The NPL detector-stabilized irradiance source

The design of a new detector-stabilized spectral irradiance source is prese nted, together with an evaluation of its performance. This new source consi sts of a tungsten-halogen lamp that is current-controlled using feedback fr om 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 li ght pollution. The lamp current is stabilized using feedback from one photo diode; the recording of the non-stabilization channels allows a real-time c heck on the effective colour temperature of the lamp, and provides an insta nt 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 performanc e of the lamp indicates ageing rates of better than 2 x 10(-5) h(-1), acros s the ultraviolet-A (UV-A) and visible spectral regions.