WO2023072325A1

WO2023072325A1 - Device for thermographic temperature measurement

Info

Publication number: WO2023072325A1
Application number: PCT/CZ2022/050111
Authority: WO
Inventors: Milan Honner; Vladislav LANG; Michal Švantner; Jan Šroub; Tomáš Kohlschütter; Jan Klepáček
Original assignee: Západočeská Univerzita V Plzni
Priority date: 2021-11-01
Filing date: 2022-10-30
Publication date: 2023-05-04
Also published as: CZ309718B6; CZ2021501A3

Abstract

The device for thermographic temperature measurement consists of at least one calibration thermal element (5) and a detector (1), which are fixed in a compact housing (6), while the calibration thermal element (5) occupies part of the field of view (7).

Description

Device for thermographic temperature measurement

Field

The invention relates to the field of quantitative thermography, in particular thermographic measurement of the temperature of persons, but also to other applications where there are increased demands on the accuracy of non-contact determination of the area distribution of the measured temperature and focuses in particular on devices for accurate thermographic temperature measurement.

Prior art

Infrared thermographic measurement is a method of measuring surface temperature distribution of objects based on the detection of infrared radiation emanating from their surface. The temperature is evaluated by the measuring system based on the knowledge of the area distribution of the infrared radiation absorbed by the detector and other values quantifying the active thermal processes of the reflection of the environment or the transmissivity of the environment.

The basic part of a thermographic system is an optical system, i.e. a lens through which infrared radiation passes and is directed so that it hits an infrared radiation detector or sensor. The latter converts this radiation into an electrical signal and is a basic element of thermographic systems. In addition, thermographic systems include electronic and software systems that ensure the processing of the electrical signal from the detector and its interpretation in the form of temperature fields displayed in a so-called thermogram and other tools, for example for setting the parameters of the measured object and surroundings, considering the parameters of the optical system, controlling the entire system and export of measured data.

From the point of view of the principle of the infrared radiation sensor, thermal detectors and photon detectors are distinguished. The most common infrared radiation detectors are currently thermal detectors based on microbolometric arrays, i.e. arrays of miniature bolometric detectors that change their electrical resistance depending on their temperature. The temperature of the sensor varies depending on the amount of infrared radiation absorbed. However, the change in temperature and thus their resistance can actually be influenced by many other factors, for example the ambient temperature. In order for the temperature change of the bolometer to be proportional only to the absorbed infrared radiation, a suitable geometric configuration is essential for the entire system, especially the insulation of the detector from the surroundings, but also the system of corrections and compensations of the entire system.

A common solution for bolometric cameras is, for example, the measurement of the temperature of the detector and its surroundings and the use of a movable aperture between the detector and the lens, while one specific solution is described in document US 006929410 B2. The aperture is closed for a short time at certain intervals and the measured values are corrected and calibrated based on its temperature and known properties. The way these corrections are made, the materials used, the geometric configuration of the layout and the algorithms used are crucial for the resulting accuracy and stability of the infrared measuring system. While the sensitivity, i.e. the temperature resolution, of thermographic cameras based on bolometric detectors can be better than 0.05 °C, while the sensitivity of cooled photon detectors is even higher, their accuracy in terms of quantitative determination of the correct temperature usually ranges from ±2 °C up to ±5 °C or even worse depending on the design of the device, the ambient conditions and the measured temperature range. Various principles, approaches and methods of solving the internal layout of thermographic systems are described in detail in professional publications and patent documents, for example in US 005994701 A, US 006267501 B l, US 006476392 Bl, US 006953932 B2, US 007105818 B2, US 008049163 B l or WO 0184118 A2.

The disadvantage of these common thermographic systems in applications with high demands on absolute measurement accuracy is their lack of accuracy and long-term temperature stability. The accuracy of temperature determination represents the difference between the actual temperature of the measured object and the temperature determined by the measuring device, typically when measuring the temperature of a reference source of radiation (the so-called black body). The temperature stability of the thermographic system expresses how the temperature determined by the measuring system changes when measuring a reference radiation source with a constant temperature, depending on the external and internal temperature conditions. In the case of conventional thermographic systems, the manufacturer specifies an accuracy of ±2 °C or worse, which also includes the effects of temperature stability. For the relevant temperature measurement of people in order to detect their elevated temperature or fever, when we require accuracy in the order of tenths of °C, typically in the range of 35 to 40 °C, a conventional thermographic system is therefore not applicable.

To increase the accuracy of thermographic systems, a calibration device is used, which is a so- called reference black body. A black body is a device that emits infrared radiation proportional to its temperature, while the accuracy and stability of its temperature is higher than the accuracy of a common thermographic camera, and it normally achieves an accuracy of ± 0.5 °C and a stability of ± 0.1 °C.

By default, the black body is positioned so that it is in one thermogram with the measured object and at a similar distance to the measured object, so that both the black body and the measured object are in focus. The thermographic record from the region of the black body, for which a known temperature is assumed, is then used to correct the entire thermogram. In the simplest case, the correction is made by subtracting the difference between the temperature of the black body measured by the thermographic camera and its actual temperature from the rest of the thermogram. It is ideal when the temperature of the black body is as close as possible to the temperature of the measured object. The black body can work either completely independently of the thermographic camera or it can be connected to the thermographic camera in one system.

The disadvantage of these thermographic systems with a black body is the need to use two devices, one of which is far from the measured object, i.e. the thermographic camera, and one is close to the measured object, i.e. the black body. This greatly complicates the operation of the entire system. A fundamental disadvantage and limitation is the requirement that the black body is always in the field of view of the camera. This usually leads to a solution with a fixed geometric configuration of the black body and thermographic camera, when even with small changes in the position of one of these devices, it is necessary to check the position of the black body in the field of view of the camera and mark the area of the black body in the camera's field of view, from which it is subsequently determined correction for the entire thermogram.

A significant disadvantage is also the need to place the black body near the measured object, which often leads to the need to place either the thermographic camera or the black body in free space, which appears to be a complication in the practical installation of the thermographic system. However, this solution is often supplied as a standard set, for example for thermographic measurement of human body temperature.

If the above solution cannot be applied, as an alternative option, a moving black body can be used, which is placed in front of the lens for the necessary time, calibration is carried out, i.e. determination of the correction, and then the black body is removed from the field of view of the camera and the temperature of the measured object is recorded. It is assumed that if the measurement is performed shortly after calibration, the conditions and the correction constant will not fundamentally change. However, this method is usually less accurate than a system with a static configuration of a thermographic camera and a black body, which is in the camera's field of view for the entire time of measurement. The disadvantage is also the more complex handling of individual parts of the system, which must be carried out throughout the measurement period. This greatly limits the universality of such a measuring system and reduces the possibilities of its use. The disadvantage of more complicated handling is removed to some extent by the solution with a black body integrated on the camera body according to WO 2005092051 A2. In this case, as well it is a moving system where the black body is attached to the camera body from the outside and is flipped out to perform calibration at certain times so that it covers the entire lens, then calibration and correction of the measured values is performed and then the lens transparency is released again and the measurement is carried out in a standard way. This solution increases the mobility of the entire system, but it does not solve the aforementioned shortcomings resulting from the periodic nature of the calibration and the necessity of mechanical manipulation of the black body.

Background of the invention

The mentioned shortcomings are largely eliminated in the device for accurate thermographic temperature measurement according to the invention, the essence of which is that at least one calibrating thermal element and the detector are fixed in the housing, while the calibrating thermal element occupies part of the field of view.

Between the detector and the calibration thermal element, the lens, aperture and protective glass are situated. The calibration thermal element is located from the detector at a distance of 20 to 300 mm.

The outer surface of the housing is in contact with the external environment.

The calibration thermal element is preferably equipped with a temperature sensor and/or a heating element.

The radiating surface of the calibration thermal element is made of material with an emissivity in the range of 0.7 to 1.

The advantages of the device for thermographic measurement according to the invention are in the accuracy of determining the temperature of the measured object and the comfort of its use, both for the person providing its operation and for the person being measured.

Since the calibration thermal element is integrated into the housing of the device and is permanently located in the field of view, during measurement, thermal processes that cause inaccuracies in determining the actual temperature of the measured object are continuously corrected. At the same time, the designed device can be fully compact, portable and suitable for both manual use and fixed attachment.

As part of the current state of the art, thermographic devices were used with calibration elements that were not permanently placed in the field of view during the measurement. Since the calibration element is permanently located in the field of view, during the measurement, thermal processes affecting the measured temperature values are continuously corrected, the result is therefore an increase in the accuracy of determining the actual temperature of the measured object.

As part of the current state of the art, thermographic devices were used with calibration elements that were not part of the actual body of the measuring system. This caused a limitation of user comfort both during the preparation of the measuring system and during the measurement itself. Since the calibration element is a permanent part of the housing, the entire thermographic device is compact, immediately ready for measurement, the need to set the exact position of the calibration element relative to the detector is eliminated, and with any change in the position of the device, the calibration element always occupies the same field of view, which is a great advantage in applications, where it is necessary to change the position of the measuring system between measurements or even during the measurement. In addition, the calibration element does not in any way interfere with the movement of measured living and non-living objects in applications where the temperature of a large number of measured objects is measured successively.

Brief description of drawings

An exemplary embodiment of the invention is shown in the attached figures, where Fig. 1 schematically shows the arrangement of individual basic parts of a device for thermographic temperature measurement, Fig. 2 schematically shows a field of view with a measured object in a device for thermographic temperature measurement with a calibration thermal element located in the corner of the field of view, Fig. 3 schematically shows the field of view with a measured object in a thermographic temperature measurement device with a longitudinal calibration thermal element located in the middle of the field of view, Fig. 4 schematically shows a field of view with a measured object in a thermographic temperature measurement device with a circular calibration thermal element located in the middle field of view, Fig. 5 schematically shows the calibration thermal element and its individual parts, Fig. 6 shows the layout of the thermographic device for temperature measurement provided by the operator from the side view, Fig. 7 shows the layout of the thermographic device for unmanned temperature measurement from the side view, Fig. 8 shows the layout of a thermographic device for unmanned temperature measurement from the front view, Fig. 9 shows the layout of a thermographic device with two calibration elements, Fig. 10 schematically shows the layout of a thermographic device for temperature measurement and the measured object during stationary temperature measurement of people in hygienic and antiepidemic applications, Fig. 11 schematically shows the layout of the device for thermographic temperature measurement and the measured object when measuring the temperature of people in healthcare and medical applications, Fig. 12 schematically shows the layout of the device for thermographic temperature measurement and the measured object when manually measuring the temperature of people in anti-epidemic and healthcare applications.

Exemplary embodiment of the invention

An exemplary embodiment of a device for accurate thermographic temperature measurement according to the invention is schematically shown in Fig. 1. The basic part of the device is a thermographic infrared detector 1, which detects the surface distribution of the infrared measured radiation 11 of the measured object 10. In front of the thermographic infrared detector 1, a lens 2 is placed, which optically defines the field of view 7, behind which the infrared measured radiation 11 falls on the thermographic infrared detector j_. The thermographic infrared detector 1_, the lens 2 and other electrical and electronic parts of the device, such as the control computer, evaluation circuits, battery power, are located inside the compact housing 6, which ensures its mechanical protection. A protective glass 4 is placed in the area of intersection of the compact housing 6 and the field of view 7, which allows the infrared measured radiation 11 to pass through and at the same time forms a mechanical protection for the optical parts of the device. Between the protective glass 4 and the lens 2, a movable aperture 3 is placed. In its one position, this movable aperture 3 optically closes the field of view 7 and does not transmit the infrared measured radiation 11 to the thermographic infrared detector j_. The movable aperture 3, the lens 2 and the thermographic infrared detector 1 are located inside the compact housing 6 in their vicinity so that they are permanently in the same thermal conditions.

The calibration thermal element 5, which emits the calibration radiation 12, is located in the part of the field of view 7 so that the calibration radiation 12 permanently hits the thermographic infrared detector 1. The calibration field 8 forms part of the field of view 7 from which the calibration radiation 12 hits the thermographic infrared detector 1. The calibration thermal element 5 is fixed in the compact housing 6 in such a way that it is permanently in contact with the external environment and therefore in the same or similar thermal conditions as the measured object 10. The distance of the calibration thermal element 5 from the thermographic infrared detector 1 is chosen so that both the calibration thermal element 5 and the measured object 10 were simultaneously in the range of distances at which it is possible to take focused thermograms.

The size of the calibration thermal element 5 is such that, for the selected distance from the thermographic infrared detector 1, the calibration field 8 occupies between 2% and 30% of the area of the field of view 7. For a thermographic device for general use, the calibration thermal element 5 can be advantageously located in the corner of the field of view 7, as shown schematically in Fig. 2. The measured object 10 is then measured in a position in the center of the field of view 7.

However, for other applications, the calibration thermal element 5 can also be located in other parts of the field of view 7. As schematically shown in Fig. 3, the calibration thermal element 5 can be in the middle of the field of view 7, which can be advantageous for comparative thermographic measurements when two measured objects 10 are simultaneously measured.

The calibration thermal element 5 can also form a continuous strip along the edge of the field of view 7 or be located in the middle of the field of view 7, as schematically shown in Fig. 4. The measured object 10 in this case is of such a shape that the infrared measured radiation 11 does not reach the thermographic infrared detector 1 through the calibration field 8 formed by the calibration thermal element 5. The actual calibration thermal element 5, as can be seen from Fig. 5, can consist of a heating element 13, a body 14, a radiating surface 15 and a temperature sensor 16. The body 14 is preferably made of copper or another well-conducting material into which the temperature sensor 16 is inserted. The calibration radiation 12 comes from the radiation surface 15, which is turned in the direction of the thermographic infrared detector 1_. The size of the radiation surface 15 then defines the calibration field 8. The radiating surface 15 is modified by a surface treatment with a high emissivity value, for example a thermographic paint. The temperature sensor 16 is in good thermal contact with the radiating surface 15 so that the temperature measured by the temperature sensor 16 and the surface temperature of the radiating surface 15 differ as little as possible. A part of the calibration thermal element 5 can advantageously also include a heating element 13, which, together with the temperature sensor 16, ensures the heating of the radiating surface 15 to the desired temperature.

The calibration thermal element 5 is outside the optical part of the system, but it is integrated in a compact housing 6, which forms a complete device for accurate thermographic temperature measurement. The thermographic infrared detector 1 with optical elements and all other electronic and control devices necessary for the function of the thermographic measuring device, for example a control processor, accessories for power or accessories for storing measured data are placed inside this compact housing 6. The control and possibly also the display elements of the measuring device, for example a switch for turning it off or a display for displaying the measured image, i.e. a thermogram, as well as inputs and outputs for power supply or data streams, or external memory cards are located on the outside part of the compact housing 6.

An essential part of the device according to the invention is the solution of the optical input, as shown in Fig. 1, which is embedded in the compact housing 6 by means of a conical opening, which gradually narrows from the surface and whose walls follows the field of view 7. The depth of this opening corresponds to the required distance of the optical part of the device from the calibration thermal element 5, which is located on the open side of the opening of the compact housing 6 at the level of its outer surface. In this way, the mechanical protection of the calibrating thermal element 5 is ensured, which does not stick out into the space and thus minimizes the risk of its mechanical damage.

Possible alternatives of the arrangement of the entire device are shown schematically in Fig. 6, Fig. 7, Fig. 8 and Fig. 9. Fig. 6 shows a device for thermographic temperature measurement performed by an operator. The infrared measured radiation 11 from the surface of the measured object 10, which is located in the field of view 7 of the measuring system, enters it on one side through a conical opening where the calibration thermal element 5 is located. The display unit 17, which shows the resulting thermogram of the measured object 10 to the operator, emits imaged radiation 18 in the visible part of the electromagnetic spectrum.

An example of the implementation of a thermographic device for unmanned temperature measurement is shown in Fig. 7 from the side view and Fig. 8 from the front view. The measured object 10, which emits infrared measured radiation 11, is in this case the face of the person being measured. The thermogram is displayed in the imaged spectrum on the display unit 17, which is located on the front side of the compact housing 6 of the thermographic device so that it is possible for the measured person in the field of view 7 to simultaneously monitor the measurement result on the display unit 17.

An example of the implementation of a thermographic device with two calibration thermal elements 5 is shown in Fig. 9. It is a device intended for non-contact temperature measurement in applications requiring extreme accuracy of temperature determination or in applications where there are relatively large changes in the ambient temperature. In such cases, the device according to the invention includes several calibration thermal elements, in the specific case in Fig. 9 two. In that case, each calibration thermal element 5 occupies a different part of the field of view 7. The temperature of the calibration thermal elements 5 is different. Either the temperature of both calibration thermal elements 5 is kept at a constant value by regulation, for example when measuring the temperature of people at temperatures of 35 °C and 40 °C. Or one of the calibration thermal elements 5 is temperature-unregulated and takes the temperature of the external environment in which the compact housing 6 is located.

The use of the device for thermographic temperature measurement according to the invention is such that the measured object 10, which is a living person or a non-living object, is placed in the field of view 7. The surface distribution of the infrared measured radiation 11 of the surface of the measured object 10 in the infrared region of the electromagnetic spectrum hits through the protective glass 4 and lens 2 to the thermographic infrared detector E Using other electrical and electronic parts of the device, the area temperature distribution is evaluated, which is displayed on the display unit 17, possibly stored on a storage medium or sent out via a data stream.

During the measurement, an internal calibration using the movable aperture 3 is started at certain intervals. When the field of view 7 is covered, the infrared measured radiation 11 from the surface of the movable aperture 3 hits the entire thermographic infrared detector 1. This is a procedure usually denoted by the abbreviation NUC (non-uniformity correction), in which the movable aperture 3 serves as an area reference source and the output is the determination of the current properties of individual parts of the area matrix detector to achieve higher accuracy of temperature determination. During the measurement, the calibration thermal element 5 is heated to a temperature higher than the ambient temperature, ideally to a temperature close to the temperature of the measured object 10, if the application allows it. At the same time, the temperature of the calibration thermal element 5 is simultaneously measured using the temperature sensor 16, which is part of it. In parallel, the measurement of the surface distribution of the infrared measured radiation 11 from the surface of the measured object 10 in the uncovered part of the field of view 7 takes place using the thermographic infrared detector 1, and within the calibration field 8 the measurement of the calibration radiation 12 from the surface of the calibration thermal element 5 takes place. Using calibration algorithms the correction of measured values is carried out throughout the recording period simultaneously and continuously, i.e. online.

Calibration procedures and algorithms may vary depending on the conditions and measurement requirements, for example depending on whether the temperature of the calibration thermal element 5 will be controlled by the temperature sensor 16 to a constant value or if the calibration thermal element 5 will be passive, i.e. without heating, or the calibration thermal element 5 will be heated with constant power and the calibration will be performed on the basis of a floating variable temperature accurately measured by the temperature sensor 16. This process of simultaneous thermographic infrared sensing and calibration takes place continuously throughout the recording period. This makes it possible to significantly eliminate both the long-term shift of the measured value, the so-called drift, and short-term fluctuations, which can generally be caused by, for example, changes in external conditions, properties of the optical system, or properties of internal control and calibration.

The result is an accurate surface distribution of the temperature of the measured object 10. It is mainly used in applications where the accuracy of temperature determination is very important. Typically, these are cases of measuring the temperature of the surface of the human face. The use of the device according to the invention in the case of temperature measurement of passing persons is schematically shown in Fig. 10. The measured person of different height approaches the field of view 7 of the measuring system and leaves after the measurement. Here, an arrangement with a calibration thermal element 5 integrated into the compact housing 6 of the device according to the invention is advantageously used, since this calibration thermal element 5 does not in any way prevent the movement of the measured persons.

Another example of use is shown schematically in Fig. 11. This is a health or medical application of thermographic measurement of the temperature of persons, where the measuring system and the person being measured move in a position where the measured part of the person is in the field of vision 7. Either the measuring system is positioned, for example manually when measuring multiple lying patients using one measuring system. Or the person being measured is positioned and the thermographic measuring system is in a fixed position, as for example with a thermographic scanner. Both options can be used in cases where the thermographic measuring system is a permanent part of the construction of a mobile bed or rescue vehicle. Also in these cases, an arrangement with a calibration thermal element 5 integrated into the compact housing 6 of the device according to the invention is advantageously used, since this calibration thermal element 5 does not in any way prevent the relative movement of the measured person and the measuring system.

Another example of use is shown schematically in Fig. 12. This is a manual anti-epidemic or medical use of thermographic measurement of human body temperature. The measured object 10 is the controlled person. The measuring system is positioned manually by a second person in a position where the measured part of the inspected person, typically the corner of the eye, is in the field of view 7. Also in these cases, an arrangement with a calibration thermal element 5 integrated into the compact housing 6 of the device according to the invention is advantageously used, since this the calibration thermal element 5 is still in the same position with respect to the field of view 7 when the position of the measuring system is changed and, moreover, it does not in any way prevent the relative movement of the person being measured and the measuring system.

Industrial applicability

The invention can be used for applications of thermographic measurement with high demands on the accuracy of determining the temperature of the measured object, for example for thermographic measurement of the temperature of people or animals or for thermographic measurement of the photo-thermal properties of materials.

In the field of measuring human body temperature, these are hygienic and anti-epidemic applications of thermographic measurement with the aim of detecting people with an elevated body temperature as a manifestation indicating an infectious disease; health and medical applications of thermographic measurement with the aim of determining the general state of health of a person using body temperature or local problems using temperature distribution on the surface of the body; security and police applications of thermographic measurement with the aim of detecting intentionally false answers of the person under investigation; or applications of thermographic measurement in the entertainment industry with the aim of non-contact measuring the emotions of people.

In the area of animal temperature measurement, it concerns veterinary and agricultural applications of thermographic measurement with the aim of detecting animals with local inflammation or other health problems manifested by increased temperature. In the field of measuring the photo-thermal properties of materials, it concerns the application of thermographic measurement within laboratory equipment for measuring the emissivity /absorption or reflectivity of material surfaces with the aim of determining their spectral, temperature, angular, temporal and area distribution, the application of thermographic measurement within industrial equipment for quality control of manufactured materials or surface treatments with functional photo-thermal properties.

List of reference marks

1 - thermographic infrared detector

2 - lens

3 - movable aperture 4 - protective glass

5 - calibration thermal element

6 - compact housing

7 - field of view

8 - calibration field 9 - measuring field

10 - measured object

11 - infrared measured radiation

12 - calibration radiation

13 - heating element 14 - body

15 - radiating surface

16 - temperature sensor

17 - display unit

18 - imaged radiation

Claims

1. Device for thermographic temperature measurement, characterized in that at least one calibration thermal element (5) and thermographic infrared detector (1) are fixed in the compact housing (6), while the calibration thermal element (5) occupies part of the field of view (7).

2. Device for thermographic temperature measurement, according to claim 1, characterized in that between the thermographic infrared detector (1) and the calibration thermal element (5) a lens (2), a movable aperture (3) and a protective glass (4) are placed.

3. Device for thermographic temperature measurement, according to claim 1, characterized in that the calibration thermal element (5) is located from the thermographic infrared detector (1) at a distance of 20 to 300 mm.

4. Device for thermographic temperature measurement, according to one of the preceding claims, characterized in that the outer surface of the compact housing (6) is in contact with the external environment.

5. Device for thermographic temperature measurement, according to one of the preceding claims, characterized in that the calibration thermal element (5) is equipped with a temperature sensor (16).

6. Device for thermographic temperature measurement, according to one of the preceding claims, characterized in that the calibration thermal element (5) is provided with a heating element (13).

7. Device for thermographic temperature measurement, according to one of the preceding claims, characterized in that the radiation surface (15) of the calibration thermal element (5) is made of a material with an emissivity in the range of 0.7 to 1.

8. Device for thermographic temperature measurement, according to one of the preceding claims, characterized in that the calibration field (8) occupies two to 30 percent of the area of the field of view (7).