US2874377A - Cathode ray oscilloscopes - Google Patents

Description

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LAWLOR CATHODE RAY OSCILLOSCOPES Feb. 17, 1959 Filed Dec. 1, 1952 INVENTO&

F61 17, 1959 R, c, LAWLQR 2,874,377

, CATHODE RAY OSCILLOSCOPES Filed Dec. 1, 1952 I :s Sheets-Sheet 2 fifiglan 44a IN TENS TY IN V EN TOR.

Feb. 17, 1959 R. c, LAWLOR 2,874,377

w CATHODE RAY OSCILLOSCOPES Filed Dec. 1. 1952 3 Sheets-Sheet 3 no 050 INVENTOR.

United States Patent Ofice i 2,874,377 Patented Feb. 17, 1959 z,s14,s17

csrnonr: RAY OSCILLOSCOPES Reed 0. Lawlor, Alhambra, cnu.

Application December I, 1952, Serial No. 323,395 I as Claims (Cl. 340-369) This invention relates to improvements in cathode ray tubes and particularly to improvements for enhancing the contrast of images formed on the screens of such tubes.

Cathode ray oscilloscopes and similar devices have been developed for generating and erasing images formed on a phosphorescent or other image-retentive screen. Such a system is disclosed and claimed in my co-pending patent application, Serial No. 619,347, filed October 30, 1956, which application is a continuationin-part of a prior application, Serial No. 558,165, filed October 11, 1944, now abandoned, which in turn was a continuation-in-part of my patent application, Serial No. 365,981, filed November 16, 1940, now Patent No. 2,363,600. In such systems, images that are formed by bombardment of the screen with accelerated electrons or otherwise are periodically erased by flooding the screen with suitable radiation such as infra-red radiation. In some cathode ray Oscilloscopes, the persistence of the image 'is varied by controlling the amount of a certain type of radiation, such as infra-red radiation, falling upon the screen while the image is being formed. Both in the cathode ray tubes employed in the aforementioned oscilloscopes and in many other cathode ray tubes, luminescent screens are employed which are affected by short wavelength radiation and by'long wavelength radiation. In both arrangements the contrast in the-image is affected adversely by such radiation from external sources.

One of the objects of the present invention is to provide a cathode ray tube with a filter for increasing image contrast by preventing any undesired external radiation from reaching the image which would reduce the sensitivity of luminescent screen, erase images formed thereon, or excite the screen.

Another obejct of this invention is to provide a cathode raytube in which deexciting energy is prevented from reaching the screen from the front side thereof.

Another object of this invention is to provide a cathode .ray tube in which exciting energy is prevented from reaching the screen from the front side thereof.

Another object of this invention is to improve the contrast of images produced by a cathode ray tube by preventing extraneous radiation from affecting the screen unnecessarily and that will at the same time prevent visible radiation from reaching the screen from the front of the tube and then being reflected to an observer at the front of the tube.

Still another object of my invention is to provide a cathode ray tube employing a cascade screen having a reflecting backing that reflects radiation employed to excite the screen but which is transparent todeexciting energy projected toward the screen from a point behind the screen. v

The foregoing and other objects of the invention will he more readily apparent from a consideration of the various embodiments of the invention illustrated in the accompanying drawings and described hereinbelow.

by the cathode rays impinging In the drawings wherein like reference characters indicate like elements throughout the several views:

Figure l is a longitudinal cross-sectional view of a Fig. 5 is a fragmentary cross-sectional view of a cascade screen to which the invention is applied;

Fig. 6 is a fragmentary side elevation of an alternate embodiment of the invention with some of the parts shown in cross-section;

Fig. 7 is a fragmentary cross-sectional view of'an al ternate embodiment of the invention;

Fig. 8 is aside elevation of an alternative embodiment of the invention;

Fig. 9 is a graph of radiation intensity versus time employed in explaining various forms of the invention; and

Figs. 10, 11, 12 and 13 are graphs representing the spectral characteristics of various parts employed in the invention. 7

In one embodiment of the invention illustrated in Figs. 1 to 4 of the drawings, a cathode ray tube 10 is provided with a luminescent screen 12, a source of erasing radiation 14 behind the screen, and a filter structure 16 in front of the screen. The screen 12 is composed of a phosphorescent material which becomes visibly excited thereon. The cathode ray tube 10 is mounted with its forward end inserted within a felt-lined mounting cylinder 17 supported upon a panel 19 of an instrument case, thus positioning the screenlZ where it is readily seen by an observer. The filter 16 is mounted in the cylinder 17 between the screen 12 and the observer. The luminescent screen 12 is of a type in which visible images are formed thereon by bombardment with cathode says. For example, the

, images may be of the oscillograph trace type in which a beam is caused to scan the screen. under the influence of sweep voltages and signal voltages. or the images may be of. the variable intensity or variable area type produced by modulating the cathode ray beam as it scans the screen 12, in a predetermined pattern.

The image formed by the impinging cathode rays causes the luminescent screen to phosphoresce visibly for a prolonged period, the length of which depends upon the persistence that characterizes the particular screen material of which the screen is composed. In this embodiment of the invention, the screen is also characterized by becoming visibly excited by short wavelength electromagnetic radiation, such as violet rays or long wavelength ultra-violet rays. The relative intensity of radiation emitted when the screen is excited by a given strength of radiation of different wavelengths is indicated by graph X of the excitation spectrum shown in Fig. 10. The relative intensity of radiation emitted at different wavelengths when the screen is' excited either by cathode rays or short wavelength radiation is indicated by the graph M of the emission spectrum in Fig. l0. When once excited, the screen continues toemit such radiation for a prolonged period. This screen 12 is also characterized by becoming deexcited upon exposure to electromagnetic radiation ina range of relatively long wavelengths, such as orange or infra-red radiation. Either the intensity of the image formed on the screen may be decreased more rapidly than otherwise or it may be momentarily increased and then decreased. The relative etficiency with which radiation known and include many of the-zinc sulphide phosphors,

not? including those that contain small portions of copper, together with minute quantities of other impurities.

The deexcitation characteristics of luminescent materials-of thetypes mentioned above may be understood by reference to Fig. 9. In this figure, the intensity of the image formed at any point of thescreen is plotted as a function of time. When a beam of cathode rays of uniform intensity strike the screen for a time interval T the intensity. of luminescence increases rapidly, along-v curve a, asymptotically approaching a saturation value 8. When the cathode ray beam is turned or is directed toward some otherpart of the screen, the intensity of the radiation emitted from the excited area decays along the curve b at a rate depending upon the exact conditions of the excitation by the cathode ray beam and also dependent upon the characteristics of the particular matesource 14 is nergized, while the dash portion b" of curve 1: indicates ow the intensity would continue to diminish if the radiation source 14 were to remain-unenergized.

However, when the radiation source-14 (see Fig. 1) is energized for a period T,, the intensity of the image decreases rapidly along the heavy curve e for certain types of materials and then after the source 14 is deenergized. continues to diminish along the light curve 0''. with this type of material the intensity on the image is less after exposure to radiation from the source 14 than .it otherwise would be if that source had remained -'unenergized.

I With other types of materials, the intensity of the image is enhanced for a limited time when exposed to radiation from the source 14 and then decays rapidly, as indicated by the curve d. After the source 14 is deenergized the intensity continues to decay along the curve d, eventually becoming less than the intensity would otherwise be if the source 14 had not been energized. It is interesting to note that if the time interval T, for which the source 14 is energized-is relatively short, then after deenergization the intensity of the image may be greater than it would have been in the absence of radiation from the source 14, as indicated by the portion of the curve 4 above the curve b. However, if the source 14 remains energized sufficiently long, the intensity of the image may be brought to a level below that of section b" of the curve b before the source 14 is deenergized.

In either event, whether curves c and c" or curve d is followed, the screen becomes deexcited by exposure to radiation from the source 14 and the image previously produced thereon by the cathode rays is erased.

October 11, 1944, now abandoned, there are described arrangements for forming and erasing images. According to the present invention, the contrast of theimages so formed is increased by employment of a filter 16. As indicated in Fig. 2, this filter consists oftwo parts or layers 18 and 20. These two filter elements 18 and 20 may be cemented together or may be separated by an air space, depending upon the characteristics of the particular materials employed.

One of these filter elements 18 is in the form of a filter which transmits visible radiation from the image formed Thus,

- rection to the screen from being transmitted'thcrethrough the excitation effects that would otherwise .be produced by such short wavelength radiation reaching the screen from the front, are eliminated.

The "filter element 18 may be of one of two types. Either it may consist of an ordinary optical filter which transmits radiation in a part of the visible range and absorbs long wavelength radiation includi g infra-red 60 In copending application Serial No. 558,165, filed a 4 radiation and also absorbs short wavelength radiation inin the visible range and to absorb or reflect radiation in the ultra-violet and infra-red regions. Characteristics of suitable filters are indicated in Fig. 11 where T represents the transmission spectrum and U and I represent the reflection or absorption spectra in the short and long wavelength regions respectively. If a semi-transparent layer of gold is employed for this purpose, it absorbs short wavelength radiation such as ultra-violet, transmits visible radiation such as the green, and reflects long wavelength radiation such as the long red and infra-red. In any event, in accordance with this invention, infra-red radiation and ultra-violet radiation from the space in front of the screen are prevented from reaching the screen while visible radiation from the image is readily seen through the filter 16. The image so produced possesses more contrast than the image that would be seen in the absence of the filter element 18.

The second filter element 20 may be an optical rectifier, that is, it may consist of a material which permits radiation to travel in one direction therethrough from in front of the screen but which prevents radiation which travels therethrough to the screen from being transmitted therethrough in the opposite direction after.

regular reflection from the screen 12. Optical rectiflers of that type are sold under the trademark Polaroid" and are manufactured by the Land Corporation. These rectifiers employ certain principles of polarized light to adapt them to prevent light that is transmitted in one diin the posite direction after regular reflection from the screen. ith this arrangement, visible light from a point in front of the screen 12 is highly attenuated, thus minimizing interference. with the image that is undergoing observation and thus enhancing the contrast of the image being viewed.

While this invention is applicable to any cathode ray tube employing luminescent screens which are excited by relatively short wavelength radiation and deexcited by relatively long wavelength radiation, it is particularly applicable to systems such as those described in my aforementioned copending patent applications, which utilize suchlong wavelength radiation to erase images or to control their persistence.

- As indicated in Fig. l, the cathode ray tube of the present embodiment of the invention has a cylindrical head 30 having a face 31 forming a base for the screen 12. The cylindrical head 30 is connected at the rear end thereof by a step 32 to a conical neck 33, connected to a cylindrical shank or leg 34. A terminal connector 35 is mounted at the rear end of the shank 34 opposite the screen. Mounted within the shank 34 and suitably connected to the connector 35 is a source of cathode rays. In addition, electrostatic deflecting plates may be mounted in that shank or magnetic deflecting coils may be mounted around the shank. In either case, the defleeting plates or coils are employed to cause a beam of accelerated electrons, that is, cathode rays, to scan the screen 12 in whatever manner is desired to. produce an image thereon. ,jg;

A first ring 40 of conductive material is'co'a ted onthe interior of the cylindrical portion andv another ring 41 of conductive material iscoated on the interior oi the conical portion 33,- the two rings being interconnected by .narrowstrips 42 of such conductive material coated along the interior of the step 32, thus providing a highly transparent window in the step, that is, a window in which the open space is much larger than the intervening opaque spaces formed by the strips. The two rings 40 and 41 of conductive material form an electrostatic Shield for thebeam directed to the screen 12.

' regions.

a plurality of rearwardly extending resilient fingers 46 that are curved inwardly at their rear ends as shown in Figs. 1 and 4. The clamping ring 44 is preferably coated with felt or other shock-absorbent material on the inner side thereof, both in the solid ring portion and in the finger portions to minimize shock. The radiation source 14 is also provided with two rearwardly directedlegs 48 in which the electrical leads are mounted as shown in Fig. 3..

The radiation source 14 may be of many different forms. For example, if infra-red radiation is being employed for era'singor for control of persistence, the tubular wall of the source may consist of glass which transmits infra-red radiation but absorbs visible and ultraviolet radiation. The transmission and absorption spectra of such a material are represented by the curves A and B respectively of Fig. 12. In any event, for maximum efficiency, the rear side of the source 14 is coated with a layer 50 of silver or other material that is highly refiective in the infra-red region.

The radiation source 14 may be in the form of an incandescent lamp or in the form of a gaseous discharge lamp. The employment of a circular source is particularly advantageous because the magnetic fields produced by current flowing through the lamp have very small or negligible components transverse to the cathode ray beam that scans the screen 12.

In order to produce images of long'persistence, it is sometimes desirable fo employ a cascade screen, that is,

a screen 12 comprising two or more layers 12a and 12b '.tion in a predetermined wavelength range such as in the visible range. A material having the characteristics of the phosphor previously described in connection with Fig. 10 is suitable for the front layer. The employment of cascade screens is particularly desirable because images formed with phosphorescent materials generally persist longer when excited by ultraviolet or other electromagnetic radiation than ode rays.

In another embodiment of the invention illustrated in Fig. 6, the front filter is incorporated in the face of the cathode ray tube. Thus, for example, a filter element 18a having the characteristics of the filter element 18 described above is mounted on the face of the tube and the luminescent screen 12c is deposited directly thereon. In this case, a front filter element in the form of an optical rectifier 20b as described above is also mounted at the front of the tube either secured directly on the filter element 18a or spaced therefrom. Again cascade screens are most suitable where long persistence is desired.

when excited directly by cathmama With this arrangement, infra-red radiation entering the screen from the source 14 is reflected rearwardly by the dichroic surface, thus enhancing any deexciting effects. At the same time, however, any infra-red radiation striking the cathode ray tube from the front is reflected outwardly by the coating 61, preventing such radiation from erasing'or inhibiting phosphorescent images. Also with thisarrangemcnt, any ultra-violet radiation produced by the cathode rays striking the rear coating of the cascade screen is refiected-by-the rear coating 63 and the dichroic coating 61 back into the screen, thus enhancing the effect of this radiation and increasing the excitation of the front arrangement, the dichroic coating 61 prevents extraneous ultra-violet radiation from reaching the luminescent screen from the front of the cathode ray tube. It is thusapparent that with this arrangement, high intensity, high contrast images are produced free of undesired background luminescence excited by external short wavelength radiation and free of undesirable deexcitation from external sources. The contrast may be further enhanced by employing an optical rectifier 20b on the outer side of the front face 31, as illustrated in Fig. 7a.

In Fig. 8 there is illustrated another form of the invention. This form of the invention is similar to the foregoing employing a cathode ray tube comprising a cylindrical head 30b, a conical step 32b, a conical neck 33b and a shank 34b attached to a connector 35b, and a tube face 31b. A filter 16b is also mounted in front of the tube. In this case, however, the screen 12d is composed of a material which at normal temperatures and under normal ambient conditions emits radiation very In Fig. 7 there is illustrated a portion of another form r of cathode ray tubeemploying this invention. In this case, the front face 31. of the tube is provided with a dichroie coating 61 on the interior thereof, and a cascade luminescent. screen 62 and a reflective rear coating 63 are dep titedfin the order named on the interior thereof. The dichroie coating 61 is adapted to reflect radiation in the infra-redr'ange' andin the ultra-violet range, but to transmit radiation in the visiblerange. The rear coating 63 is adapted tq'retlect'radiation in the visible and ultraviolet ranges but to transmit radiation in theinfra-red The luminescent screen 62 is of the cascade type described above.

slowly, if at all, even through a latent image is formed thereon by cathode ray beam excitation. With such a screen, after an image is formed, it is excited by suitable radiation, say infra-red radiation, to deexcite-the screen, thus causing radiation in-the visible range to be emitted, so that the image may be observed. Subsequently, when desired, the deexcitation may be accelerated by radiation in another range, such as orange light, effectively erasing the latent image.

In this embodiment of the invention two tubular radiant energy sources 14a and 14bare employed of the types hereinbefore described, both being mounted behind a the luminescent screen. One of the sources 14a emits infra-red radiation and the other source 14b emits orange radiation, both being of annular tubular configuration and being mounted behind the screen by means of a clamping ring 44a and compound fingers 46a. In this cathode ray tube the screen 12c may also be of the cascade type and filters of the type hereinbefore described may be employed.

Though the invention has been described hereinabove with reference to the employment of a source 14 of radiation for erasing images formed thereon, it is useful in other ways. For example, the combination of screens and filters described are applicable where the source 14 of radiation is energized continuously in order to control the persistence characteristic of the screen. Furthermore, it will be clear that many of the features of the combinations of filters and luminescent screen may be applied in ordinary cathode ray Oscilloscopes in which no such source 14 is present. For example, if the luminescent screen is composed of material which emits visible radiation when excited bycathode rays or by ultra-violet rays and which is deexcited by infra-red few forms of the invention have been specifically def scribed, the invention is not limited thereto but is capable of a wide variety of mechanical embodiments. Various changes which will now suggest themselves to those skilled in the art may therefore be made in the material, form, details of construction and arrangement of the various elements, without departing from the invention. In particular, it is to be understood that the luminescent screens, filters and coatings need not have all of the characteristics described, but that materials having some: of the characteristics described may be combined in accordance with the present invention in order to increase the contrast of images produced with cathode ray tubes. Reference is therefore to be had to the appended claims to ascertain the scope of the invention.

The invention claimed is:

1. In a cathode ray tube, a luminescent screen characterized by emitting radiation in a first wavelength region when excited either by cathode rays or by radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region,

' said screen; afilter in front of said screen characterized by selectively attenuating transmission of radiation ,of such short wavelengthsand such long wavelengths and also characterized by selectively transmitting radiation in said first wavelength region, and a separate source for emitting radiation in said relatively long wavelength region mountedbehind said screen.

- 2; In a' cathode ray tube, a luminescent screen characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region; a source of cathode rays for exciting said screen; a filter in front of said screen characterized by'selectively attenuating transmission or radiation of such long wavelengths and also characterized by selectively transmitting radiation in said first wavelength region; and a separatesource for emitting radiation in said long wavelength region mounted behind said screen.

3. In a cathode ray tube, a luminescent screen characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first rectifier in front of said screen for preventing radiation in said first region that is transmitted therethrough to said screen from being transmitted therethrough in the opposite direction after reflection from said screen; and a separate source for emitting radiation in said second wavelength region mounted behind said screen.

4. In a cathode ray tube, a luminescent screen characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region,

a wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region; a source of cathode rays for exciting said screen; a reflecting coating behind said screen characterized by selectively reflecting radiation in said first wavelength region and also characterized by selectively transmitting radiation in said second wavelength region; and a separate source for emitting r adiation in said second wavev length region mounted behind said screen.

5. In a cathode ray tube, a luminescent screen characterized by emitting radiation in a first wavelength region when excited bycathode rays and by becoming deex-.

cited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region; a source of cathode rays for exciting said screen; a filter in front of said screen-characterized by selectively attenuating transmission of radiation of such long wavelengths and also characterized by selectively transmitting radiation in said first wavelength region; a reflecting. coating behind said screen characterized by selectively reflecting radiation in said first wavelength region'and also characterized by selectively transmitting radiation in'said second wavelength region; and a separate source for emitting radiation in said second wavelength region mounted behind said screen.

6. In a cathode ray tube, a transparent plate, a luminescent screen on the rear side of said plate and characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region; a source of cathode rays for exciting said screen; a filter in front of said screen characterized by selectively attenuating' transmission of radiation of such long wavelengths and also characterized by. selectively transmitting radiation in said first wavelength region; an optical rectifier in front of said screen for preventing radiation in said predetermined region thatis transmitted therethrough to said screen from being transmitted therethrough in the opposite direction after reflection from said screen; and a separate source for emitting radiation in said second wavelength region mounted behind said screen.

7. In a cathode ray tube, a cascade screen comprising a front layer and a back layer, said front layer being characterized by emitting radiation in a first wavelength region when excited by radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region, wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in said first region, said back layer being characterized by emitting radiation in said second wavelength region when excited by cathode rays; a reflecting coating behind said screen characterized by selectively reflecting radiation in said first wavelength region and said radiation in said second wavelength regionand also characterized by selectively transmitting radiation in said third wavelength region; a source of cathode rays for exciting said screen; and a separate source for emitting radiation in said third wavelength region mounted behind said screen.

8. In a cathode ray tube, a cascade screen comprising a front layer and a back layer, said front layer being characterized by emitting radiation in a, first wavelength region when excited by radiation in a second wavelength region and by becoming deexcited. upon exposure to radiation in a third wavelength region, wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in said first region. said back layer being characterized by emitting radiation in said second wavelength region when excited by cathode rays; a source of cathode rays for exciting said screen;

a filter in front of said screen characterizedby selectively attenuating transmission of radiation of such short wavelengths and'such long wavelengths and also characterized by selectively transmitting radiation in said first wavelength region; a reflecting coating behind said screen characterized by selectively reflecting radiation in said first wavelength region and said radiation in said second wavelength region and also characterized by selectively transmitting radiatiqn in said third wavelength region;

and a separate source for emitting radiation in said third wavelength region mounted behind said screen.

9. In a cathode ray tube, a cascade screen comprising a front layer and a back layer, said front layer being characterized by emitting radiation in a first wavelength region when excited by radiation in a second wavelength a dichroic mirror in front of said screen characterized byselectively transmitting radiation in said first wavelength range and selectively reflecting radiation in said other wavelength regions; a reflecting coating behind said screen characterizedby selectively reflecting radiation in said first wavelength region and said radiation in said second wavelength region and also characterized by selectively transmitting radiation in said third wavelength region; and a separate source for emitting radiation in said third wavelength region mounted behind said screen.

10. Ina cathode ray tube, a transparent plate,- a cascade screen on the rear side of said plate and comprising a front layer and a back layer, said front layer being characterized by emitting radiation in a first wave-length region when excited by radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region, wavelengths in .said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in saidfirst region, said back layer being characterized by emitting radiation in said second wavelength region when excited by cathode rays; a source of cathode rays for exciting said screen; a filter in front of said screen characterized by selectively attenuating transmission of radiation of such short wavelengths and such long wavelengths and also characterized by selectively transmitting radiation in said first wavelength region; an optical rectifier-in front of said screen for preventing radiation in said first wavelength region that is transmitted therethrough to said screen from being transmitted therethrough in the opposite direction after reflection from said screen; and a separate source for emitting radiation in said third wavelength region mounted behind said screen.

insaid first wavelength region that is transmitted therethrough to said screen from being transmitted therethrough in the opposite direction after reflection from said screen; a reflecting coating behind said screen characterized by selectively reflecting radiation in said first wavelength region'and' said radiation in said second wavelength region and also characterized by selectively transmitting radiation in said third wavelength region; and a separate source for emitting radiation in said third wavelength region mounted behind said screen.

12. In a cathode ray tube, a screen of luminescent material thereon characterized by emitting radiation in a first wavelength region when excited either by cathode rays or by radiation ina second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region; wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in said first region and a dichroic mirror in front of said screen characterized by transmitting radiation in said first wavelength region and reflecting radiation in said other wavelength regions.

13. In a cathode ray tube, a screen of luminescent material thereon characterized by emitting radiation in a first wavelength region when excited either by cathode Y raw or by radiation in a-second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region, wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in said first region; a dichroic mirror in front of said screen characterized by transmitting radiation in said first wavelength region and reflecting radiation in said other wavelength regions; and a reflecting layer behind said luminescent screen characterized byreflecting radiation in said first wavelength region and radiation in said second wavelength region and by transmitting radiation in said third wavelength region.

14. In a cathode ray tube, a screen of luminescent material thereon characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength 'region being long compared with wavelengths in said first wavelength region, a dichroic mirror in front of said screen characterized by transmitting radiation in said first wavelength region and reflecting radiation in said second wavelength region, and a reflecting coating behind said screen characterized by reflecting radiation in said first wavelength region and by transmitting radiation in said second wavelength region.

15. In a cathode ray tube, a screen of luminescent material thereon characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region, and a transmitting radiation in said first wavelength region region being long compared with wavelengths in said I rectifier in .front of said screen. for preventing. radiation and reflecting radiation in. said. secondwavelength region. g t

16. In a cathode ray tube, a luminescent screen, said screen comprising a front luminescent layer and a back luminescent layer acterized by cmitti radiation in a first wavelength region when excited b radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region, wavelengths in -saidsecond region being short compared with wavesaid first region, said back layer,, being characterized sligereon, said front layer being charation in said other wavelength regions, and a reflecting coating behind said screen characterized by reflecting radiation in said first wavelength region and radiation in said second wavelength region and by transmitting radiation in said third wavelength region.

17. In a cathode ray tube, a luminescent screen, said screen comprising a front luminescent layer and a back luminescent layer thereon, said front layer being characterized by emitting radiation in a first wavelength region when excited by radiation in a second wavelength region, said back layer being characterized by emitting radiation in said second wavelength region when excited by cathode rays, wavelengths in said second wavelength region being short compared with wavelengths in said first wavelength region, a dichroic mirror in front of said screen characterized by transmitting radiation in said first wavelength region and reflecting radiation in said second wavelength region, and a reflecting coating behind said screen characterized by reflecting radiation in said first wavelength region and radiation in said second wavelength region.

18. in a cathode ray tube screen, a transparent plate, a luminescent screen parallel to said plate and charactcrizcd by emitting radiation in a first wavelength region when excited either by cathode rays or by radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region. wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths in said first region, and a filter on the same side of said screen as said transparent plate characterized .by transmitting radiation in said first wavelength region therethrough and preventing radiation in said other wavelength regions from being transmitted therethrough.

19. In a cathode ray tube screen, a transparent plate, a luminescent screen parallel to said plate and charac' terized by omitting radiation in a first wavelength region when excited by cathode rays and by becoming dccxcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region, and a filter on the same side of saidscreen as said transparent plate characterized by transmitting radiation in said first wavelength region therethrough and preventing radiation in said second wavelength region from being transmitted therethrough. I

20. A filterfor a cathode ray tube employing a luminescent screen on the rear side of a transparent plate and characterized by emitting radiation in a first wavelength region when excited by cathode rays or by radiation in a'second wavelength region and by becoming deexcited upon exposure to radiation in a third wavelength region, wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region being long compared with wavelengths insaid first region, said filter comprising a first filter element characterized by transmitting radiation in said first wavelength region but also characterized by preventing transmission of radiation in said second wavelength region and in said third wavelength region, and a second filter element comprising an optical rectifier characterized by transmitting radi ation therethrough in one direction from a source on one side thereof but not in said direction after being transmitted therethrough in the opposite direction and then being regularly reflected from a surface n the same side thereof as such source.

12 21. A filter for a cathode ray tube employing -a luminescent screen on the rear side of a transparent plate and characterized by emitting radiation in a first wavelength region when excited by cathode rays or by radiation in a second wavelength region, wavelengths in said second wavelength region being short compared with wavelengths in said first wavelength region, said filter comprising a first filter element characterized by transmitting radiation in said first wavelength region but also characterized by preventing transmission of radiation in said second wavelength region, and a second filter element com prising an optical rectifier characterized by transmitting radiation therethrough in one direction from a source on one side thereof but not in said direction after being transmitted therethrough in the opposite direction and then being regularly reflected from a surface on the same side as such source.

22. A filter for a cathode ray tube employing a luminescent screen on the rear side of a transparent plate and characterized by emitting radiation in a first wavelength region when excited by cathode rays and by becoming deexcited upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region, saidfilter comprising a first filter element characterized by transmitting radiation in said first wave length region but also characterized by preventing transmission of radiation in said second wavelength region, and a second filter element comprising an optical rectifier characterized by transmitting radiation therethrough in one directionfrom a source on one side thereof but not in said direction after being transmitted therethrough in the opposite direction and then being regularly reflected from a surface on the same side thereof as such source.

23. In a cathode ray tube, a luminescent screen having a characteristic that is altered in a first wavelength region when excited by cathode rays and that is restored upon exposure to radiation in a second wavelength region; a source of cathode rays for exciting said screen; a filter in front of said screen characterized by selectively attenuating transmission of radiation in said second wavelength region and also characterized by selectively transmitting radiation in said first wavelength region whereby altered areas may be distinguished from unaltered areas by viewing said screen through said filter; and a separate source for emitting radiation in said second wavelength region mounted behind said screen.

24. In a cathode ray tube, a luminescent screen having a characteristic that is altered in a first wavelength region when excited by cathode rays and that is restored upon exposure to radiation in a second wavelength region, wavelengths in said second wavelength region being long compared with wavelengths in said first wavelength region; a source of cathode rays for exciting said screen; a filter in front of said screen characterized by selectively attenuating transmission of radiation of such long wave lengths and also characterized by selectively transmitting radiation in said first wavelength region whereby altered areas may be distinguished from unaltered areas by viewing said screen through said filter; and a separate source for emitting radiation of such long wavelengths mounted behind said screen.

25. In a cathode ray tube having an envelope, a luminescent screen mounted within said envelope adjacent one end of said envelope, the material forming said screen having a characteristic that is altered in a first wavelength region when excited by cathode raysand that is restored upon exposure to radiation in a second wavelength region, said end of said envelope being transparent in said first wavelength region whereby alterations in the screen in said first wavelength region are observable through said end of said envelope; means including a source of cathode rays mounted within said envelope for directing a beam of cathode rays at said screen and for causing said beam to scan said screen; and a source for emitting radiation 13 in said second wavelength screen and said cathode ray'source.

26. In a cathode ray tube, a tubular envelopehaving an enlarged face atone end and a small neck at the other end; a luminescent screen mounted within said envelope adjacent said,face,.the material composing said screen having a characteristic that is altered in a first wavelength" region .when excited-by cathode rays and that is restored upon exposure toradiation in a second wavelength region, said end of said envelope being transparent in said first wavelength region whereby alterations in the screen in region mounted between in said second wavelength region encircling said tubular member behind said step.

.connected to said shank by a conical neck and a step,

said first wavelength region are observable through said end of said envelope; means including a source of cathode rays mounted within said neck for directing a beam of cathode rays at said screen and for causing said beam to scan said screemand a source for emitting radiation in'said second wavelength region encircling the axis of said tubular envelope between said screen and said cathode.ray source.

27. In a cathode ray tube comprising a tubular envelope, a luminescent screen mounted within said envelope at one end thereof, the material composing said screen having a characteristic that is altered in a first wavelength region when excited by cathode rays and that is restored upon exposure to radiation in a second wavelength region, said end of said envelope being transparent in said first wavelength region wherebyalterations inthe screen in said first wavelength region are observable through said end of said envelope; means including a source of cath-' oderays mounted within said envelope at a 'position remote from said screen fordirecting a beam of cathode rays at said screenand for causing said beam to scan said screen; an electrostatic shield within said envelope encircling the region between said cathode ray source and said screen, said shield having a window formed therein;

'and a source for emitting radiation in said second wavelength region positioned to radiate through said window onto said screen.

28. In a cathode ray tube comprising a tubular mem her having an enlargedface at one end and a small shank at the other end, said face and said shank being connected by a wall member; a coating of conductive material applied to the interior of said wall member from a point adjacent said face to a point adjacent said shank, said coating forming an electrostatic shield, said coating having a said cylindrical head having a face thereon, a ring of con-1 ductive material coated on the interior of the cylindrical portion of said tubular member and a ring of conductive material coated on the interior of the conical portion, leaving a transparent window in said step; a luminescent screen within said tubular member adjacent said face, the material. forming said screen having a characteristic that is altered in a first wavelength region whenexcited by cathode rays and that is restored upon exposure to radiation in a second wavelength region, said face being transparent in said first wavelength region whereby alterations in the screen in said first wavelength region are observable through said face; means including a source of cath-.

ode rays mounted within said shank for directing 2 beam of cathode rays at said screen and for causing sa' bean: to scan said screen; and an elongated source for emitting radiation in said second wavelength region-encircling said transparent window between the ends thereof, a luminescent screen mounted on said face, said screen having a characteristic that is altered in a first wavelength region when excited by cathode rays and that is restored upon exposure to radiation in a second wavelength region, said face being transparent in said first wavelength region whereby alterations in the screen in said first wavelength region are observable through said face; means including a source of cathode rays mounted within said shank for directing a beam of cathode rays atsaid screen and for causing said beam to scan said screen; and a source for emitting radiation in said second wavelength region positioned to radiate through said window onto said screen.

29. In a cathode ray tube comprising a tubular member having an enlarged cylindrical head at one end and a small shank at the other end, said cylindrical head being connected to said shank by a conical neck and a step, said i cylindrical head having a face thereon, a luminescent of cathode rays at said screen and for causing said beam toscan said screen; and an elongated source for emitting tubular member behind said step for projecting radiation through said window onto said screen.

31. In a cathode ray tube, a transparent plate, a cascade screen on the rear side of said plate and comprising a front layer and a back layer, said'front layer being characterized by emitting radiation in a first wavelength region when excited by radiation in a second wavelength region and by becoming deexcited upon exposure to radiation in athird wavelength region, wavelengths in said second region being short compared with wavelengths in said first region and wavelengths in said third region'beinglong compared with wavelengths in said first: region, said back layer being characterized by emitting radiation in said second wavelength region when excited by cathode rays;

a source of cathode rays for exciting said screem'and a length region mounted behind'said-screem' separate source for. emitting radiation insaid third wave- 32. In a cathode rayhtu esscreemffla transparent-plate, a luminescent screen the rear "side {of 1-,;sstapmsma; characterized by emitting radiation in a t'ljtstwavelengthregion when excited by cathod e rays-and by'becomitig deexcited upon exposure to radiation in; asecon'dwavelen'gthi;

seeondwavelength region beregion, wavelengths in said ing long comparedwith wavel engths'in aid first wave 1' length region, and a filter on same-side of said screen: as said transparent plate characterized;by transaiitting" radiation in said first wavelength region therethrough and-f. preventing radiation in saidsecond wavelength-region) from-being transmitted therethrough.

33. In a cathode ray tube having an envelope a luminescent screen mounted within said envelope adjacent. one end of said envelope, the material forming said screen having a characteristic that is visibly altered inafirst wavelength region when excited by' cathoderays and that is restored upon exposure to radiation in a second-wavelength region, said end of said envelope being-transparent in said first wavelength region whereby alterations in the screen in said first wavelength region are visible through said end of said envelope; means including a source of cathode rays mounted within said envelope for directing a beam of cathode rays at said screen and for'ca'using said beam to scan said screen; and a source for emitting radiation in said second wavelength region mounted between said screen and said cathode ray source.

References Cited in the file ofthis patent UNITED STATES PATENTS 2,124,225 Batchelor "i July 19, 1938 2,247,112 Batchelor June 24, 1941 (Other references on following page) 30. In acathode ray tube comprising a tubular "ntemhaving an enlarged cylindrical head at one end and a small shank at the other end, said cylindrical head being Aronstcin Feb. 8, 1949 Buaigniea May 31, 1949 Rosenthal Sept. 13, 1949 Swcdlund Maylfi, 1950 Blout Oct. 10, 1950 Cage May 15, 1951 Leverenz Aug. 7, 1951

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