RESEARCH PAPERS: Experimental Techniques

Thermochromic Liquid Crystal Thermography: Illumination Spectral Effects. Part 2: Theory

[+] Author and Article Information
M. R. Anderson

 Calpine Corporation, 104 Woodmere Road, Folsom, CA 95630michael.anderson@calpine.com

J. W. Baughn

Department of Mechanical and Aeronautical Engineering,  University of California, Davis, One Shields Avenue, Davis, CA 95616jwbaughn@ucdavis.edu

J. Heat Transfer 127(6), 588-597 (Jan 12, 2005) (10 pages) doi:10.1115/1.1915388 History: Received June 27, 2004; Revised January 12, 2005

A theoretical model of a Thermochromic Liquid Crystal (TLC) imaging system was developed to aid in understanding the results of experiments on spectral effects and to investigate the various factors affecting the hue-temperature calibration of TLC’s. The factors in the model include the spectral distribution of the illumination source and UV filter, surface reflection of both the TLC and background, and the sensing device (camera) spectral characteristics and gain settings. It was found that typical hue-temperature calibration curves could not be entirely explained by a TLC reflectivity model with either a monochromatic spike or a narrow bandwidth reflectivity, which is often assumed. Experimental results could be explained, however, by a model that reflects over a relatively large band of wavelengths. The spectral characteristics of the five illumination sources (those for which experiments were performed) were considered. Background reflection, which commonly accounts for 30%–50% of the reflected light, was found to significantly attenuate the hue-temperature calibration curves toward the background hue value. The effect of the illumination source on the hue-temperature calibration curves is demonstrated and several experimentally observed phenomena are explained by the results of the theoretical calculations, specifically the spectral reflective properties of the liquid crystals and the transmissivity of the R, G, and B filters in the image capture camera.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Diagram of liquid crystal thermography system with illumination model

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Figure 2

Normalized spectral distribution of several light sources

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Figure 5

Spectral characteristics of a (27–35°C) microencapsulated liquid crystal (from Akino (11))

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Figure 3

Spectral reflection coefficient for light normally incident on a chiral nematic planar film (a) semi-infinite slab and (b) finite slab (from Coles (17))

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Figure 4

Intensity of selectively reflected light as a function of wavelength. Material Cholesteryl oleyl carbonate (from Ennulat (18)).

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Figure 6

Square function and tapered edge surface reflectivity models

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Figure 7

Estimates of the R, G, and B filter spectral transmissivities for the camera used in the present work

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Figure 8

Theoretical RGB output vs temperature (untapered model)

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Figure 9

Theoretical HSV output vs temperature (untapered model)

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Figure 10

Theoretical RGB output vs temperature (tapered model)



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