Abstract
THE gas-filled, incandescent tungsten lamps used as photometric and radiometric standards have good long term stability, but their performance is frequently marred by fluctuations in light output resulting from changes in the convective cooling of the filament. Typical fluctuations are shown in Fig. 1a, which is a record of the output from a ribbon filament lamp of the type used at the National Physical Laboratory (NPL) as a standard of spectral radiance (Fig. 2a). In this situation, an accuracy of 0.1% (a common requirement in standards work) can only be obtained by averaging for several seconds, a tedious restriction where a large number of measurements is required.
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