A spatial filter that includes multiple opaque regions and multiple transparent regions; wherein at least a pair of opaque regions out of the multiple opaque regions are much longer than a cross sectional dimension of a light beam that impinges onto the spatial filter; and wherein an axial distance between the pair of opaque regions alters as a function of a rotational angle of the spatial filter. A spatial filter that includes multiple opaque regions and multiple transparent regions; wherein multiple groups of transparent regions are substantially shaped as portions of different spirals; wherein the different spirals are characterized by different separation distances between successive spiral turnings.