an enlargement of the focal spot caused 1)in a cathode-ray tube, by the differences in the electron velocity distribution through the beam and 2)in an optical lens system using white light, by the refrective index of the glass varying with the wavelength of the light, resulting the image
Optical system consisting of a lens with a focal length, f, of at least 500 mm and corrected for chromatic aberrations. The full aperture of the lens must not exceed f/20.
