Neil J. Harrison

Investigation of high-temperature black body BB3200

During an international comparison of radiation temperature measurements performed at the AllRussian Institute for Optophysical Measurements (VNIIOFI), Moscow, in June 1997 by participants from the VNIIOFI, the National Physical Laboratory (NPL), Teddington, and the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, detailed measurements of the characteristics of two different types of high-temperature black-body source took place. Both of the black bodies consisted of a pyrolytic-graphite cavity and differed only in the design of the electrodes, which were either axial or coaxial. All investigations were carried out covering a temperature range from 1380 K to 3100 K. The electrical characteristics of the black bodies were investigated at all temperature points; and measurements of the temperature drift were performed to determine the stability of the systems in constant-current mode and in optical-feedback control mode. The uniformity of the black-body radiation field was measured in the radiance mode by using narrowband, interference-filter-based radiometers with imaging optics from the NPL and in the irradiance mode by using broadband-filter detectors from the PTB to scan the irradiated area. The results confirm the suitability of the BB3200 for both radiance and irradiance measurements at national metrological institutes.

Intercomparison of radiation temperature measurements over the temperature range from 1600?K to 3300?K

An intercomparison of radiation temperature measurements was performed at VNIIOFI during October 2000 using a pyrolytic graphite blackbody operating over the temperature range from 1600?K to 3300?K. A pyrometer and two photometers from VNIIOFI, a pyrometer and four broadband glass filter detectors from PTB, and two narrow-band interference filter based radiometers and a broadband glass filter radiometer from NPL were used to perform the temperature measurements in either radiance or irradiance mode. Across almost the entire temperature range the VNIIOFI, NPL and PTB instruments showed results within the combined standard measurement uncertainties.

Evaluation of spectral irradiance transfer standards

The performance of spectral irradiance transfer-standard lamps has long been a concern in the radiometry and photometry communities. The adoption of a modified FEL-type lamp has recently been proposed as the most suitable transfer standard for spectral irradiance measurements. While data have been published on the performance of these lamps when operated continuously and in a laboratory environment, however, little information is available on their performance under typical usage. The results are presented of real-life testing of over thirty FEL lamps both pre-selected, according to published procedures, and direct from the manufacturer. When operated vertically all of the lamps tested suffered terminal coil collapse after between 140 h and 250 h burn time. The reproducibility of the FEL lamps after transportation is also investigated.

International comparison of radiation-temperature measurements with filtered detectors over the temperature range 1380 K to 3100 K

A series of black-body radiation-temperature measurements has been made over the temperature range 1380 K to 3100 K using two different designs of pyrolytic-graphite black bodies with calculated emissivities of 0.999. All measurements were performed at the All-Russian Research Institute for Optophysical Measurements (VNIIOFI) during June 1997. A filter photometer from the VNIIOFI, broadband glass-filter detectors from the Physikalisch-Technische Bundesanstalt (PTB) and narrowband interference-filter-based radiometers from the National Physical Laboratory (NPL) were used to perform the temperature measurements in either radiance or irradiance mode using both black bodies. Across the entire temperature range, the NPL and PTB instruments showed consistent results with both black bodies and differing geometrical arrangements. Results from the VNIIOFI photometer were also broadly consistent for a wide temperature range.

Preliminary results of the investigation of a 3500 K black body

A new ultra-high-temperature black body designed and constructed at the All-Russian Research Institute for Optophysical Measurements (VNIIOFI, Russian Federation) has recently been delivered to the National Physical Laboratory (NPL, UK). The black-body cavity consists of a series of pyrolytic-graphite rings and is designed to operate at temperatures of up to 3500 K. Preliminary results show that the cavity uniformity varies with operating temperature and change in shape of cavity bottom. Results of active and passive temperature control are also given.

New facility for the high-accuracy measurement of lens transmission

A high-accuracy lens-transmission measurement facility has been constructed and tested. The facility uses a tuneable laser source to allow measurements of lens transmission at discrete monochromatic wavelengths in the range 200 nm to 10 µm. Lenses of focal lengths from 300 mm to 750 mm can be measured to between 1 part in 104 and 1 part in 103, depending on wavelength.

Improved transfer standard sources for calibration of field spectrometers used for Earth observation applications

NPL, in conjunction with many NMIs, has been seeking to improve the accuracy of its primary scales of spectral irradiance and radiance. In common with other laboratories this has been done through the use of an ultra-high temperature blackbody characterized using filter radiometers calibrated against a cryogenic raediometer. While such work is of importance to the Earth Observation community, it is also recognized that of at least equal importance is an improvement in the quality of the scales that are provided to the end user. This paper will describe new transfer standard sources of both spectral radiance and iradiance that have been developed not only to improve accuracy to the end user, but also to provide it in a form that is both robust and convenient to use. For example the radiance source has a spatial non-uniformity of <0.05% over a 50 mm diameter aperture and can maintain its accuracy for more than 100 hrs of operation.

Intercomparison of characterization techniques of filter radiometers in the ultraviolet region

Narrow-band filter radiometers at 248?nm, 313?nm, 330?nm and 368?nm wavelengths were used to compare calibration facilities of spectral (irradiance) responsivity at HUT, NPL and BNM?INM. The results are partly in agreement within the stated uncertainties. Use of demanding artefacts in the intercomparison revealed that the wavelength scales of the participating institutes deviate more than expected. Such effects cannot be seen in typical intercomparisons of spectral responsivity or spectral transmittance, where spectrally neutral samples are used.

High-accuracy characterization and applications of filter radiometers

The paper describes the high-accuracy radiometric calibration of filter radiometers using both laser and Fourier transform spectrometer based methods to uncertainty levels < 0.1%. The paper also describes the development of detector-based transfer standards and their use for the spectral calibration of radiometric sources. It discusses how these techniques can improve the accuracy and reduce the cost of source calibration.

The NPL detector-stabilized irradiance source

The design of a new detector-stabilized spectral irradiance source is presented, together with an evaluation of its performance. This new source consists of a tungsten-halogen lamp that is current-controlled using feedback from an array of filtered photodiodes. The bandpass and central wavelength of these filtered detectors may be selected specifically to meet the required application of the source. The lamp is fully enclosed (except for the exit aperture) in an air-cooled, light-tight housing that minimizes unwanted light pollution. The lamp current is stabilized using feedback from one photodiode; the recording of the non-stabilization channels allows a real-time check on the effective colour temperature of the lamp, and provides an instant diagnosis of the state of calibration of the lamp. The calibration data for each lamp (in its associated housing) are stored on a microchip within the lamp housing, which allows the lamp to be transported independently of its associated control electronics for calibration purposes. The performance of the lamp indicates ageing rates of better than 2 × 10−5 h−1, across the ultraviolet-A (UV-A) and visible spectral regions.