  X-ray dark-field imaging enables a spatially-resolved visualization of
ultra-small-angle X-ray scattering. Using phantom measurements, we demonstrate
that a material's effective dark-field signal may be reduced by modification of
the visibility spectrum by other dark-field-active objects in the beam. This is
the dark-field equivalent of conventional beam-hardening, and is distinct from
related, known effects, where the dark-field signal is modified by attenuation
or phase shifts. We present a theoretical model for this group of effects and
verify it by comparison to the measurements. These findings have significant
implications for the interpretation of dark-field signal strength in
polychromatic measurements.
