Two samples of orpiment (As2S3) were prepared, one deliberately concentrated to induce pronounced fluorescence self-absorption, with a calculated post-edge absorbance of 2.0 (measured 2.02), the other more dilute and appropriate for HERFD-XAS, but still showing some self-absorption. Measured absorbances at the As Kα1 energy, 𝐴 in eq. 1, were 1.191 and 0.621 for the concentrated and dilute samples, respectively. We note here that absorbance measurements should be as close as possible to the actual sample absorbance, with correction relevant to detectors (e.g. lengths of gas ionization chambers and gains of current-to-voltage amplifiers) and additional absorbing components in the experiment (e.g. flight paths, thick windows and other contributing absorbers that are not part of the sample). If the sample composition is known precisely, then even if the absorption has not been measured with care, the measurements can in principle be scaled to a computed absorption coefficient, based on the cross-sections of the components, and a value for 𝐴 can also be calculated. indistinguishable from the transmittance spectrum (Fig. ). For the dilute sample, only subtle self-absorption correction is needed, with a substantial correction for the concentrated sample.