Advances in spectrophotometric transfer standards at the National Physical Laboratory

Abstract

The paper summarises research into the development of spectrophotometric transfer standards at NPL in the eight years since the first Oxford Conference. The principal areas of interest are transmittance spectrophotometry over the wavelength range 200 nm to 3000 nm and diffuse reflectance over the range 320 nm to 2500 nm, including surface colour standards. Results on the uniformity and temperature coefficients of transmittance transfer standards are given. There is a need for a neutral transmittance standard over the range 200 nm to 400 nm with a surface reflectance similar to that of silica. Crystals have been introduced as transmitting wavelength standards. These have much narrower absorption bands than holmium or didymium glasses. The transmittance reading at absorption peaks varies with instrument bandwidth and it is thus possible for the crystals to be calibrated as bandwidth standards. Uniformity, thermochromism and translucency of diffuse reflectance and surface colour standards have been studied in detail and results are presented. Surface colour and diffuse reflectance standards are now issued in both glossy and matt forms. Glossy transfer standards are calibrated against glossy masters and matt transfer standards against matt masters. The accurate calibration of a glossy master against a matt master is a difficult problem and advances in this area are discussed. Rare earths have been incorporated into ceramic tile glazes to give diffusely reflecting transfer standards. Pairs of metameric colour standards have been developed which give a good colour match under illuminant D65 but a mismatch under illuminant A. They are useful for testing the performance of tristimulus colorimeters. The near infrared is a region of growing interest and possibilities for new standards in that area are discussed.