Luminescent screen having a mosaic structure

Abstract

In a luminescent layer which is composed of spatially separated areas for reduction of lateral light dispersion in the layer, this subdivision is realized by provision of a crack structure in the luminescent layer. This crack structure is realized by thermal treatment of a layer which is provided on a substratum, and can be made optimum by an appropriate choice of the layer thickness, so that deposition in a plurality of sublayers can be advantageous. The crack structure can also be caused by a pattern which is provided in the substratum surface or by means of a gauze which serves as a substratum.

What is claimed is: 
     
       1. A method for forming a luminescent screen, comprising the steps of: adhering a luminescent layer to a substratum having a coefficient of thermal expansion different from said luminescent layer; and   changing the temperature of both said substratum and said adhering luminescent layer from the temperature at which said adhering occurred in a direction which due to the different coefficients of thermal expansion creates laterally directed tensile forces in said luminescent layer sufficient to crack said layer into a closely spaced mosaic pattern of discrete luminescent areas.   
     
     
       2. A method for forming a luminescent screen as claimed in claim 1 wherein said luminescent layer comprises grains of luminescent material and binder material to form said grains into a cohesive and adhering layer. 
     
     
       3. A method for forming a luminescent screen as claimed in claim 2 wherein said luminescent layer has a thickness of approximately 400 microns. 
     
     
       4. A method for forming a luminescent screen as claimed in claim 1 wherein said luminescent layer is substantially composed of luminescent material. 
     
     
       5. A method for forming a luminescent screen as claimed in claim 4 wherein said luminescent layer has a thickness of approximately 200 microns. 
     
     
       6. A method for forming a luminescent screen as claimed in claim 5 wherein said luminescent material is cesium iodide. 
     
     
       7. A method for forming a luminescent screen as claimed in claim 1 wherein said substratum is composed of a metallic material. 
     
     
       8. A method for forming a luminescent screen as claimed in claim 1 wherein the coefficient of thermal expansion of said luminescent layer is greater than the coefficient of thermal expansion of said substratum and in the temperature changing step the temperature is lowered. 
     
     
       9. A method for forming a luminescent screen as claimed in claim 1 further comprising the steps of: adhering to said previously cracked luminescent layer an additional luminescent layer having a coefficient of thermal expansion different from said substratum; and   changing the temperature of said substratum and said previously cracked layer and said additional luminescent layer all from the temperature at which the second adhering step occurred in a direction which due to the different coefficients of thermal expansion creates laterally directed tensile forces in said additional luminescent layer sufficient to crack said additional layer substantially along the crack lines of said previously cracked layer.   
     
     
       10. A method for forming a luminescent screen as claimed in claim 9 wherein said second luminescent layer has a different composition from said first deposited luminescent layer. 
     
     
       11. A method for forming a luminescent screen as claimed in claim 1 further comprising the additional step of impressing on the surface of said substratum a desired mosaic pattern before adhering said luminescent layer thereto in order to concentrate said tensile forces within said layer along said desired pattern, thereby substantially cracking said layer into a pattern corresponding with said desired mosaic pattern. 
     
     
       12. A method for forming a luminescent screen as claimed in claim 1 further comprising the additional steps of: depositing on said cracked luminescent layer a chemical contamination isolating layer of material; and   depositing on said isolating layer of material a conductive layer of material suitable for use as a photocathode.   
     
     
       13. A method for forming a luminescent screen as claimed in claim 12 wherein said isolating layer comprises a layer of partially oxidized aluminum. 
     
     
       14. A method for forming a luminescent screen as claimed in claim 12 wherein said isolating layer is composed of aluminum oxide and said photocathode layer is composed of cesium antimony. 
     
     
       15. A method for forming a luminescent screen as claimed in claim 1 further comprising the additional step of filling the spaces between said closely spaced mosaic pattern of discrete luminescent areas with electromagnetic radiation absorbing material. 
     
     
       16. A method for forming a luminescent screen, comprising the steps of: mounting a metallic gauze on a substratum; and   depositing a luminescent material onto said metallic gauze by vapor deposition or cathode sputtering, thereby forming columns of luminescent material perpendicular to said gauze, said columns at least to some extent remaining distinct from one another.   
     
     
       17. A method for forming a luminescent screen as claimed in claim 16 further comprising the additional step of heat treating said screen to crack apart any columns which have been joined in the forming step. .Iadd. 18. A luminescent screen comprising: a substratum,   a luminescent layer adhered to said substratum and having a coefficient of thermal expansion different from that of said substratum,   said luminescent layer having a plurality of discrete luminescent areas separated by cracks and forming a closely spaced mosaic pattern, said mosaic pattern being produced by changing the temperature of both said substratum and said layer adhered thereto from the temperature at which the adhering occurred in a direction which due to the different coefficients of thermal expansion creates laterally directed tensile forces in said layer sufficient to form said cracks therein. .Iaddend..Iadd. 19. In an X-ray image intensifier including a screen for converting X-radiation into visible radiation, a photocathode responsive to the visible radiation for producing an electron-optical image therefrom, and a receiving screen for converting the electron-optical image into a visible image, the improvement wherein the screen for converting X-ray radiation into visible radiation consists essentially of a layer of cesium iodide having closely spaced mosaic pattern of discrete areas adhered to a substrate by vapor deposition, and produced by changing the temperature of both the substrate and the cesium iodide from the temperature at which said layer was adhered in a direction which due to the different coefficients of thermal expansion creates laterally directed tensile forces in said layer sufficient to crack said layer. .Iaddend. .Iadd. 20. An X-ray image intensifier as claimed in claim 19 in which the layer has a thickness of approximately 200 microns. .Iaddend..Iadd. 21. An X-ray image intensifier as claimed in claim 20 in which the layer has a crack frequency of approximately 125 lines per cm. .Iaddend..Iadd. 22. An X-ray image intensifier as claimed in claim 19 in which the screen for converting X-ray radiation into visible radiation includes an additional layer of cesium iodide having a closely spaced mosaic pattern of discrete areas adhered to the first-named layer by vapor deposition, said mosaic pattern of said additional layer being formed by changing the temperature of said substrate and said first layer and said additional layer all from the temperature at which adhering of said additional layer occurred in a direction which due to the different coefficients of thermal expansion creates laterally directed tensile forces in said additional layer sufficient to crack said additional layer substantially along the crack lines of said first-named layer. .Iaddend.