  The strong enhancement of the ultraviolet emission during solar flares is
usually taken as an indication of plasma heating in the lower solar atmosphere
caused by the deposition of the energy released during these events. Images
taken with broadband ultraviolet filters by the {\em Transition Region and
Coronal Explorer} (TRACE) and {\em Atmospheric Imaging Assembly} (AIA 1600 and
1700~\AA) have revealed the morphology and evolution of flare ribbons in great
detail. However, the spectral content of these images is still largely unknown.
Without the knowledge of the spectral contribution to these UV filters, the use
of these rich imaging datasets is severely limited. Aiming to solve this issue,
we estimate the spectral contributions of the AIA UV flare and plage images
using high-resolution spectra in the range 1300 to 1900~\AA\ from the Skylab
NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600~\AA\
is { dominated by} the \ion{C}{4} 1550~\AA\ doublet (26\%), \ion{Si}{1}
continua (20\%), with smaller contributions from many other chromospheric lines
such as \ion{C}{1} 1561 and 1656~\AA\ multiplets, \ion{He}{2} 1640~\AA,
\ion{Si}{2} 1526 and 1533~\AA. For the AIA 1700~\AA\ band, \ion{C}{1} 1656~\AA\
multiplet is the main contributor (38\%), followed by \ion{He}{2} 1640 (17\%),
and accompanied by a multitude of other, { weaker} chromospheric lines, with
minimal contribution from the continuum. Our results can be generalized to
state that the AIA UV flare excess emission is of chromospheric origin, while
plage emission is dominated by photospheric continuum emission in both
channels.
