US2073522A - Photoelectric tube - Google Patents

Description

March 9, 1937. K. H. KINGDON ET AL 2,073,522

PHOTOELECTRIC TUBE Filed NOV. 13, 1931 Inventors:

- Kenneth H. Kingdom Hugh E.Thomson',

f I Their Attorney.

Patented Mar. 9, 1937 UNITED STATES PATENT OFFICE.

- 2.073.522 rrlO'lOELECTRIC 'runn Application November 13, 1931', Serial No. 574,804

1 6Claims. (cl. 250 -275);

rection that will intercept the tube. The im- The present invention relates to light-sensitive devices, more particularly to photo-electric tubes.

Objects of the present invention are to provide a light-sensitive device of extreme sensitiveness; 5 also one which presents an unusually large field of view to a' light source and which responds to light reaching the tube from any and all directions. Photo-electric tubes ordinarily are made, in 10. two types: (1) Those in which the envelope is coated on the interior with a silver layer which serves as a foundation for the light-sensitive ma-- terlal, and (2) those in which the light-sensitive surface is formed on an electrode physically separate from the envelope. I While these various forms of photo-electric tubes have given satisfactory service inthe translation of light energy into electrical energy, they are open to certain objections. The first meng tioned type has the disadvantage of being opaque to visible light except when there is an opening termed a window in the silver layer. The field of view presented by a tube of this type is thus limited to the area of the window, ordinarily quite small. The optimum size of such a window can never exceed the greatest rectilinear,

dimension of the envelope, so that a tube'of this" sort responds to light propagated only in certain directions.

' In the second typepof photo-electric tube, the

envelope is transparent to light and the lightsensitive' member is contained within the tube physically separate from the envelope. Inasmuch as the position ofthe light-sensitive cathode is fixed, a tube of this kind is also directive.

The disadvantages of these tubes are at once apparent. As anexample, assume it is desired to measure the intensity of rapidly moving sources of light, such as the heavenly bodies, over a 'considerable period oftime, it is clear that the di-. rective. type of photo-electric tube must be moved through the same angle as the moving light source if the latter is tc be kept within the small held of view. Moreover, in the determination sources of large area over which the light intensity is not uniform; for example daylight, a

photo-electric tube of limited viewmay give an erroneous indication on account of being respon lighted area.

In accordance with the objects of the present invention, we provide. a tube which is, not open to the objection of being directive, but, on the 56 contrary, responds tolight propagated in any diof the integratedamdunt of light produced by sive to light emanated by only a portion of the proved tube thus presents an unusually large and unlimited field of view. The invention con-' templates 2. light-sensitive layer formed on the interior of the envelope, preferably by a sputtering process, said layer being so thinas readily to allow the admission of visible light, 1. e., radiations of wave lengths between about 4000 Angstroms and 7000 Angstroms, through the envelope. It has been found that a light-sensitive layer which is sufliciently'thin to be transparent .to visible light is more stable than layers oLordi nary thickness; that is, the improved thin layer exhibits less tendency toward having spontaneous changes occur in the structure of the active surface, as is evidenced by the constancy of its.

photo-sensitivity;

In the drawing, numeral l designates an envelope of vitreous material,- such as glass or quartz, and preferably of spherical shape. The envelope terminates at. the lower portion in a cylindrical elongation 2 which is provided with a, rentrant stem 3. Within the envelope there is a pair of electrodes 4, 5 mounted rigidly in the stem. The electrode maytake the form of a washer of any base material, such as copper; plated with silver and positioned flatwise above the stem by means of a vertical support wire 6. The electrode 5 constitutes a rod'or wire of stiff character, arranged concentrically with the washer. There is a capsule 1 positioned on either side of the .stem'by means of a framework secured to the rodelectrode. These capsules may contain pellets of an alkali metal such-as caesium dlchromate or other activating material, together with a suitablereducing' agent such as-silicon.

Theimproved tube may be manufactured in derstood that various changes in the process and construction will suggest themselves to those. skilled in the art, without departing from the' spirit of the invention. Thesilvercoating on the washer 4 is first oxidized. This is conveniently accomplished by admitting oxygen into the envelope at a pressure oi. aboutmicrons of mercury and then glowing the'bulb byapplying the following manner,-altho ughit is 'to be un-.

heated, preferably byinduced high ,irequency '1 currents, in order to release the'caesium or otherreference character 9, has been shown. .These conductors may be positionedv at opposite points on the bulb so that the resistance of the lightsensitive material which spans the conductors maybe measured from time to time.. The resista light-sensitive material. The caesium vapor reacts with the oxidized washer and forms thereon a light-sensitive layer. A glass pant-leg 8 may be provided about the wire which. supports the washer, in order to prevent electrical leakage between the'electrodes 4 and 5;

'After the application of light-sensitive material to the oxidized surface of the washer, the bulb is baked out at a temperature of about 300 C., for. a period of time which may vary from a few minutes to about two hours, depending upon the amount of alkali metal present in the bulb, the latter being in the meantime connected to a suitable evacuating pump. In this manner all excess caesium or lightesensitive material that did not stick is driven out, leaving a thin layer of light-sensitive material covering all the free' surface of the oxidized silver. The feature of removing'the excess alkali metal isdescribed and claimed in the Bainbridge application, Serial No.

244,533, filed January 4, 1928, and entitled Photo-electric tube'sand a method for preparing the light-sensitive surface therefor. This application has matured into Patent N9. 1,901,577, granted'March 14, 1933. As stated in the said application, the removal of the excess alkali metal adds to the stability of operation and sensitivity of the lay .The' next step is to transfer the light-sensitive material from the washer onto the envelope, and

this conveniently is accomplished by a sputtering operation. For this purpose, the envelope is filled with an inert gas, such as argon, at a pressure of about 100 microns, and a voltage of about 300, alternating current or direct current being.

appliedbetween the washer and the rod. In the case of direct current, the washer is made the cathode or negative member. Th'latter' is bom barded during, this glowing stage with argon ions,

40 which causes thelight-sensitivematerial to leave the washer and to travel toward the envelope, on which it condenses as an extremely thin sensitive layer extending overthe entire, perimeter of the envelope interior. .The duration of the sputteringprocess depends upon the required thickness of layer," but we have found that when this layer is of minute thickness, in fact so thinas to be substantially transparent to light of wave lengths between about 4000Angstroms and 'z ooo Aug strozns, the sensitivity of the material may be. quite large, and at-the same time stable, over long" periods of operation. The degree of thinness made possible by this process in order to; obtain these qualities lends itself to thefabrication of a photo-electric tube which shows only a slight discoloration of envelope, and, therefore, is readily transparent to visible light.- Thus, there is no need or providing a window in the tube, as the entire ,envelope, serves the purpose of 0 light.

In order accurately to determine when a sumcient amount of light-sensitive material has collected on the envelope during the sputtering process,it may be desirable to provide a pair ofilush seal conductors, of which only one, designated by ance will be observed to decrease suddenly when the film is of suflicient thickness, and the sputtering treatment should be stopped immediately.

The flush seal conductors referred to may be retained and later connected together to constitute sensitive film. It will be we are unable to state the exact nature of the light-sensitive material produced on the envelope in this manner, it is probable that it consists of a complex compound involving caesium,

.electrictubes provided with opaque films. While oxygen and silver. Regardless of the correctness of this belief, it is sufiicient to state that this sensitive material may be derived by asputtering process from a base metal which has been silverplated, oxidized, and then coatedwitha light-v sensitive material such as caesium. It also ap-' pears that a transparent light-sensitive material having the minimum thickness consistent with a substantial photo-electric effect resists photode-activation with time to a marked extent. It. is possible that the material is so thin under the conditions stated that it is of fairly uniformcomposition throughout, so that diffusion of material between the surface and the body of the film does not ,cause the sensitiveness to change with time.

The portion of the light-sensitive material which remains on the washer 4 after the film has been formed on the envelope-does not deleterious-- 1y affect the operation of the device. This washer together with the rod electrode may be connected together to constitute the positive electrode of a circuit in which the photo-electric tube is-connectedso that any photo-electrons which normal- 1y would be produced at the washer by a light source are unable to leave, the washer. The

emanation of photo-electrons is thus confined to the transparent film of light-sensitive material: From the foregoing, it is evident that a photoelectric tube improved in accordance with our invention and provided with a transparent photoelectric surface offers not only advantages in its adaptation to certain uses which require an unlimited fieid 01' view, but also offers a greater stability of operation, due to the thinness of layer, thanphoto-electric tubes of the prior art.

- It isalso apparent that by reason of the spherical configuration of the light-sensitive cathode, the

improved photo-electric tube is particularlywell suited, from the standpoint of minimum distortio'n,-to measure the intensity of light sources which move over a large field of view.

-What we claim as newand desire to secure by LettersPatent of the United States, is:

1.- A photo-electric tube comprising a hermetie cally tight envelope coated with light-sensitive material of which all the component parts are intimately mixed to form a homogenous layen.

isaid light-sensitive material covering an area within the envelope which is perimetrically complete and substantially transparent to light, and

a cooperating electrode in said envelope.

2. A photo-electric tube comprising a herinet icallytight envelope coated with light-sensitive material, of which all the component parts'are intimately mixed to form a homogeneous layer,

said light-sensitive material coverlngan area within the envelope which is perimetrically com-* plete and tra sparent to light, said envelope also containing a plurality of electrodes adapted to pass a discharge therebetween.

3. In the art of fabricating a photo-electric tube having an envelope, the method which includes the steps of coating a foundation member in said envelope with an oxidizable metal, oxidising said metal, then depositing an alkali metal onto said envelope to form a light-sensitive film substantially transparent to light.

s. A photo-electric tube an enveiope', said envelope being coated on the interior with a thin transparent mm of light-sensitive material, and containing a plurality of electrodes includinta rod member and a metal Washer, said washer surrounding said rod and being moun normalwithrespecttotheazls oisaidrod.

6. A photo-electric tube comprising a hermetimaterial of which all the component parts are intimately mixed to form .a homogeneous layer. the mm and envelope together being substantialiv transparentto 1181 1 and a cooperating electrode in said envelope.

. a. memoir.

HUGH; a. women.

0 cally tight envelope coated with light-sensitive

Images