Determination of Emissivity by Using Reflected Thermal Radiation

Frank Liebmann

Learning Objectives

• Learn about the effects of emissivity uncertainty• Learn test methods to determine emissivity• Help laboratory customers to determine emissivity

© 2014 Fluke Calibration Liebmann – Emissivity Determination 2

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 3

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 4

Introduction

• Emissivity is essential to accurate IR thermometry measurement• Lack of credible information on emissivity• Existing methods to determine emissivity• Reflective method – practical for field usage

– Yamada and Ishii (2011)

© 2014 Fluke Calibration Liebmann – Emissivity Determination 5

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 6

Effects of Error in Estimating Emissivity

• Effects due to emissivity estimation error can be large• Conditions for this graph

– εSURF = 0.96

– 8 – 14 µm radiation thermometer

– εIRT = 0.95

– TAMB = 23 °C

© 2014 Fluke Calibration Liebmann – Emissivity Determination 7

-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650-2

-1

0

1

2

3

4

5

6

Surface Temperature / °C

erro

r /

K

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 8

Methods for Determining Emissivity

• Comparison method– Part of surface must have known emissivity

• Contact versus non-contact method– Contact temperature may not be true surface temperature

• Blackbody method– Destructive

• FTIR testing– Laboratory measurement (for the most part)

• Emissivity tables– No uncertainties– Values vary greatly from table to table

• Reflective method – Novel method discussed here

© 2014 Fluke Calibration Liebmann – Emissivity Determination 9

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 10

Theory

• Vary TW

• Observe change in S(TRT)

• Solve for εS

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Measurement Equation IR ThermometryεIRT = 1.00

S(TRT): IR Thermometer ReadoutS(TS): Surface temperatureS(TW): Reflected temperatureεS: Surface emissivity

WSSSRT TSTSTS 1

Theory

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Theory

• A portion of the reflected temperature (α) is coming from the heat source

• For a reference measurement the source is blocked making α = 0.

© 2014 Fluke Calibration Liebmann – Emissivity Determination 13

AMBHSW TSTSTS 1

Theory

• What is α?

© 2014 Fluke Calibration Liebmann – Emissivity Determination 14

AMBHS

REFRTS TSTS

TSTS

11

Reflective Method with Specular Surfaces

• For specular surfaces α = 1• This is the equation Yamada used

© 2014 Fluke Calibration Liebmann – Emissivity Determination 15

AMBHS

REFRTSPECS TSTS

TSTS

1

Reflective Method with Lambertian Surfaces

• Lamberts Cosine Law

© 2014 Fluke Calibration Liebmann – Emissivity Determination 16

cos0II

Reflective Method with Lambertian Surfaces

© 2014 Fluke Calibration Liebmann – Emissivity Determination 17

22

0

20

21

1sin

sin

HS

HS

SS

TOT

S

D

dI

I

F

F

Reflective Method with Surfaces of Unknown Diffusivity

• Initial measurements did were not as equation for Lambertian surfaces predicted

• However, this equation’s shape was preserved α(dHS)

© 2014 Fluke Calibration Liebmann – Emissivity Determination 18

30 35 40 45 50 55 60 65 70 75 800.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

dHS / cm

alp

ha

21

11

HS

SAPP ad

2

2

0

20

21

1sin

sin

HS

HS

SS

TOT

S

D

dI

I

F

F

Process

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AMBHS

REFRTSPECS TSTS

TSTS

1

(6) (12)

21

11

HS

SAPP ad

Uncertainty

• Uncertainty analysis elements– Radiation Thermometer Readout– Reference Readout– Heat source Calibration– Ambient Temperature Readout– Heat Source Distance– Heat Source Diameter– Radiation Thermometer Alignment– Repeatability

© 2014 Fluke Calibration Liebmann – Emissivity Determination 20

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 21

Practical Measurements

• Heat Source– Fluke Calibration 4181– Generally used at 200 °C

– dHS 2.5 to 5 times that of DHS

• Radiation Thermometer– Fluke 568– 1.9 cm spot size

• 11 surfaces measured

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Results

© 2014 Fluke Calibration Liebmann – Emissivity Determination 23

Data Set Sample Type εS

U(ε) (k = 2)

Comparison Value

ε Type

1 Painted Wall 0.9466 0.0033 0.88 – 0.96 Table

2 Stainless Steel 0.2993 0.0253 0.16 – 0.79 Table

3 Cardboard 0.9436 0.0032 0.81 – 0.94 Table

4 Glass 0.8617 0.0032 0.68 – 0.95 Table

5 Aluminum 0.1530 0.0030 0.09 – 0.31 Table

6 White Board 0.9507 0.0008 0.84 – 0.95 Table

7 Flat Plate 0.9495 0.0123 0.9394Contact v

Radiometric

8 Flat Plate 0.9505 0.0157 0.9448Contact v

Radiometric

9 Flat Plate 0.9463 0.0081 0.9459Contact v

Radiometric

10 Flat Plate 0.9501 0.0122 0.9459Contact v

Radiometric

11 Flat Plate 0.9275 0.0107 0.9187Contact v

Radiometric

Results

© 2014 Fluke Calibration Liebmann – Emissivity Determination 24

20 30 40 50 60 70 80 90

0.850

0.855

0.860

0.865

0.870

dhs / cm

εs

40 45 50 55 60 65 70 75 80 85

0.150

0.151

0.152

0.153

0.154

0.155

0.156

dhs / cm

εs

25 30 35 40 45 50 55 60 65 70

0.940

0.945

0.950

0.955

0.960

dhs / cm

εs

Set 4Glass

Set 5Aluminum

Set 7Flat Plate

Results

© 2014 Fluke Calibration Liebmann – Emissivity Determination 25

Set 7 Set 8 Set 9 Set 10 Set 110.91

0.92

0.93

0.94

0.95

0.96

0.97

Refl

R v C

emis

sivi

ty

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 26

Conclusion

The reflective method…• Works well on large surfaces• Works fair on smaller surfaces• In-situ type measurement

– Can be done in field– Quick

© 2014 Fluke Calibration Liebmann – Emissivity Determination 27

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 28

Acknowledgements

The author would like to thank the many people who have made this research possible. • Yoshiro Yamada

– Provided the author with details of his research

• Fluke Calibration – Provided the author with time, laboratory space, and equipment– Time to analyze the resulting data.

© 2014 Fluke Calibration Liebmann – Emissivity Determination 29

Overview

• Introduction • Effects of Error in Estimating Emissivity• Methods for Determining Emissivity• Applying the Reflective Method to Diffuse Surfaces

– Reflective Method with Specular Surfaces– Reflective Method with Lambertian Surfaces– Reflective Method with Surfaces of Unknown Diffusivity– Uncertainty

• Practical Measurements• Conclusion• Acknowledgements• Questions

© 2014 Fluke Calibration Liebmann – Emissivity Determination 30

Questions

?

© 2014 Fluke Calibration Liebmann – Emissivity Determination 31