7/27/2019 Size-of-Source Effect Sensitivities In Filter Radiometers

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QueensPrinterandControllerof

HMSO

,2013

.10643/1013

www.npl.co.uk

Size-o-source eect sensitivitiesin flter radiometers

M. R. Dury1, T. C. Arneil2, G. Machin1 and T. M. Goodman1

1National Physical Laboratory, Teddington, United Kingdom2University o Cambridge, Cambridge, UK

email: martin.dury@npl.co.uk

The prototype achieved a SSE o the order o < 6 10-5, when using a Lyot stop with adiameter slightly smaller than the theoretical optimum.

THOR is expected to have a SSE o 8 10-5, at this level temperature measurements wouldrequire corrections o 3 mK at Cu and 11 mK at Re-C to account or its SSE.

Summary

Objective lens comparisonWe measured the SSE o the prototype or dierent objective lenses, using a 6 mmdiameter Lyot stop. Only achromatic doublet lenses were measured due to their superiorimage quality when compared with singlet lenses. Newport Corporation and CVI MellesGriot lenses were compared or SSE; we measured two o each lens type. For all oour measurements, we used a Newport Corporation plano-convex singlet lens as thecollimating lens.

IntroductionWhen perorming high accuracy radiationthermometry, the size-o-source eect (SSE) o a flterradiometer can provide a signifcant contribution tothe uncertainties associated with the measurements.During the development o a new flter radiometer, theTHermodynamic Optical Radiometer (THOR), which wedesigned specifcally to measure the melting pointso High Temperature Fixed Point (HTFP) cells, indirectSSE measurements were perormed on a prototypeinstrument to aid selection o optical components andtheir optimum positions with the aim o minimising itsSSE. We perormed tests to choose the objective lens,and to understand the eects o the collimating lensand Lyot stop positions, and the Lyot stop diameter.

Experimental setupOur prototype comprised a 50.8 mm diameter objectiveachromatic doublet lens with a 200 mm ocal length, whichwas stopped by an aperture to 40 mm. We positioned a1 mm diameter eld stop 333 mm rom the objective lens tocreate a system with a 2:3 magnication. A collimating lenswas positioned 50 mm behind the eld stop aperture anda 6 mm diameter Lyot stop was placed 57.5 mm behind thecollimating lens, to reduce SSE.

Our source or the SSE measurements was a 250 mmdiameter integrating sphere illuminated by a tungstenlament lamp. We placed a transparent acrylic plate with a

3 mm diameter blackened cavity over the spheres 60 mm diameter output port to approximate the aperture o a HTFP cavity.We aligned the radiometer centrally with the plates blackened cavity, and ocussed the image o the cavity onto the eld stop.

Lyot stop position sensitivityFor our radiometer geometry, the opt imum Lyot stop position is 57.5 mm behind thecollimating lens. When moved away rom this optimum position, the aperture no longerbehaves as a Lyot stop. To understand the instruments sensitivities to this misalignment,SSE measurements were perormed using the radiometer with the Lyot stop moved by 5 mm, 10 mm, 15 mm and + 30 mm rom the deault (57.5 mm) position.

Lyot stop diameter sensitivityFor our radiometer, the optimum Lyot stop diameter is 6.66 mm. We measured the SSE othe prototype with Lyot stop diameters o approximately 2 mm, 4 mm, 5 mm, 6 mm, 7 mm,8 mm, 10 mm and 12 mm, to measure sensitivity to diameter.

Collimating lens position sensitivityTo produce a parallel beam o light rom the eld stop, the collimating lens should beplaced behind the eld stop at a distance equal to its ocal length. As the collimating lenswill be aligned by eye, it may be difcult to judge when the beam is exactly collimated.

To determine the instruments SSE sensitivities to lens position, we measured SSE with thecollimating lens at dierent distances ( 5 mm, 10 mm, and 15 mm) rom its optimum

position, 50 mm behind the eld stop.

Small variations rom optimum Lyot stop and collimating lens positions were insignicant,although it was better or the Lyot stop to be slightly too ar rom the lens than too close.

There was no measured increase in the radiometers SSE when the collimating lens wasmoved towards the eld stop; however, the SSE was seen to increase as the lens was movedaway rom the eld stop.

AcknowledgmentsThe authors wish to acknowledge the advice that Howard Yoon o NIST provided in theinitial stages o THORs design. This work was unded by the UK National MeasurementSystem, an Executive Agency o the UK Department o Business, Innovation and Skills.