Calibration of a zone plate for an EUV solar irradiance monitor

A 4 mm diameter zone plate was calibrated in the 7 nm to 18.5 nm wavelength range using synchrotron radiation. The efficiency in the focused 1 st order was measured using the scanning monochromator at the Naval Research Laboratory beamline X24C at the National Synchrotron Light Source. The measured efficiencies were compared to efficiencies calculated by accounting for the partial transmittance through the molybdenum zone plate rings and the resulting phase enhancement of the efficiencies. Accurate absolute efficiency calibrations enable the use of zone plates in EUV solar irradiance monitors having excellent stability against contamination and oxidation.