  Given a total sequence in a Hilbert space, we speak of an upper (resp. lower)
semi-frame if only the upper (resp. lower) frame bound is valid. Equivalently,
for an upper semi-frame, the frame operator is bounded, but has an unbounded
inverse, whereas a lower semi-frame has an unbounded frame operator, with
bounded inverse. For upper semi-frames, in the discrete and the continuous
case, we build two natural Hilbert scales which may yield a novel
characterization of certain function spaces of interest in signal processing.
We present some examples and, in addition, some results concerning the duality
between lower and upper semi-frames, as well as some generalizations, including
fusion semi-frames and Banach semi-frames.
