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Experiments in Fluids

, Volume 43, Issue 6, pp 929–937 | Cite as

Angular effects on thermochromic liquid crystal thermography

  • Paul M. KodzwaJr
  • John K. EatonEmail author
Research Article

Abstract

This paper directly discusses the effects of lighting and viewing angles on liquid crystal thermography. This is because although thermochromic liquid crystals (TLCs) are a widely-used and accepted tool in heat transfer research, little effort has been directed to analytically describing these effects. Such insight is invaluable for the development of effective mitigation strategies. Using analytical relationships that describe the perceived color shift, a systematic manner of improving the performance of a TLC system is presented. This is particularly relevant for applications where significant variations in lighting and/or viewing angles are expected (such as a highly curved surface). This discussion includes an examination of the importance of the definition of the hue angle used to calibrate the color of a TLC-painted surface. The theoretical basis of the validated high-accuracy calibration approach reported by Kodzwa et al. (Exp Fluids s00348-007-0310-6, 2007) is presented.

Keywords

Liquid Crystal Spectral Reflectance Color Component Viewing Angle Thermochromic Liquid Crystal 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols

CCD

charge coupled device

B

blue component value

BL

referring to black reference

c

arbitrary constant

G

green component value

i

integer value

max

maximum value

min

minimum value

MV

measured voltage

N

number of calibration curves

nair

index of refraction for air

\(\overline{n}_{\rm TLC}\)

average index of refraction for TLC

0

initial conditions

Q

hue angle

R

red component value

REF

reference

ROI

region of interest

S

saturation

S

partial saturation

\({{\mathcal{T}}}\)

time required to collect calibration curves (h)

TLC

Thermochromic Liquid Crystal

V

voltage or color parameter (Hay and Hollingsworth 1996)

V

color parameter (Hacker and Eaton 1995)

WH

referring to white reference

x

spatial coordinate

y

spatial coordinate

\({{\mathcal{Z}}}\)

channel value

Δ

spacing

Δx|min

minimum cell size in x-direction

Δy|min

minimum cell size in y-direction

δ

change

δu

uncertainty

ɛ

difference between mean component values

ϕTLC,i

angle of illumination

ϕTLC,s

angle of scattering or viewing

λ

wavelength (nm)

λn

wavelength of maximum scattering for normal incidence and observation (nm)

τ1

time necessary to convert \({{\mathcal{Z}}}\) for each pixel (s)

τ2

time necessary to set temperature of calibration surface (s)

\(\vartheta\)

number of pixels in a given ROI

Notes

Acknowledgments

This research was sponsored by General Electric Aircraft Engines, through their University Strategic Alliance Program. The first author received support from the National Science Foundation via a 3-year Graduate Fellowship. The authors would like to express their sincere gratitude to F. Buck, B. Bergholz and D. Wisler (at GEAE) and D. Dods (at Instrument Technology Incorporated) and V. Davison at (QC/NDT) for their assistance and advice in the course of this experimental program. S. Carver, J. Hammer, J. Glassman, T. Hasler and L. Johal are gratefully acknowledged for their high level of expertise in manufacturing various parts in this experimental program.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Department of Mechanical Engineering, Flow Physics and ComputationStanford UniversityStanfordUSA

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