  The strongest remotely detectable signature of life on our planet today is
the photosynthetically produced oxygen (O2) in our atmosphere. However, recent
studies of Earth's geochemical proxy record suggest that for all but the last
~500 million years, atmospheric O2 would have been undetectable to a remote
observer--and thus a potential false negative for life. During an extended
period in Earth's middle history (2.0 - 0.7 billion years ago, Ga), O2 was
likely present but in low concentrations, with pO2 estimates of ~0.1 - 1% of
present day levels. Although O2 has a weak spectral impact at these low
abundances, O3 in photochemical equilibrium with that O2 would produce notable
spectral features in the UV Hartley-Huggins band (~0.25 um), with a weaker
impact in the mid-IR band near 9.7 um. Thus, taking Earth history as an
informative example, there likely exists a category of exoplanets for which
conventional biosignatures can only be identified in the UV. In this paper, we
emphasize the importance of UV capabilities in the design of future space-based
direct imaging telescopes such as HabEx or LUVOIR to detect O3 on planets with
intermediate oxygenation states. We also discuss strategies for mitigating
against 'false positives'--that is, O3 produced by abiotic processes. More
generally, this specific example highlights the broad implications of studying
Earth history as a window into understanding potential exoplanet biosignatures.
