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RESEARCH PAPERS: Experimental Techniques

The Effects of Film Thickness, Light Polarization, and Light Intensity on the Light Transmission Characteristics of Thermochromic Liquid Crystals

[+] Author and Article Information
Timothy B. Roth

Department of Mechanical Engineering, Union College, Schenectady, NY 12302

Ann M. Anderson1

Department of Mechanical Engineering, Union College, Schenectady, NY 12302andersoa@union.edu

1

Corresponding author.

J. Heat Transfer 129(3), 372-378 (Jun 15, 2006) (7 pages) doi:10.1115/1.2430724 History: Received December 15, 2005; Revised June 15, 2006

Thermochromic liquid crystal materials change their crystalline structure and optical properties with temperature, making them useful in temperature measurement applications. This paper presents the results of a study to develop a temperature measurement system that uses light transmission through thermochromic liquid crystals instead of light reflection. We painted Hallcrest R25C10W sprayable liquid crystals on a clear surface and placed it in a spectrophotometer. The amount of light transmitted at monochromatic wavelengths from 400nm to 700nm was measured for temperatures from 25°C to 55°C under conditions of nonpolarized, linearly polarized, and cross-polarized light, for three light intensity levels, and three liquid crystal layer thicknesses. As the temperature was increased the amount of light transmitted through the liquid crystal layer increased. When the liquid crystals are in their active range the transmission spectra exhibit an s-curve shape and the percent of light transmitted through the liquid crystals at a fixed temperature increases with increasing wavelength. We detected significant changes in the transmission spectra for temperatures from 27°C to 48°C, whereas when used with reflected light the thermochromic liquid crystals are useful over a significantly smaller range. As the thickness of the thermochromic liquid crystal layer increases or as the incoming light intensity decreases, the amount of light transmitted through the liquid crystals decreases. We also investigated the effects of temperature overheat on the transmission spectra and found that heating the thermochromic liquid crystals above their active range increases the amount of light transmission. However, when the liquid crystals are cooled below their active range they return to their original state. We have analyzed the spectrophotometer data in a number of ways including: (a) total amount of light transmitted, (b) amount of red, green, and blue light transmitted; and (c) spectral curve shape characteristics (peak transmission, inflection wavelength and wavelength for peak transmission) all as a function of temperature. A linear relationship exists between temperature and all of these variables which we believe can be exploited for the development of a charge coupled light camera based light transmission system for temperature measurement.

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Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of test setup used in light transmission tests

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Figure 2

Schematic of test setup used in light reflection tests

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Figure 3

Transmission spectra for Surface 1 under linear polarized lighting conditions

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Figure 4

Transmission spectra for Surface 1 under linear cross polarized lighting conditions

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Figure 5

RGB values for reflected light conditions for Surface 2. These data show a red peak at 29°C, a green peak at 34°C, and a blue peak at 45°C.

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Figure 6

Temperature overheat effects under linear polarized lighting conditions show that the overheated TLCs transmit more light at low wavelengths

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Figure 7

A comparison of the transmission spectra at 36°C for Surfaces 1, 3, and 4 shows decreasing transmission with increasing layer thickness

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Figure 8

A comparison of the transmission spectra at 36°C for Surface 3 under three different light intensity levels. The shapes of the spectra are similar.

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Figure 9

Total intensity of transmitted light as a function of temperature for Surfaces 1, 3, and 4 for linear polarized and linear and cross-polarized lighting conditions

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Figure 10

Intensity of red, green, and blue transmitted light as a function of temperature for Surface 3 under linear polarized lighting conditions

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Figure 11

The normalized green signal for Surfaces 1, 3, and 4 shows a larger relative change in the signal for the thicker surface

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Figure 12

Characteristics of the spectral shape as a function of temperature for Surface 1

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Figure 13

Inflection point wavelength for three light intensity levels on Surface 3 and 100% light on Surface 4

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