Workshop on uncertainties in radiation thermometry Paris, September 7, 2001

1. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 The size-of-source effect (SSE) and its influence on the uncertainties in radiation thermometry M.Battuello Istituto di Metrologia G.Colonnetti (IMGC), Italy Istituto di Metrologia G.Colonnetti

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 Source Blackbody aperture (black spot) Istituto di Metrologia G.Colonnetti

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

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

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 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). Istituto di Metrologia G.Colonnetti

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

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 • 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 Istituto di Metrologia G.Colonnetti

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

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

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

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

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

17. Workshop on uncertainties in radiation thermometry Paris, September 7, 2001 • SSE correction in the calibration of a transfer standard thermometer • Two calibration approaches: • fixed-points calibration • with a blackbody cavity as transfer source: • radiation thermometer as a standard • contact thermometer as a standard Istituto di Metrologia G.Colonnetti

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

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

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

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

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

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

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