  Fluorescent excitation of spectral lines is demonstrated as a function of
temperature-luminosity and the distance of the emitting region from the central
stars of planetary nebulae. The electron densities and temperatures are
determined, and the method is exemplified through a detailed analysis of
spectral observations of a high excitation PN, NGC 6741, observed by Hyung and
Aller(1997). Fluorescence should also be important in the determination of
element abundances. It is suggested that the method could be generally applied
to determine or constrain the luminosity and the region of spectral emission in
other intensively radiative sources such as novae, supernovae, and active
galactic nuclei.
