  Enhanced continuum brightness is observed in many flares (''white light
flares''), yet it is still unclear which processes contribute to the emission.
To understand the transport of energy needed to account for this emission, we
must first identify both the emission processes and the emission source
regions. Possibilities include heating in the chromosphere causing optically
thin or thick emission from free-bound transitions of Hydrogen, and heating of
the photosphere causing enhanced H$^-$ continuum brightness. To investigate
these possibilities, we combine observations from IRIS, SDO/HMI, and the
ground-based FIRS instrument, covering wavelengths in the far-UV, near-UV,
visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody
spectra to infrared and visible wavelengths are reasonable, yielding radiation
temperatures $\sim$6000-6300 K. The NUV emission, formed in the upper
photosphere under undisturbed conditions, exceeds these simple fits during the
flare, requiring extra emission from the Balmer continuum in the chromosphere.
Thus, the continuum originates from enhanced radiation from photosphere
(visible-IR) and chromosphere (NUV). From the standard thick-target flare
model, we calculate the energy of the nonthermal electrons observed by RHESSI
and compare it to the energy radiated by the continuum emission. We find that
the energy contained in most electrons $>$40 keV, or alternatively, of
$\sim$10-20% of electrons $>$20 keV is sufficient to explain the extra
continuum emission of $\sim4-8 \times 10^{10}$ erg s$^{-1}$ cm$^{-2}$. Also,
from the timing of the RHESSI HXR and the IRIS observations, we conclude that
the NUV continuum is emitted nearly instantaneously when HXR emission is
observed with a time difference of no more than 15 s.
