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Surface temperature measurement using thermochromic liquid crystals and ratiometric analysis of spectral intensities of scattered light

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Abstract

This study presented a new method for surface temperature measurement using thermochromic liquid crystals. In the proposed method, the temperatures over the surface of a sheet of thermochromic liquid crystals were measured using the ratio of the spectral intensities of two specific wavelengths less than 450 nm. The measurable temperature range was broadened by approximately three times that of the conventional methods based on color change. In the experiment, the target object for measuring the surface temperature was a planar copper plate with a sheet of thermochromic liquid crystals on its surface having a color-change range of 10–20 °C; a white light emitting diode was used as the light source. Two monochrome charge-coupled device cameras were used to acquire images of the spectral intensities to eliminate the effect of spatial fluctuations of the light emission on measurement accuracy. The results demonstrated that a measurable temperature range of 21–51 °C with a temperature resolution higher than 0.04 °C was obtained using the ratio of the spectral intensities of two wavelengths, i.e., 442 nm and 436 nm. The uncertainty of the measured temperature was also analyzed. The minimum temperature uncertainty of 0.06 °C was obtained through image processing analysis using a spatial filtering window of size five times five pixels and 500 sequential images for time averaging.

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Abbreviations

\({F}_{0}\) :

Geometric factor [–]

\(I\) :

Spectral intensity of scattered light [–]

\({I}_{0}\) :

Intensity of the light source [W/m2]

\(L\) :

Optical factor [–]

\(N\) :

Number of images [–]

\(n\) :

Number of pixels [–]

\(R\) :

Temperature resolution [°C]

\(S\) :

Sensitivity of the camera [–]

\(T\) :

Temperature [°C]

\({T}_{R}\) :

Measurable temperature range [°C]

\({T}_{T}\) :

Thermocouple temperature [°C]

\({T}_{U}\) :

Uncertainty of measured temperature [°C]

\({T}_{UT}\) :

Uncertainty of thermocouple temperature [°C]

\(t\) :

Time [s]

\({t}_{S}\) :

Exposure time of the camera [s]

\(x,y\) :

Coordinates of the image [–]

\(\alpha\) :

Confidence coefficient [–]

\(\epsilon\) :

Absolute reflectance [–]

\(\lambda\) :

Wavelength [nm]

\(\sigma\) :

Standard deviation [–]

\(\phi\) :

Ratio of scattered light intensities [–]

\(i\) :

Number of pixels

\(\lambda\) :

Wavelength

\({\lambda }_{2}/{\lambda }_{1}\) :

Wavelength ratio

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Funding

This work was supported by JSPS KAKENHI Grant Numbers JP16K06113, JP20K04306.

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Authors and Affiliations

  1. Graduate Faculty of Interdisciplinary Research, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi, 400-8511, Japan

    Koji Toriyama, Koichi Ichimiya & Shumpei Funatani

  2. Department of Applied Physics, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa, 239-8686, Japan

    Shigeru Tada

  3. Toyota Auto Body Co., Ltd, 100 Kanayama, Ichiriyama-Cho, Kariya, Aichi, 448-8666, Japan

    Daisuke Kokui

Authors
  1. Koji Toriyama
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  2. Shigeru Tada
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  3. Koichi Ichimiya
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  4. Shumpei Funatani
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  5. Daisuke Kokui
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Correspondence to Shigeru Tada.

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Toriyama, K., Tada, S., Ichimiya, K. et al. Surface temperature measurement using thermochromic liquid crystals and ratiometric analysis of spectral intensities of scattered light. Heat Mass Transfer 59, 1049–1058 (2023). https://doi.org/10.1007/s00231-022-03310-2

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