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Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
Calibration
Calibration Details
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Calibration of Liquid Crystal Thermography (LCT) with Isothermal & Gradient Methods, Slides of Mechanical Engineering
An in-depth understanding of the calibration process for liquid crystal thermography (lct), specifically focusing on the successive isothermal and gradient methods. The importance of calibration, the procedures for each method, and the factors affecting the accuracy of lct measurements. It also mentions recent investigations and recommendations for best practices.
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2012/2013
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Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
Calibration
Calibration Details
Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
Calibration
The color-temperature response of the surface coated with a liquid crystal sheet or painted with slurry should be known prior to its use for temperature measurement. This step, called calibration is carried out to develop the intrinsic color-temperature response of the liquid crystals. During calibration, the color image of the LCs is acquired when the surface is held at a known, spatially uniform and steady temperature. The calibration effort can be either successively isothermal or a gradient technique.
Successive isotherm method
In the successive isothermal method, a temperature controlled test surface and the imaging system are used to generate liquid crystal color-to-temperature calibration data. The color image of the test surface is acquired after bringing the test surface and the liquid crystal to its event temperature. An average color value is computed and stored along with the temperature of the test surface. This process is repeated at subsequently higher temperatures until the clearing point temperature is reached. Though it appears to be simple, the successive isotherm method can be very time consuming since the amount of data is large and the processing requirement is acute.
Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
A majority of researchers recommend the in-situ mode of calibration. Here, calibration is carried out within the main experimental apparatus itself. A wide-band calibration technique (covering the bandwidth of the liquid crystal material) applied point-wise is proposed. Wide-band calibration employed by the previous researchers mostly calibrate the LCs using a single reference point on the test surface against temperature, and then apply this single-point calibration over the entire test surface (Smith, 2001). The illumination (source) as well as imaging (camera) positions is close to normal with respect to the test surface. For the micro-encapsulated thermochromic LCs, the sensitivity of hue to the viewing and illumination angles is found to be insignificant (Camci, 1992, 1993).
It is worth mentioning here that recent studies have opted for pixel-level calibration- namely generate calibration curves of hue vs temperature for each of the 1024 X 1024 pixels.
For the images discussed in the present chapter, an in-situ , single-point calibration has been performed under no-flow condition. The lighting arrangement and camera position are kept identical for the calibration stage and the full experiment. The parameters of the image processing system including color capturing settings are locked. The imaging camera is mounted at the top of the test section. The illumination system was also located on the camera side of the test surface (called on-axis lighting). With this configuration, the maximum permissible deviations of the viewing inclination and illumination angle from the normal are found to be less than 10 o^ and 20 o^ respectively.
Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
The image acquisition and processing system used in the present study consist of a high-resolution 3- CCD video camera (SONY XC-003P) with 16 mm focal length lens (VCL-16WM), a 24-bit true color frame grabber board (Imaging Technology Inc., IC-PCI) and a high speed PC. The model XC-003P (Figure 6.5) is a 3-CCD RGB color camera module designed primarily for process control and image processing applications.
Figure 6.3 High resolution 3-CCD camera (SONY XC-003P)
Camera XC-003P works in accordance with EIA video norm of NTSC (National Television Systems Committee of the Electronic Industries Association) which has prepared the TV standard for the USA , Canada , Japan , Central America , half of the Caribbean and half of South America ). When referring to NTSC video what is normally meant is 525 line 60 Hz. PAL (phase alternation line) is the TV format used in most of Western Europe , Australia and other countries including India. When referring to PAL video, what is normally meant is the 625 line 50 Hz video. The new 3 CCD C-mount optical prism block achieves three times higher resolution compared to a 1-CCD system. Moreover, the standard C-mount lens system allows the use a wide variety of lenses.
Module 6: Liquid Crystal Thermography
Lecture 37: Calibration of LCT
The Imaging Technology frame grabber, IC-PCI is a half-slot PCI bus image capture card, which is used in conjunction with the ITEX -IC software. The latter is a generic term for Imaging Technology software functions, callable and linkable with C language application programs. ITEX -IC refers to the aggregate sum of sub-libraries or software modules for the Image capture family. ITEX -IC is supported under Windows and protected mode DOS with the WATCOM 32-bit C/C++ compilers. It provides the full support for Microsoft Visual C/C++ and Visual Basic, allowing Windows programmers the environment of their choice. It features one of the fastest sustained image data transfer rates to PC memory (up to 120 MB/sec to the PCI bus and more than 90 MB/sec to the host memory, depending on the host PC). Transfer rates faster than real time (< 33 ms) allow the good use of the CPU for host based processing, as it is not tied up with arbitrating bus operations. Display is performed completely by the host display processor. Data is transferred out of the IC-PCI over the PCI bus to the display card or the host memory. The frame grabber captures the image at the camera speed of 38 frames per second. The resulting image retains a majority of the characteristic intensities and spatial frequencies of the original. Subsequently, the image is stored on the hard-disk of the computer.
In the experiments reported in the present module, 45 sequential images have been recorded from short- duration transient experiments. The precision timer function allows correct setting and estimation of the intermediate time between sequential images. Using the image processing software these images are digitized frame by frame and converted into a hue matrix. The digitized data becomes the basis for the measurement of temperature. The temperature data can be used for the evaluation of the local heat transfer coefficient.
Module 6: Liquid Crystal Thermography
Lecture 37:
Calibration details
For calibration, the surface plate mounted with the LC sheet is heated under no flow conditions to a temperature just above its active range. Here, the color of the LC sheet will appear to be uniformly blue (Figure 6.4(a)).
Figure 6.4(a) Test surface heated to the clearing point temperature of the LCT
sheet
The heater is turned off and the surface is allowed to cool slowly by natural convection. Several K-type thermocouples can be embedded inside the aluminum plate from underneath. One of these pre-calibrated thermocouples can be utilized for the calibration purpose.
Module 6: Liquid Crystal Thermography
Lecture 37:
Subsequently, the average hue for the chosen region along with the measured temperature forms a data point of the calibration curve (Figure 6.4(c).
Figure 6.4(c) Calibration curve of the LC sheet relating hue (H), saturation (S),
and intensity (I) with temperature
In the experience of the authors, a typical calibration curve consists of 40-50 points. One can expect this data set to be sufficient to resolve the hue-temperature relationship. Further, a suitably high order polynomial can be used to analytically represent the calibration data. This approach can be used for saturation and intensity as well.
Figure 6.4(c) shows a typical calibration curve drawn for a liquid crystal sheet used by the authors.
Module 6: Liquid Crystal Thermography
Lecture 37:
A companion plot of the variation of the individual colors R,G, and B with temperature is shown in Figure 6.4(d).
Figure 6.4(d): Variation of individual colors R,G, and B with temperature.
It is to be observed that hue has a monotonic variation with respect to temperature. The uniform level of intensity is itself an indication of the uniform level of illumination over the test-surface. It confirms the quality of lighting arrangement used for the experiments.
To estimate the spread of H, S, and I within the sample region of the LC image, their variance is shown in Figure 6.5.
Module 6: Liquid Crystal Thermography
Lecture 37:
Earlier investigations (Baughn, 1999; Sabatino, 2000; Smith, 2001) have reported hysteresis in the liquid crystal response, the hue-temperature behavior during cooling being significantly different from that during heating. Hysteresis is likely to occur if the heating cycle takes the liquid crystal material beyond its clearing point temperature. Conversely, the transient experiment may have been initiated below the event temperature. To eliminate hysteresis effects, proper care is taken to ensure that the test surface operates strictly between the pre-defined temperature limits. In the studies conducted by the authors, calibration of the liquid crystal sheet has been performed during the cooling of plate within the end point temperatures. By performing the calibration test several times, the authors have seen that the calibration curve is repeatable.