US3278326A - Method of coating fluorescent layer of electron discharge tube - Google Patents

Description

Oct. 11, 1966 J, CGEE 3,278,326

METHOD OF COATING FLUORESCENT LAYER OF ELECTRON DISCHARGE TUBE Original Filed May 2, 1962 BVOLS/OA United States Patent 3,278,326 METHOD OF COATING FLUORESCENT LAYER OF ELECTRON DISCHARGE TUBE James Dwyer McGee, London, England, assignor to National Research Development Corporation, London, England Original application May 2, 1962, Ser. No. 191,844. and this application July 7, 1965, Ser. No.

Claims priority, application Great Britain, May 4, 1961,

36/61 Claims. c1.117-33.s

This application is a division of my copendin-g application Serial No. 191,844, filed May 2, 1962 now abandoned.

This invention relates to electron discharge tubes employing a phosphor screen which is backed by a thin aluminum film.

It is well-known technique in such electron discharge tubes to provide a fluorescent screen by applying a thin layer of crystalline phosphor powder to the output endface of the tube, through which end-face the phosphor screen is viewed. A thin, electron-transparent but optically nearly opaque aluminum film is then applied to the phosphor screen surface remote from the tube end-face.

Such an aluminum backing film is intended to have three functions.

First, when the phosphor screen is excited by incident electrons, some light is emitted from the screen in the direction away from the observer. An aluminum backing layer reflects such light back to the direction in which the screen is viewed. Provided that the aluminum layer is in intimate contact with the phosphor layer, little light dispersion occurs and image definition is but little impaired.

Second, as the aluminum layer is nearly opaque, light is substantially prevented from leaving the back surface of the phosphor screen. Otherwise, :such light, if presented in a tube using a photocathode, would result in a spurious, background illumination of the photocathode and unwanted consequent photo-electron emission. In a cathode ray tube, such light would be scattered back to the phosphor screen and thereby reduce image contrast.

Third, the aluminum film serves as an electrically conducting electrode to maintain the screen potential uniform over its area. In the absence of an aluminum backing film, the screen potential has to be maintained by secondary emission from the screen itself. This process is unreliable because, at not very high potentials, the secondary emission coeflicient may already have fallen to less than unity.

The conventional aluminum film backing, as provided by the conventional method, is subject to disadvantages which are discussed in detail below.

The object of the present invention is to provide an improved method of applying an aluminum backing film to a phosphor screen and an improved electron discharge tube having an aluminum backing film thus applied.

Accordingly, one form of the invention provides an electron discharge tube having a fluorescent layer disposed on a transparent carrier and first and second electron-permeable backing layers disposed in succession on the face of the fluorescent layer remote from the carrier, the first backing layer being a discontinuous, high lightreflecting, metallic layer in intimate local contact with said fluorescent layer and the second backing layer being, continuous and electrically conductive.

Another form of the invention provides a method of providing light-reflecting and electrically-conductive backing layers for the fluorescent layer of an electron tube as described above, comprising evaporating a first metallic backing layer onto discontinuous areas of said fluorescent layer and subsequently disposing a second, continuous, metallic, backing layer over the first backing layer.

In order that the invention may readily be carried into effect, the problem with which the present invention deals and a preferred embodiment of the invention, by way of example, will now be described with reference to the accompanying drawings, of which:

FIG. 1 is a diagram showing, not to scale, a small section of the end-face of a known electron discharge tube, and

FIG. 2 is a similar diagram showing a variant provided by the present invention.

FIG. 1 shows a small section of the glass end-face 1 of the envelope of an electron discharge tube. On the inner surface A of the end-face 1 is deposited a crystalline phosphor screen 2. On the inner face of the phosphor screen 2, is provided a thin aluminum film 3.

I By way of example, the screen 2 is viewed by a photographic emulsion layer 4, arranged in close contact with the outside surface B of the tube face 1.

The path of an incident electron is shown by the full line 5. This line is produced through the tube face 1 and through the emulsion layer 4 by a dotted line 5. A divergent path, from the phosphor screen 2 to the emulsion layer 4, of a ray of light produced by an electron incident along the path 5, is shown by the dash-line 6.

The conventional method of applying the aluminum backing film 3 is as follows. After the phosphor for the screen 2 has been settled on the inner face of tube 1, a thin film of organic material is laid down in very intimate contact with the phosphor screen. When dry, the tube is evacuated and a layer of aluminum metal about 0.1 thick is evaporated onto the continuous organic film. Thus, the thin aluminum film is continuous and highly reflecting. It is almost opaque optically and is a good conductor.

Air is then admitted to the tube and it is backed to a temperature of about 350 C., when the organic film material is oxidised to gas and substantially all removed from the space between the screen 2 and the aluminum film 3. The aluminum film 3 is thus left in very intimate contact with the screen 2 and should perform all the functions required of it, as enumerated above.

However, it is found in practice that it is difiicult to remove all the organic material. A residue is left which either impairs the reflection efficiency of the aluminum film or absorbs some of the energy of the incident electrons, or usually both. This reduces the efficiency of the screen by a serious factor, of perhaps 40%.

This reduction in efliciency has been proved by comparing the efliciency of a screen prepared with an aluminum backing film 3, by the conventional method described, with that of a screen for which the aluminum backing film 3 was prepared separately, kept very clean and highly-reflecting and then floated over the phosphor screen 2 and settled on it. Other things being kept constant, a screen prepared by this latter method is some 50% more eflicient than a screen prepared by the conventional method.

This latter method is not a practicable method, however, since although this aluminum film prevents light leaving the inner surface of the screen 2 and also acts as a good electrical conductor, it causes dispersion of light reflected by it.

The reason appears to be that the film 3 cannot be laid on the screen 2 in sufficiently intimate contact with the screen surface and hence, because of a gap between the phosphor screen 2 and the reflecting aluminum film 3, the reflected light is able to spread laterally by an amount that is sufficient to impair the image definition.

It is not practicable to evaporate the aluminum film 3 directly onto the surface of phosphor screen 2, because the granular nature of the screen prevents a coherent film from forming. The aluminum film so formed is consequently not satisfactory for preventing light leaving the back surface of the screen 2 or for maintaining uniform the electrical potential of the screen.

Furthermore, the aluminum, if evaporated normal to the surface of the screen 2, tends to penetrate between phosphor grains and reach lower levels of the screen 2 where, by absorbing light, it can do more harm than good.

The reason for the deterioration of the fluorescent im age produced by the screen 2, as viewed by the photographic emulsion layer 4, is also shown in FIG. 1.

If fluorescent light, produced by an electron incident along the path 5, were to continue in the same direction, it would travel along the path and be absorbed by the emulsion layer 4 at, say, a point X. The distance travelled by the light ray from the point of its generation to the point of its use is indicated in the figure by the distance A. This distance is largely determined by the thickness of the glass end-face 1. This thickness is naturally reduced to the minimum value consistent with the mechanical strength of the tube envelope as a whole.

Any divergence of the light ray from the path 5', such as by the path 6 shown, results in the light ray being utilised at the point X. The resultant dispersion of the light image in the emulsion layer 4 is indicated by the distance B.

The improved method of applying an aluminum backing film to a phosphor screen will now be exemplified with reference to FIG. 2, in which figure corresponding parts to those of FIG. 1 are indicated by the same reference numerals.

The inner surface of the end-face 1 of the tube is prepared and the crystalline phosphor screen 2 deposited thereon in known manner.

A layer of aluminum is next evaporated directly onto the inner surface of the phosphor screen 2, without an intermediate organic film, in such a way that it reaches substantially only the upper surfaces of the surface layer of crystals of the screen 2. This is done according to a preferred method, by evaporating from several separate sources, for example, source A and source B, of aluminum from, in this example, four directions separated by 90 and directed at a small angle to the surface, in this case less than 30.

Thus, a large part of the upper surfaces of the crystals of screen 2 are coated with a very clean efficient reflecting aluminum layer which will perform the light-reflecting function very efliciently. However, it is not a continuous conducting layer, and it does not suppress all the backwardly-directed light. This discontinuous, intimate aluminum layer on the upper surfaces of the phosphor crystals is shown by the broken heavy line 7 in FIG. 2.

It is estimated that this aluminum reflecting layer 7 will reflect some 80% of the reversely-directed fluorescent light from screen 2 back into the required direction for viewing with, possibly, less unwanted lateral spread than when the conventional aluminium backing method is used.

To provide for the requirements mentioned of preventing light from leaving the inner face of screen 2 and of providing a continuous conducting layer for main taining uniform the screen potential, a further film 3' than when the conventional aluminum backing method is deposited on top of the film 7, either by the conventional technique described or by the floating technique described above. This provides a continuous conducting aluminum film which prevents light from leaving the screen 2 in the reverse direction. Even if the film 3' is a poor reflector this fact will not greatly matter because the film 3' is only required to reflect a small proportion, say 20% of the total backwardly-directed light.

The floating-on technique described, which provides the continuous aluminum film 3' directly onto the discontinuous layer 7, is preferred to the conventional evaporating method, which would require first an intermediate organic layer to be provided on top of the layer 7, since the floating on method provides a very clean film free from residual organic matter which would result from the evaporating method and which would absorb energy from the incident electrons.

The method according to the invention for applying the composite aluminum backing film 7, 3' is considered to have limited importance only as applied to cathode ray tubes, but to have great importance as applied to electron image intensifying tubes, particularly tubes employing several stages of image intensification in cascade in the one tube.

I claim:

1. A method of providing electron-permeable backing layers for a crystalline fluorescent layer disposed on a transparent carrier in an electron discharge tube, said method comprising the steps of evaporating a metallic material from a plurality of spaced sources disposed at a small angle to the fluorescent layer surface to form a discontinuous, high light-reflecting, first backing layer solely on the surface of said crystals remote from the carrier, and subsequently disposing a second continuous planar, metallic, backing layer over and in local contact only with said first backing layer.

2. A method according to claim 1, in which each of said two backing layers consists of aluminum.

3. A method according to claim 1, using four said spaced sources separated by and directed to the fluorescent layer surface at an angle of less than 30.

4. A method according to claim 1, in which a thin layer of organic material is disposed on said first backing layer, said second backing layer is evaporated onto the surface of the organic material layer and the organic material is subsequently removed by heating.

5. A method according to claim 1, in which said second backing layer is formed apart from said fluorescent layer and is floated over and settled onto the fluorescent layer and said first backing layer.

References Cited by the Examiner UNITED STATES PATENTS 2,960,416 11/1960 Reed 313-92 X 3,058,842 10/1962 Kahan 117107 3,087,085 4/ 1963 Turner 313-92 3,095,319 6/1963 Williams 117-107 3,141,106 7/1964 Kapany 313-92 ALFRED L. LEAVITT, Primary Examiner.

MURMY KATZ, Examiner.

A. H. ROSENSTEIN, Assistant Examiner.

Claims (1)

1. A METHOD OF PROVIDING ELECTRON-PERMEABLE BACKING LAYERS FOR A CRYSTALLINE FLUORESCENT LAYER DISPOSED ON A TRANSPARENT CARRIER IN AN ELECTRON DISCHARGE TUBE, SAID METHOD COMPRISING THE STEPS OF EVAPORATING A METALLIC MATERIAL FROM A PLURALITY OF SPACED SOURCES DISPOSED AT A SMALL ANGLE TO THE FLUORESCENT LAYER SURFACE TO FORM A DISCONTINUOUS, HIGH LIGHT-REFLECTING, FIRST BACKING LAYER

Images