Research in the Electromagnetic Metrology Area

This page contains more information for the following projects:

Development of a method for refractive index measurements on liquids and gloss measurements
Primary spectral irradiance facility
Development of a primary standard for therapy level photon dosimetry
Frequency stabilization of lasers
Thermometry comparison in SADC

Development of a method for refractive index measurements on liquids and gloss measurements

Refractive measurements are mostly done on, amongst others, liquids like cold drinks; wine; and anti-freeze for motor vehicles. In most of these cases refractive index liquids or a standard liquid are used for the calibration of the equipment. A survey revealed that there is an urgent need for 20°, 60° and 85° gloss measurements traceable to international standards in the paint and automotive industry. Standard tiles are currently used for these calibrations. These tiles are either sent overseas for calibration; or new tiles with calibration certificates are purchased.

The initial phase in 2006/7 established a method for refractive index measurements on prisms. The current project aims to develop a facility for the measurement of gloss and the refractive index of liquids.

For more detail contact Ms. Elsie Coetzee.

Primary spectral irradiance facility

A primary spectral irradiance facility is being designed in order to realise the spectral irradiance scale by directly linking it to the cryogenic radiometer through the use of calibrated filter radiometers. The project started in 2005/6, and is being continued in 2007/8. The filter radiometers are used to determine the temperature of a high temperature black body which is then used as a primary standard source whose spectral radiance can be determined from Planck’s equation. The spectral irradiance scale is transferred to standard spectral irradiance lamps, which are used to disseminate the scale to industry.

The aim of the project during the current year is to further improve phase transition plateaux and the accuracy of the temperature measurement. Measurements during the first year of this project showed that the quality of the plateau shapes depends on the temperature uniformity of the black body. The temperature uniformity can be improved by adding baffles and graphite felt to the cavity rings, and by optimising the resistance distribution of the cavity rings. Experience gained during a visit to VNIIOFI (Russia) during 2006/7 will be implemented to improve the uniformity of the NMISA BB3200pg black body.

In order to perform thermodynamic temperature measurements, the size-of-source effect, lens transmittance, aperture areas, distance settings and spectral responsivity of the filter radiometer will be determined and corrected for where applicable.

For more detail contact Ms. Natasha Nel-Sakharova.

Development of a primary standard for therapy level photon dosimetry – Phase 1

The development of primary standards is a strategic objective of the NMISA, particularly considering the leading role that it plays in SADC and Africa. The Ionising Radiation Laboratory maintains standards for therapy level photon dosimetry (air kerma in Co-60 and medium energy x-rays and absorbed dose to water in Co-60), providing traceability for the measurement of radiation dose delivered to patients during radiotherapy in all radiotherapy centres throughout South Africa.

Currently these standards are all secondary standards. This implies that, although traceability is obtained from primary standards facilities such as the BIPM, the best uncertainties that the laboratory can achieve, only lie between 1,5% and 2%. In comparison, primary standard laboratories are generally able to achieve uncertainties of below 0,7%.

The ultimate aim of the project is to develop a primary standard for therapy level photon dosimetry. The project is planned to be implemented as a studentship for an MSc degree over a period of three years. Three main phases (each spanning a period of one year) are planned:

Phase one includes an investigation of existing therapy level photon dosimetry primary standards that are maintained by recognized primary standards facilities; a study of the theory on which modern primary standards are based; training and skills development in Monte Carlo modelling techniques; the design of a measurement system and the development of a detailed project plan for the construction, characterization, validation and commissioning of a therapy level photon dosimetry primary standard.

Phase two starts the implementation of the detailed project plan that was developed during phase one. This phase consists of the acquisitioning and manufacturing of equipment (electrometers and chambers), the assembly, setting up and testing of the measurement system, the initial calibration of the system against the existing secondary standard and initial Monte Carlo modelling of the radiation beams and the measurement system.

Phase three will be the final phase of the project. During this phase the initial models of the radiation beams and the measurement system are refined through iterative cycles of measurements and adjustment until the system is capable of achieving a predefined uncertainty of measurement. On completion of the characterization of the measurement system, the project is concluded with a bilateral intercomparison with a suitable primary standards laboratory such as the BIPM.

For more detail contact Dr. Jaco Mostert.

Frequency stabilization of lasers

A requirement was identified in NMISA for the ability to measure optical wavelengths of laser light sources. Laser light sources are used in the areas of Dimensional metrology and Optical metrology.

In Dimensional metrology, lasers are used in interferometer systems where the wavelength of the light is used to measure a linear displacement. The Dimensional Laboratory have Iodine stabilised He-Ne red lasers for this purpose. This means that only lasers with wavelengths close to the He-Ne laser can be calibrated. Any other laser, e.g. the green laser, must be calibrated at an overseas laboratory at a high expense with long turn around times.

In optical metrology, the wavelength of a light source is important in some measurement systems. This is especially evident in Fibre Optic metrology where communication systems work at approximately 1550 nm (infra-red). Different options have been considered for this problem. These include the building of external cavity diode lasers (ECDL) that could be locked to an absorption line of a gas- or metal vapour cell. In 2005, a decision was made to purchase a femtosecond frequency comb for the calibration of lasers in the visible spectrum.

A position is currently being advertised for a full-time laser physicist, and will be filled in 2007/8. The project will be combined with chemical metrology in 2007/8, due to the overlap in the technology of optical frequency standards with cavity ring down spectroscopy.

For more details, contact Shravan Singh.

Thermometry comparison in SADC

Ten SADCMET national metrology institutes (NMIs) presently have thermometry capabilities. Interlaboratory comparisons are necessary for these labs to test their Best Measurement Capabilities (for accreditation to ISO 17025) and to evaluate equivalence of their national measurement standards (for submission of their BMCs for inclusion in the CIPM Mutual Recognition Arrangement (MRA)). A pilot comparison of mercury-in-glass thermometer calibrations from 0 to 50 °C was piloted by NMISA from 2001 to 2004, comparing measurement capabilities of eight SADCMET NMIs to better than 0.1 °C over this limited temperature range. Comparisons at higher accuracy and over wider temperature ranges are necessary to more fully support SADCMET NMIs’ Thermometry calibration capabilities.

A Regional Metrology Organisation (RMO) comparison of industrial platinum resistance thermometer (PRT) calibration capabilities (classified as “supplementary” in the terminology of the MRA) is required in SADCMET: it should evaluate equivalence of Thermometry measurement standards at the 0.02 °C level over the range -50 to 450 °C. The industrial PRT comparison will cover the full temperature range of six SADCMET NMIs (those in italics below).

The participation of NMISA in APMP.T-K3, comparing standard PRT calibration capabilities from -39 to 420 °C at the 0.001 °C level, will provide an indirect link to the global comparison CCT-K3. It is hoped that a second NMI that has participated in an RMO comparison of PRT calibrations will agree to join the SADCMET comparison (this is not essential for a Supplementary comparison, but is desirable).

Participants: NMISA (South Africa), BOBS (Botswana), MSB (Mauritius), INNOQ (Mozambique), TBS (Tanzania), SIRDC-NMI (Zimbabwe), UNBS (Uganda), KEBS (Kenya), QSAE (Ethiopia), NIS (Egypt), a 2nd linking lab to an RMO PRT comparison (preferably APMP.T-K3 or APMP.T-S3, as NMISA results will be traceable to at least APMP.T-K3.