Optical systems used in space and military applications require proof of performance of components at acceptance of contract and an active QA system in use throughout manufacture. To underpin these and other requirements the National Physical Laboratory (NPL) provides reference standards of regular reflectance that are individually calibrated to a known uncertainty of absolute measurement under conditions that match the customer's needs. These standards are necessary because of the significant errors that often occur when reflectance accessories are installed in IR spectrophotometers. The nature and causes of the errors are discussed, and why they are worse in IR instruments than in UV/visible ones. The NPL techniques, equipment and special modifications are described. Measurements can be made for angles of incidence up to 80 degrees, but for angles above 15 degrees it is essential to use polarizers, and NPL uses these in mutually reinforcing pairs at accurately set azimuth angles to achieve an adequate degree of polarization. The transfer standards supplied consist of overcoated or non-overcoated aluminized mirrors, or of uncoated glass to give a range of low reflectances. Stable coatings submitted by customers are also standardized, and in some case these have the advantage of giving an advantageous like-with-like measuring situation for the customer.
Frank J.J. Clarke,
"Infrared regular reflectance standards from NPL", Proc. SPIE 2776, Developments in Optical Component Coatings, (19 August 1996); doi: 10.1117/12.246813; https://doi.org/10.1117/12.246813