  IR emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 um are generally attributed
to IR fluorescence from (mainly) FUV pumped PAHs. As such, they trace the FUV
stellar flux and are a measure of star formation. We examined the IR spectral
characteristics of Galactic star forming regions, normal and starburst
galaxies, AGNs and ULIRGs. The goal is to analyze if PAH bands are a good
qualitative and/or quantitative tracer of star formation and hence the
application of PAH bands as a diagnostic in order to identify the dominant
processes contributing to the IR emission from Seyfert's and ULIRGs. We develop
a MIR/FIR diagnostic and compare it to known diagnostics, with these also
applied to the Galactic sample. This diagnostic is based on the FIR normalized
6.2 um PAH flux and the FIR normalized 6.2 um continuum flux. The Galactic
sources form a sequence spanning a range of 3 orders of magnitude, from
embedded compact HII regions to exposed PDRs and the (D)ISM. The variation in
the 6.2 um PAH/continuum ratio is relative small. Normal and starburst galaxies
ressemble exposed PDRs. While Seyfert-2's coincide with the starburst trend,
Seyfert-1's are displaced by at least a factor 10 in 6.2 um continuum flux.
ULIRGs show a diverse spectral appearance (AGN hot dust continuum,
starburst-like or strong dust obscuration in the nucleus). ULIRGs also seems to
have more prominent FIR emission than either starburst galaxies or AGNs. We
discuss the observed variation in the Galactic sample in view of the
evolutionary state and the PAH/dust abundance and the use of PAHs as
quantitative tracers of star formation activity. We find that PAHs may be
better suited as a tracer of B stars, which dominate the Galactic stellar
energy budget, than as a tracer of massive star formation (O stars).
