The absolute calibration of mid-infrared transmittance standards

Abstract The National Physical Laboratory (NPL) uniquely has 40 years’ experience in pioneering, developing instruments and exploiting the spectroscopic advantages of the Fourier transform technique. This has allowed an objective and knowledgeable appraisal of the Fourier transform technique for ordinate standards work. At present not enough is known of the ways in which the many and complicated categories of systematic error in such instruments can be identified, separated, evaluated and corrected for. Thus at NPL grating spectrophotometers are still used to determine, maintain and disseminate transmittance standards for the mid-infrared spectral region. This allows well-established methods developed in UV/visible region work to be adapted and used to minimise and where necessary correct for the residual systematic errors. The paper expands on the above considerations, and goes on to describe the techniques used to explore possible systematic errors and to give an absolute regular transmittance calibration to a suitable secondary and transfer standard. This last was designed at NPL and consists of a Schott NG11 optical glass filter mounted in a protective milled frame that allows for reproducible alignment in the spectrophotometer, and this is described, together with the particular properties that were required of it. Seven levels of transmittance are provided, ranging from about 73% down to just above zero. The uncertainties of calibration are of order ±0.1%, varying with transmittance level. The pros and cons of highly reflecting filters are also discussed, and it is concluded that these last should be used initially to explore and minimise the interreflection errors in an instrument by adding stops, etc. and then the NPL-certified NG11 filter should be used to actually calibrate the ordinate scale.