The angular and wavelength selectivity of noise gratings in doped strontium barium niobate crystals for samples with two different thicknesses is investigated. We found, that an offset in the optimum reconstruction angle with respect to the recording geometry occurs as a function of the thickness. This is a consequence of the unidirectional amplification. Thus the scattering intensity for the thinner sample is considerably lowered at the very recording conditions. We analyze the experimental results in terms of the kinematic theory of diffraction.