a beam of light rays in which either the rays themselves or their extensions intersect at a single point. The wave surface of homocentric pencils of light is spherical; its center is precisely the point of intersection of the homocentric pencil of rays. The image obtained using an optical system reproduces the size of the object precisely only if the homocentric pencil of rays is again transformed into a homocentric pencil of rays after passing through a given system; only under these conditions does a single definite image point correspond to each point of the object.