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ABSTRACT: Photodiode-array-based spectrometers are increasingly being used in a wide variety of applications. However, the signal measured by this type of instrument often is not what is anticipated by the user and is often subject to contamination from stray light. This paper describes an efficient and low-cost stray light correction approach based on a relatively simple system using a monochromator-based source. The paper further discusses the limitations of using a monochromator instead of a laser, as used by previous researchers, and its impact on the quality of the stray light correction. The reliability and robustness of the stray light correction matrix generated have been studied and are also reported.
Applied Optics 09/2011; 50(26):5130-8. · 1.41 Impact Factor
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ABSTRACT: This paper describes a thorough investigation of the NPL Vega BB3500 furnace to improve the quality and duration of the melt plateaux of a Re–C eutectic cell. The results of this investigation have shown that the position of the cell within the furnace is the most critical factor for optimizing fixed-point performance. It was found that better performance could be obtained from a furnace with uniform ring resistance than with highly non-uniform resistance profiles, but this effect was relatively small. When high resistance (hot) rings were deliberately introduced on either side of the cell, at a distance corresponding to several rings beyond the position of the cell, considerable improvement in the plateaux was observed. Nevertheless, the cell (or the cell performance or even plateaux) is very sensitive to the positioning of these hot rings. A new cell holder was designed and introduced. Raising the cell on supporting rings provided a radiative gap that increased the thermal uniformity of the cell itself. Carbon–carbon composite material (CC sheet) wrapped loosely around the cell further improved the uniformity of the cell wall and hence the plateau quality. Following these changes, the plateau duration was increased by a factor of 5.
Metrologia 12/2008; 46(1):33. · 1.75 Impact Factor
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ABSTRACT: A small palm-sized, reference spectrometer, mounted on a remote-controlled model helicopter is being developed and tested
by the National Physical Laboratory (NPL) in conjunction with City University, London. The developed system will be used as
a key element for field vicarious calibration of optical earth observation systems in the visible-near infrared (VNIR) region.
The spectrometer is hand held, low weight, and uses a photodiode array. It has good stray light rejection and wide spectral
coverage, allowing simultaneous measurements from 400 to 900nm. The spectrometer is traceable to NPL’s primary standard cryogenic
radiometer via a high-temperature metal-carbon eutectic fixed-point blackbody. Once the fixed-point temperature has been determined
(using filter radiometry), the eutectic provides a high emissivity and high stability source of known spectral radiance over
the emitted spectral range. All wavelength channels of the spectrometer can be calibrated simultaneously using the eutectic
transition without the need for additional instrumentation. The spectrometer itself has been characterized for stray light
performance and wavelength accuracy. Its long-term and transportation stability has been proven in an experiment that determined
the “World’s Bluest Sky”—a process that involved 56 flights, covering 100,000km in 72days. This vicarious calibration methodology
using a eutectic standard is presented alongside the preliminary results of an evaluation study of the spectrometer characteristics.
International Journal of Thermophysics 11/2007; 28(6):2041-2048. · 0.95 Impact Factor
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ABSTRACT: Since 1999, when the first high temperature fixed-points based on the metal–carbon eutectic phase transitions were realized, more than 60 papers have been published on this topic. Eutectic based fixed-points are already being considered as secondary reference points for the International Temperature Scale and have been introduced into industrial laboratories. This rapid progress has been possible through the combined effort of scientists around the world, from national metrology institutes, universities and industry. It has been proposed that these fixed-points should be officially adopted as a way to improve the realization and dissemination of temperature scales above the silver point. In radiometry, the availability of stable high temperature fixed-points will give greater flexibility and at some wavelengths the potential for greater accuracy for spectral radiance and irradiance scale realization. This paper summarizes the major progress in eutectic research so far.
Metrologia 11/2006; 43(6):R11. · 1.75 Impact Factor
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ABSTRACT: The CCPR K1-a key comparison of spectral irradiance (from 250 nm to 2500 nm) was carried out to meet the requirements of the Mutual Recognition Arrangement by 13 participating national metrology institutes (NMIs). Because of the fragile nature of the tungsten halogen lamps used as comparison artefacts, the comparison was arranged as a star comparison with three lamps per participant. NPL (United Kingdom) piloted the comparison and, by measuring all lamps, provided a link between participants' measurements. The other participants were BNM–INM (France), CENAM (Mexico), CSIRO (Australia), HUT (Finland), IFA–CSIC (Spain), MSL–IRL (New Zealand), NIM (China), NIST (United States of America), NMIJ (Japan), NRC (Canada), PTB (Germany) and VNIIOFI (Russian Federation). Before the analysis was completed and the results known, the pilot discussed with each participant which lamp measurements should be included as representative of their comparison. As a consequence of this check, at least one measurement was excluded from one third of the lamps because of changes due to transportations. The comparison thus highlighted the difficulty regarding the availability of suitable transfer standards for the dissemination of spectral irradiance. The use of multiple lamps and multiple measurements ensured sufficient redundancy that all participants were adequately represented. In addition, during this pre-draft A phase all participants had the opportunity to review the uncertainty budgets and methods of all other participants. This new process helped to ensure that all submitted results and their associated uncertainties were evaluated in a consistent manner. The comparison was analysed using a model-based method which regarded each lamp as having a stable spectral irradiance and the measurements made by an NMI as systematically influenced by a factor that applies to all that NMI's measurements. The aim of the analysis was to estimate the systematic factor for each NMI. Across the spectral region (250 nm to 2500 nm) there were 44 wavelengths at which a comparison was made. These were treated entirely independently and thus the report describes 44 comparisons. For wavelengths from 250 nm to 800 nm (apart from 300 nm) all participants had unilateral degrees of equivalence (DoEs) with values consistent with their uncertainties for a coverage level k = 2. At all other wavelengths (apart from 1400 nm) all participants achieved consistency at the k = 4 level for the unilateral DoEs and the vast majority within k = 3. The results are a significant improvement over those of the previous comparison in 1990, especially considering that the declared uncertainties of most participants have been substantially improved over the intervening decade. These results are evidence of the value of the effort devoted to the development of improved spectral scales (and of the evaluation of their uncertainty) by many NMIs in recent years. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).
Metrologia 10/2006; 43(1A):02003. · 1.75 Impact Factor
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ABSTRACT: The CCPR K1-a key comparison of spectral irradiance (from 250 nm to 2500 nm) was carried out to meet the requirements of the Mutual Recognition Arrangement by 13 participating national metrology institutes. Because of the fragile nature of the tungsten halogen lamps used as comparison artefacts the comparison was arranged as a star comparison with many more lamps than participants. The National Physical Laboratory (UK) piloted the comparison and, by measuring all lamps, provided a link between participants' measurements. The comparison was analysed using a model-based method, which ensured that all participants, including the pilot, were treated equitably. This paper presents the comparison philosophy, methodology, analysis and results.
Metrologia 03/2006; 43(2):S98. · 1.75 Impact Factor
