  From ISO [Ne V] 14.3/24.3 micron line flux ratios, we find that 10 out of 20
planetary nebulae (PNs) have measured ratios below the low-electron density
(N_e) theoretical predicted limit. Such astronomical data serve to provide
important tests of atomic data, collision strengths in this case. In principle,
well-calibrated measurements of the [Ne V] 14.3/24.3 flux ratio could improve
upon the existing atomic data.
