Thin films of Si deposited by R.T.C.V.D. on (001) Si wafer were investigated on cross sectional sample using a JEOL 3010 high resolution microscope, in order to study the epitaxy and extended lattice defects formed during the deposition process in the film. H.R.T.E.M. images of extended defects were investigated by optical Fourier analysis in different areas of the picture; the results are interpreted in terms of twins and periodicity of twin lamellae. Further analysis from digital processing of the H.R.T.E.M. pictures was done by Gabor filtering and is interpreted in terms of strain field in the matrix.