Patent GB-788661-A (GB)
Title: Improvements in or relating to methods of forming screen structures for cathode ray tubes
IPC: H01J9/227
Filed: 1954-08-24
Abstract: 788,661. Luminescent screens for colour television tubes. PHILCO CORPORATION. Aug. 24, 1954 [Aug. 25, 1953], No. 24592/54. Class 39(1). Method of forming a luminescent screen for a colour television C.R.T. comprises depositing unexposed photosensitive material, e.g. a water solution of polyvinyl alcohol mixed with ethyl alcohol and a solution of ammonium or potassium dichromate on a translucent or transparent substrate, the photosensitive material having substantially different solubility in pre-determined solvents, e.g. water, when exposed than when unexposed and being retentive of substances deposited thereon at least in its condition of lesser solubility, exposing, e.g., through a mask of strip or dot form, selected portions of the photosensitive material, thereby modifying the solubility of the exposed portions in the predetermined solvents, depositing a luminexcent substance on the exposed and unexposed portions of the photosensitive material and applying at least one of the solvents to the photosensitive material, thereby removing selectively from the substrate the portions of the photosensitive material having the greater solubility and the luminescent substance deposited thereon while leaving on the substrate the portions of the photosensitive material having the lesser solubility and the luminescent substance deposited thereon. A second and third luminescent substance can be deposited on the substrate in displaced positions by the same method using a movable mask. A thin aluminium film is deposited on the luminescent substances and then an organic film as described in Specification 761,180. A substance having a secondary electron emission ratio substantially in excess of unity, e.g., MgO, silver, gold, tungsten and other high atomic weight elements, can be deposited on the organic film by the same method as for the luminescent substances to form indexing strips. The indexing substances may be deposited in the form of narrow parallel strips superimposed on every third luminescent strip, e.g., having the same colour. In a modification the indexing strips can be made of a luminescent material having a light emissivity different from that of the aluminium film. The structure is finally baked to substantially completely vaporize the organic - film and remaining photosensitive material. Apparatus for exposing the photo. sensitive material is shown in Fig. 2 and comprises a box-like supporting frame 16 provided with an apertured frame 16a for receiving the face plate 17 of the C.R.T. 18. Alternatively the face plate 17 alone can be inserted in the frame 16a. A light source 20, a mask having opaque and translucent portions, and a micrometer adjustment 22 for the mask 21 are provided. A point light source can be used for luminescent and/or indexing material elements in the form of dots and a line light source for elements in the form of strips. The photosensitive material can be applied to the face plate 17 by spraying or by flowing down the wall of the flared portion 19 to a thickness of 0À005 to 0À025 inches after which it is dried by an air blast at a temperature of less than 75‹F. The luminescent layers are applied by spraying in a water suspension or dusting to a surface density of 0À5 to 6 mgrs. per sq. cm. In an alternative arrangement the light source is disposed above the narrow end of the flared portion 19 and a scaled-down mask is disposed within this narrow end, the neck 19a being removed and replaced at the end of the process. In cases where the photo-sensitive material is rendered more soluble by exposure to light the above apparatus can be used provided that the mask is aligned with the C.R.T. face plate during each exposure so that those portions to be eventually covered with substance are shielded from illumination. Specification 713,908 also is referred to.