EP3191812A1

EP3191812A1 - Device and method for controlling the calibration of thermochromic liquid-crystal matrices

Info

Publication number: EP3191812A1
Application number: EP14789377.0A
Authority: EP
Inventors: Jacek REINER; Mariusz MRZYGLÓD; Dariusz TRYBA; Marcin TRZYNA; Wojciech CIESZYNSKI; Henryk Jaremek; Janusz ZMYWACZYK
Original assignee: Politechnika Wroclawska; Braster SA
Current assignee: Politechnika Wroclawska; Braster SA
Priority date: 2014-09-10
Filing date: 2014-09-11
Publication date: 2017-07-19
Also published as: PL230748B1; PL409449A1; WO2016038425A1

Abstract

The subject of the invention is a device for controlling the calibration of thermochromic liquid- crystal matrices characterized in that it comprises a heating table (2) with stabilized thermally active means (3) and (4) placed at the ends and temperature sensors (5), stretching-pressing means (9), a lighting (9), a detector (7) and a screen (11). The invention also relates to a method of controlling the TLC matrix using the above-mentioned device and the using the said device for testing thermal-chromatic calibration of the thermochromic liquid-crystal matrices.

Description

DEVICE AND METHOD FOR CONTROLLING THE CALIBRATION OF THERMOCHROMIC LIQUID-CRYSTAL MATRICES

The subject of the invention is a device for controlling the calibration of thermochromic liquid crystal matrices, a method of controlling the calibration of thermochromic liquid- crystal matrices and use of the device for controlling the calibration of thermochromic liquid-crystal matrices.

Liquid crystals, whose light filtration capacity depends on the temperature, thereby changing their colour, have been known for many years (ABDULLAH et al "Film thickness effects on calibrations of a narrowband Thermochromic liquid crystal," Experimental Thermal and Fluid Science, vol. 33, No. 4, p. 561 -578, 2009), they are applicable to imaging temperature fields of technical objects (SMITH et al "Temperature sensing with thermochromic liquid crystals Experiments in Fluids", vol. 30, No. 2, p. 190-201 , 2001 ), flow and stress (IRELAND et al "Liquid crystal measurements of heat transfer and surface shear stress", Measurement Science and Technology, vol. 1 1 , No. 7, p. 969, 2000), as well as in medical diagnostics (POCHACZEVSKY et al "Liquid crystal contact thermography of deep venous thrombosis", AJR. American Journal of Roentgenology, vol. 138, No. 4, p. 717-723, 1982).

Medicine is seeking the possibility to differentiate the disclosed thermal anomalies resulting from pathophysiological processes. Tumoral pathologies (of neoplastic nature) have a significant advantage over anabolic processes associated with intense neoangiogenesis (FOLKMAN, "Tumor angiogenesis: Therapeutic implications", The New England Journal of Medicine, vol. 285, No. 21 , p. 1 182-1 186, 1971 ), which is associated with the appearance of changes in the characteristics of the thermogram of hyperthermic lesions. In the case of breast cancer such lesions were previously observed using high- resolution infrared cameras (DIAKIDES et al "Medical infrared imaging" CRC press, 2007). However, imaging of these lesions using thermochromic liquid-crystal matrices is characterized by a favourable thermodynamic process (YAHARA et al "Relationship between microvessel density and Thermographic hot areas in breast cancer", Surgery Today, vol. 33, No. 4, p. 243-248, 2003). Furthermore, such matrices would be more accessible than infrared cameras, addressing the essential diagnostic problem of the XXI century diseases, especially breast cancer in women.

Liquid-crystal matrices (TLC - Thermochromic Liquid Crystals) are multi-layered polymer systems comprising liquid crystals, which selectively scatter light having a wavelength dependent on the temperature. TLCs are manufactured by means of microencapsulation or by continuous piling of microlayers, so-called CLCF method - Continuous Liquid Crystals Film (EP2528994). Both technologies are susceptible to interference, which results in temperature/colour response (calibration) and the interference is bigger if the range of the matrix is narrower (so-called narrow temperature range matrices with the scope below 5°C). Hence, an important problem is controlling calibration accuracy of the produced matrix, that is, establishing the relation between the input of the system (temperature) and the output (chrominance) of the scattered light.

Literature discloses the steady state calibration methods of TLC matrices using a flat heating table of a fixed temperature or a flat gradient table. In the case of flat tables there is a problem of providing "good contact" between the heating surface and the TLC matrix. This contact can be guaranteed by introducing liquid (e.g. glycerine) between the surface of the matrix and the heating table. However, this approach does not guarantee reproducible liquid film thickness, spatio-temporally, which in the case of narrow operating ranges of the TLC matrices causes additional measurement errors. In industrial environments, the use of fluid to increase thermal contact introduces an additional activity of applying the liquid before the testing process and the activity of removing the residue after the completion of the test.

An advantage of the present invention is the ability of testing the calibration of liquid- crystal matrices and inspecting their quality in respect of the reference model with higher accuracy by:

minimizing matrix contact resistance - heating table,

lighting minimizing specular reflections,

correction of the matrix angular response characteristics and

system calibration method based on stable reference models.

The aim of the present invention was to provide a new, more efficient device for controlling the calibration of thermochromic liquid-crystal matrices and the method of its operation, which may be used in the manufacturing process of the liquid-crystal matrices used in the detection of the temperature anomalies associated with pathological processes, such as malignant tumours, especially breast cancer.

This aim is achieved by a device for controlling the calibration of thermochromic liquid- crystal matrices of the present invention.

Thus, the objective of the present invention is a device for controlling the calibration of thermochromic liquid-crystal matrix characterized in that it comprises a heating table with stabilized thermally active means placed at the ends and temperature sensors, stretching- pressing means, lighting, detector and screen.

Preferably, the above-mentioned device is connected to a computer with appropriate interfaces and software.

Preferably, the table has a convex shape and is made of a material with high thermal conductivity, for example, selected from aluminium, copper.

Preferably, thermally active means are Peltier pads or resistance heaters, and the temperature can be set independently.

Preferably, lighting is positioned such that there is no light reflection due to the reflection from the film surface in the direction of the observation by the detector.

Preferably, lighting is disposed along the opposite edges of the table, or omnidirectionally.

Preferably, the detector is placed over the table.

Preferably, the screen prevents reflections from the surroundings.

Further objective of the invention is to provide a method of controlling the TLC matrix using the above-mentioned device, characterized in that it comprises the steps of placing the TLC matrix on the heating table and pressing against the surface of the table by stretching-pressing means; activating the lighting system; activating the recording system, wherein the order of the above-mentioned steps is optional, and then the relationship between the input of the system (temperature) and the output (chrominance) is established; and the sensors determine the temperature at selected points of the table and the temperature values are extrapolated along the resulting temperature gradient.

Preferably, using a convex shape of the table the film is stretched using stretching- pressing means, ensuring full contact between the table and the matrix.

Preferably, colour image of the matrix placed on the table is registered using a detector.

Preferably, lighting which is placed below the surface defining a direct light reflection from the TLC matrix in the direction of the camera imaging the colour of the matrix is used.

Preferably, chrominance data of the tested matrix is compared with the reference data collected from the reference model in the same system under varying thermal conditions.

Further objective of the invention is to provide a device according to the present invention for testing thermal-chromatic calibration of thermochromic liquid-crystal matrices. The terms used above and throughout the specification and claims have the following meanings:

Chrominance - component of the analogue or digital colour image signal corresponding to the colour hue and colour saturation;

Metrological function- means the measurement function defining the value of the measured quantity;

Inspection function- determines the outcome of comparing the values of the measured quantity with the reference model in terms of tolerance;

Matrix conversion range - is a temperature range in which a colour response occurs in the visible optical range (approximately 380 nm - 780 nm)

Matrix responsiveness - in other words it means a matrix conversion range.

The invention is illustrated in the drawings and the following non-limiting embodiment, wherein:

Fig. 1 is a schematic illustration of a device for controlling the calibration of thermochromic liquid-crystal matrices of the invention;

Fig.2 shows the results of a heating table simulation with gradient and symmetric with a regression function;

Fig. 3 shows a compensation algorithm and determining the chrominance temperature profile;

Fig.4 illustrates a workstation for testing a thermochromic response of the TLC matrix. Designations used:

1 - TLC matrix

2 - heating table

3 - thermally active means

4 - thermally active means

5 - temperature sensor

5 - white-light illuminator

6 - colour camera

7 - computer with appropriate interfaces and software

8 - stretching-pressing elements

9 - screen Example:

a) The device and its construction

A device according to the invention illustrated in Fig. 1 comprises a gradient heating table 2 with built-in temperature sensors 5, on which a tested TLC matrix 1 is applied, a white light illuminator 6, an imaging detector in the form of a colour camera 7 and is connected to a computer 8 with appropriate interfaces and software.

In a gradient embodiment a heating table 2 comprises a plate of a material with high thermal conductivity (e.g. aluminium, copper) with stabilized thermally active means 3 and 4 (e.g. Peltier pads or resistance heaters) placed at the ends and temperature sensors, on which the temperature can be set separately. By setting the temperature at one end of the plate in the lower range of the TLC matrix conversion, and at the other end in the upper part of the conversion, full temperature gradient on the surface of the table is achieved across the plate. Temperature sensors 5 located at the heating plate allow to determine the temperature value at selected points of the plate and extrapolate the temperature value along the resulting temperature gradient. Surface of the plate has a convex shape which allows to stretch the film using stretching-pressing means 9, to ensure full contact between the table and the matrix, without the need for additional means to improve the thermal contact. The applied lighting 6 is arranged in a way that guarantees the absence of light reflections due to direct reflection from the film surface in the direction of the observation by the detector 7. This can be done either by placing the lighting along the opposite ends of the plate or omnidirectionally. The screen 10 prevents reflections from the surroundings. The detector 7 is installed above the heating plate and allows the registration of the colour image of the matrix placed on the heating table.

The TLC thermochromic liquid-crystal matrix 1 is placed on the heating table 2 and is pressed to the profiled surface of the table with the aid of the stretching-pressing means 9. After activating the heating-cooling, lighting and recording system the calibration control may start, which in this case consists in determining the relationship between the input of the system (temperature) and the output (chrominance). From the data collected that way many characteristic film parameters can be determined, including thermal responsiveness ranges of the thermochromic films. In turn, the inspection procedure of TLC matrix calibration is based on a comparison of the calibration curves with the reference calibration map.

b) The calibration method of the thermal excitation

Ensuring full compliance of the excitation and observation conditions (by in-situ) is not possible due to the medical nature of the sensor. Thus, a thermal excitation system is replaced by a stabilized heating table 2, and a visual observation by imaging with a camera 7. The correct designation of colorimetric parameters in this case requires taking into account the spectral characteristics of the detector 7 and the illuminator 6, and directional characteristics of light scattering on the thermochromic matrix test surface. There are two known calibration methods: steady-state and dynamic method. In the case of steady-state method, the method of uniform temperature and temperature gradient method can be distinguished (CUKUREL at el "Color theory perception of steady wide band liquid crystal Thermometry", Experimental thermal and fluid science, vol. 39, p. 1 12- 122, 2012).

c) Thermal excitation

The following requirements were set with respect to the proposed heating table 2:

the working area for examining the matrix 120 mm wide and minimum 200 mm long (ribbon type),

the ability to set gradient temperature distributions,

the temperature stabilization AT < 0.01 °C,

the rate of temperature stabilization Tstab < 100 sec,

the uniform thermal surface contact resistance.

In order to achieve the above requirements a block of heat conducting material was suggested (aluminum λ (T=25°C) = 237 (W/m/K)), heated from two opposite sides by means of Peltier pads 3 and 4. Two TEC PID controllers were used for temperature stabilisation, controlling the actuators with a PWM signal, with a feedback from the thermo-resistance sensors. Two sets of Peltier pads with a power of 7x17W were used to achieve high system dynamics, while directing excess heat via a water cooling system.

Analysing the transfer of heat from the heating table to the insulator (matrix) attention has been paid to the resistance of the contact layer, which depends on the pressing force between the layers, the surface roughness and the intermediate material. To minimize the impact and to ensure uniformity, the surface of the heating table 2 is formed as convex, which facilitates stretching the matrix thereon. Moreover, the surface roughness was decreased through the surface polishing, reaching Sa < 20 μηι.

It is also noted that the applied matrix TLC 1 introduces a much larger share of the surface radiation, as the surface of polished aluminium has a low emissivity coefficient (ε ~ 0.06), in contrast to the black absorbent layer of the TLC matrix. As shown in the temperature profiles of the heating table 2 in the simulation model (Fig. 2), temperature distribution between the heating elements is not linear as a result of heat dissipation to the environment. The value of the temperature drop is dependent on the set temperature and ambient temperature. For its designation multi-temperature measurement was introduced, based on which non-linear regression model was determined (quadratic function) to give the adjustment r2 > 0.999.

Dynamic testing of the system was also performed, setting the time to reach the steady state. Table surface reaches a certain temperature, with an error below 1 % after 100 sec. d) Image acquisition and temperature measurement

The following requirements were set for the image acquisition module:

RGB imaging of the TLC matrix surface,

the field of view (FOV): length min. 120 mm, width min. 10 mm in the central part of the table,

spatial resolution min. 0.1 mm/pix., dynamic resolution min. 14 bits,

minimizing distortion and aberration,

minimizing the impact of camera sensor dark current, and

polychromatic illumination (white) with CRI > 80.

Design constraints were observed, resulting from a strong surface gloss of the matrix and table convexity, which causes the variation of viewing angles and lighting, resulting in a change in the colour response. Moreover, since the illuminator used cannot warm up the surface of the matrix, duration of the test should be minimized to a few minutes, so that there is no falsification of the measurement results.

Simulation model of light propagation, based on two rows of LEDs (REINER Jdentyfikacja i modelowanie optyczne systemow wizyjnej kontroli jakosci wytwarzania", Wroclaw: Oficyna Wydawnicza Politechniki Wroctawskiej, 2013) was developed for verifying and optimizing the lighting system along with the acquisition of images.

CMOS camera with the matrix size of 1 " and a pixel size of 5.5 μηι was chosen for achieving high sensitivity and minimizing noise. Dependence of dark current on temperature, which has been used in the process of compensation was determined for the camera. Distortions and aberrations of the chosen lens were determined, stating that their share of the error budget is negligible.

Processed multi-channel temperature measurement of the heating table and the camera sensor temperature and ambient temperature reading were realised concurrently with the acquisition of images, as shown in Fig. 3.

Camera image is reduced by leaving the central portion with a height of 100 pixels, and then converted to RGB profiles by averaging the intensity in individual columns and channels. Then, the effect of temperature on the value of the transducer's dark current is adjusted based on the experimentally determined dependence. Then the adjustment is made to the effect of the heating table's convexity and uneven lighting. It is performed on the basis of a reference profile of the directional film scattering and lighting heterogeneity, registered for excitation outside the matrix responsiveness. The final step is to normalize the need which results from variable intensity light source.

Temperature profile adjustment for 12 measuring points is performed concurrently with the image processing. Registered temperature values are the basis for determining approximation coefficients using a quadratic function and the quality of it adjustment is evaluated. In the final stage of processing, both profiles are synchronized, which allows to create a dependence of the chrominance on the temperature excitation - i.e. a calibration curve. This curve is then transformed to the other colour space e.g. HSV.

e) Matrix calibration

Checking the accuracy of the calibration involves determining characteristic parameters of the chromatic and brightness data recorded by the image camera of the tested matrix at various points of the heating table and at a given temperature after adjusting them according to the information collected from the reference matrix. Reference data is collected in the corresponding measurement conditions (ambient temperature, the time of applying on the heating table), wherein multiple image data recordings are carried out using different settings of the thermally active circuits of the heating table. Set values are changed stepwise in the environment of the target matrix conversion range. In this way, a map of chromatic and brightness data changes is developed depending on the angle of camera observation/lighting and matrix temperature. Thermal transfer of the controlled matrix is determined based upon shifting the chrominance the median value of the tested matrix relative to the data from the reference model and matrix sensitivity is determined based on the tilt angle of the chrominance function value change relative to the temperature.

Suggested system allows the rapid assessment of the accuracy of the calibration of TLC matrices in a full-range conversion.

Inspection procedure of TLC matrix calibration is based on a comparison of the calibration curves with a reference calibration map. Depending on the parameters describing the calibration, characteristics of the various chrominance components are used (e.g. Hue for middle temperature and its scope, and Green component for intensity and saturation).

The key task of preparing the system for inspection is determining a reference calibration map. It involves scanning the temperature range at a gradient excitation with a gradient corresponding to the scope of the matrix. The temperature range of the reference calibration map results from the nominal value of the TLC matrix, adopted for the field of tolerance (e.g. ± 0.5°C) and resolution (e.g. 0.1 °C).

Different matrices are examined in a gradient way for nominal calibration. Moreover, in thermodynamic conditions, duration of the examination corresponds to a recording time of the reference calibration curve.

f) Testing workstation

According to the above discussed results of research and development, a detailed draft of the workstation was prepared, which is shown in Fig. 4.

Control, acquisition and processing functions of images and temperature measurements were implemented on a PC connected to the individual components via the Ethernet communication interface. The algorithms were implemented in the LabView environment. Proper operation of the workstation is guaranteed also by a stretching system of the tested matrices and shields to minimize external interference against lighting and temperature.

According to the simulation model, after applying the matrix to the surface of the table the transition state occurs. Terms of equilibrium for a temperature error ΔΤ = 0.01 °C requires stabilization time t min = 200 s. Hence, calibration tests are conducted under dynamic conditions (transient state).

g) Application

The device is used to study thermal-chromatic calibration of the thermochromic liquid- crystal matrices. Therefore it allows the control of the incorrect colour response resulting from incorrect preparation of liquid-crystal emulsion, and emulsion application or crosslinking, etc. Based on the measurement results a qualitative decision is taken on the correctness of the thermal range of the tested matrix.

Developed workstation help to detect defective TLC matrices, with abnormal middle temperatures and narrow temperature range, and matrices with lower brightness or saturation of the colour response.

Claims

1 . A device for controlling the calibration of thermochromic liquid-crystal matrices, characterized in that it comprises:

- a heating table (2) with a stabilized thermally active means (3) and (4) placed at the ends and temperature sensors (5),

- a stretching-pressing means (9),

- a lighting (6), a detector (7) and a screen (1 1 ).

2. The device according to claim 1 , characterized in that is connected to a computer (8) with appropriate interfaces and software.

3. The device according to claim 1 or 2, characterized in that the table (2) has a convex shape.

4. The device according to claim 1 or 2 or 3, characterized in that the table (2) is made of a material with high thermal conductivity, selected from aluminium, copper.

5. The device according to any of the claims 1 -4, characterized in that the means (3) and (4) are Peltier pads.

6. The device according to any of the claims 1 -4, characterized in that the means (3) and (4) are resistance heaters.

7. The device according to any of the claims 1 -6, characterized in that the temperature can be set independently on these means (3) and (4).

8. The device according to any of the claims 1 -7, characterized in that the lighting (6) is positioned such that there is no light reflection due to the reflection from the film surface in the direction of the observation by the detector (7).

9. The device according to any of the claims 1 -8, characterized in that the lighting (6) is disposed along the opposite edges of the table (2).

10. The device according to any of the claims 1 -8, characterized in that the lighting

(6) is disposed omnidirectionally.

1 1 . The device according to any of the claims 1 -10, characterized in that the detector

(7) is placed over the table (2).

12. The device according to any of the claims 1 -1 1 , characterized in that, the screen (1 1 ) prevents reflections from the surroundings.

13. A method of controlling the TLC matrix using the device as defined in claims 1 -12, characterized in that it comprises the steps of:

- placing the TLC matrix (1 ) on the heating table (2) and pressing against the surface of the table by the stretching-pressing means (9);

- activating the lighting system;

- activating the recording system,

wherein the order of the above-mentioned steps is optional, and then

- the relationship between the input of the system (the temperature) and the output (the chrominance) is established;

and thanks to the sensors (5) determine the temperature at selected points of the table and the temperature values are extrapolated along the resulting temperature gradient.

14. The method according to claim 13, characterized in that using a convex shape of the table (2) the film is stretched using stretching-pressing means (9), ensuring full contact between the table (2) and the matrix (1 ).

15. The method according to claim 13 or 14, characterized in that the colour image of the matrix (1 ) placed on the table (2) is registered using the detector (7).

16. The method according to claim 13 or 14 or 15, characterized in that the lighting (6), which is placed below the surface defining the direct light reflection (1 1 ) from the TLC matrix (1 ) in the direction of the camera imaging the colour of the matrix is used.

17. The method according to claim 13-16, characterized in that the chrominance data of the tested matrix are compared with the reference data collected from the reference model in the same system under varying thermal conditions.

18. A use of the device according to claims 1 -12 for testing thermal-chromatic calibration of the thermochromic liquid-crystal matrices.