An in-line method for optically inspecting transparent or translucent containers (3) comprises illuminating each container with a light source that presents light intensity variation in a periodic pattern along at least a first variation direction. A number N greater than or equal to three of images of the container traveling in front of the light source and occupying N different respective positions along the travel path is taken. Between taking successive images, a relative shift between the container and the periodic pattern is created. A geometrical transformation is determined and applied in order to put the pixels belonging to the container in the N successive images of the same container into coincidence. A phase image for each container is constructed using the N registered images of the container. The phase image is analyzed in order to deduce therefrom at least the presence of defects or the quality of the container.