Improving branching fraction calibration methods: The optical to ultraviolet bridge

•Re-measurement of critical Ar II branching ratios.•First measurements of branching ratios using a NIST traceable detector.•Calibration technique for response of spectrometer using a NIST traceable detector.•Improvement of the optical (tungsten filament lamp) to UV (deuterium lamp) calibration. Herein we report new measurements of key Ar ii branching ratios (BRs) that span the optical to ultraviolet (UV) wavelength region. Our new measurements are the first to use a standard detector, a NIST calibrated Si photodiode, instead of standard sources. The use of a NIST calibrated Si photodiode requires a stable source with sufficient output to yield power measurements with a good signal-to-noise ratio from the NIST calibrated Si photodiode after filters are used to isolate wavelength regions from the source. In this work we used a Hg pen lamp which has good stability, but is a line source instead of a continuum source. The Hg i lines fell near the Ar ii lines of interest and were easy to isolate using filters. A complete calibration of the filter transmission as a function of wavelength is not necessary when a line source is used. The selected Ar ii BRs are the best for testing and/or improving a bridge between an optical calibration based on a tungsten standard lamp and an UV calibration based on a deuterium lamp.