Beamline 4 is a general purpose radiometric beamline consists of a 2 m monochromator to disperse radiation from SURF III. An Absolute Cryogenic Radiometer (ACR) at the end of the beamline accurately determines the radiant flux of the dispersed beam which, in turn, is used to calibrate the response of photodetectors. We have achieved an uncertainty of less than 1% (k=2) from 135 nm to 325 nm in measuring the spectral response of a typical silicon detector. In addition to detector power response calibration, the flexible design of the beamline enables a wide range of measurements such as reflectivity, internal quantum efficient, irradiance responsivity and radiation degradation of photodetectors as well as reflectivity and transmission of optical materials. Such works allow us to understand and model the behavior of various solid-state photodetectors and resulted in great impact on accurate UV detection for industries such as UV curing and semiconductor photolithography.
The CaF2 window between the monochromator and the end station has been replaced by a MgF2 window, which extends the usable wavelength range down to about 115 nm.