  The newly-commissioned University of New South Wales Infrared Fabry-Perot
(UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25
microns in the reflection nebula Parsamyan 18. P 18 is known to exhibit low
values of the (1-0)/(2-1) S(1) ratio suggestive of UV-pumped fluorescence
rather than thermal excitation by shocks. Our line ratio mapping reveals the
full extent of this fluorescent emission from extended arc-like features, as
well as a more concentrated thermal component in regions closer to the central
exciting star. We show that the emission morphology, line fluxes, and gas
density are consistent with the predictions of photodissociation region (PDR)
theory. Those regions with the highest intrinsic 1-0 S(1) intensities also tend
to show the highest (1-0)/(2-1) S(1) line ratios. Furthermore, variations in
the line ratio can be attributed to intrinsic fluctuations in the incident
radiation field and/or the gas density, through the self-shielding action of
H_2. An isolated knot of emission discovered just outside P 18, and having both
an unusually high (1-0)/(2-1) S(1) ratio and relative velocity provides
additional evidence for an outflow source associated with P 18.
