analysis is defined in the thermogram. Determining its temperature field is the goal of the measurement. In area A3, the same unknown emissivity is assumed as in analysis A2, since it is the same material of the measured object. However, the temperature is generally different.

The evaluation of the thermographic measurement then takes place automatically for each image from the thermal imaging camera according to the following procedure:

1. Intensities of infrared radiation II, 12, 13 from areas Al, A2, A3 are measured radiometrically. The functional dependence of the measuring system I = f(e, T) between the intensity of infrared radiation I, temperature T and emissivity ε is known.

2. The temperature Tl on the Al area is evaluated from the known intensity II and the emissivity of the reference thermographic color εΐ:II = ί(ε1, Tl), II and εΐ are known => Tl.

3. The emissivity ε2 on area A2 is evaluated from the known intensity 12 and temperatures T2 = Tl : 12 = f(a2, T2), 12 and T2 are known =í> ε2.

4. The temperature T3 on area A3 is evaluated from the known intensity 13 and emissivity ε3 = ε2: 13 = ί(ε3, T3), 13 and ε3 are known => T3.

5. Areas Al, A2 and A3 are shown in the thermogram with corresponding temperatures Tl, T2 and T3.

Using the mentioned procedure, it is possible to evaluate and display the entire thermogram. Analysis A3 typically covers the entire rest of the measured object, with the exception of analyzes Al and A2. Areas Al, A2 and A3 can be made up of one or more pixels in the thermogram, they can have different shapes and sizes.

The resulting thermogram is displayed on area Al with temperature evaluation according to the emissivity of the reference thermographic color and on areas A2 and A3 with temperature evaluation according to emissivity from area A2. The temperature field thus corresponds more closely to reality.

The advantage of non-contact measurement of temperature fields using the thermographic paint according to the invention is the fact that the entire thermogram is evaluated without the need to determine the unknown emissivity of the measured object in advance. Another advantage is that the time courses of temperature fields can be automatically evaluated online, directly during measurement, even under changing conditions (temperature-dependent emissivity), or offline using recorded sequences of thermograms.

The described method should be implemented as a software function of a measuring device, i.e. a thermal imaging camera or a computer that records and evaluates data.

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• ······ ······ · * · · · ·· · · ·' Industrial applicability

Non-contact measurement of temperature fields using the thermographic paint according to the invention can be successfully used by users of thermal imaging cameras for measuring temperature fields and as a software function of the measuring device by manufacturers of thermal imaging cameras.

Claims (1)

1. Non-contact measurement of temperature fields using thermographic paint, characterized by the fact that a reference thermographic paint with a known emissivity εΐ is applied to the measured surface and the intensities of infrared radiation II, 12 and 13 from the measurement areas Al are measured radiometrically by a thermal imaging camera, A2 and A3, while from the known intensity of infrared radiation II and the known emissivity of the reference thermographic color εΐ, the temperature TI is determined using the function II = f(sl, TI), which, assuming that the temperature T2 = the temperature TI, using the function 12 - f( s2, T2) will enable the determination of the emissivity ε2, the use of which, under the condition that emissivity ε3= emissivity ε2, will enable the determination of the measured temperature T3 using the function 13 = ί(ε3, T3).