US2575033A - Image converter tube - Google Patents

Description

Nov. 13, 1951 c. s. SZEGHO 2,575,033

IMAGE CONVERTER TUBE Filed May 1, 1948 x 6 5 Ti 4 3am A 2 x f "33 T'lil'lTT'l'l'IIfil'P'T INVENTOR.

CON STANTIN S SZEGHO 4 7' TORNFY.

ment time.

Patented Nov. 13, 1951 lConstantin .S. 'Szegho, Chioago,.-Ill., assignor to 'TheTRauland Corporation, Chicago, 111., .a zcorjporation pf "Illinois Application 1, 1948, Serial No. 24,631

2 'Claims.

This invention'relates to :new and useful improvements inimage converter tubes and more particularly to screens for such tubes.

One well known use of image converter tubes is to reproduce a picture of a poorly illuminated object on a viewing screen. This is accomplished by1=focusing whatever light :comes from -the (object onto a photocathode which weleases saifiood of electrons varying in density accordance with the light distribution from the object. Accelerating and focusing means are provided within the tube to direct the electrons towardsa fluorescent screen where a visible image is produced by the impinging electrons. Usually the density and velocity of the electron flood are low, despite the use of sensitive photocathodes and strong accelerating means, and the image produced on the fluorescent screen is not surficiently bright.

Various suggestions have been made for overcoming this shortcoming of image converter tubes. According to one, the electrons released from the cathode are multiplied by secondary emission to increase the brightness of the image as, for example, described in British Patent No.

460,579 to Szegho and Lance.

One of the disadvantages of the above is that the tube is unnecessarily complicated as a result of the need for means for producing secondary electrons.

According to the invention these and other disadvantages of image converter tubes have been overcome and the tube which is the subject of this invention is as simple as any image converter tube, yet produces bright clear images of even the most poorly illuminated objects.

This is accomplished by replacing the conventional fluorescent screen with a screen which is cumulatively responsive to electron impingement.

The length of time a fluorescent screen is exposed to electron impingement, except for the initial short excitation period, does not vary its luminous output. However, in the case of 2. cu-

mulatively responsive screen it is possible to improve the brightness of an image produced thereon simply by lengthening the exposure time.

' Such a screen may be formed out of the socalled ionic crystals which when struck by electrons develop a deposit of opaque material. The

" density of this deposit on any given area of the screen varies directly with th density and velocity of the electrons and grows with impinge- The opacity of the screen will, therefore, be enhanced the longer it is bombarded. The electrons insteadof prodncingdig'htnn the screen produce .darhness, :tenebrescence. The ,most satisfactory screens have been .made of potassium chloride vandare .frequentlyreferred to as scotophorscreens.

The decay time of such a screen may run from hours to days and therefore, it .is possible .to make a semi-permanent recording .of .an object. However,-.if this is not desirable, according to another feature of the invention it is possible to erase an image produced on the screen by applying, heat or an electric field to the screen. In this way as soon as the usefulness of one image has ceased, the screen may be wiped clean of it and made ready for another image.

According to a feature of the invention, an image may be produced in the darkness without employin visible light. In this case a special photocathode is used which is sensitive to infrared light used to illuminate the object. Such a photocathode is the so-called caesiated silver surface or other special emitters of great sensitivity. The image may be projected onto a viewing surface or it can be seen directly on the screen of the tube as a black-White pattern. If there is insuflicient ambient light to illuminate the screen, light from an auxiliary source may be used for this purpose.

The invention will now be described in greater detail, reference being had to the accompanying drawing which shows, by way of example. one embodiment of the invention.

A semi-transparent photocathode l is mounted on end wall 2 of image converter tube 3. Light from an object 4 may be focused by lens 5 through end wall 2 upon photocathode End wall 6 of tube 3 carries a screen I which is cumulatively responsive to electron impingement. As the exposure time of screen I to the electrons emitted from photocathode I is increased, the brightness of the image on the screen improves. The screen may be formed of ionic crystals such as potassium chloride. An aluminum film 8 may cover the screen and also part of the side walls of tube 3. Film 8 receives positive potential from source 9 and functions as an accelerating electrode for the photoelectrons and as a collecting anode for slow-moving electrons.

Two pairs of electrodes I0 and H, to which positive potentials are applied from sources l2, l3, are situated between photocathode l and screen 1, and accelerate and focus the photoelectrons emitted from cathode I; instead of electrostatic means electromagnetic coils may be used for focusing.

In certain cases the decay time of the screen may be too long and therefore, it is advisable to use heating means such as a source of current H! which is connected by suitable leads to end wall 6, to erase the image on the screen. Wall 6 may be formed of conductive glass or have a conductivelayer supporting the screen to which is connected source I4. However, other ways of erasing screen 1 may be employed.

A poorly illuminated object 4 may be illuminated by infra-red light and projected by lens 5 onto a semi-transparent photocathode I. The electrons emitted from photocathode I are distributed according to the distribution of the light values of object 4. An electric field which 7 is set up on rings 10 and II and on aluminum film 8, accelerates the electrons so'that they travel through tube 3 towards end wall 6 where they produce an image'of gradually increasing clarity. Naturally there will come a point in the exposure time of the screen to the electrons when increasing definition and clearness give way to obscurity because the screen becomes completely covered by the opaque deposits. Therefore, after the image is sufficiently clear on the screen the electron impingement should be discontinued, as by masking or turning the photocathode away from the object.

Since the image produced on screen 1 is a dark trace and needs illumination from an external source, such a source of light, generally indicated at l5, may be provided and positioned to shine on end wall 6 and illuminatescreen 1.

What is claimed is:

1. In an image converter tube for producing still images of objects, a photocathode exposed to light rays emanating from the object, a screen comprising ionic crystals aligned with said photocathode and cumulatively responsive to the impingement of electrons therefrom, means for accelerating and. focusing the electrons onto the' REFERENCES CITED' The following references are of record file of this patent:

UNITED STATES PATENTS Number Name Date 2,074,226 Kunz et a1. Mar. 16, 1937 2,189,322 Flory Feb. 6, 1940 2,330,171 Rothenthal Sept. 21,1943

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