A method to determine the in-plane cathodoluminescence (CL) probe response function (PRF) (i.e., the function characterizing the in-plane luminescence intensity distribution within the electron probe volume) is proposed, which is based on "perturbing" the spectral position of a selected luminescence band using a highly graded stress field. The method is applied to the stress field developed ahead of the tip of an equilibrium crack in three different cases of CL bands, which arise from different structural phenomena: (i) the F(+) (oxygen excess) defect band in a nominally stoichiometric sapphire (alpha-Al(2)O(3)) single crystal; (ii) the chromophoric R-line in ruby lattice (alpha-Al(2-x)Cr(x)O(3)); and (iii) the near band-gap line in n-type GaN semiconductor crystal. A computer-aided data restoration procedure was applied to rationalize data retrieved from crack-tip line scans performed at different acceleration voltages. For the excitonic band-gap in GaN and for F(+) emission in sapphire the CL probe in the electron focal plane was found to be comparable, but not necessarily coincident, in size to the electron probe. On the other hand, the occurrence of self-absorption in the case of R-line photons in ruby resulted in a significantly broadened CL probe with respect to the average scattering length of electrons.