US2441217A - Process of coating cathode-ray tube screens - Google Patents

Description

y 1948- R. B. WINDSOR 2,441,217

PROCESS OF COATING CATHODE RAY TUBE SCREENS Filed July 9, 1945 2 Sheets-Sheet 1 F|G.l I3

5 Ill 0! [LI & Ill a0 5 Ill l h i I II: u 20 i 4 l E x I I o I n: l0 I i I 0 1 l I I 1 -200 -|00 0 +35 200 300 400 500 TEMPERATURE IN C INVENTQR ROBERT BEACH WINDSOR ATTORNEY May 11,

1948. R. B. WINDSOR 2,441,217

PROCESS OF COATING CATHODE RAY TUBE SCREENS Filed July 9, 1945 2 Sheets-Sheet 2 FIG.3

conoauseo AT 35c p CONDENSED AT -50C z g? l6 m E 53' l2- a. If '6 g so l I I MAXIMUM CONTRAST OF 36OTH. PUL$E(PERCENT) 20 FIG. 4

CONDENSED AT C CONDENSED AT-C 'z' 8 IS 5 I 4- 2 IZ- 0. DJ m a 2 IO 5 8- 56 8 si- 5 I 6 A o 2 z z I TE SE I 2 4 6 8 IO l2 l4 l6 I8 20 22 24 26 28 3O 32 MAXlMUM CONTRAST OF IST. PULSE (PERCENT) ATTORNEY Patented May 11, 1948 -ur OPFFICE' PROCESS F COATING CATHODE-RAY TUBE SCREENS Robert Beach Windsor, Cambridge, Mass, assigner, by mesne assignments, to the United States of America as representedbythe Secretary of War Application July 9, 1945, Serial No. 604,044

2 Claims.

This invention relates in general to cathoderay tube faceplate coating processes, and more particularly to such processes as are applied to so called Skiatrons, or dark trace cathode ray tubes.

The Skiatron is a color trace or dark trace cathode ray tube in which an image appears upon its screen by virtue of the areas of light absorption which are set up by electron bombardment, rather than areas of fluorescence as in conventional cathode ray tubes. The screen of a Skiatron normally consists of a coating of a film of a microcrystalline alkali halide, with potassium chloride being one of the substances which is used extensively. A television projection tube making use of the peculiar properties of microcrystalline alkali halides under simultaneous electron excitation and illumination by light is described in an article in the Proceedings of the Institute of Radio Engineers, volume 28, page 203 (1940) entitled A system of large-screen television reception based on certain electron phenomena in crystals, by A. H. Rosenthal.

One of the characteristics of the alkali halide screen used in the Skiatron tube is that a-narea excited by electron bombardment will partially absorb any light to which it is exposed. Thus external light will cause such an area to show up colored in a manner visible in normal room illumination.

Previous fSkia-tron screens were unsatisfactory because of the long persistence of the coating to repeated excitation. This is undesirable because detection of new signals after the 'bom-' bardment has stopped must wait for the old si nals to die out to some extent. The persistence of the screen for a single signal was satisfactory for some purposes in previous Skiatron tubes,-

one embodiment .of the screen allowing a single signal to be'visible for 5 to seconds.

Among the objects of this invention, therefore, are:

1. To provide a Skiatron screen in which the coating has a relatively long persistence to single signal excitation.

2. To provide a Skiatron screen in which the coating has a relatively short persistence to repeated excitation.

3. To provide a process for applyin a. coating.

such as used-in the Skiatron.

In accordance with the present invention, there isprovided a Skiatron tubewhose facelhaslbeen cooled to from 0 C. to -90 C. by a contiguous that noted when the screen was condensed at room temperatures. Several cups'use'd to evaporate the alkali halide are placed at an optimum angle with respect to the screen and the process of condensation ofthe coatingis made to last the most favorable amount of time. Theangle ofv condensation process;

Fig. 2 is a graph of -t'he'approximate iight reflectance from a'screenoffixed-thickness which has had its' coating process applied at various condensationtemperatures;

Fig. 3 is a graphof the decay of the screen darkness for repeated excitations in screens con;-

densed' at .5'0 C(and at +35 0.; and

Fig. 4 is a graph of the decay .of the screen darkness fora single excitation in screens condensedat 50 andat +35 C.

Referring now'to a'description of the process, it was found after research on many variables that the condensing under controlled conditions of potassium chloride on a Skiatron faceplate after it has beencooled to .a temperature between 0C. and .90 C. :is one desirable embodiment. Fig. 1 shows one arrangement that may be used in the condensation-process. The Skiatron tube in has its race all contiguous to a copper block-l2. This copp r block 12 is cupped to-fit the curvature of the face of the tube. A cup 13 has small amounts of liquid air added to it, and by this means the temperature may be :held constant within i2 C, A--thermocou-ple-in contact with the faceplate of the tube will indicate when the temperature has become stabilized.- evaporator cup t l holdingthe potassium chloride is shown in position; The cup may be held in place-by supportsleading from the Skiat-ron high voltage electrode lead (not shown) In order that the screen have comparative y peated excitations the temperature of the screen.

must be .betWeenO" -.C. and C. while condensationistaking place. The reflectance of the screen, however, which is a measure of the amount of signal lightereflected from it, varies in this temperature region as is shownby the graph .of Fig. .2. This graph gi-ves reflectancegin percent 9f a given'value .for an angle betweenta normal tothe screen and-the direction ofeaim of the evaporator.rcup;of;25. In oneisatisfactory embodiment :of the 'process the screen was Z-hel'd at .a temperature of T5U:C. 'duringthe condensa tion process.

The

Fig. 3 shows the variation in decay of the 360th pulse applied after a quiescent non-excitation conditionof the screen, with 10 second intervals between pulses, as a function of maximum contrast, or maximum coloration, of the 360th pulse for screens condensed at 35 C. and at -50 C. The curves are labeled as to the time after the 360th pulse maximum that the data was taken, and the decay is measured as a percent of a given value. The contrast of the darkening, or coloration, of the screen is defined as (Lo-L) [L0, where L is the light reflected from an undarkened, or uncolored, area, and L is the light reflected from the same area after being darkened. It is seen from this graph that the contrast for repeated excitation of the screen after a given time from the maximum point of the pulse, say 1000 seconds later, is less for screens condensed at -50 C. than for screens condensed at +35 C. The graph of Fig. 3 is for a Skiatron operating temperature of 35 C.

Fig. 4 shows the variation in decay of the first pulse applied after a quiescent non-excitation condition of the screen, as a function of maximum contrast of the first pulse, for screens condensed at 35 C. and at -50 C. The curves are labeled as to the time after the pulse maximum that the data was taken, and the decay is measured as a percent of a given value. It is seen from the graph. that the persistence for single signals, which is satisfactory in screens condensed at 35 0., is not appreciably altered in screens condensed at 50 C. This is shown by the closeness of the two curves for say 100 seconds after the pulse maximum for screens condensed at +35 C. and those condenser at 50 C. The graph of Fig. 3 is for an operating temperature of 35 C.

In judging the amount of alkali halide to be evaporated on the screen, a compromise must be made between reflectance, which is better the thicker the screen, and volume scattering of the light incident upon the screen, which increases with thickness of the screen. If the thickness of the screen is increased beyond a certain point, an undesirable number of comparatively large crystals of halidewill form between the face of the tube and the comparatively thin layer of halide crystals which are excited by the electron bombardment. Hence some of the light impinging on the face of the tube will be scattered by these large crystals, and will be reflected without containing the intelligence imparted to the screen by the electron bombardment.

A type of evaporator cup which has been found to be most satisfactory for a uniform screen is one which may have its evaporation product aimed to a certain extent. For screens condensed at low temperatures, the reflectance increases with the angle between the vapor stream and a normal to the face of the tube. One desirable embodiment of the process entails the use of two evaporator cups aimed so that the minimum reflectance area given by the stream from one of the cups may be covered by the maximum reflectance area from the stream of the other. The angles of aim of the two cups, in this instance, were 90 apart, and 1200 mg. of potassium chloride was used in each cup.

The face of the tube must be kept free from impurities during the condensation process, because potassium chloride tends to form large crystals around small particles, with the result being an undesirable amount of light scattering. If a large amount of persistence is desired, the

4 Skiatron must be operated with the screen at as low a temperature as practicable. Higher operating temperatures increase the. rate of decay of signals from both single and repeated excitations of all Skiatron screens. In order to obtain a desired contrast after a given time when high operational temperatures must be used, it is V necessary to increase the initial contrast, usually done by sending a higher current to the screen. The result is that the pattern becomes less clear due to the larger spot diameter.

A summing up of one combination of conditions for screen condensation for a four-inch tube which give desirable results is as follows:

1. Faceplate held at C.

2. Two evaporator cups used with 1200 mg. of potassium chloride in each, and placed about 90 apart.

3. Each cup aimed at the area of the faceplate directly above the other.

4. Pressure in the tube just prior to condensation less than 10- mm. of mercury. Condensation period for each evaporator cup made to last at least 10 minutes.

While there has been described what is at present considered the preferred embodiment of th invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In the manufacture of cathode ray tubes, the process of producing a reproducing screen within the tube, which process comprises cooling, to a temperature substantially between 0 and 90 0., the screen portion of the tube, and condensing thereon the vapor of' an alkali halide while the screen portion of the tube is held within said temperature range during the entire condensation step whereby the persistence of the screen, under repeated signals, is diminished, and the persistence under single signals is substantially unchanged.

2. In the manufacture of cathode ray tubes,

'the process of producing a reproducing screen within the tube, which process comprises cooling,

to a temperature substantially between 0 and C., the screen portion of the tube, and condensing thereon the vapor of potassium chloride while the screen portion of the tube is held within said temperature range during the entire condensation step whereby the persistence of the screen, under repeated signals, is diminished, and the persistence under single signals is substantially unchanged.

ROBERT BEACH WINDSOR.

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

UNITED STATES PATENTS Number

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