Radiometric Comparison of the Primary Synchrotron Radiation Source Standard Metrology Light Source with Calibrated Filter Radiometers in the Visible and NIR Spectral Range

The Metrology Light Source (MLS), the electron storage ring of the Physikalisch-Technische Bundesanstalt (PTB), is operated as a primary radiometric source standard from the near infrared (NIR) to the vacuum ultraviolet spectral region with calculable synchrotron radiation according to electromagnetic theory (Schwinger equation). The operational parameters of the MLS can be varied in a wide range to adjust the spectral distribution and the intensity of the resulting spectrum to the specific measurement requirements. The electron beam energy can be set to values between 105 MeV and 630 MeV, and the electron beam current can be varied from 1 pA (one stored electron) up to 200 mA.Using two calibrated filter radiometers with centre wavelengths of 476 nm and 1595 nm as transfer standards, the calculated spectral radiant power of the MLS into a well-defined aperture was compared with the spectral irradiance responsivity scale of PTB realized by cryogenic radiometers in the visible and NIR spectral range. The measurements were performed with the MLS operated at various electron energies. Good agreement was found within the combined relative uncertainties.