Absolute radiant flux measurement of the angular distribution of synchrotron radiation

We have measured the absolute radiant flux of synchrotron radiation as a function of the angle above and below the orbital plane with high accuracy at the Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST), and the results were compared with theoretical calculations. The radiant flux of synchrotron radiation was measured at effective wavelengths of 256.5, 397.8, and 799.8 nm using three calibrated narrow-band filter radiometers with electron energies ranging from 180 to 380 MeV at SURF III. The filter radiometers were positioned inside a beamline with an unobstructed view of synchrotron radiation. The measured radiant flux agrees with theoretical Schwinger formulation to better than 0.5% for angles up to several milliradians.