<p>The scattering of a non-monochromatic Gaussian beam of light on a two-dimensional photonic crystal is studied. The field has been calculated and the spectral dependence of the radiation intensity has been analyzed. Noticeable differences in the scattering of a quasi-monochromatic wave have been revealed in comparison with two limiting cases of a monochromatic wave and a wave with a broad spectrum. The conditions have been determined under which a simple method, often used in photonics, where the incident field is considered as a plane monochromatic wave, can lead to a significant distortion of the diffraction pattern, which is expressed in a change in the wavelength and in the number of radiation intensity maxima. The role of the non-planar (Gaussian) wavefront and the effect of the size of the spot illuminated on the surface of the grating by the incident beam on the characteristics of scattered radiation have been discussed.</p>