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Color-based image processing to measure local temperature distributions by wide-band liquid crystal thermography

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This study presents a color-image-processing procedure for non-intrusive local temperature measurements by thermochromic liquid crystals (TLCs). The image evaluation software is completely independent of the color detection and acquisition hardware. This allows to use a wide variety of hardware solutions. An easy reproducible calibration of camera and light source is presented. The dependence of the detected hue values on intensity is investigated and further the hueversus temperature relation is studied.

Sprayable TLC formulations and TLC-coated polyester sheets are studied and compared with regard to their signal-to-noise ratio and the dependence of their hue values on illumination and viewing angle. Furthermore, a method to investigate the hue resolution is presented. The relation between the resolution of hue values and the illumination intensity and its influence on signal noise is discussed for the first time for TLC applications. Different techniques of signal noise reduction are implemented in the image processing system. Their effects on the signal noise level are discussed. As an example the two dimensional temperature distribution caused by wing-type vortex generators in a channel flow is given.

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Δ (H,S,I):

smallest change ofH,S orI for variation of R, G or B by one amplitude step

Δ(T rep):

reproducibility error of temperature

ΔT :

usable calibrated temperature range, K


hue, saturation, intensity

k :


N :

number of averaged images

n′ :

index of refraction of TLCs, ≅ 1.5

R, G, B:

red, green, blue intensities


resolution ofH,S orI, bit

t :

time, s

T :

temperature, °C

x,y :

spatial coordinates

x :


Y/C :

luminance and chrominance signal

φ i :

angle, degree


heat conductance, W/(mK)


dominant wavelength, 10−6·m

i :


i :

calibration no.i

i :


j :

calibration no.j

M :

object (model)

n :

viewing- and illumination angle =0°C (perpendicular to the surface)

s :



thermochromic liquid crystals


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Behle, M., Schulz, K., Leiner, W. et al. Color-based image processing to measure local temperature distributions by wide-band liquid crystal thermography. Appl. Sci. Res. 56, 113–143 (1996). https://doi.org/10.1007/BF02249377

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Key words

  • liquid crystal thermography
  • color image processing
  • hue-to-temperature—calibration