  Absorbing boundaries are frequently employed in real-time propagation of the
Schr\"odinger equation to remove spurious reflections and efficiently emulate
outgoing boundary conditions. These conditions are a fundamental ingredient for
an implicit description of observables involving infinitely extended continuum
states. In the literature, several boundary absorbers have been proposed. They
mostly fall into three main families: mask function absorbers, complex
absorbing potentials, and exterior complex-scaled potentials. To date none of
the proposed absorbers is perfect, and all present a certain degree of
reflections. Characterization of such reflections is thus a critical task with
strong implications for time-dependent simulations of atoms and molecules. We
introduce a method to evaluate the reflection properties of a given absorber
and present a comparison of selected samples for each family of absorbers.
Further, we discuss the connections between members of each family and show how
the same reflection curves can be obtained with very different absorption
schemes.
