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National Institute of Standards and Technology, Gaithersburg, MD 20899-0001 As radinmctric lamp ■ttandards improve, the need to set lamp current to specific values bccotncs more important. Commercially available power supplies typically provide 12 bit internal digiialto-analog logic which permits current control with a relative expanded uncertainty of about 1 part in 4096, corresponding to an expanded uncertiiinty of the current of about 2 mA at 8 A (in this paper, expimded uncertainties are given as 2 standard deviations). For an FEL-typc standard spectral irradiance lamp, this corresponds to a spectral irradiance difference of 0.12% at 6SS nm. We have developed a technique using 16 bit digital-to-ana log conversion which permits current control with a relative expanded unccrtaintj' of about 1 part in 65536, corresponding to an expanded uncertainty of the current of about 0.1 mA at 8 A. This corresponds to a spectral irradiance difference of approximately 0.006% for an FEL lamp at 655 nm. We describe the technique used to achieve this improvement and wc show data from a lamp demonstrating the effect of the improvement. Wc also describe the limitation provided by the uncertainty of the resistance of the current measuring shunt.

Improved automated current control for standard lamps