In most experiments, the spectrometer will be a bent-crystal Johann geometry device, comprising an array of spherically bent crystals whose bend radii match the diameter of the Rowland circle, arranged with each Rowland circle intersecting at both sample and detector. For optimal energy resolution, the crystal analyzers must provide a near-90° Bragg geometry for the fluorescence line being examined , which constrains the crystal cut used for the analyzer. Away from the near-90° Bragg condition, the resolution becomes sensitive to the X-ray spot size on the sample, and improvements over conventional XAS can be marginal (La . The Rowland circle radius also matters, since 0.5-meter devices collect a better solid angle, while the 1.0-meter devices give slightly better resolution. X-ray fluorescence self-absorption is a well-known artifact of X-ray fluorescencedetected XAS . Arising from the non-linear relationship between the X-ray fluorescence and X-ray absorption, self-absorption, sometimes known as fluorescence thickness effects, can dampen intense features in the XAS. Self-absorption is pronounced with thick, concentrated samples. With conventional XAS self-absorption can be avoided by measuring the XAS with transmittance rather than using fluorescence; for HERFD-XAS of concentrated samples, X-ray fluorescence is integral to the technique. We previously showed that fluorescence self-absorption distortions of HERFD-XAS depend upon the absorption coefficient, and not on the HERFD-XAS itself and can be removed by applying the correction factor in eq. 1. Here, 𝐻′ 𝐸 is the corrected, distortion-free, HERFD-XAS and 𝐻 𝐸 the experimentally measured data with self-absorption distortions present. The correction assumes samples are oriented at 45° to the incident beam and requires measurements of the sample absorption 𝐴 𝐸 along with 𝐻 𝐸 .