Design and Construction of a New Primary Standard Pyrometer at NPL

For many years the temperature scale at NPL has been realized and maintained using the NPL primary pyrometer. This instrument has reached the end of its useful life, and a replacement instrument has been built and is currently being validated. This new instrument will give reduced uncertainties in disseminating the temperature scale. The new pyrometer uses lens optics, designed to be achromatic at the two operating wavelengths of 650 nm and 900 nm. The wavelengths are defined by interference filters. The target size of 0.75 mm has been chosen to allow for calibration of tungsten ribbon lamps. The filters, stops, detector and amplifier are all temperature stabilised. The following tests have been carried out: linearity of the detector and amplifier, the temperature coefficient of the instrument, size-of-source effect measurement, determination of the effective wavelength of each interference filter and the short-term stability. This paper gives a detailed description of the instrument design and the results of the validation tests carried out so far.