GB2121070A - Phosphors - Google Patents

Abstract

A blue phosphor, suitable for use in a cathode ray tube with high intensity electron beam energisation, has the general formula: ZnS.xZnTe.yAl2S3 where x is in the range of 1 x 10<-2> to 8 x 10<-2>, and y is 0 or in the range of 5 x 10<-7> to 5 x 10<-4>, per mol of ZnS.

Description

SPECIFICATION Phosphors This invention relates to phosphors.

The only blue phosphor which will emit light of high brightness by electron excitation in a cathode ray tube, and which has been used to any extent, is ZnS:Ag. Such phosphors have poor linearity of brightness relative to excitation current such that they display a brightness saturation characteristic. In a high brightness cathode ray tube, suitable in particular for colour image projectors, ZnS:Ag is usually employed at a high excitation current.

The brightness saturation occurring in the blue phosphor causes a disorder of the colour balance with the other luminous colours, namely the red and green of the other phosphors.

Recently, it has become desirable to employ in a cathode ray tube or a viewfinder in a video camera a phosphor which can provide high brightness at a low accelerating voltage, for example 6kV, and which displays a fast decay in light emission. Although the phosphor ZnS:Ag provides relatively high brightness at relatively low accelerating voltages, its decay in light emission is relatively slow such that it takes 30 microseconds or so for the luminescent level on the decay curve to decrease to 1/10 of the peak value.

According to the present invention there is provided a phosphor having the general formula: Zn S .xZnTe.yAI2S3 where x is in the range from 1 X 10-2 to 8 x 10-2, and y is in the range from 5 x 10-7 to 5 X 10-4, per mol of ZnS.

The invention also provides a phosphor having the general formula: ZnS .xZnTe where x is in the range from 1 x 10-2 per mol of ZnS.

Phosphors embodying the present invention and described hereinbelow can obviate or at least alleviate the defects inherent in the usual known blue phosphor in that they can be capable of exhibiting a brightness as high as that of the conventional ZnS:Ag by electron beam excitation, yet be able to withstand stronger excitation, and have the characteristic of displaying a fast decay of light emission.

The invention further provides a screen structure arranged to be subjected to electron excitation in an evacuated cathode ray tube, the screen structure comprising a phosphor which emits blue light under electron excitation and forms at least a portion of the screen structure, the phosphor having the formula: ZnS.xiCnTe where x = 1 x 10-2 to 8 X 10-2 per mol of ZnS.

Also, the invention provides a screen structure arranged to be subjected to electron excitation in an evacuated cathode ray tube, the screen structure comprising a phosphor which emits blue light under electron excitation and forms at least a portion of the screen structure, the phosphor having the formula: ZnS.xZnTe.yAl2S3 wherein x = 1 X 10-2 to 8 X 10-2, and y= 5 x 10-7 to 5 X 10-4, per mol of ZnS.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view illustrating rather schematically an apparatus used to produce phosphors embodying the present invention; Figure 2 is a graph of the luminous spectra of phosphors embodying the present invention and of known phosphors; Figure 3 is a table detailing measured results of luminous characteristics of the phosphors embodying the present invention; Figure 4 is a graph plotting luminescent intensity against excitation current for a phosphor embodying the present invention and a known phosphor; Figure 5 is a graph of luminous spectra of phosphors whose preparation is described in various examples described herein as compared with a conventional phosphor;; Figure 6 is a table comparing luminous characteristics of materials embodying this invention with known materials; Figure 7 is a graph plotting various luminous characteristics against firing temperature; Figure 8 is a table illustrating changes in luminous characteristics of phosphors occurring when amounts of added sulphur are changed; Figures 9 and 10 are graphs of luminous spectra of phosphors in which the added amounts of sulphur are changed; Figure 11 is a table setting forth measured results of the relationship between heating conditions and luminous characteristics; Figure 12 is a graph of luminous decay characteristics possessed by the phosphors embodying the present invention; Figure 13 is a table setting forth measured values of luminous characteristics possessed by phosphors embodying the present invention;; Figure 14 is a face view of a projection cathode ray tube employing a phosphor embodying the present invention; Figure 15 is a side elevational view of the tube shown in Fig. 14, partially broken away to illustrate its interior construction; and Figure 16 is a fragmentary view of a cathode ray tube screen embodying the invention that employs a three phosphor screen.

Phosphors embodying the present invention and described hereinbelow have the general formula: ZnS.xZnTe.yAl2S3 where x is in the range of 1 X 10-2 to 8 x 10-2, and y is0 or in the range of 5 x 10-7 to 5 X 10-4, per mole (mol) of ZnS.

Phosphors embodying the present invention can be manufactured using the following method.

A raw material for making up the phosphor defined by the above general formula is combined with sulphur powder in an amount of 0.5 to 2 weight percent for the raw material and then put into the bottom of a furnace tube 2, such as a quartz tube, inserted vertically into a vertical type furnace 1 as shown in Fig. 1. The resultant layer of the raw material 3 is covered by a layer of carbon 4, for example granular activated charcoal or carbon, so as to isolate the material 3 from the air. A lid 5, made, for example, of quartz, is mounted on the tube 2 at its upper open end to close the tube 2 by its dead weight.

This provides a predetermined space 6, within the upper portion of the quartz tube 2, extending between the layer of carbon 4 and the lid 5. The partially filled tube 2, or at least the bottom portion thereof, is inserted into the furnace 1, which is kept in a predetermined heated state or, after the tube 2 is inserted into the furnace, the furnace is heated to fire the raw material 3 at a temperature in the range from 830C to 1 200'C, and preferably from 830'C to 930'C. In Fig. 1, reference numeral 7 denotes generally a heating means for the furnace 1, the heating means 7 affording such a temperature distribution that the portion of the tube 2 which is filled with the raw material 3 is kept at its highest temperature in the direction of the axis of the furnace tube.As indicated, the upper portion of the quartz tube 2 can project above the furnace 1.

If the raw material ZnS contains excess sulphur as compared to the required stoichiometric amount, it can be considered equivalent to the raw material which has been supplemented with sulphur powder. Thus, in this case, the addition of sulphur powder can be omitted.

Phosphors embodying the present invention will now be described in detail. For ease of understanding, a fundamental or basic composition of phosphor embodying this invention has a composition ZnS.xZnTe, that is, y is equal to 0 and x is in the range from 1 X 10-2 to 8 x 10-2. Examples of the luminous characteristics of this basic phosphor will be described in conjunction with a method of manufacturing it and examples of its luminous characteristics.

Example 1 Zinc telluride (ZnTe) of 99.99% purity in an amount corresponding to the factor x in the above general formula was added to 32 grams of conventional zinc sulphide of high purity (luminescence grade), mixed well in a mortar and then put into a container of 100 ml volume made of polyethylene. Balls composed of agate measuring 5 mm in diameter and pure water were added to the mixture in the container in amounts of three times and twice as much as the ZnS, respectively. The material was subjected to a ball mill treatment for extended periods of time, for example, 24 hours. The raw material thus made was subjected to a suction filtration treatment by a vacuum pump or subjected to a forced filtration treatment and then dried at 1 20eC for five hours.After drying, it was combined with 320 mg of 99.999% pure sulphur powder and then mixed sufficiently in the mortar. This mixture was then fired. The firing treatment was performed in a vertical type furnace 1 as shown in Fig. 1. In this example, the mixture was put into the bottom of a closed end quartz tube of 30 mm diameter and 50 mm length. About 1 0g of the granular activated charcoal or carbon 4 was put on top of the raw material 3 so as to isolate the raw material 3 from air or oxygen. A lid 5 made of quartz glass was mounted on the quartz tube 2 at its upper open end. The quartz tube 2 was inserted into the vertical type furnace 1 and kept at 930 C.

The temperature in the furnace 1 was lowered to about 700 C momentarily by the insertion of the quartz tube 2, but was restored to 930etc after five minutes or so. After baking for three hours at 930etc, the quartz tube 2 was withdrawn from the furnace 1 and put into water to be cooled. Thereafter, the material 3 was withdrawn from the quartz tube 2 and non-reactive material adhering to the surface of the material 3 was washed out and removed. About 30g of phosphor was produced.

Curves 11 to 15 in the graph of Fig. 2 show the luminous spectra from the phosphors due to electron beam excitation of 1 5 kV and 4 nA when the amounts of ZnTe in the phosphor were varied in the range from 1 X 10-2 to 8 x 10-2 mol. The curves 11 to 1 5 indicate the luminous spectra generated from phosphors where the amounts of ZnTe (the value x) were 1 x 10-2 mol, 2 X 10-2 mol, 3 X 10-2 mol, 4 X 10-2 mol and 8 x 10-2 mol, respectively. A curve 10 indicates, for the purpose of comparison, the luminous spectrum of the conventional phosphor ZnS:Ag.

Fig. 3 is a table indicating measured results of luminous characteristics (lumen equivalent, energy conversion efficiency, relative brightness, emission peak wavelength and colour coordinates x and y) of these phosphors in which the relative brightness and the energy conversion efficiency are given as relative values with respect to ZnS:Ag, the corresponding characteristics of which are assumed to be 100.

As will be clear from Figs. 2 and 3, as the amount of ZnTe is increased the emission peak wavelengths of the luminous spectra of the phosphors are gradually moved to the long wavelength side. Thus, when the amount of ZnTe is 1 X 10-2 mol, the wavelength is 4140.0 Angstroms. When the added amount of ZnTe is 8 x 10-2 mol, the wavelength is 4950.0 Angstroms. The values of the colour coordinate y in the table of Fig. 3 are increased, whereby the brightness afforded to the viewer's visual sense is increased. In this case, although the energy conversion efficiency is lower, the phosphors with amounts of ZnTe in the range from 1 x 10-2 to 8 x 10-2 can be used in practice.From Figs. 2 and 3 it may be supposed that the added amounts of ZnTe existing in the range from 1 X 10-2 to 2 x 10-2 mol would have the same emission peak wavelength as those of fhe conventional phosphor ZnS:Ag. If the amount of ZnTe is in this range, the brightness thereof cannot be made as high or higher than the conventional phosphor. A curve 8 in Fig. 4 shows the measured results of brightness versus excitation current of the phosphor having an added amount of ZnTe of 2 X 10-2 mol and formed by the above-described manufacturing method. In the graph of Fig. 4, the abscissa indicates a relative value of excitation current and the ordinate indicates the luminous intensity resulting from converting light emitted from each of the measured samples into an electromotive force.In this graph, a curve 9 shows the results obtained from the conventional phosphor ZnS:Ag for comparison. In the graph of Fig. 4, the initial value for brightness of each phosphor was selected to be the same. As will be clear from this graph, the new phosphors have a better linearity of brightness associated with an increase in excitation current than that displayed by the conventional phosphor ZnS:Ag.

Example 2 A manufacturing method similar to that of Example 1 was used, but the firing temperature and the duration of firing were changed to 1 200 C and one hour, respectively.

Fig. 5 is a graph indicating the luminous spectra exhibited by phosphors where the amount of ZnTe was varied in the range from 1 X 10-2 to 8 X 10-2 mol due to electron beam excitation of 14 kV, at 4 nA. In the graph of Fig. 5, curves 21 to 28 indicate the luminous spectra of phosphors having amounts of ZnTe of 1 x 10-2 mol; 2 x 10-2 mol; 3 x 10-2 mol; 4 x 10-2 mol; 5 x 10-2 mol; 6 x 10-2 mol; 7 x 10-2 mol; and 8 X 10-2 mol, respectively. A curve 20 indicates the properties of the conventional phosphor ZnS:Ag for comparison.

Fig. 6 is a table showing the measured results of luminous characteristics of these phosphors. The relative brightness and the energy conversation efficiency are indicated by a relative value based upon the conventional phosphor ZnS:Ag, the corresponding characteristics of which are assumed to be 100. As will be clear from Figs. 5 and 6, as in the case of Figs. 3 and 4, the emission peak wavelength, the value of the colour coordinate y and the relative brightness are increased as the amount of ZnTe increases.

The firing temperatures were 930"C and 1 200 C in Examples 1 and 2, respectively.

Comparison of Figs. 2 and 3 with Figs. 5 and 6 shows that Examples 1 and 2 have the same added amount of ZnTe but have different luminous characteristics. The reason for this is that the method of Example 1 forms a cubic system ZnS which is stable at low temperatures, while the method of Example 2 forms a hexagonal system ZnS which is stable at high temperatures. Since the added ZnTe does not form a hexagonal system but a cubic system with a high temperature firing system, the added ZnTe of a cubic lattice system is difficult to assimilate into the ZnS of a hexagonal lattice system formed by the high temperature firing treatment.

Example 3 The phosphors were prepared by methods similar to those of Example 1, but the amount of ZnTe was fixed at 0.025 mol and the firing temperature was varied from 830"C to 1250'C. Curves 30, 31 and 32 in the graph of Fig. 7 indicate measured results of lumen equivalent, energy conversion efficiency, and the relative brightness possessed by the phosphors treated at the various firing temperatures. The energy conversion efficiency and the relative brightness were determined as a relative value in which each value possessed by the phosphor at a firing temperature 830"C was taken as 100.As shown in the graph of Fig. 7, the lumen equivalent exhibits the highest value when the firing temperature is about 880"C. Thereafter, it decreases rapidly as the firing temperature reaches about 1 030 C, at which temperature the ZnS host in the phosphor is changed from a cubic system to a hexagonal system. The lumen equivalent changes only slightly at firing temperatures ranging from 1 050 C to 11 50'C and decreases significantly as the firing temperature exceeds 1 200 C. The increase and/or decrease of the lumen equivalent depends on the crystal structure of ZnS as a host in the phosphor.In the low temperature region where the system is cubic, the value of lumen equivalent changes substantially, while in the region of high temperature where the amount of hexagonal system is high, the value of lumen equivalent does not change substantially.

The energy conversion efficiency has peak values in both the low and high temperature regions. The maximum value achieved occurs near the range from 1 050C to 1 00oC in the high temperature region. The product of lumen equivalent and the energy conversion efficiency which constitutes the relative brightness exhibits its maximum value at a temperature near 880"C.

The graph of Fig. 7 shows that if the firing temperature is in the range from 830 C to 1 200 C, a relatively high energy conversion efficiency can be obtained. From the point of view of the lumen equivalent and the relative brightness, it is desirable that the firing temperature be in the range from 830 C to 1030"C, and preferably from 830 C to 930"C. It was confirmed that a phosphor formed at a firing temperature ranging from 830"C to 930"C has high relative brightness as compared with the conventional phosphor ZnS:Ag.When the firing temperatures were 980"C and 880"C, the relative brightness was increased to 120% and 182%, respectively.

Example 4 The phosphors were formed by a method similar to that of Example 1, but the amount of ZnTe was held at 0.02 mol and the temperature and duration of firing were fixed at 950"C and three hours, respectively. The amount of sulphur in Example 1 was changed to a range of 0.5 to 5 weight percent for the raw material of the phosphor. Fig. 8 is a table indicating the measured results of luminous characteristics of the phosphors, namely, the lumen equivalent, the energy conversion efficiency, the relative brightness, the emission peak wavelength, the colour coordinates and the relative brightness as compared with the conventional phosphor ZnS:Ag. These values were obtained with amounts of sulphur ranging from 0 to 5.5 weight percent.In the table of Fig. 8, the energy conversion efficiency and the relative brightness are relative values, each of which is assumed to be 100 when the added amount of sulphur powder is one percent by weight.

Figs. 9 and 10 are graphs indicating the luminous spectra of the phosphors produced according to this example. In Fig. 9, a curve 40 indicates the luminous spectrum of the phosphor where no sulphur powder was added into the phosphor raw material. Curves 41, 42 and 43 indicate the luminous spectra of phosphors when 1.0 weight percent, 2.5 weight percent, and 5 weight percent of sulphur powder, respectively, were mixed into the phosphor raw material and fired.

In the graph of Fig. 10, curves 44, 45, 46 and 47 indicate the luminous spectra of the phosphors when 0.5 weight percent, 1.0 weight percent, 1.5 weight percent, and 2.0 weight percent, respectively, of sulphur powder were mixed into the phosphor raw material and then fired. As shown by the curve 40, when no sulphur powder is added the substituted amount of Te in ZnTe for sulphur in ZnS as the host material is increased, the impurity contained in the added ZnTe causes luminescence on the red side thereby decreasing the colour purity of the blue phosphor.

On the other hand, if the amount of sulphur powder is increased, the substituted amount of Te in ZnTe for S in ZnS is decreased thereby shifting the luminous spectrum to the snorter wavelength side of the blue spectrum.

The curves 40 to 47 show that the amount of sulphur added for producing the blue phosphor should desirably be in the range from about 0.5 to 2 weight percent. In this case, when the ZnS used as the raw material contains excess sulphur as compared with the stoichiometric amount, if the amount of excess sulphur is in the range of 0.5 to 2 weight percent, the addition of the sulphur powder can be omitted.

Example 5 Raw material of the same composition as in Example 1 was fired at 930"C for three hours.

The firing was performed on the basis of a quick-heating method wherein the raw material was rapidly heated up to 930"C and a slow-heating method wherein the raw material was gradually heated from room temperature at a rate of increase of temperature of 10 deg C per minute. Fig. 11 is a table showing measured values of luminous characteristics of the phosphors obtained according to the quick-heating method and the slow-heating method, respectively. The table of Fig. 11 shows that the phosphor produced according to the slow-heating method exhibits a relatively large lumen equivalent and energy conversion efficiency as compared with the phosphors produced according to the quick-heating method, but the difference is not large. Consequently, the manner of heating the raw material to reach the firing temperature is not considered important.

Fig. 12 is a graph illustrating the luminous attenuation characteristic, namely the luminescent decay of a cubic-based phosphor sample of ZnS.0.02.ZnTe produced according to the method of example 1. Electron beam excitation was created by a pulse of one microsecond and a frequency of 1 kHz. In the graph of Fig. 12, the abscissa is graduated in microseconds. As shown from the graph of Fig 1 2, it takes three microseconds for luminescence to decay to 1/10 of the peak height of luminescent intensity. With respect to the decay time of 30 microseconds in the case of the conventional ZnS:Ag, the decay time according to the phosphor produced in Example 5 was reduced to about 1/10.

In the aforementioned manufacturing method, if the fired phosphor is washed in sodium hydroxide or potassium hydroxide, the relative brightness could be improved further.

The above-described examples of the present invention provide phosphors which do not cause brightness saturation easily, and retain the fundamental improvements of the system ZnS.xZnTe where x is in the range from 1 x 10-2 to 8 X 10-2 mol, namely, a higher brightness and a much faster decay characteristic as compared with the conventional blue phosphor ZnS:Ag. In particular, the above-described phosphors embodying the invention display an energy conversion efficiency without deteriorating the aforementioned characteristics substantially.

Other phosphors embodying the present invention will now be described by way of another Example.

Example 6 Zinc telluride of 99.99% purity and aluminum sulphate in amounts corresponding to 0.025 mol as x and 5 x 1007 to 5 X 10-4 mol as y in the above general formula were added to 329 of conventional zinc sulphide of high purity (luminescence grade), mixed well in a mortar, and then put into a 100 ml container made of polyethylene. Balls of agate of 5 mm in diameter and pure water were added to the mixture in the container in amounts of three times and twice as much as the ZnS, respectively. These materials were subjected to a ball mill treatment for a long period of time, such as 24 hours. The radv material thus made was subjected to a suction filtration treatment by a vacuum pump or subjected to a forced filtration treatment and then dried at 1 20 C for five hours.After drying 320 mg of 99.999% purity sulphur were added and mixed sufficiently in the mortar. This mixture was then fired. The firing was performed in the vertical type furnace shown in Fig. 1. The mixture was put into the bottom of a single end closed quartz tube of 30 mm in diameter and 50 mm in length.

About 1 0g of granular activated charcoal or carbon was put on top of the material so as to isolate the material from air or oxygen. The lid made of quartz glass was mounted on the quartz tube as its upper open end. The quartz tube was inserted into the vertical type furnace maintained at 930"C.

The temperature in the furnace was lowered to about 700"C momentarily by the insertion of the quartz tube, but was restored to 930'C after five minutes or so. After baking for three hours at 93Q"C, the quartz tube was withdrawn from the furnace and put into water to be cooled. The material was then withdrawn from the quartz tube and non-reactive material which had adhered to the surface of the filled material was washed out and removed. About 309 of phosphor were produced.

Fig. 1 3 is a table indicating measured results of the energy conversion efficiency and the relative brightness as compared with the conventional ZnS:Ag, as well as phosphors obtained by-firing raw material containing no aluminum sulphate. In this case, the energy conversion efficiency was measured using the phosphor without the aluminum sulphate as a reference (100%). As will be apparent from the table of Fig. 13, the addition of the aluminum sulphate improves the energy conversion efficiency, particularly when y= 5 X 10-7 to 2.5 x 10-4. As the amount of aluminum sulphate is increased, the emission peak wavelength of the luminous spectrum is shifted to the short wavelength side.

Thus, although the phosphors with ZnTe alone tend to decrease the lumen equivalent, that is, the relative brightness, phosphors which have been supplemented with aluminum sulphate of less than 5 X 10-4 mol exhibit sufficiently high relative brightness when compared with the conventional ZnS:Ag.

As described above, embodiments of this invention can provide blue phosphors which are difficult to saturate, which exhibit high brightness characteristics at high electron beam excitation, which have fast decay characteristics, and which have high energy conversion efficiency.

In the above-described preferred method of manufacturing phosphors embodying this invention, since the phosphor is fired in a vertical type furnace 1, and the raw material 3 within the furnace tube 2 can be isolated from air by placing activated charcoal or carbon 4 thereon and merely mounting a lid 5 onto the upper open end of the tube 2, handling can be carried out quite easily. More specifically, since the raw material 3 within the quartz tube is covered with activated carbon 4, air in the quartz tube is absorbed by the activated carbon and never reaches the raw material 3.

Furthermore, since there is a space 6 within the tube 2 over the activated carbon 4, upon firing the space 6 is filled with a gas containing sulphur mixed into the raw material 3.

Thus, when firing, the sulphur of the ZnS host material can be prevented from escaping therefrom and the sulphur can be prevented from dropping below the stoichiometric amount. Therefore, it is possible to manufacture phosphors having excellent luminous characteristics with excellent reproducibility.

Figs. 14 to 1 6 illustrate structures in which phosphors embodying the present invention find great utility. Figs. 14 and 1 5 illustrate a video projection tube of the type used in projection systems wherein separate tubes are provided for blue, red, and green emitting phosphors. The signals from each projection tube are passed through suitable lenses and focused on to a projection screen in proper synchronisation.

In Figs. 14 and 15, reference numeral 51 indicates generally a projection tube having a dace 52. The tube itself includes a cylindrical neck portion 53 which merges into a conical section 54 terminating in the face 52. A phosphor layer 55 of the type described hereinabove is applied to the inner surface of the face 52. The layer 55 receives energisation by electron excitation from an electron gun 56 located in the neck portion 53.

Blue phosphors embodying the present invention are also useful in television type tubes including three types of phosphors, regardless of the geometrical arrangement of the phosphors. In this respect, a particular structure shown by way of example in Fig. 16 includes strips 57, 58 and 59 of red, blue and green phosphors, with guard bands 60 of substantially the same width as the strips 57 to 59 disposed therebetween. In this instance, the blue phosphor strip is composed of a phosphor embodying the invention as described hereinabove.

Claims (14)

1. A phosphor having the general formula: ZnS.xZnTe.yAI2S3 where x is in the range from 1 x 10-2 to 8 x 10-2, and y is in the range from 5 x 10-7 to 5 X 10-4, per mol of ZnS.

2. A phosphor according to claim 1, wherein y is in the range from 5 X 10-7 to 2.5 x 10-4 mol.

3. A phosphor according to claim 1, wherein y is approximately 5 X 10-7 mol.

4. A phosphor according to claim 1, wherein y is approximately 5 x 10-6 mol.

5. A phosphor according to claim 1, wherein y is approximately 5 x 10-5 mol.

6. A phosphor according to claim 1, wherein y is approximately 2.5 X 10-4 mol.

7. A phosphor according to claim 1, wherein y is approximately 5 X 10-4 mol.

8. A phosphor having the general formula: ZnS.xZnTe where xis in the range 1 x 10-2 to 8 x 10-2 per mol of ZnS.

9. A phosphor substantially as set forth in any one of Examples 1 to 6 hereinabove.

10. A screen structure arranged to be subjected to electron excitation in an evacuated cathode ray tube, the screen structure comprising a phosphor which emits blue light under electron excitation and forms at least a portion of the screen structure, the phosphor having the formula: ZnS.xZnTe where x = 1 x 10-2 to 8 x 10-2 per mol of ZnS.

11. A screen structure according to claim 10, which is part of a video projection tube.

1 2. A screen structure arranged to be sub jected to electron excitation in an evacuated cathode ray tube, the screen structure com prising a phosphor which emits blue light under electron excitation and forms at least a portion of the screen structure, the phosphor having the formula: ZnS.xZnTe.yAl2S3 wherein x = 1 x 10-2 to 8 x 10-2, and y=5x 10-7 to 5X 10-4, permol of ZnS.

13. A screen structure according to claim 12, which is part of a video projection tube.

14. A screen structure substantially as herein described with reference to Figs. 14 I and 1 5 or Fig. 1 6 of the accompanying drawings.