Method and device for determining the focal length of a long focal length electron optical lens

摘要

A method and a device used for determining the focal length of a long focal length electron optical lens particularly used with a microprojector characterized by projecting a bundle of parallel rays along the axis of the long focal length lens, said bundle of rays having a ring-shaped cross section with a radius R, providing a pinhole diaphragm having a diameter which corresponds approximately to the diameter of the circle of least confusion of the lens for a ring-shaped bundle of parallel rays, either positioning the diaphragm axially relative to the lens or holding the diaphragm fixed and changing the excitement of the lens to obtain the least influence of the diaphragm on the bundle of rays passing through the pinhole diaphragm, detecting the image of the rays passing through the diaphragm on a plane at a distance z.sub.2 from the diaphragm, measuring the radius r of the image of the ring-shaped bundle of rays on the plane and then determining the focal length f from the magnitude of R, r, z.sub.2 according to the formula f=(R/r)·z.sub.2. The device includes an annular field stop for producing the bundle of rays having a ring-shaped cross section, means for positioning the field stop in that path of the rays prior to their reaching the lens, a diaphragm having an aperture of the circle of least confusion for the lenses of the ring-shaped bundle of parallel rays and means for recording the image passing through the diaphragm.