the preferred embodiment , shown in the drawing , has the structure and mode of operation now discussed . the drawing is a vertical sectional view , somewhat diagrammatical , and with a small portion shown enlarged , through the preferred embodiment . there is shown in the drawing , indicated generally at 10 , an imaging tube according to the invention . the tube 10 includes a metal housing 12 surrounding a cryogenic portion 14 and a vacuum - tight imaging portion 16 , the two being separated by integral heat - conducting metal wall 18 . portion 16 is closed in a vacuum - tight way by means of infrared (&# 34 ; ir &# 34 ;)- transmissive window 20 . mounted in housing 12 is optical lens 22 , which is secured in housing 12 by means of ring 24 therearound . extending through lens 22 is an optical unit indicated generally at 26 and which includes electron lens 28 ( with a resolution of 3 microns , and a minification ratio of 4 : 1 ), concavoconvex microchannel plate (&# 34 ; mcp &# 34 ;) 30 ( with channels on 10 micron centers ), fiber optic bundle 32 of the general character disclosed in my u . s . pat . no . 4 , 202 , 599 , &# 34 ; nonuniform imaging &# 34 ;, granted may 13 , 1980 , phosphor layer 34 ( with proximity focus between mcp 32 and phosphor screen 34 of 3 micron resolution ), and prism 36 . phosphor layer 34 and fiber optic bundle 32 are , toward microchannel plate 30 , provided with spherical surfaces parallel with the surface toward them of the microchannel plate , phosphor layer 34 being thinly coated on fiber optic bundle 32 ( with fibers on 5 micron centers ) and being spaced from microchannel plate 30 . window 38 transmissive to visible light is provided in housing 12 for viewing by eye 40 . infrared radiation thus enters imaging tube 10 through window 20 along a first axis , and visible light leaves imaging tube 10 through window 38 along a second axis that makes an angle of 90 ° with the first . the angle between the two is thus less than 180 °, so that the light to be viewed is not directed back to the source of light being imaged . coated on wall 18 in imaging portion 16 is a continuous electrode 42 which carries on it a multiplicity of separate semiconductor phototransistor elements ( indicated collectively at 44 ), as a mosaic . the elements 44 are about 75 microns square , and spaced apart with gaps of about 5 microns . each semiconductor element carries on its face away from continuous electrode 42 an electrode 46 in contact only with its respective semiconductor element of the mosaic . overlying the electrodes 46 is photocathode 48 . extending across portion 16 adjacent photocathode 48 is mesh grid 50 . mounted in portion 16 adjacent ring 24 is emission source 52 , of wavelength of 850 nanometers . in operation , infrared radiation 60 , 10 microns in wave length and defining an image , enters tube 10 through window 20 . the image is focused by lens 22 on the semi - conductor - electrodes - photocathode assembly 42 , 44 , 46 , 48 . impact of rays of 10 - micron infrared on particular semiconductor transistor elements 44 causes them to go to a negative 100 millivolt potential . at the same time , source 52 continuously supplies to photocathode 48 radiation at an emission wavelength of 850 nanometers ; photocathode 48 has a photoemissive threshold of 900 nanometers , so that the radiation from source 52 causes photocathode 48 to emit photoelectrons 60 at a kinetic energy of about 80 millivolts . the potential on mesh grid 50 is minus 125 millivolts , so that an electron at a potential energy of 80 millivolts is unable to go through it . however , where an area of photocathode 48 is in contact with an electrode element 46 which is in contact with a semiconductor element 44 which has been exposed to the ir , that area of photocathode 48 has its potential reduced to minus 100 millivolts , making the voltage drop between it and grid 50 only 25 millivolts , enabling electrons from that area of photocathode 48 to penetrate the grid , in a patterning corresponding with the patterning of the ir beam incident on the tube . electrons 62 thus leaving photocathode 48 are focused by electron lens 28 onto the concavo surface of microchannel plate 30 , in which the signal is amplified , and whence it goes through a vacuum gap onto phosphor layer 34 , coated on the concavo surface of fiber optic bundle 32 , the phosphor converting the electrons to visible light , which is turned by prism 36 to be viewed through window 38 as at 40 . distortion is modified by fiber optic bundle 32 . use of a reflective photocathode provides many advantages . temperature and electrical potential of the photocathode may be easily controlled . cooling may be direct and efficient . the optical element employed may be , instead of an optical lens , an optical mirror , for example at 45 ° to incident radiation focused on it by an optical lens to reflect it onto the photocathode . the mirror may be segmented to increase resistance along the mirror and prevent distortion of the electronic or electrostatic lens field . the semiconductor elements in mosaic may be photoconductive , photovoltaic , or mis elements . alternatively , an electron beam may be used to produce a varying potential in the photocathode . the radiation to the photocathode to cause it to release electrons may be intermittent or continuous .