Image orthicon target

An image orthicon target is made by applying to a nickel-iron alloy support ring a sealing glass frit having a thermal expansion over the temperature range 0-300 DEG C. within 0.0005 in./in./ DEG C. of that of the ring, heating to 600-700 DEG C. to fuse the frit to the ring and then placing over the ring and in contact with the glass frit a membrane of a semi-conducting glass having a softening point at least 120 DEG C. higher than that of the sealing glass and a thermal expansion similar to that of the ring, heating the resulting assembly to 650-700 DEG C. so causing the membrane to sag over the ring and a sealing glass fillet to form between the ring and the membrane, the assembly finally being cooled. The sealing glass composition consists essentially in weight per cent, of 10 to 25 BaO, 15 to 30 B2O3, 25 to 40 V2O5, and 25 to 40 ZnO. Th BaO may be replaced in part by CaO, SrO, or MgO, and the ZnO by BeO, or CdO. A suitable semi-conducting glass contains 63 mol per cent CaO.2B2O3, 10 mol per cent Fe3O4 and 27 mol. per cent V2O5.