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Intercomparison of spectrophotometric measurements of regular transmittance


Spectrophotometry is the measurement of the fraction of incident optical radiation transmitted or reflected from a material sample as a function of the wavelength of the radiation. It is widely used for analysis in the pharmaceutical and chemical industries and for the measurement of the optical properties of materials such as glass. Previous international intercomparisons of regular transmittance have been limited to the visible region. This intercomparison covers the ultraviolet, visible and near infrared regions. This necessitates the use of transfer standards not previously used in an international intercomparison.

There are two specific objectives:
1. to harmonise the scales of spectrophotometric measurements of regular transmittance in the European Union over the spectral range from 200nm to 3000nm with a targeted level of agreement of 1 part in 1000.
2. to explore the limitations of currently available transfer standards so as to define the technical requirements for improving their performance.
A Summary Report was presented to the European Commission in November 1996. Because of technical difficulties not all of the repeat measurements were completed by that date. However, the measurements are now complete. The main conclusion is that the degree of agreement is limited by the uniformity of the filters used even though these were the best that are currently available. The agreement is a factor of 10 less than can be achieved by glass filters in the visible region. (Glass filters could not be used for this intercomparison because they have a neutral spectral transmittance only in the visible region.) Better agreement might have been achieved if the area of filters measured had been more equal. The final report is now being written and will include a recommendation for research into improved transfer standards.
The intercomparison is between 7 European Union national standards laboratories. Metal on silica filters were chosen as the transfer standards. These have been used for many years as transfer standards between national laboratories and industry. Each participant measured 6 filters with transmittances in the range 92 % to 1%. Participants measured as many wavelengths as possible from a list of 16 wavelengths between 200nm and 3000nm. The filters were selected to be the most uniform available from commercial manufacturers.


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