• theoretical background• design of interference filters• measurement of spectral responsivity• blocking problems• stability• polarisation• scattered light • references• summary

Determination of spectral responsivity in radiation thermometry

J. Fischer, Physikalisch-Technische Bundesanstalt, Berlin, Germany

Basic theory of radiation thermometry

0

0

)(),(

)(),(

dsTL

dsTL

i

iQ

refs

s

ref

0

0

)(),(

)(),(

1

dsTL

dsTL

s

s

eff

ratio Q of spectral radiance :

limiting effective wavelength ( Tref T), (Wien´s approximation) :

Propagation of uncertainties

ref

ref

refref T

dT

T

Td

T

T

Q

dQ

c

T

T

dT

12

measurement of photocurrent ratio

determination of wavelength

realisation of reference temperature

Influence of wavelength uncertainty

-0,05

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

1000 1500 2000 2500 3000

T / K

T

/ K

Tref = 1337,33 K

Tref = 2300 K

Tref = 1800 K

= 650 nm = 0.05 nm

Determination of spectral responsivity

refref i

iss

Measurement by comparison to reference detector :

sref () : spectrally flat responsivity (thermopile, pyroelectric detector)

sref () : calibrated responsivity (silicon photodiode, trap detector)

: monochromator with wavelength calibration, tunable laser

i (iref (: sensitive to out-of-band radiation, scattered light, etc.

Spectral comparator (IMGC)

Interference filters

10-6

10-5

10-4

10-3

10-2

10-1

100

900 940 980 1020 1060Wavelength

nm

Transm

itta

nce

10-6

10-5

10-4

10-3

10-2

10-1

100

580 620 660 700Tr

ansm

itta

nce

Wavelength

740nm

Spectral transmittance of the interference filter with effective wavelength 974 nm and halfwidth 41 nm

Spectral transmittance of the interference filter with effective wavelength 653 nm and halfwidth 39 nm

Out-of-band error without blocking

Error in silver-gold ratio of 2.1 K

Additional Schott glass KG3

Error in silver-gold ratio reduced to 3.9 mK

Confirmation by sharp-cut filter

Out-of-band signal of 1.4x10-4 measured

Stability, definitions

For monitoring the stability of the calibration, two quantities areused:

Integral of the spectral responsivity ( 1, 2 bandpass limits)

dsI s 2

1

Center wavelength

SSP I

ds

2

1

800 nm filter radiometer

0,0E+00

5,0E-04

1,0E-03

1,5E-03

2,0E-03

2,5E-03

3,0E-03

19

94

-12

-07

19

95

-12

-07

19

96

-12

-06

19

97

-12

-06

19

98

-12

-06

19

99

-12

-06

20

00

-12

-05

20

01

-12

-05

date of calibration

rel.

ch

an

ge

of

inte

gra

l sp

ec

tra

l re

sp

on

siv

ity

0

0,05

0,1

0,15

0,2

0,25

0,3

dri

ft o

f c

en

ter

wa

ve

len

gth

/ n

m

1 year

Stability of filter 800 nm

900 nm filter radiometer

-4,0E-03

-3,5E-03

-3,0E-03

-2,5E-03

-2,0E-03

-1,5E-03

-1,0E-03

-5,0E-04

0,0E+00

19

99

-02

-27

19

99

-08

-28

20

00

-02

-26

20

00

-08

-26

20

01

-02

-24

20

01

-08

-25

date of calibration

rel.

chan

ge

of

inte

gra

l sp

ectr

al

resp

on

sivi

ty

0

0,05

0,1

dri

ft o

f c

en

ter

wa

ve

len

gth

/ n

m

1 year

Stability of filter 900 nm

Measurement of filters

1,0E-12

1,0E-10

1,0E-08

1,0E-06

1,0E-04

1,0E-02

1,0E+00

1,0E+02

400 600 800 1000 1200 1400 1600

wavelength / nm

spec

tral

res

po

nsi

vity

/ A

W-1

800 nm

900 nm

1000 nm

Line above responsivity curve indicates the bandpassfrequently calibrated, covering four orders of magnitude

Polarisation

||2

1sssunpol

Monochromator system emits partly polarised light

radiation thermometer and reference detectors sensitive to polarisation

required : calibration for use at randomly polarisedblack body sources

polariser behind exit slit, 2 measurements :

Scattered light in monochromator

0,996

0,997

0,998

0,999

1,000

1,001

1,002

1,003

4 9 4 9 9 9 2

grating

inte

gra

l of

spec

tral

res

po

nsi

vity

, n

orm

alis

ed

0°

90°

with pre-disperser

with pre-disperser

Calculatedfor randomlypolarisedlight

Principle of spectral comparator

TUNGSTENHALOGEN

LAMP GLAN-THOMPSON

PRISM

GRATING MONOCHROMATORZEISS SPM2 (f=400 mm)

TRANSLATION UNIT

TRAP-DETECTOR

FILTERRADIOMETER

f=450 mm

f=300 mm

FILTER WHEEL(SUPPRESSION OFHIGHER ORDERS)

SPECTRAL ORTUNGSTEN HALOGEN LAMP

LEISS PRISMMONOCHROMATOR

f=150 mm

LIGHTTIGHT ENCLOSURE

SPECTRAL LAMP

Example for uncertainty

Source of uncertainty explanatory note TAu (mK)

Spectral Transmittance of interferencefilter (passband)

= 14 pm 2.9

Blocking of interference filter OBS(TAu)=1.5•10-4 6.0

Relative spectral response of photodiode = 25 pm 5.1

Non-linearity of photodiode NL = 5•10-5 2.9

Impurity of silver sample 6N 3.0

Impurity of gold sample 5N 6.0

Random uncertainty includingrealization of freezes

see table I 4.8

Addition in quadrature 12.1

uncertainty contributions (k=1) for measurement of thesilver-gold interval :

• Battuello M., Girard F., Ugur S., Aytekin S.H.,. Measurements of spectral responsivity at IMGC (Italy) and UME (Turkey). a bilateral comparison,to be published in Proceedings of TEMPMEKO 2001

• Battuello M., Uncertainty budgets for realisation of scales by radiation thermometry, CCT-WG5 working paper, 2001

• Fischer J., Hartmann J., Calibration of tungsten strip lamps as transfer standards for temperature, In Proceedings of TEMPMEKO ‘99, edited by M. de Groot and J. Dubbeldam, Delft 1999, pp 561...566

• Fischer, J., Jung, H.J., Friedrich, R., A new determination of the freezing temperature of gold relative to that of silver by radiation thermometry, Proceedings of Temperature, Its Measurements and Control in Science and Technology, Vol. 6, edited by J.F. Schooley, New York, 1992, pp 53..57

• Gibson, C.E., Tsai, B.K., Parr, A.C., Radiance temperature calibrations, Natl. Inst. Stand. Technol. Spec. Pub. 250-43, 1998

• Sakuma F., Kobayashi M., Interpolation equations of scales of radiation thermometers, In Proceedings of TEMPMEKO ‘96, edited by P. Marcarino, Torino 1997, pp 305...310

• Saunders P., General interpolation equations for the calibration of radiation thermometers, Metrologia 1997, 34, pp 201...210

References

• Uncertainty depends crucially on effort for calibration

• re-calibration interval• main contributions :• monochromator wavelength calibration• calibration of reference detector• scattered light (single/double monochromator)• out-of-band blocking of filters• stability of filters and detectors• temperature coefficient of thermometer

Conclusions