GB2102197A - Current-sensitive colour cathode ray tubes - Google Patents

Abstract

A current-sensitive colour cathode ray tube has a phosphor screen 4 scanned by an electron beam from an electron source 5, the current density of the beam being varied at a substantially constant accelerating voltage. The screen 4 comprises a red- emitting phosphor having a sub-linear luminance to current density characteristic, and a different colour- emitting phosphor other than red having a super-linear or at least linear luminance to current density characteristic. The red-emitting phosphor has the formula: (Ln1-xEux)2O2S where Ln is Y, La, Gd or Lu, and x is in the range from 0.05 to 0.10. The concentration of rare-earth impurity such as terbium Tb and praseodymium Pr is less than 10 ppm. In an example 35 wt % of red- emitting Y2O2S:Eu is mixed with 65 wt % of green-emitting (Zn0.64Cd0.36)S:Ag, Ni and coated on a CRT faceplate. Alternatively the phosphors may be applied in separate superposed layers. <IMAGE>

Description

SPECIFICATION Current-sensitive colour cathode ray tubes This invention relates to current-sensitive colour cathode ray tubes.

In a colour cathode ray tube as used in an ordinary colour television receiver, there is located adjacent to its phosphor screen a means such as a shadow mask or aperture grill which functions to determine the landing position of an electron beam on the phosphor screen. In this way the electron beams corresponding to the respective colours land on the respective colour phosphor dots or stripes of the phosphor screen thereby to cause respective coloured light emissions and hence reproduce a colour picture on the phosphor screen.

On the contrary, in a current-sensitive colour cathode ray tube, no means is provided to determine the landing position of the electron beams, but the phosphor screen is formed of at least two colour phosphors, which are mixed and then coated. Each colour phosphor emits coloured light within a predetermined limited range of colour in dependence on the beam current density.

Since such a current-sensitive colour cathode ray tube has no means to determine the electron beam landing position, it is lighter in weight, its manufacturing and assembling processes are simpler and its resolution is improved. Moreover, mislanding caused by the relative positional displacement of the phosphor screens and the means to determine the electron beam landing position is avoided.

A previously proposed current-sensitive colour cathode ray tube includes a phosphor screen made of mixed colour phosphors including a green phosphor showing a so-called sub-linear characteristic represented by a curve 1 in the graph of Figure 1 of the accompanying drawings, which shows the luminance characteristic of the phosphor relative to the electron beam current density, and a red phosphor showing a so-called super-linear characteristic represented by a curve 2 in the graph of Figure 1. In this case, within a range where the beam current density is small, the green emission is dominant, while within a range where the beam current density is large, the red emission is dominant, so a colour image having colours synthesized from these two colours in response to the beam current density is produced.

For the green-emitting phosphor with the sublinear characteristic, phosphor of Zn2SiO4:Mn is usually used, while as the red-emitting phosphor with the super-linear characteristic, (CdZn)S:Ag,Ni is usually used. To make the greenemitting phosphor Zn2SiO4:Mn have the required sub-linear characteristic, it is necessary to make the amount of Mn small. However, when the amount of Mn is small, burning by the beam irradiation tends to occur. On the other hand, to obtain the super-linear characteristic of the redemitting phosphor (ZnCd)S:Ag,Ni, the amount of Cd must be increased. However, when the amount of Cd is increased, the luminance becomes low and the colour purity also becomes low. In practice, therefore, the range of colours that can be reproduced by the colour cathode ray tube is relatively narrow.

According to the present invention is provided a current-sensitive colour cathode ray tube having a phosphor screen scanned by an electron beam from an electron source, the current density in said beam being varied at a substantially constant accelerating voltage, said screen comprising a red-emitting phosphor having a sub-linear characteristic in the luminance to current density relationship and a different colour-emitting phosphor other than red having a super-linear or linear characteristic in the luminance to current density relationship, wherein said red emitting phosphor has the following formula: (Ln1-xEux)2o2s where Ln is Y, La, Gd or Lu, and x is in the range from 0.05 to 0.10, the concentration of rare earth impurity such as terbium Tb and praseodymium Pr being less than 10 ppm.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like references designate like elements, and in which: Figure 1 is a graph showing a beam current density versus luminance characteristic and used to explain a previously proposed current-sensitive colour cathode ray tube: Figure 2 is a schematic diagram showing an embodiment of current-sensitive colour cathode ray tube according to the invention; Figures 3, 5, 9, 10 and 11 are graphs showing beam current density versus luminance characteristics; Figure 4 is a chromaticity diagram; Figure 6 is a graph showing a measured curve representing the relation between the amount of Eu in a red-emitting phosphor and the luminance thereof; Figure 7 is a graph showing the amount of Eu and the chromaticity value; and Figure 8 is a cross-sectional view of the phosphor screen used in the cathode ray tube of Figure 2.

In general, in embodiments of the invention, by using the combination of one colour-emitting phosphor, which has a sub-linear current density versus luminance characteristic and another colour-emitting phosphor which has a superlinear or at least a linear current density versus luminance characteristic, there is generated light emission with a hue or colour range represented by the synthesized colour of the light emitted from both the colour phosphors, by controlling the current density, for example, by modulating the cathode current.

In embodiments of the invention, as the phosphor with the sub-linear characteristic a redemitting phosphor, which is high in colour purity and has the composition formula of (Ln,~xEux)202S is used. In this formula Ln is formed of at least one of the group consisting of yttrium Y, lanthanum La, gadolinium Gd and luletium Lu, and x is selected to satisfy the condition of 0.05 is less than or equal to x is less than or equal to 0.10. Moreover, in this redemitting phosphor, rare-earth impurities such as terbium Tb and praseodymium Pr are less than 10 ppm.

The phosphor screen used in embodiments of colour cathode ray tube according to the invention may be made by mixing this redemitting phosphor with the sub-linear characteristic with another colour, for example, a green-emitting phosphor or a blue-emitting phosphor with a super-linear or at least linear characteristic, and the mixed phosphor is formed as a layer, or each phosphor is respectively and sequentially formed as a layer.

The reason why the value of x in the redemitting phosphor (Ln,,Eu,),O,S is selected to satisfy 0.05 is less than or equal to x is less than or equal to 0.10, is that if x is less than 0.5 the colour purity is lowered and yellowish light emission is caused, while ifxexceeds 0.10, the luminance becomes low. Moreover, the reason why the rare-earth impurity concentration is less than 10 ppm is that when this impurity concentration exceeds 10 ppm, the luminance is difficult to saturate with a high beam current density and hence the sub-linear characteristic is not achieved.

Now, some specific examples will be explained.

Example 1 The red-emitting phosphor is made as follows: Rare-earth oxide Ln2O3 and europium oxide Eu203 are mixed in predetermined amounts to prepare a mixed rare-earth oxide; to the mixed rare-earth oxide is further added sulphur S at 30 to 80 weight %, and sodium carbonate Na2CO3 at 30 to 80 weight %, which serves as flux, and they are mixed; and the mixture is then burned in air at 900 to 1 3O00C for 0.5 to five hours to form a phosphor represented by the composition formula (Ln,~xEux)202S. By a hydraulic elutriation method, small size particles are removed from the phosphor and, by use of a sieve, large size particles are removed from the phosphor. Greenemitting phosphor (Zn084Cd0,36)S:Ag,Ni which contains 50 ppm of Ag and 10 ppm of Ni is-also prepared.Then, 35 weight % of the red-emitting phosphor and 65 weight % of the green-emitting phosphor are mixed.

Then, the mixed phosphor is coated on the inner surface of a panel 3a of a cathode ray tube envelope 3 by a sedimentation method to form a phosphor screen 4 as shown in Figure 2. An electron gun 5 will emit an electron beam at the phosphor screen 4 and horizontal and vertical deflection means 6 are provided. This currentsensitive colour cathode ray tube has no means to determine the landing position of the electron beam on the phosphor screen 4.

Figure 3 is a graph showing the measured current density versus luminance characteristics of the phosphor screen 4 for the case of Example 1 in which the red-emitting phosphor is Y202S: Eu. In the graph of Figure 1, a broken line curve 11 represents the current density to luminance characteristic of the red light emission by the redemitting phosphorY2O2S:Eu, a solid line curve 12 shows that of the green light emission by the green-emitting phosphor (ZnO.64CdO.36)S:Ag,Ni, and a one-dot chain line curve 13 represents the current density to luminance characteristic of this phosphor screen, namely the total luminance versus current density characteristic of respective coloured lights of the colour cathode ray tube.As will be apparent from the graph of Figure 3, the red-emitting phosphorY2O2S:Eu has a sub-linear characteristic while the green-emitting phosphor (ZnO.64CdO.36)S:Ag,Ni has a super-linear characteristic.

Now, the colour range of the light emission by the red and green-emitting phosphors will be considered with reference to the graph of Figure 4 which is a MacAdam's u,v chromaticity diagram.

In the graph of Figure 4, reference letters R, G and B respectively shows the chromaticities of red, green and blue. In this case, the line R,G, from a point R1 to a point G-shows a colour gamut within which the light emission of the colour cathode ray tube with the phosphor screen made by the mixture of the red-emitting phosphor Y2O2S:Eu and the green-emitting phosphor (ZnO 64CdO.36)S:Ag,Ni according to Example 1 is possible.In this case, in dependence on the beam current density, namely as the beam current density becomes large, the hue of the light emitted goes to the point G, in the line R,G,, while as the beam current density becomes low the hue of light emitted goes to the point R. In the graph of Figure 4, letters R,' and G,' respectively designate chromaticity points of red and green colours of the previously proposed current-sensitive colour cathode ray tube mentioned previously. In this case, the colour gamut which can be reproduced rests on the line connecting the points R1, and G1,.As will be apparent from the comparison of the lines R,G, and R1'G1', the colour gamut of the embodiment is wider, since the line RG, is longer than the line RWGW ,' and hence the colour purity, especially of the red colour is improved.

In the graph of Figure 5, a solid line curve 21 shows the current density to luminance characteristic of Y2O2S:Eu which corresponds to the broken line curve 11 in the graph of Figure 3, and a solid line curve 22 in the graph of Figure 5 shows a similar characteristic of Y202S:Eu,Tb, from which it will be clear that when Tb is added to the phosphor, it becomes difficult to obtain the sub-linear characteristic.

The graph of Figure 6 shows relative values of the luminance for the amount of Eu in the Y2O2S: Eu phosphor (namely the value x in the composition formula (Ln1~xEux)202S) which were measured, and curves 31 and 32 in the graph of Figure 7 respectively show values of x and yin the x-y chromaticity diagram of the amount of Eu in similar phosphors. In the x-y chromaticity value, when the amount of Eu is less than 5 mole %, the phosphor represents yellow.Therefore, it will be easily understood that when the amount of Eu is more than 5 mole % and less than 10 mole % so as to select the relative value of the luminance more than 50% in view of the luminance characteristic in the graph of Figure 6, the x value in the above composition is selected to satisfy 0.05 is less than or equal to x is less than or equal to 0.10.

In Example 1, although the red and green phosphors are mixed to form the phosphor screen 4, it is possible for the colour phosphors to form respective layers on the phosphor screen 4. An example of such case will now be described.

Example 2 90 weight % of the green-emitting phosphor used in Example 1 is first coated on the inner surface of the panel 3a of the cathode ray tube envelope 3 by a sedimentation method and the phosphor layer thus formed is dried. Thereafter, 10 weight % of the red-emitting phosphor used in Example 1 is coated on the green-emitting phosphor layer by a similar sedimentation method. Thus, the phosphor screen 4 consisting of the green and red phosphor layers 4G and 4b laminated in this order on the inner surface of the panel 3a is provided as shown in Figure 8 and hence a current-sensitive colour cathode ray tube similar to that of Figure 2 is obtained.

The current density to iuminance characteristics of the red and green colours in Example 2 become as indicated by broken line and solid line curves 41 and 42 in the graph of Figure 9 and the total characteristic thereof is shown by a one-dot chain line curve 43 in the same graph. In this case, as will be apparent from the comparison of the curve 12 in the graph of Figure 3 with the curve 42 in the graph of Figure 9, in the case of the laminated phosphor screen of Example 2, the current density versus luminance characteristic of the lower layer, namely the green-emitting phosphor layer 4G located on the side opposite to the electron beam impinging side has a lower luminance in the low current range so that the purity of the red can be much enhanced.

The invention can of course be applied to the combination of a red-emitting phosphor having a sub-linear characteristic and another colour phosphor having a super-linear or linear characteristic, such as a blue/green-emitting phosphor. An example of this will now be described.

Example 3 Similarto Example 1 but in place of the greenemitting phosphor, a blue/green-emitting phosphor (ZnO 82Cdo ,8)S:Ag,Ni is used. The measured current density versus luminance characteristics of red light emission, blue/green light emission and total light emission of this case are respectively shown by broken, solid and onedot chain line curves 51, 52 and 53 in the graph of Figure 10.

Example 4 Similar to the laminated structure in Example 2, but in place of the green-emitting phosphor, a blue-emitting phosphor (ZnO 82Cdo ,8)S:Ag,Ni is used. The measured current density versus luminance characteristics of red light emission, bluish/green.light emission and total light emission of this case are respectively as indicated by broken, solid and one-dot chain line curves 61, 62 and 63 in the graph of Figure 11.

As will be apparent from the foregoing explanation, with the embodiments the colour purity is enhanced as compared with the previously proposed colour cathode ray tube of this kind and accordingly, the range of coloured light emission can be widened.

Claims (9)

Claims

1. A current-sensitive colour cathode ray tube having a phosphor screen scanned by an electron beam from an electron source, the current density in said beam being varied at a substantially constant accelerating voltage, said screen comprising a red-emitting phosphor having a sublinear characteristic in the luminance to current density relationship and a different colouremitting phosphor other than red having a superlinear or linear characteristic in the luminance to current density relationship, wherein said red emitting phosphor has the foliowing formula: (Ln1-xEux)2o2s where Ln is Y, La, Gd or Lu, and x is in the range from 0.05 to 0.10, the concentration of rare earth impurity such as terbium Tb and praseodymium Pr being less than 10 ppm.

2. A cathode ray tube according to claim 1 wherein said different colour is green.

3. A cathode ray tube according to claim 1 wherein said different colour is blue/green.

4. A cathode ray tube according to claim 1 wherein said different colour-emitting phosphor and said red-emitting phosphor are sequentially coated onto the inner surface of said cathode ray tube to provide a plurality of layers.

5. A cathode ray tube according to claim 1 wherein said red-emitting phosphor and said different colour-emitting phosphor are mixed to provide a single layer on the inner surface of said cathode ray tube.

6. A cathode ray tube according to claim 1 and substantially as hereinbefore described with reference to Example 1.

7. A cathode ray tube according to claim 1 and substantially as hereinbefore described with reference to Example 2.

8. A cathode ray tube according to claim 1 and substantially as hereinbefore described with reference to Example 3.

9. A cathode ray according to claim 1 and substantially as hereinbefore described with reference to Example 4.