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Technical Briefs

A Light Transmission Based Liquid Crystal Thermography System

[+] Author and Article Information
Timothy B. Roth

Department of Mechanical Engineering,  Union College, Schenectady, NY 12308

Ann M. Anderson1

Department of Mechanical Engineering,  Union College, Schenectady, NY 12308andersoa@union.edu

1

Corresponding author.

J. Heat Transfer 130(1), 014503 (Jan 28, 2008) (4 pages) doi:10.1115/1.2780187 History: Received August 29, 2006; Revised April 23, 2007; Published January 28, 2008

Abstract

This paper presents results from a study aimed at developing a novel thermochromic liquid crystal (TLC) temperature measurement system that uses light transmission instead of light reflection to measure surface temperature fields. In previous work, we reported on the effect of temperature on light transmission through TLCs as measured with a spectrophotometer [Roth, T. B., and Anderson, A. M., 2005, “Light Transmission Characteristics of Thermochromic Liquid Crystals  ,” Proceedings of IMECE2005, Orlando, FL, Paper No. IMECE2005-81812;Roth, T. B., and Anderson, A. M., 2007, “The Effects of Film Thickness, Light Polarization and Light Intensity on the Light Transmission Characteristics of Thermochromic Liquid Crystals  ,” ASME J. Heat Transfer, 129(3), pp. 372–378]. Here we report on results obtained using a charge coupled device (CCD) camera and polychromatic light setup that is similar to the type of equipment used in TLC reflection thermography. We tested three different light sources, a white electroluminescent light, a green electroluminescent light, and a halogen fiber optic light, using both direct and remote lighting techniques. We found that the green signal (as detected by the CCD camera) of the green electroluminescent light makes the best temperature sensor, because under remote lighting conditions it showed a 500% linear signal increase as the temperature of the R25C10W TLCs was raised from $30°to48°C$. We further found that the angle of the CCD camera relative to the light did not significantly affect the results for angles up to $10deg$ for remote lighting and $15deg$ for direct lighting. The effect of light intensity variation was not significant for intensities up to 40% of the original level when normalized on the intensity at $19°C$ (a temperature outside the active range of the TLCs). The use of light transmission results in a larger range of temperature over which the TLCs can be calibrated and offers opportunities for more uniform lighting conditions, which may help overcome some of the problems associated with light reflection.

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Figures

Figure 1

Test setup for liquid crystal thermography in (a) reflection mode and (b) transmission mode. For the transmission tests, we used both direct lighting (as shown) and remote lighting, in which the light source was placed a distance below the TLC layer.

Figure 2

The CCD RGB and I signals for light transmitted through the 0.4mm TLC layer using the green electroluminescent light under remote lighting conditions

Figure 4

Normalized intensity through a 0.40mm and a 0.23mm TLC layer with the green electroluminescent light source under remote and direct lighting conditions. The intensity values are normalized by the intensity value at 30°C.

Figure 5

The normalized green signal for a range of green light intensity levels. The green values are normalized by the green value at 19°C, which is a temperature outside the active range of the TLCs.

Figure 3

The CCD RGB and I signals for light transmitted through the 0.4mm TLC layer using the green electroluminescent light under direct lighting conditions

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