US2418779A - Alkali metal halide and luminescent screens of substantially coincident spectral absorption - Google Patents

Description

April 8, 1947. H. w. LEVERENZ ALKALI METAL HALI'DE AND LUMINESCENT SCREENS OF SUBSTANTIALLY COINGIDENT SPECTRAL ABSORPTION Flled July 22 1942 2 Sheets-Sheet 1 INVENTOR filnrZoUfiWL erecenz P" 8, 1947- H. w. LEVERENZ 2,418,779

ALKALI METAL HALIDE AND LUMINESCENT SCREENS OF SUBSTANTIALLY COINGIDENT SPECTRAL ABSORPTION Filed July 22, 1942 2 Sheets-Sheet 2 L Q Q o v w as c 3 -g" .8 f? 33 Q6 '0 Q g I VENTOR ss/voassg 910407.94 4 RN Patented Apr. 8, 1947 ALKALI METAL HALIDE AND LUMINESCENT SCREENS OF SUBSTANTIALLY COINCI- DENT SPECTRAL ABSORPTION Humboldt W. Leverenz, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 22, 1942, Serial No. 451,870

Claims. (Cl. 250-164) My invention relates to methods and means for portraying-intelligence and particularly to targets comprising materials which change color under electron bombardment, such as targets incorporating alkali halide crystals and their method of operation.

It is-known that alkali halides, notably potassium chloride, have the property of developing color centers under electron bombardment. For example, when such an alkali halide target is scanned by an electron beam, electrons are injected into the crystal or crystals in the scanned area thereby developing a group of color centers of a density depending upon the instantaneous intensity of the electron beam. This coloration has been used to produce images for television and oscillograph purposes. The recent development of aircraft position and indicating equipment utilizing cathode ray tubes wherein the electron beam of the cathode ray tube is sequentially pulsed to form on the target a trace portraying intelligence, such as the trajectory of the aircraft,

necessitates the development of high contrast between the areas of a target indicating aircraft position and distance with respect to the surrounding areas of the target. In addition it is required to observe the trajectory as a visible trace during a relatively long period of time. Luminescent materials have been used for this application although the majority of such materials have a flash characteristic which tends to reduce the dark-adaptation of the eye, whereas the alkali halide target is ideally suited for this application although greater contrast and efficiency as Well as controllable persistence of the dark trace is desired. One method of persistence control is described in my copending application,

Serial No. 451,871, filed concurrently herewith.

It is an object of my invention to provide methods and means for increasing the usable light output from an alkali halide target scanned by a cathode ray beam. It is another object to intensify the lightoutput in pre-determined spectral regions from an alkali halide target and to provide a method of portraying intelligence and of my invention will become apparent when considered in view of the following description and the accompanying drawing, wherein:

Figure 1 shows a cathode ray tube made in accordance with my invention;

Figure 2 shows the characteristics of a, target as shown in Figure 1; and

Figures 3a to 3e, inclusive, are schematic arrangements of a number of structures made in accordance with my invention symbolizing various modes of operation.

I have shown in Figure 1 one type of a cathode ray tube utilizing a target made and operated in accordance with my invention wherein the effect of the electron beam trace on the target may be viewed either by reflected or transmitted light and it should be understood that this showing of a tube is merely exemplary and various other modifications and arrangements may be utilized to an equal advantage as hereinafter explained. Referring to Figure 1, the tube comprises a highly evacuated envelope or bulb l of cylindrical shape with a neck or armsection enclosing a conventional electron gun. The cylindrical portion of the bulb l is provided at one end with a window 2 if the tube is to be utilized for viewing by transmitted light so that a light from a substantially constant light source 3 may be formed into parallel light rays by a lens system 4, projected through the cylindrical portion of the bulb and upon a target 5 which may be supported independently of the envelope wall as shown or deposited directly upon a second window 6. The effects of the trace on the target may be viewed preferably from a position as at 1 although the target may be viewed from a position such as at 1a. Alternatively the trace may be viewed at la utilizing a constant intensity light source 30. developing light projected on the target 5 through the lens system 4a. Although for this position of the light source, the target is preferably viewed on the side thereof scanned by the electron beam such as from the position 1.

Alkali halide targets have a relatively narrow spectral absorption characteristic so that only a portion of a light from a relatively wide spectral range light source is absorbed by the color centers developed in the halide target by the scanning operation. Therefore, it is essential that the light source have a spectral emission band falling at least partially, or preferably entirely, within the spectral absorption band of the halide target so that the differences between the light transmiss'ion or reflection of excited areas and that of nonexcited areas of the target may be distinct.

. The electron gun assembly 8 may be of any one of the conventional types either of the magnetic focus or of the electrostatic focus type as shown. The electron beam developed by the electron gun 8 is modulated in intensity, such as by grid control from a receiver 9 and scanned over the target by horizontal and vertical deflection coils H and V supplied with operating currents of the desired wave form depending upon the type of trace whether of circular, radial or rectangular form.

In accordance with my invention I utilize a light source capable of exciting a luminescent material, or phosphors, to luminescence and I so choose the phosphor material or materials to have a spectral emission characteristic falling within, corresponding to or substantially overlapping the spectral absorption characteristics of the alkali halide target. An alkali halide target of the potassium chloride type has a peak spectral absorption at approximately 5500 A. and in accordance with my invention I provide a luminescent screen exposed to the halide target, the said screen having an emission characteristic which peaks preferably at a corresponding frequency, such as at 5500 A.

Referring to Figure 2 the full line curve represents the absorption characteristic of an alkali halide, such as potassium chloride, the absorption extending over the range from 4900 A. to 7000 A. with its peak at 5500 A. I have shown also in Figure 2 in dashed outline a curve representative of the emission characteristic of a luminescent material, chosen in accordance with my invention, such as an alpha willemite, having an emission spectrum extending from approximately 4900 A. to 6500 A. and peaked at 5230 A.

It will be noted from an examination of the two curves shown in Figure 2 that the spectral emission curve of the phosphor, such as alpha willemite, is-substantially coincident and sub stantially overlaps the spectral absorption curve of the halide'screen. In addition as indicated above for this mode of operation the light source, such as land 3a, should have a spectral emission band including the spectral absorption band of the halide target so that a component of the light from the source 3-3a may be absorbed by color centers developed upon scanning the electron beam over the target. A light source having a predominate-emission in the absorption band and in the emission band of the halide and phosphor respectively is desired. A medium pressure mercury lamp, such as the commercial H-4 type, having substantial emission at 5461 A. and 5790 A. is particularly suitable as a light source although other light sources such as a fluorescent lamp or screen having the required frequency band may be used. However, in one embodiment of my invention hereinafter described, no auxiliary light source, such as sources 3 and 3a is required.

Referring again to Figure l the target 5 may be deposited in one of several manners either on the window 6 of the-envelope I or upon a carrier such as a sheet'of glass, not shown, to support the target 5 clear of the window. More particularly as indicated above the target 5 com prises in addition to the support, not shown, a crystal or layer of crystals ofan alkali halide l0 and a phosphor screen'l2. While 'I have shown in Figurel the halide l0 as being directly exposed to the electron beam developed by the. electron gun 8 and also the'phosphor screen I 2 on the opposite side of the halide target 10 from the electron beam, it will be appreciated hereinafter that these elements may be interchanged so that the electron beam is first incident upon the phosphor screen and becomes incident on the halide target only after complete- 1y penetrating the phosphor screen.

A number of embodiments of my invention are shown in Figures 3a., 3b, 3c, 3d, and 3e, and referring particularly to Figure 3a, I have shown a structure wherein the halide target I0 is directly exposed to the scanning beam. Incidence of the electron beam produces a discoloration or a group of color centers l4 within the halide crystal or crystals which, depending upon the halide characteristics, are dissipated following an interval of time. In the mode of operation shown in Figure 3a, the screen is viewed from the halide side and likewise is subjected to radiation on the halide side. Since the halide has a light absorption characteristic over the portion of the emission band of the source 3, that quantity of light corresponding to the halid absorption band will be absorbed by the color centers l4 so that the halide reflects less light over the color centers area and this area appears dark in comparison with the reflection from adjacent areasof the halide Hl. Since the halide I0 is relatively thin and translucent over these adjacent areas, the light from the source 3 not reflected to the observer at 1 will be incident upon the phosphor screen 12 at all areas except those directly beneath the color centers l4. As indicated above the phosphor is likewise chosen to have an energy absorption or energy conversion band within the spectral emission band of the source 3 such that it may become excited to luminescence. The luminescence, however, is in a spectral band coincident with or at least overlapping the absorption band of the halide; and consequently, light liberated by the phosphor screen I2, may be viewed at the position I except for that light from the screen which is absorbed by the color centers I 4. However, since the color centers of the halide absorb not only a portion of the light from the source 3 over the effective color centers area but also light from the phosphor screen l2, the contrast between the excited area and the non-excited adjacent area is greatly increased.

It is not necessary that the light from the light source be incident upon the viewing side of the target but the light source may be onthe opposite side as shown in Figure 3?: wherein the halide is viewed by wholly transmitted light. Furthermore, it is not necessary that the halide l0 and the screen 12 be contiguous either in that of Figure 31) or in any of the other modifications.

,- Therefore, I have shown in Figure 3b the halide l0 and the screen i2 as being separated b a finite distance IE, it being understood that, in fact, the screen l2 may be located outside of the envelope I with or without intervening lens systems for focusing the light therebetween, In the mode of operation of Figure 3b the color centers l4 not only absorb that portion of the light from source 3a which may be transmitted through the phosphor screen but also the light liberated by the phosphor screen, in both cases this absorbed light being over the absorption range of the halide.

My target combination may likewise be viewed from the phosphor side as shown in Figure 3c.

- Here the light from the source 3a is incident ditent, some oi. this light being absorbed by the color centers so that less is reflected over the color center area to the observer at la. Obviously, this mode of operation is not as efficient as those shown in Figures 3a and 31) because only a minor portion of the light in the absorption band of the col-or centers and over their area is actually absorbed.

A somewhat different mode of operation is examplified by Figure 3d wherein the position of halide l and the phosphor screen I2 is interchanged with respect to the electron beam direction, the beam being first incident upon the phosphor screen. In this mode of operation the velocity of the electron beam must be somewhat higher than for the beam used with the preceding structures so that it may penetrate the phosphor screen and develop color centers in the halide target. Obviously the viewing position may be at Ia, Figure 3d, although the preferred arrangement is to view the target on the phosphor halide side as at I. It will be noted that no auxiliary source of light is shown in Figure 3d, the light from the phosphor screen I2 being the only source. Consequently, somewhat better contrast may be obtained with this mode of operation especially when the point of viewing is on the halide side as at Ia.

A further advantage possessed of all of the modifications shown in Figures 3a3d resides in the fact that the luminescent material comprising the phosphor screen I2 may be such as to have long phosphorescence persistence characteristics wherein the persistence is longer than the persistence of the human eye. The phosphor screen material may thus be made to provide useful illumination of the color-trace screen material in the interval between successive scansions by the electron beam or other excitation means. Furthermore, the rate of decay of persistence of the color centers is directly proportional to the intensity of incident light falling within the absorption band of the color center material. Hence, the decay rate of the color centers is accelerated during the initial strong phosphorescence, and decreases during the later weak phosphorescence. being terminated by the next strong luminescence from the'phosphor screen.

It will be appreciated that the color centers developed under a moving or scanning beam are gradually dissipated as no additional colorcenters are being developed following the movement of the beam to previously non-excited areas.

Consequently the persistence characteristic depends upon the combined delay occasioned b the dissipation of the color centers coupled with the phosphorescence decay characteristic of the phosphor. This combined delay may be accentuated by util z ng a cascade phosphor screen in place of the single layer phosphor screen I2, Such cascade screens are described in my copending application Serial No. 383,893, filed March 18, 1941, and comprise two or more individual phosphor layers of highly phos horesc nt materials having difierent spectral emission characterist cs and different energy absorption charac eristics, such that u on excitation of one laye-rthe light liberated by that layer excites an adjacent layer. In this arrangement of layers on the cascade principle. the period of decay of phosphorescence may be increased and for applications requiring longer decay periods than those obtainable by a single phosphor layer I2, I prefer to utilize a multiple layer as shown in Figure 32. Referring to Figure 3e I have shown two phosphor screens I2 and I5 although a greater number of phosphor screens, wherein the materials are chosen with respect to the teaching contained in my said cope'nding' application, may be utilized. In the arrangement of Figure 3e the phosphor screen I2 may be of material previously described whereas the phosphor screen I5 may be of a luminescent material having an energy absorption spectrum overlapping the emission spectrum of the screen I2 and an emission spectrum of longer wavelength than that of screen 12. In this mode of operation the observer is preferably on the side of screen I 5 as shown at la, the auxiliary light source preferably being as shown at 3. Under these conditions the efiects of the beam trace or color centers are intensified and likewise made of longer duration in that opposite the color centers the light from the screen I2 is absorbed, thereby developing less light in the screen I5 over areas opposite the color centers. Furthermore, the observed trace is more persistent than when determined by the color centers and by the persistence of the screen I2 alone. Obviously the structure of Figure 3e may be operated in the manner wherein the beam penetrates the halide layer It and becomes incident upon the phosphor screen I5 in which case the use of the auxiliary source 3 becomes optional, although this arrangement is not well adapted to operation in accordance with the showing of Figure 31) wherein the halide side of the target is viewed.

Above I referred specifically to alpha willemite as having a spectral emission characteristic suitable for use with potassium chloride as the halide target although it will be appreciated that other phosphors having similar spectral emission characteristics may be used. Although a number of phosphors may be used, further phosphor compositions are cited herewith merely as examples for use with potassium chloride. Copper-activated zinc cadmium sulphides having a molar ratio of zinc sulphide to cadmium sulphide con stituent components of approximately 86/ 14, manganese-activated zinc aluminate, manganeseactivated beta zinc silicate and manganese-activated zinc beryllium silicate, with or without tin as a major constituent, wherein the molar proportions of zinc oxide to beryllium oxide to silicon dioxide as constituents are 4-2-3, this material preferably being thermo-synthesized from the oxides for a period of 60 minutes at 1200 C.

While I have described my invention with respect to matching the emission spectrum of the phosphor or phosphors to the absorption band of the coloration material, such as alkali halide or magnesium oxide, in order to obtain maximum contrast, it 'will be appreciated that the structures shown in the accompanying drawings allow advantageous operation in the case in which the absorption spectrum of the coloration material and the excitation spectrum of the phosphor substantially overlap. In the latter case. the colorations may be produced in an invisible region of the spectrum, such as in ultraviolet light, and may be made visible by projecting light, which will excite the phosphor, through the ultraviolet absorbing color centers area and upon the phosphor or phosphors. The image or images of the ultraviolet absorption areas will appear as dark spots on the luminescent screen. Obviously, the colorations may be in any region of the spectrum having an average wavelength less than the emission peak wavelength of the phosphor as long as the absorption spectrum of the coloration material and the excitation spectrum of the phosphor substantially overlap.

While I have described my invention with particular reference to a halide comprising potassium chloride, it will be appreciated that other halides or other reversibly colorable materials may be used to substantially equal advantage and the principles above set forth as to the choice of the phosphor emission band with respect to the halide absorption band is equally valid for other halides in addition to potassium chloride and for other substances, such as magnesium oxide, which may be similarly reversibly colored by corpuscular or undulatory energy. Consequently, I do not wish to be limited to the specific structures and constituents or to the mode of operation except as specifically set forth and limited in the appended claims.

I claim:

1. The combination comprising an alkali metal halide target and a luminescent screen in light receiving relation with said target, the spectral absorption band of said halide target and the spectral emission band of said luminescent screen being substantially coincident.

2. The combination comprising an alkali metal halide target having a spectral absorption to light of a wavelength between 4900 A. and 7000- A. when subjected to energy, developing color centers therein, and a luminescent screen having a spectral emission entirely within said wavelength range optically exposed to said target whereby the effect of said color centers is accentuated and made more readily visible.

3. The combination comprising a substantially translucent film of an alkali metal halide, a luminescent screen adjacent thereto, said film and said screen having substantially the same spectral absorption and spectral emission bands respectively and a source of light having a spectral emission band substantially the same as the spectral absorption band of said film and positionedto project light upon said screen.

4. The combination comprising a substantially translucent film of an alkali metal halide having the property of developing color centers of stantially the same as the spectral opacity wave length of said film and positioned to direct light through said film upon said luminescent screen and excite said screen to luminescence.

5. The combination comprising a substantially translucent film of an alkali metal halide, a plurality of phosphorescent luminescent screens optically exposed to said film, said screens lying at one side of said film, the screen nearest adjacent the said film having a spectral emission characteristic substantially correspoding in wavelength to the spectral absorption characteristic of said film and the other screen having a spectral emission characteristic outside the said spectral absorption characteristic, each of said screens havinga phosphorescence persistence greater than that of the human eye, and a source of light at the remaining .side of said film having substantially the same emission characteristic as said absorption characteristic.

HUMBOLDT W. LEVERENZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,351,889 Strubig June 20, 1944 2,330,171 Rosenthal Sept. 21, 1943 2,263,149 Vargas, G. Nov. 18, 1941 2,243,828 Leverenz May 27, 1941 2,275,290 Dreyer Mar. 3, 1942 2,152,353 Lewin Mar. 28, 1939 831,591 Ayl'sworth Sept. 25, 1906 2,163,918 Schwartz June 27, 1939 1,448,456 Levy et al Mar. 13, 1923 1,925,546 Sheppard Sept. 5, 1933 2,096,986 Von Ardenne Oct. 26, 1937 2,155,465 Behne Apr. 25, 1939 2,303,563 Law Dec, 1, 1942 2,247,112 Batchelor June 24, 1941 2,306,407 Rosenthal Dec. 29, 1942 2,072,115 Leverenz Mar. 2, 1937 2,243,828 I Leverenz May 27, 1941 FOREIGN PATENTS Number Country I Date 432,432 British July 26, 1935 OTHER REFERENCES Nichols Cathode-Luminescence, Carnegie Institution of Washington 1928, pg. 106 (copy in Div. 54.).

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