[mark-sev]

If i understand correctly both methods employ passing a beam of light through the sample, but what is the difference in output that they get then?

Thank you for the answers

[ARGOS++]

Dear mark-sev;

With a “photoacoustic spectrometer” you are able to measure a spectrum of a gas with the dependency of the wavelength, similar to a normal optical spectrometer.
You may also read on:   http://en.wikipedia.org/wiki/Photoacoustic_spectroscopy
(Also non-dispersive spectrometers are possible.)

With the “Brillouin spectrometer” you measure a physical property of a (“mostly”) solid: over light scattering you measure the acoustic wave behaviour of the solid in different directions.
You may start with:   http://www.spectroscopynow.com/coi/cda/list.cda?catId=2611&type=Link&sort=az&chId=6

I hope to have been of some help to you.
Good Luck!
                    ARGOS⁺⁺