Report on the long-term stability of filter radiometers developed in the previous (98-01) optical programme.

This report presents the findings of an investigation into the long-term stability of two types of filter radiometer, one type based on a high specification custom 800 nm interference optical bandpass filter and the other on a proprietary single glass 710 nm bandpass filter. Spectral responsivity measurements made over a three-year period showed that the 800 nm filter radiometers exhibited excellent long-term stability, of the order of the spectral responsivity measurement uncertainty of ± 0.1 %. This, combined with their high out-of-band blocking, makes these 800 nm filter radiometers a notable improvement on the 800 nm NPL primary filter radiometers used previously. Unfortunately, high spatial non-uniformity and low optical transmission of the 710 nm glass filters resulted in insufficient measurement accuracy to reliably evaluate their long-term stability.