US3444413A - Cathode ray tube provided with a continuous phosphor layer for producing indexing signals - Google Patents

Description

May 13, 1969 R. T. HARROLD CATHODE RAY TUBE PROVIDED WITH A CONTINUOUS PHOSPHOR LAYER FOR PRODUCING INDEXING SIGNALS FIG. 2

WITNESSES XMAV' /1)- MAJ 2% Z INVENTOR Ronald T. Horrold BY 2 I 4 CL ATToRN US. Cl. 31392 Claims ABSTRACT OF THE DISCLOSURE A cathode ray tube in which an index signal generating system is provided. The screen structure of the cathode ray tube consists of a display phos hor with a continuous layer of a sensing material disposed on the back surface of the display phosphor. Certain areas of the sensing layer are modified to either activate or de-activate the area to provide a sensing layer with an active sensing pattern.

This invention relates to cathode ray tubes in which an indexing signal is derived from the display screen in order to correlate modulation of the electron beam with the position of impingement of the beam on the display screen.

One particular application of this invention is in the reproduction of color television images in which a sensing signal is derived from the display screen in order to modulate the electron beam with the proper color video information.

One particular sensing color television tube utilizes a display screen made up of a plurality of vertical strips of phosphors imprinted on the face plate of the cathode ray tube and in which the color strips include a plurality of recurring groups, each of the groups including one red emitting, one green emitting and one blue emitting strip. The normal practice is to provide an electrical conductive coating on the back surface of the display phosphor screen and then provide sensing strips on the conductive coating. These sensing strips may be of the type which emit light in response to electron bombardment which is detected by a photosensitive device giving a representative signal of the position of the electron beam in scanning the display screen. A general description of such a color sensing tube may be found in the British Patent 793,351 by Brian Clifford Fleming-Williams.

It is found in the prior art type of color television tube of the type described above that the indexing phosphor has a tendency of creating a beat pattern in the image. This is primarily due to the electron transmission variation between the sensing strips and the intermediate portions and as a result the energy of the electrons after penetrating through the indexing phosphor strip is less or more than that of the electrons penetrating through the intermediate region between the indexing strips and as a consequence a reduction or increase in image intensity is noted on those areas covered by the indexing strips. In view of the fact that the indexing phosphors are quite often spaced at varying intervals with regard to specific color strips so that they may cover a first red color strip but then not cover the next red color strip then, as a result one would obtain a less or more bright color from the first red strip then from the second red strip. In other words a beat pattern or brightness difference pattern will occur and will of course occur for the other colors.

It is accordingly the primary object of this invention to provide an improved sensing structure for a cathode ray tube.

3,444,413 Patented May 13, 1969 It is another object to provide an improved sensing system for a color television tube in which the position of the sensing strips is not noticeable to the human eye.

It is another object to provide an improved method of fabricating a display screen so as to provide a sensing strip arrangement on the back of a display screen such that uniform properties will be obtained across the entire screen as far as transmission of the electron beam through the sensing and intermediate layers and secondary emission therefrom and all other physical and chemical properties.

Briefly, the present invention accomplishes the abovecited objects by providing a uniform layer of material which exhibits the property of radiation in the form of light or secondary electron emission in response to electron bombardment over the back surface of the display screen and then de-activating those portions of the layer that are not required for emission of radiation in response to electron bombardment. Areas not de-activated are referred to as the sensing areas.

These and other objects of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIGURE 1 is a sectional view of a cathode ray tube according to the invention and illustrating an embodiment of the invention;

FIG. 2 is a fragmentary enlargement of a portion of the display screen illustrated in FIGURE 1.

In FIGURE 1 a color television tube is illustrated. The tube includes an envelope 12 having a neck portion 14, a flared portion 16 and a faceplate portion 18. An electron gun assembly 20 is provided in the neck portion 14 of the envelope 12 for generating and directing the electron beams onto a screen structure 22 provided on the inner surface of the face-plate portion 18. The screen structure 22 includes in effect three layers. A first layer 24 of display phosphor is deposited on the faceplate 18. -An electrical conductive layer 26 is provided on the back surface of the display phosphor layer 24. A layer 28 of sensing phosphor is provided on the back surface of the conductive layer 26. A photosensitive detector 30 which is responsive to light emitted from the sensing phosphor layer 28 is provided on the flared wall portion 16.

The display phosphor layer 24 consists of a plurality of vertical phosphor strips R, G and B and may be deposited by techniques well known in the art. This utilizes a type of photographic process. In this process, use is made of a lacquer which is polymerized and hardened under ultraviolet radiation. The lacquer is usually a solution of polyvinyl alcohol in water, sensitized with a dichromate. The first phosphor to be applied to the faceplate is suspended in the lacquer and the resultant slurry is coated uniformly over the tube face. After drying, the coating is irradiated only in those places where the phosphor such as the red strips R are required. Depending on the type of tube, this is done by producing an optical image of a particular negative on the faceplate by means of ultraviolet radiation or by the shadow effect obtained by irradiating a negative with a point source of ultraviolet. These negatives of course contain the appropriate pattern of lines. The lacquer is hardened only at the places irradiated, so that a latent image is produced which is developed in the next operation. This operation is water treatment, the unwanted phosphor being removed by causing the lacquer of the non-irradiated areas to swell up and dissolve. At the irradiated places the lacquer is insoluble and remains adhering to the glass together with the phosphor. The result, after drying, is the first phosphor pattern still mixed with the polyvinyl alcohol. The process is then repeated with a second phosphor such as the green and then again with the third phosphor such as the blue. The irradiation must be done with the other negatives or from other points, since of course the three patterns must be displaced with respect to one another.

Once the display phosphor layer 24 is completed, it is aluminized in the same way as that well known in the black and white art. The phosphor layer 24 is coated with some suitable organic material generally a methacrylate and the conductive layer 26 such as aluminum is then vacuum deposited. The aluminum layer 26 serves, among other things, as a light reflector. Next a nitrocellulose layer is applied to the layer 26 and the coating 28 of a sensing phosphor is deposited on this nitrocellulose coating. Here again, the phosphor coating 28 may be provided thereon by a slurry process so as to provide a uni form layer. This layer is then hardened by ultraviolet irradiation and developed by a water treatment as described previously but this time a continuous even layer of sensing phosphor remains over the screen area. A suitable index phosphor is calcium magnesium silicate, cerium activated, known as P16. A suitable photoresist material such as polyvinyl alcohol in water sensitized with dichromate is then provided over the uniform coating 28 of index phosphor and this photoresist coating may be a lacquer which is polymerized and hardened by ultraviolet radiation in a similar manner as that described with the vehicle used with regard to the display phosphors. The photoresist layer is then exposed to ultraviolet radiation so that the resist layer will remain in only those portions that are irradiated by ultravoilet light. The resulting photoresist coating 36 includes what is known in the art as a phosphor fluorescent killer such as cobalt, magnesium, nickel, cadmium or iron. The exposure to the ultraviolet radiations will be in those areas in which a sensing phosphor is not desired. A suitable additive solution for this resist coating 36 is magnesium chloride, nickel chloride or ammonium dichromate in sufficient quantities. When the tube is air baked, the index phosphor, where the resist coating 36 is, will not emit radiation under electron bombardment. The optical and the electron transmission properties will be the same as the portion of the index phosphor layer 28 which fluoresces. In this manner, the fluorescence of the sensing layer 28 has been killed where fluorescence is not desired. The coating 36 may be fused into the phosphor layer 28. The thickness of the coating 36 is only a few hundred angstroms while the layer 28 may be 250,000 angstroms. The result is a screen in which patterning due to difierent electron transmission characteristics of the index and non-index phosphor areas is substantially eliminated.

It is also obvious that the fluorescent killer could be applied to the aluminum backing prior to depositing the color sensing phosphor. Another possible modification is to include in the resist pattern a dye or suitable material so that the crystals in the areas where the index signals are not required are converted from white to yellow body color for example, so that ultravoilet radiation from the crystals will not penetrate the yellow layer and reach the photosensitive detector 30. The requirement is that the coating 36 is around the phosphor crystals or forms a layer over the crystals. The coating 36 should be transparent to electrons and act as a cutoff filter to wavelengths below about 4,500 angstroms, there being negligible radiation from P16 phosphor at wavelengths greater than about 4,500 angstroms. Different index phosphors would of course require coatings that cut off radiation at different wavelengths.

Where more than one index phosphor is desired, it may be even more difficult to avoid beat patterns as the different index phosphors will probably have different crystal sizes, secondary emission ratios, etc.

The following describes a method to overcome this problem:

(1) The required color line or dot screen is printed by the photoresist or other applicable techniques and filmed and aluminized, etc., in preparation for index phosphor application.

(2) Two index phosphors are mixed together to form a slurry and applied onto the aluminum layer to form a uniform even coating.

(3) Now if the two index phosphors used were short afterglow blue phosphor, such as calcium magnesium silicate, and short afterglow zinc oxide, green phosphor for example, then a pattern of lines or dots as required would be applied with a suitable filter to pass green light and a pattern with a suitable filter for blue light. These materials would act as light filters and be transparent to electrons at the C.R.T. operating potential and remain the same colors after processing. A suitable green filter would be ammonium dichromate and a suitable blue filter is a cobalt chloride applied in suitable amounts by the photoresist technique.

Thus if two photomultipliers were employed as light sensors, one for blue and one for green light, with appropriate filters in front of them, then from the blue dyed areas only information from the blue fluorescing phosphor would be received and similarly from the green dyed areas only information from the green fluorescing phosphor. Of course more than two index phosphors could be mixed and employed with appropriate dyes.

For more complex index screens, various combinations of index phosphor mixes and different filters together with phosphor fluorescent killer resists could be employed, giving for example a pattern in sequence, blue light, green light, no light and so on.

An additional advantage of mixing index phosphors and employing filters instead of separately applying index phosphors is that unwanted signals due to index phosphor cross contamination are avoided; also by carefully selecting the filters for the appropriate cutoff frequency unwanted signals due to index phosphor spectral overlap are avoided.

Various modifications may be made within the spirit of the invention.

I claim as my invention:

1. An electron beam device comprising an electron beam source and a beam interception structure, said beam intercepting structure comprising a display screen consisting of a display layer of material of electron sensitive material capable of emission of light in response to electron bombardment, a substantially light opaque layer disposed upon the back side of said display layer of phosphor material, a continuous sensing layer disposed upon the back side of said opaque layer of electron sensitive material capable of emission of radiation in response to electron bombardment and selected areas of said sensing layer being modified to vary the emission radiation therefrom in respect to the remaining areas of said sensing layer, and means for detecting radiation from said sensing layer.

2. The invention described in claim 1 in which said selected areas of said sensing layer are provided with a coating substantially transparent to electrons and modifying the light emission from said selected areas.

3. The invention in claim 1 in which said selected areas of said sensing layer are de-activated to substantially prevent radiation emission therefrom in response to electron bombardment.

4. A color television display tube comprising a display screen including a display phosphor layer consisting of a plurality of vertical strips representative of three colors, an electrically conductive coating provided on the back surface of said display phosphor layer, a continuous sensing phosphor layer of uniform thickness provided over the back surface of said conductive coating and a photosensitive detector, said sensing phosphor layer having selected areas thereof capable of activating said detector and remaining areas thereof unable to activate said detector in response to electron bombardment of said sensing phosphor layer.

5. A color television display tube comprising a display screen including a first phosphor layer consisting of a plurality of vertical strips representative of three colors and an electrically conductive coating provided on the back surface of said display phosphor and a continuous layer consisting of a mixture of two or more sensing phosphors of uniform thickness provided over the back surface of said conductive coating, said layer of sensing phosphor mixture having selected areas thereof capable of emission of light of selected wavelengths in response to electron bombardment and other selected areas thereof capable of emission of light of different selected wavelengths in response to electron bombardment.

6 References Cited UNITED STATES PATENTS 2,633,547 3/1953 Law 313-92 2,892,123 6/1959 Sunstein 315-10 X 3,041,391 6/1962 Chatten 315-10 X 3,164,744 1/1965 Bowkel.

JAMES W. LAWRENCE, Primary Examiner.

10 V. LAFRANCHI, Assistant Examiner.

U.S. Cl. X.R. 178-5.4; 315-10

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