  Nearby interstellar clouds at high Galactic latitudes are ideal objects in
which the interaction of interstellar dust with photons from the
well-characterized interstellar radiation field can be studied. Scattering and
UV-excited photoluminescence at optical wavelengths as well as thermal emission
at mid- and far-infrared wavelengths are observable manifestations of such
interactions. Here we report initial results from an optical imaging survey of
optically thin high-Galactic-latitude clouds, which is designed to study the
surface brightness, structure, and spectral energy distribution of these
objects. The primary aim of this paper is to study the extended red emission
(ERE) that has been reported at high Galactic latitudes in earlier
investigations and which is attributed to ultraviolet-excited photoluminescence
of an as yet unidentified component of interstellar dust. We find strong
evidence for dust emission in the form of a broad (>1000 A FWHM) ERE band with
peak emission near 600 nm wavelength and peak intensity of ~ 5x10^-9 (erg cm^-2
s^-1 A^-1 sr^-1) in optically-thin clouds. This amounts to about 30% of the
total optical surface brightness of these clouds, the remainder being
consistent with expectations for dust-scattered light. This supports claims for
the ubiquitous presence of the ERE carrier throughout the diffuse interstellar
medium of the Milky Way Galaxy. We suggest that the ERE carrier is involved in
the radiative processing of about 20% to 30% of the dust-absorbed UV/optical
luminosity of the Milky Way galaxy, with the bulk of this energy being emitted
in the near- to mid-infrared spectral regions.
