Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia...

Workshop on uncertainties in radiation thermometryParis, September 7, 2001

Istituto di Metrologia

G.Colonnetti

The size-of-source effect (SSE) and its influence on the uncertainties in radiation thermometry

M.Battuello

Istituto di Metrologia G.Colonnetti (IMGC), Italy


Size of source effect (SSE):Sources of equal temperature but different size give rise to different apparent temperatures

The SSE originates from:•diffraction (largely by the aperture stop)•scattering by dust particles and at imperfections of optical icomponents•reflection at surfaces of lenses•optical aberrations


G.Colonnetti



G.Colonnetti

Measurement of the SSE and its representation

Two different schemes can be adopted for representing the SSE, according to the method usedfor its measurement

Scheme 1 (indirect method)

The SSE is measured aiming the thermometer at a blackspot that simulates the target

The spot is located at the centre of a bright circle whosediameter d can be changed by replacing the limitingdiaphragm

The SSE corresponding to any d value is calculated as theratio of the signal obtained from the black spot to the signalobtained from the bright region around it

In this way, the SSE represents the contribution of strayradiation originated from the surroundings of the target

Blackbody aperture

Source

(black spot)



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IMGC apparatus for the measurement of the SSE



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Source

(clear spot)

Blackbody aperture


Graphical representation of the SSE measured according to Scheme 1 (indirect method)


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Graphical representation of the SSE measured according to Scheme 2 (direct method)


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G.Colonnetti

Are the two methods equivalent ?

They should be equivalent, but recent results indicate large differences between the SSE obtained with an integrating sphere (indirect method) and with a blackbody cavity (direct method).



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Figure 2a:SSE of MC52 by both techniques compared

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0 20 40 60

Aperture /mm

SS

E SSE(IS)

SSE(hp)

Figure 2b: SSE of M190 by both techniques compared

0.000

0.002

0.004

0.006

0.008

0.010

0 20 40 60

Aperture /mmS

SE SSE(IS)

SSE(hp)

NPL results of a comparison between direct and indirect techniques



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Figure 4: Temperature difference between the two corrections

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1000 1500 2000 2500 3000

Radiance temperature /°C

MC52

M190



G.Colonnetti

Major sources of uncertainty in the measurement of the SSE

direct method (blackbody source)•variation of the effective emissivity•unaccounted for variation of the source temperature•source uniformity•spectral radiance distribution of the source

indirect method (integrating sphere)•source uniformity•inter-reflection between the thermometer and the integrating sphere•spectral radiance distribution of the source



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SSE of two InGaAs based radiation thermometers measured with the indirect method (integrating sphere)Effect of different spectral distributions of the source



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InGaAs-based thermometer TS1 without filterTemperature differences in the SSE corrections due to the assumption of SSE curves obtained with different spectral distribution of the radiation source



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To harmonise the results of calibrations performed using different target sizes with different temperature distributions in their surroundings, it is necessary to refer the calibrations to a reference target diameter.

SSE correction apply to:

•ITS-90 realisation•calibration of a transfer standard thermometer•inter-laboratory comparisons using a transfer standard radiation thermometer



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SSE correction in the ITS-90 realisation

SSE correction must be applied when a fixed-point calibration is transferred to a pyrometric lamp. In this way, comparisons will not be affected by the different SSE of the thermometers

Typically, the corrections are from 0.02°C to 0.2°C



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SSE correction in the ITS-90 realisation

Major sources of uncertainty:

•temperature distribution of the blackbody surroundings•temperature distribution of the lamp filament•uncertainty in the measured SSE curve

In the recent CCT-K5 the uncertainty estimates for the SSE correction ranged from 2 m°C to 48 m°C



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SSE correction in the calibration of a transfer standard thermometer

Two calibration approaches:• fixed-points calibration

• with a blackbody cavity as transfer source:1radiation thermometer as a standard2contact thermometer as a standard



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Major sources of uncertainty in the fixed-points calibration

• temperature distribution of the blackbody surroundings• uncertainty in the measured SSE curve

The contribution to the total uncertainty is limited in extent, provided that:• the SSE of the transfer standard is low in extent• the temperature distribution can be measured



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Major sources of uncertainty in the calibration with a blackbody cavity as a transfer source (radiation thermometer as a standard)

•temperature distribution of the blackbody surroundings•uncertainty in the measured SSE curves

The contribution to the total uncertainty may be large in extent, because:•the SSE of the transfer standard may be large in extent•difficulty in the measurement of the temperature distribution



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SSE corrections in the recent TRIRAT (Traceability in Infrared Radiation Thermometry) comparison

• with TRT1 (IMGC thermometer): <0.13 °C

• with TRT2 (HEITRONICS thermometer): <0.45 °C



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The results of recent comparison exercises, e.g., the TRIRAT project and the CEM-IMGC-NPL comparison of Zn point, suggested that some components of stray radiation could be not correctly accounted for



G.Colonnetti

In the CEM-IMGC-NPL comparison of Zn point it was found that the geometry and the temperature distribution in front of the blackbody aperture can affect the results

• out-of-focus SSE that is difficult to evaluate because the temperature distribution along the furnace tube is generally unknown and also because the behaviour of this SSE may differ from that of the in-field SSE that is usually measured.

• stray radiation emitted by the regions in front of the blackbody cavity and collected by the cavity itself



G.Colonnetti

Differences in the Zn point determination with two furnaces of different geometry and dimensions:

with a “low-SSE” thermometer (total SSE: 0.1 %)32 mK

with a “high SSE” thermometer (total SSE: > 3 %)1.4 K



G.Colonnetti

More studies are needed

• to clarify the mechanism of the out-of-focus SSE

• to set proper guidelines for the design of the fixed-point iiequipment.

Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia...

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