GB770238A - Improvements in or relating to image intensifying devices - Google Patents

Abstract

770,238. Cathode ray tubes. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. March 8, 1955 [March 17, 1954], No. 6766/55. Class 39(1). An image intensifier comprises between the photocathode 2, Fig. 1, and fluorescent screen 21, a. target structure 12, Fig. 2, comprising an electron,a bombardment induced conductivity layer 15 and a photo-emissive mosaic 16, and a control grid or grids 17, 18 is or are arranged between the target structure 12 and the fluorescent screen 21, the electron BIC layer controlling the photo-emission from layer 16, The BIC material may be arsenic trisulphide, selenium, cadmium sulphide or anthracene. In operation an X-ray image is projected onto TuCdS fluorescent layer 6 the light from which passes through glass support 4 and transparent conductive layer 3 and excites photo-emissive (CS-Sb) layer 5 to produce an electron image which charges up BIC layer 15. The BIC layer 15 controls the photo-emission from CS-Sb mosaic 16 illuminated by light source 23 through shutter 24 so that a greatly amplified photoemission excites the fluorescent screen 19 which consists of ZnCdS layer 21 and electron permeable Al layer 22. Magnetic focusing means such as solenoids can focus the electron images on 12, 19, and grid 18 can be omitted or connected to grid 17. To obtain reduction of average image brightness without loss of detail brightness variations the light from 23 is pulsed or made intermittent by operating abutter 24, the charged areas of 16 discharging in the dark periods. The time interval between the light pulses can be chosen to increase the contrast at the desired brightness level. The potential of grid 18 can be varied to vary the minimum brightness level and the brightness range within which greatest contrast occurs can be adjusted. It is stated that by helping grid 18 at a few volts negative to grid 17 the contrast can be increased as well as the background brightness eliminated. In a modification, Fig. 3, the DC source 25 is replaced by a pulsating or AC source e.g. supplying a rowtooth wave, and the abutter 24 is dispensed with. The islands of mosaic 16 discharge when the voltage of the source is small and photoelectrons pass to fluorescent screen 19 when the voltage of the source is above a certain critical value. In further modifications the BIC layer may be heated e.g. to 500‹K. to increase its efficiency especially where the islands of 16 are thermionically emissive instead of photo-emissive ; 16 can be heated by an electrical and/or radiant heater. If fluorescent layer 6 is omitted the device can be used for intensifying light images e.g. in front of a pick up tube ; it is also suggested that fluorescent layer 6 can be used to emit light when struck by radiations other than X-rays.