The NPL detector-stabilized irradiance source

Abstract

The design of a new detector-stabilized spectral irradiance source is presented, together with an evaluation of its performance. This new source consists of a tungsten-halogen lamp that is current-controlled using feedback from an array of filtered photodiodes. The bandpass and central wavelength of these filtered detectors may be selected specifically to meet the required application of the source. The lamp is fully enclosed (except for the exit aperture) in an air-cooled, light-tight housing that minimizes unwanted light pollution. The lamp current is stabilized using feedback from one photodiode; the recording of the non-stabilization channels allows a real-time check on the effective colour temperature of the lamp, and provides an instant diagnosis of the state of calibration of the lamp. The calibration data for each lamp (in its associated housing) are stored on a microchip within the lamp housing, which allows the lamp to be transported independently of its associated control electronics for calibration purposes. The performance of the lamp indicates ageing rates of better than 2 10 -5 h -1 , across the ultraviolet-A (UV-A) and visible spectral regions.

Primary radiometric quantities and units

“…A possible solution to this problem has been developed by the NPL [29,30] and others [31], which makes use of a group of filter radiometers in conjunction with a tungsten halogen lamp to monitor and correct changes in the output of the lamp. In this way, drifts and step changes in the spectral irradiance output can be corrected for, relying only on the stability of the filter radiometer.…”