  Better knowing the precision on the measured value of the Brillouin peak
frequencies is essen-tial in order to use it to deduce parameters related to
studied materials. Modern methods for evaluating uncertainties are based on the
recommendations of the Guide to the expression of uncertainty in measurement.
After checking the agreement between the measured 15.7 GHz shift on a Brillouin
signal on Poly-(methyl methacrylate) and the expected value, the elementary
un-certainty terms are evaluated in two groups, by statistical methods and by
other means. We de-scribe the general principle of operation of Brillouin light
scattering setup with a high power laser and evaluate the elementary terms of
uncertainty, according to international standards en-acted for metrology. The
global uncertainty on frequency shift is then calculated and we find
+-1.20x10-2 at 2 {\sigma}.
