National Physical Laboratory

The Absolute Radiation Thermometer

The Absolute Radiation Thermometer (ART) is a filter radiometer designed to determine the thermodynamic temperature of blackbody sources to the highest accuracy. It has been developed for our work for the improvement of the international temperature scale.

Absolute Radiation Thermometer
Absolute Radiation Thermometer.

ART has a simple design. It is based on a filter radiometer consisting of a silicon detector, an 800 nm narrow-band spectral filter and precision aperture mounted in a frame that maintains a constant geometry relative to a single lens and a thin-film aperture. The lens focuses light from within a blackbody cavity onto the filter radiometer and the two apertures define the measurement geometry for radiance.

This simple design allows the instrument to be characterised in several different ways – providing a detailed understanding of the systematic uncertainties. The excellent agreement we have seen between these different calibration approaches, gives us the confidence to calibrate more complex instruments using just one of the methods.

The primary filter radiometer calibration facility calibrates the spectral responsivity of the filter radiometer traceably to the cryogenic radiometer by direct comparison to the trap detector. The geometrical system can be measured using interferometry to provide traceability to the metre. For an ideal system this would be sufficient, however a real system suffers from the “Size-of-Source Effect” and needs a lens transmittance measurement. Furthermore the traceability from the cryogenic radiometer through the trap detector to the filter radiometer requires the transition from collimated laser light to a diverging beam from a Lambertian source.

Calibration Chain

ART Calibration Chain

A three-element reflection trap detector is used to transfer the spectral responsivity calibration from a cryogenic radiometer to the ART filter radiometer. The trap is calibrated at a few wavelengths in the region of interest, using a laser beam that underfills the detector, and its responsivity (in A W-1) is interpolated between these wavelengths. A calibrated diamond-turned brass aperture of the same nominal diameter as that of the filter radiometer is then added to the trap for the filter radiometer calibration, which requires the detectors to be overfilled as for the filter radiometer measurement. The spectral responsivity of the filter radiometer (in A W-1 m-2) is then determined with this trap and the filter radiometer calibration facility.

We have recently carried out a thorough investigation of a critical step in this chain – the addition of the aperture to the trap detector. We were concerned about potential interreflections within the trap introduced by this aperture. However, investigations using different measurement geometries and comparing the trap detector to a transmission trap detector showed no noticeable effect at the 10-4 level.

Calibration of the ART Instrument

The spectral responsivity of ART’s filter radiometer is determined using the filter radiometer calibration facility. The spectral responsivity of the entire ART instrument also requires a determination of the lens transmittance and a correction for the Size-of-Source Effect.

The simplicity of ART has allowed us to calibrate the instrument “as a whole” and “in parts”. The two techniques agreed to the 10-4 level, giving confidence that the methods are robust. In future the “as a whole” technique can be used to calibrate more complex instruments, for example those designed to have a smaller Size-of-Source Effect.

The Size-of-Source Effect

The filter radiometer and lens apertures define a solid angle of light imaged by the lens from a larger source. For an ideal lens, only light from within this solid angle is imaged. If the lens suffers from any imperfections, then light can be scattered into this solid angle from other parts of the source, or light can be scattered out of this solid angle. The effect of these two components is known as Size-of-Source Effect.

If the source aperture is equal to the ‘ideal object diameter’, then the instrument will detect less light than with an ‘ideal lens’ as only the scattering out process takes place. This light is gradually regained when the source aperture increases beyond this ideal diameter, through light scattered in.

The Size-of-Source Effect is significant when an instrument is calibrated using one source and then used to measure a source of a different size or uniformity. This relative Size-of-Source Effect is corrected for by determining the spatial uniformity of the calibration and measurement sources and the response of the instrument to a uniform source of varying sizes. This is achieved using a set-up similar to Figure 2b below, where the aperture size is varied. Often a central obstruction, equal to the size of the observed object, is added to reduce the effect of source instability.

Calibration of ART ‘as a whole’

When ART is placed in front of a monochromatic source of known radiance and known size, its spectral responsivity at that wavelength can be determined directly. The filter radiometer calibration and the known spectral variation of lens transmittance can then determine its spectral responsivity.

This is achieved using a large integrating sphere, as shown in Figure 2. The upper picture (a) shows the setup with ART being calibrated. The intermediate aperture provides the source of known size and this is illuminated by the integrating sphere (itself illuminated using a despeckled laser, as for the filter radiometer calibration facility).

The ART instrument, but without the lens, determines the radiance of this source, as in Figure 2b. Without a lens there is no lens transmittance and no size-of-source effect, although the aperture’s diffraction must be modelled. The integrating sphere is placed such that ART is illuminated by the same amount of the sphere in both set-ups. This significantly reduces non-uniformity effects from the source.

ART Calibration
Figure 2: ART calibration.

Calibration of ART ‘in parts’

The ‘in parts’ calibration of ART required separate measurements of the lens transmittance and the ‘absolute Size-of-source Effect’. The lens transmittance set-up compared the signal of a detector before and after the second lens in Figure 3. To determine an ‘absolute Size-of-Source Effect’, the response of the instrument to an infinite source is required – as for this the light scattered out by the lens is fully balanced by light scattered into the optical path from the larger source. A source of 60 mm diameter was ‘sufficiently infinite’ for this purpose.

ART Calibration In Parts
Figure 3: ART calibration in parts.

Last Updated: 25 Mar 2010
Created: 23 Jul 2007

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