1. Workshop on uncertainties in radiation thermometry Paris, 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

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

3. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti Blackbody aperture Source (black spot)

4. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti IMGC apparatus for the measurement of the SSE

5. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti Source (clear spot) Blackbody aperture

6. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Graphical representation of the SSE measured according to Scheme 1 (indirect method) Istituto di Metrologia G.Colonnetti

7. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Graphical representation of the SSE measured according to Scheme 2 (direct method) Istituto di Metrologia G.Colonnetti

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

9. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti NPL results of a comparison between direct and indirect techniques

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

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

12. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti SSE of two InGaAs based radiation thermometers measured with the indirect method (integrating sphere) Effect of different spectral distributions of the source

13. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti InGaAs-based thermometer TS1 without filter Temperature differences in the SSE corrections due to the assumption of SSE curves obtained with different spectral distribution of the radiation source

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

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

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

17. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 Istituto di Metrologia G.Colonnetti 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 standard 2contact thermometer as a standard

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

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

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

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

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

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

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