Method of manufacturing a color display tube having a magnetic quadrupole post-focusing mask and a color display tube made by the method

A method of manufacturing a colour display tube having a magnetic quadrupole post-focusing mask (17) in which magnetic field shaping means (30,31) of soft iron, or the magnetic equivalent, having raised portions, for example strips (32), are placed against opposite sides of a mask sheet (17) and are then subjected to an external strong magnetic field by an electromagnet (34,35) to induce magnetic poles through the thickness of the mask sheet. The geometric arrangement of the raised portions of the field shaping means (30,31) is such that cyclically a north pole, a south pole, a north pole and a south pole are induced into the sheet material surrounding each aperture. Using this technique the entire area of a mask sheet can be magnetised in one operation. Also masks for small tubes and high definition tubes can be made. Quadrupole lens fields which are of variable predetermined strengths can be produced by either varying the applied magnetic field across the mask sheet or selectively partially demagnetising an already magnetised mask sheet.