The probability distribution of the amplitude (ΔM) of Barkhausen jumps during magnetization reversal in thin films can be measured with optical techniques since each jump produces a variation (ΔI) of the laser beam intensity used to probe magnetization. Here we present a theoretical model which demonstrates that no distortion takes place if P(ΔM) is a power law [P(ΔM)=ΔM−α] with (α>=1.0). This prediction has been experimentally confirmed by measuring P(ΔI) in the same experimental conditions but in two different ways: first with a Gaussian and then with a constant intensity laser profile. In both cases the same power-law distribution has been observed with α=1.0.