  X-ray absorption spectroscopy of thin films is central to a broad range of
scientific fields, and is typically detected using indirect techniques. X-ray
excited optical luminescence (XEOL) from the sample's substrate is one such
detection method, in which the luminescence signal acts as an effective
transmission measurement through the film. This detection method has several
advantages that make it versatile compared to others, in particular for
insulating samples or when a probing depth larger than 10nm is required. In
this work we present a systematic performance analysis of this method with the
aim of providing guidelines for its advantages and pitfalls, enabling a wider
use of this method by the thin film community. We compare and quantify the
efficiency of XEOL from a range of commonly used substrates. Our measurements
demonstrate the equivalence between XEOL and x-ray transmission measurements
for thin films. Moreover, we show the applicability of XEOL to magnetic studies
by employing XMCD sum rules with XEOL-generated data. Lastly, we demonstrate
that above a certain thickness XEOL shows a saturation-like effect, which can
be modelled and corrected for.
