We examined the lateral atomic order and the correlation length of crystalline domains in ultra thin Ca/Cd-arachidate Langmuir Blodgett (LB) films by means of a surface sensitive Grazing Incidence Diffraction (GID) technique with a conventional Cu X-ray tube (E(tube) = 8.05 keV), as well as a synchrotron radiation source (E(syn) = 9.1 keV). The conventional X-ray tube GID and the synchrotron radiation GLD yield comparable results for the lateral lattice parameter. We determined a centered rectangular lattice for 25 layers of Ca-arachidate and for 9 layers of Cd-arachidate. The crystalline lattice parameters for 9 layers of Cd-arachidate amount to a = 0.478 nm/b = 0.733 nm (synchrotron radiation GID) and a = 0.486 nm/b = 0.729 nm (X-ray tube GLD). For the lateral lattice constants of 25 layers of Ca-arachidate, the values a = 0.493 nm/b = 0.727 nm (synchrotron radiation GID) and a = 0.489 nm/b = 0.721 nm (X-ray tube GID) were determined. All length scale parameters were calculated with an accuracy of 2.5 percent. For 9 layers of Cd-arachidate, the correlation length of crystalline domains is greater than that of 25 layers of Ca-arachidate. Furthermore, we introduced a deconvolution procedure for the correction of the experimental collimation error for this GID set-up. This correction influences mainly the results for the correlation length.