  This paper presents optical night sky brightness measurements from the
stratosphere using CCD images taken with the Super-pressure Balloon-borne
Imaging Telescope (SuperBIT). The data used for estimating the backgrounds were
obtained during three commissioning flights in 2016, 2018, and 2019 at
altitudes ranging from 28 km to 34 km above sea level. For a valid comparison
of the brightness measurements from the stratosphere with measurements from
mountain-top ground-based observatories (taken at zenith on the darkest
moonless night at high Galactic and high ecliptic latitudes), the stratospheric
brightness levels were zodiacal light and diffuse Galactic light subtracted,
and the airglow brightness was projected to zenith. The stratospheric
brightness was measured around 5.5 hours, 3 hours, and 2 hours before the local
sunrise time in 2016, 2018, and 2019 respectively. The $B$, $V$, $R$, and $I$
brightness levels in 2016 were 2.7, 1.0, 1.1, and 0.6 mag arcsec$^{-2}$ darker
than the darkest ground-based measurements. The $B$, $V$, and $R$ brightness
levels in 2018 were 1.3, 1.0, and 1.3 mag arcsec$^{-2}$ darker than the darkest
ground-based measurements. The $U$ and $I$ brightness levels in 2019 were 0.1
mag arcsec$^{-2}$ brighter than the darkest ground-based measurements, whereas
the $B$ and $V$ brightness levels were 0.8 and 0.6 mag arcsec$^{-2}$ darker
than the darkest ground-based measurements. The lower sky brightness levels,
stable photometry, and lower atmospheric absorption make stratospheric
observations from a balloon-borne platform a unique tool for astronomy. We plan
to continue this work in a future mid-latitude long duration balloon flight
with SuperBIT.
