A cathode ray tube

Abstract: A cathode ray tube (1) comprises an envelope (2) including a faceplate (5) with inner and outer surface and an antistatic layer (10) covering the outer surface of the faceplate for discharging static charges accumulating in the faceplate. The antistatic layer (10) is formed by using a solution containing an alcoholate of silicon as main constituent. The antistatic layer (10) contains a stabilising substance for maintaining antistatic characteristics. The antistatic layer (10) also has light filtering characteristics when the layer contains xanthene dye as filtering substance.

Description: This invention relates to a cathode ray tube and more particularly, to a light filtering layer which can have antistatic properties provided on or in front of a faceplate of the cathode ray tube. It is known that a cathode ray tube can reproduce letters and pictures by electron beam bombardment of phosphor screen formed on an inner surface of a faceplate of glass. The electron beam is emitted from an electron gun assembly placed inside a neck of an envelope including the faceplate. The phosphor screen includes dot-shaped or stripe-shaped red, green and blue phosphors which are distributed regularly on the inner surface of the faceplate. The cathode ray tube has a defect that contrast of the reproduced images deteriorate under bright ambient light. In order to improve the contrast, modification to reduce the light transmissivity of the faceplate has been generally employed. For example, it has been proposed that a glass plate (neutral filter), which has an almost uniform transmissivity for light in the visible light region, is fitted on the front surface of the faceplate. It is, however, undesirable for the reproduced images to use the neutral filter, since brightness of the reproduced images is reduced in spite of improvement of the contrast. That is, when the transmissivity of the plate is designated as T, brightness of the reproduced images through the faceplate is reduced propotional to the transmissivity T. On the contrary, ambient light reflected to viewers is reduced propotional to T². Thus, the contrast of the reproduced image is improved. However, it is inevitable to reduce the brightness of the reproduced images. Another cathode ray tube having a faceplate or a glass plate in front of the faceplate containing neodymium oxide (Nd₂O₃) for improving the contrast without reduction of the image brightness has been proposed in U.S. patent No.4,728,856 and Japanese Patent Disclosures No.57-134848, 57-134849 and 57-134850. Since the faceplate and the glass plate containing Nd₂O₃ act as a light filter, which has a steep main absorption band at 560nm∼615nm and a secondary absorption band at 490nm∼545nm, because of selective light absorption characteristics of neodymium oxide, the red and blue color purity of the reproduced images are improved and thus the contrast is improved to some extent. The reader is also referred to US-A-4,521,524 which discloses CRT contrast-enhancing glass filters which contain, inter alia, neodymium oxide, other metal oxides and colouring components, and 1980 SID International Symposium-Digest of technical papers (1980) April, Coral Gables, Florida, USA pages 172-173 which is concerned with contrast enhancing glass filters with a transmission peak at 545-550nm for colour CRT displays. However, a remarkable improvement of the contrast has not been achieved in the cathode ray tube in spite of utilization of selective light absorption characteristies. Namely, when the contrast improvement of the light filter containing neodymium oxide is evaluated by using BCP (Brightness Contrast Performance) as an index, the BCP of the filter is 1 ≦ BCP ≦ 1.05. It is clear from the value of the BCP that the contrast is not sufficiently improved. The BCP represents the contrast improvement ratio to the contrast improvement in case of using the neutral filter mentioned above as the standard. And the BCP can be also expressed as BCP = ΔB/ ΔRf when the brightness reduction ratio is designated by ΔB and the reduction ratio of the ambient light reflectivity is designated by ΔRf. Also, since the filter containing neodymium oxide has the main absorption band in the wavelength range of 560 nm∼615nm and, moreover, the main absorption band has the steep region, having a width of 5nm∼10nm in the wavelength region of 560nm∼570nm, the colour of the glass plate and the faceplate (so called as body colour) change due to the ambient light. In particular, the body colour becomes red under the ambient light from incandescent lamps. As a result, the parts of the images with low brightness, such as the black colour and shadows take on a reddish tinge, and thus, quality of the images deteriorate. Moreover, the cost of the filter increases due to the high cost of neodymium. The cathode ray tube has another problem due to the glass faceplate. Since the surface resistance of the faceplate is high, static charges due to the electron beam accumulate on the faceplate during tube operation. Because of the accumulation of the static charges, dust and fluff in the atmosphere are absorbed on to the outer surface of the faceplate. Also, when someone touches the faceplate during tube operation, they receive an electrical shock. In order to solve the problems due to the accumulation of the static charges, it has been proposed that the outer surface of the faceplate is covered with an antistatic layer which can discharge static charges accumulated on the faceplate during tube operation. For example, it is disclosed in U.S. patent No. 4,563,612 issued on January 7, 1986 that a cathode ray tube has an antistatic, glare-reducing, image-transmitting coating on an external viewing surface of a glass viewing window. The coating has a rough surface for imparting the glare-reducing characteristics and is composed essentially of a silicate material and a metallic compound in proportions to impart the desired antistatic characteristics without substantially degrading the image-transmitting capability of the coating. Further, it is also disclosed that the formulation may contain pigment particles and/or dyes to reduce the brightness up to about 50 percent of its initial value and/or to modify the spectral distribution of the transmitted image. However, the coating can not exhibit a satisfactory antistatic effect in practical use. Since the silicate material composing the coating substantially has no conductivity, the resistance value of the coating is not sufficiently reduced even if the small amount of metal compounds are contained in the coating. Further, when the amount of the compound added is increased to reduce the resistance value, strength and optical characteristics of the coating deteriorate. Another cathode ray tube for solving the accumulation of static charges is disclosed in Japanese Patent Disclosure No.61-118946. An outer surface of a faceplate is covered with double layers, which consists of an antireflection layer and an antistatic layer formed on the antireflection layer. The antireflection layer consists of transparent SiO₂ and has rough surface for improving the contrast of the reproduced images. The antistatic layer is formed on the outer surface of the faceplate by spraying a solution which contains an alcoholate of silicon as its main constituent and contains silanole radical. Since the antistatic layer can absorb moisture in the atmosphere due to the silanole radical, the resistance value of the layer can be effectively reduced. However, when using the antistatic layer, the silanol radical is reduced with the passage of time through the progressive glassification of the silicon forming the basis of the layer. Because of reduction of the silanol radical, the resistance value of the layer increases in accordance with reduction of the moisture absorption capability. As a result, the antistatic effect deteriorates. Accordingly, the antistatic layer lacks stability of antistatic characteristics. An object of this invention is to provide a cathode ray tube with a thin layer provided in front of a faceplate for improving reproduced images. The invention provides a cathode ray tube comprising an envelope including a faceplate with inner and outer surfaces and a sidewall portion, a neck, and, a cone connecting the faceplate to the neck, an electron gun provided inside the neck for emitting at least one electron beam, a phosphor screen which includes red, green and blue phosphors provided on the inner surface of the faceplate for emitting red, green and blue light by bombardment of the electron beam, and light filtering means provided on or in front of the faceplate, for selectively transmitting light. The light filtering means includes at least one light filtering substance comprising pigment(s) and/or dye(s) and has a maximum absorption wavelength in the wavelength range of 575±20nm within the wavelength range of 400nm to 650nm and satisfys the relationships: Tmin ≦ T₅₅₀ < T₅₃₀, 1 ≦ T₄₅₀/T₅₃₀ ≦ 2, 1 ≦ T₆₃₀/T₅₃₀ ≦ 2, and 0.7 ≦ T₄₅₀/T₆₃₀ ≦ 1.43 wherein T₄₅₀, T₅₃₀, T₅₅₀, T₆₃₀, and Tmin represent the transmissivities for lights of wavelength of 450nm, 530nm, 550nm, 630nm, and the maximum absorption wavelength, respectively. Accordingly to the invention, since the thin layer for preventing accumulation of static charges contains a stabilizing substance, the resistance value of the antistatic layer may not increase with the passage of time. Accordingly, a stable antistatic layer can be obtained. A non-limiting theoretical explanation can be considered for illustration only. The antistatic layer, which is formed by using a solution of an alcoholate of silicon, is composed of a SiO₂ film partially having a silanol radical. In the conventional antistatic layer, the silanole radical will cause a dehydrating condensation reacting with passage of time, and thus, moisture absorption capability due to the silanole radical will disappear through the glassification of the layer. On the contrary, since the antistatic layer which may be the light filtering means of the invention may contain stabilizing substance, the glassification mentioned above can be effectively prevented. It is assumed that the stabilizing substance is present in such a way that it separates neighbouring silanol radicals and thus prevents the reaction of the silanol radicals in the layer. As a result, the dehydrating condensation reaction can be prevented and thus the increase in the resistance value of the layer with the passage of time can be prevented. The said stabilizing substance, which can act as a light filter, is preferably an organic substance, which is solid at normal temperature, can be dissolved in water or an organic solvent such as alcohol, and has a molecular weight of 100 to 5000. For example, one or more dyes, such as anthraquinone group dyes composed of anthraquinone and its derivatives, azo group dyes and carbonium dyes, can be used. Other dyes, such as xanthene dyes and phthalein dyes including Sulpho Rhodamine B (colour Index 45100) and Rhodamine B (colour Index 45170), Kayanol Milling Red 6BW(Acid Violet 97), and Kayaset Blue K-FL (Solvent Blue 70), can be used as the light filtering substance. These dyes of Sulpho Rhodamine B, Rhodamine B, Kayanol Milling Red 6BW, and Kayaset Blue K-FL are marketed by Nippon Kayaku Co., Ltd. The amount of the light filtering substance in the layer can be adjusted depending on the molecular weight and specific gravity of the substance. The amount of the substance is preferably between 0.01 wt% and 75 wt%. If the amount is less, prevention of deterioration of antistatic properties can not be expected. Also, if the amount is more, transmissivity and adhesion of the layer is reduced for practical use. The antistatic layer of this invention can contain metal salts, such as Li, Na, Ba, Sr and Ca, as moisture absorbent. The present inventors found that the antistatic layer, which contained a small amount of particular dyes, acted as a light filter having excellent light filtering characteristics for improving contrast of reproduced images of the cathode ray tube. Namely, the inventors developed a new light filter based on a novel concept. The filter took into account the radiation spectrum of the light emitted from the phosphor screen of the cathode ray tube and spectral luminous efficacy characteristics, and considerably improved even optimised light absorption characteristics for the cathode ray tube. The reason for only slight contrast improvement of glass plates containing Nd₂O₃ with BCP such that 1 ≦ BCP ≦ 1.05, in spite of the selective absorption filter, was established. As shown in Figure 1, the glass plate as the light filter had high transmissivity near the wavelength of 550nm where the spectral luminous efficacy characteristic is highest, but near the radiation peak of the green light at wavelength of 530 nm, the transmissivity was lower. Finally, the inventors optimised the light filter for the cathode ray tube by adjusting the transmissivity of each characteristic wavelength in the relationship between the radiation spectrum characteristics of the phosphor screen of the cathode ray tube and spectral luminous efficacy characteristics. The light filter according to the invention preferably has maximum absorption wavelength in a range of 575±20nm within the wavelength range from 400nm to 650nm and may satisfy the following equations (1) to (4). Tmin ≦ T₅₅₀ < T₅₃₀ 1 ≦T₄₅₀/T₅₃₀ ≦2 1 ≦T₆₃₀/T₅₃₀ ≦2 0.7 ≦T₄₅₀/T₆₃₀ ≦1.43 In the equations, T₄₅₀, T₅₃₀, T₅₅₀, T₆₃₀ and Tmin represent transmissivity for lights of wavelength of 450nm, 530nm, 550nm, 630nm and the maximum absorption wavelength, respectively. The following is offered as a non-limiting explanation of the operation of the light filtering layer used in the cathode ray tube of this invention. In Figure 2, emission spectra of the typical phosphors for emitting blue (ZnS: Ag, Cl phosphor), green (ZnS: Cu, Al phosphor) and red (Y₂O₂S: Eu³⁺ phosphor) used in the phosphor screen of the cathode ray tube are shown. Also, Figure 3 shows the spectral distribution (a), the luminosity curve (b) and the product of the spectral distribution and the luminosity curve (C), when the light from a fluorescent lamp is taken as the ambient light. As can be seen from the graphs, the ambient light can be most efficiently absorbed near the peak of the curve (C), namely, light of the wavelength 575nm±20nm can be interrupted. However, at the same time, every effort must be made to avoid a reduction in brightness. Consequently, the characteristics of the light filtering layer has maximum transmissivity, in other words, maximum ambient light absorption efficiency near 450nm and 630nm where the luminosity is lowest and emission energy is large; the minimum transmissivity, in other words, increased luminosity near 575nm where the emission energy of the phosphor is small; and an intermediate transmissivity near 530nm where emission energy of green phosphor peaks. In addition, the transmissivity of the filtering layer between 530nm and 575nm is smaller than the transmissivity at 530nm, since energy of the ambient light near 550nm is larger than energy of the ambient light at 530nm, and the emission energy of green phosphor is small. That is to say, if the filtering characteristics is taken as satisfying Tmin ≦ T₅₅₀ < T₅₃₀, and T₅₃₀ ≦ T₆₃₀, the maximum efficiency for contrast improvement can be obtained. Regarding the body colour of the light filtering layer, there are cases where it takes on a slightly reddish tinge when an incadescent lamp is used as the ambient light . However, the body colour can be corrected. Figure 4 shows the spectral distribution (d), the luminosity curve (e) and the product of the spectral distribution and the luminosity curve (f) in the case of ambient light from the incadescent lamp. As seen from the Figure 4, the longer the wavelength of the light, the greater the emission energy of the light. Consequently, the body colour can be corrected by adjusting the transmissivity of the filtering layer in the region of 650nm∼700nm, where the reddish tinge is stronger, to be smaller than the transmissivity near 630nm, where the emission energy of the red phosphor peaks. In detail, the body colour of the faceplate could be corrected by adjusting the characteristics of the filtering layer according to the invention for satisfying the following equations (5) to (7). T₄₅₀/T₅₃₀=1∼2 T₆₃₀/T₅₃₀=1∼2 T₄₅₀/T₆₃₀=0.7∼1.43 In the above relationships, if the value of equation (5) exceeded 2 or the value of equation (7) exceeded 1.43, the body colour showed a strong bluish tinge. If the value of equation (6) exceeded 2 or the value of equation (7) fell below 0.7, the body colour showed a strong reddish tinge which was not practical. Furthermore, if the values of the equations (5) and (6) fell below 1, the filter was not practical since the contrast improvement reduced and the BCP value was small. The light filter of the invention may contain xanthene dye(s) and/or phthalein dye(s) including Sulpho Rhodamine B (colour Index 45100) and Rhodamine B(colour Index 45170) of the following formulae, respectively, and kayanol Milling Red 6BW (Acid Violet 97) to confer the above mentioned filter characteristics. Sulpho Rhodamine B Rhodamine B In order to correct the body colour mentioned above, the filter of this invention preferably contains other dye(s) in addtion to the dye(s) mentioned above, such as Kayaset Blue K-FL (Solvent Blue 70) marketed by Nippon Kayaku Co., Ltd. which has maximum absorption wavelength at 675 nm and near infra-red absorption agents of a type which have a near infra-red absorption, for example, a maximum absorption wavelength at 675nm and the end of the light absorption extending to the range of wavelength between 650nm and 700nm. The filter of this invention preferably contains 2.0g to 0.02g of dye(s) for satisfying the basic relationship shown by the equations (1) to (4). Furthermore, not only dyes, but also pigments, and particularly organic pigment can be used in the filter. In the colour cathode ray tube of the invention, a BCP value of the light filter increased up to 1.05∼1.50, which varied according to radiation spectrum of the phosphor screen and the concentration of the filter material, such as dye, and thus excellent contrast characteristics can be obtained. The light filtering layer of this invention can be formed by coating a solution, conveniently prepared by mixing suitable dyes and pigments with the selective light transmissivities mentioned above into an alcohol solution containing ethyl silicate as a main constituent, directly on the faceplate of the cathode ray tube by suitable means, such as by spin coating or spray methods. Also, the light filtering layer can be obtained by producing a filtering plate composed of a transparent base plate, such as acrylic resin(s), dye(s) and/or pigment(s) which are contained in the plate. The filtering plate can be attached to the faceplate. Furthermore, in the case of telepanel cathode ray tubes, the filtering layer can be formed by mixing the dye(s) into the adhesive resin(s), which are used for sticking the telepanel acting as a colour at the faceplate. In order that the invention may be more readily understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph showing a transmissivity curve and a luminosity curve of a conventional light filter containing neodymium oxide together with the spectral characteristics of the green phosphor shown in Figure 2, Figure 2 is a graph showing the emission spectra of typical blue, green and red phosphors used for the phosphor screen of the cathode ray tube, Figure 3 is a graph showing spectral characteristics, a luminosity curve and the product of the spectral characteristics and the luminosity curve for a typical flouorescent lamp, Figure 4 is a graph showing spectral characteristics, a luminosity curve and the product of the spectral characteristics and the luminosity curve for a typical incandescent lamp, Figure 5 shows a side view of a cathode ray tube in accordance with one embodiment of the invention, Figure 6 is an enlarged diagram showing part of the molecular structure of an antistatic layer shown in Figure 5, Figure 7 is a graph showing a transmissivity curve of a light filtering layer according to another embodiment of the invention, and Figure 8 is a graph showing a transmissivity curve of a light filtering layer according to the other embodiment of the invention. Prefered embodiments of this invention will be explained with reference to the drawings. In Figure 5, a cathode ray tube 1 includes an envelope 2 which is hermetic and is made of glass. The envelope 2 has a neck 3 and a cone 4 as a continuation of the neck 3. The envelope 2 also has a faceplate 5 sealed with the cone 4 by frit glass. A metal tension band 6 for preventing explosion is wound around the outer periphery of a sidewall portion 7 of the faceplate 5. An electron gun 8, which emits three electron beams, is provided in the neck 3. On the inner surface of the faceplate 5, there is provided a phosphor screen 9 which consists of a plurality of phosphor stripes for emitting red, green and blue lights and light absorbing stripes between the phosphor stripes. A shadow mask (not shown), which has a plurality of apertures for bombarding the phosphor stripes by the electron beams, is placed adjacent to the phosphor screen 9. A deflection yoke (not shown) is attached to the outside of the cone 4 for deflecting the electron beams to scan the phosphor screen 9. The outer surface of the faceplate 5 is covered with an antistatic layer 10 to reduce the surface resistance of the faceplate 5. As shown in Figure 6, the antistatic light filtering layer 10 contains stabilizing substances 11, which is composed of methyl violet and separates the silanol radicals. Although the antistatic layer 10 is shown as a two-dimensional structure in Figure 6, the actual antistatic light filtering layer is three dimensional. Since the antistatic light filtering layer 10 contained stabilizing substances 11 separating the silanol radicals, the resistance value of the layer 10 did not increase with the passage of time and the layer 10 could maintain stable antistatic characteristics. Also, since the layer 10 contained methyl violet as the stabilizing substances, acting as a light filter, the external light reflectivity was reduced by 20 % and the contrast was also improved. The antistatic, light filtering layer 10, of course, was electrically connected to the metal band 6 to effectively discharge the static charges which would be accumulated on the faceplate 5. The antistatic layer was formed as follows. Embodiment 1 A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Methyl violet 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating After coating, the antistatic layer was formed by drying. The resistance value of the layer was 5×10⁹ Ωcm, by measurement. A heat-resistance test was carried out by leaving the cathode ray tube with the antistatic layer for 500 hours at a temperature of 80°C to evaluate the the stability of the antistatic layer with the passage of time. As the result of the test, the resistance value did not increase to more than 5×10¹⁰ Ωcm, and the antistatic layer maintained satisfactory antistatic characteristics. On the contrary, after the heat-resistance test mentioned above, an antistatic layer which did not contain the stabilizing substance deteriorated and was accompanied by an increase in resistance from 5×10⁹ Ωcm to 1×10¹³ Ωcm. Embodiment 2 An antistatic layer according to another embodiment contained lithium chloride as a moisture absorbent in addition to violet dye as the stabilizing substance. A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Lithium chloride 1 wt% Violet dye 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating. After coating, the antistatic layer was formed by drying. The resistance value of the layer was 1×10⁸ Ωcm, by measurement. As mentioned above, a heat-resistance test was carried out under the same conditions, after the test, the resistance value did not increase to more than 1×10⁹ Ωcm, and this result indicating the antistatic layer maintained satisfactory antistatic characteristics. Embodiment 3 An antistatic layer according to a further embodiment contained saccharin with a molecular weight of 183 as the stabilizing substance. A coating solution having the following composition was prepared. Ethyl silicate 7 wt% Hydrochloric acid 3 wt% Saccharin 0.2 wt% Water 2 wt% Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate of the assembled cathode ray tube by spin coating. After coating, the antistatic layer containing the stabilizing substance saccharin was formed by drying. The resistance value of the layer was 5×10⁹ Ωcm, by measurement. A heat-resistance test was carried out under the same condition mentioned above. After the test, the resistance value did not increase to more than 5×10¹⁰ Ωcm. This result meant that the antistatic layer had an excellent stability. According to further embodiments of the invention, an antistatic layer with not only antistatic characteristics but also light filtering characteristics is explained. In other words, the antistatic layer is a light filtering with antistatic characteristics by containing a filtering substance of particular organic dye(s) which can act as the stabilizing substance for maintaining antistatic characteristics. Embodiment 4 A coating solution having the following composition was prepared. Ethyl silicate (Si(OC₂H₅)₄) 7 g Hydrochloric acid (HCl) 3 g Water 2 g Sulpho Rhodamine B 0.02g∼4.0 g Isopropyl alcohol Remainder The solution was coated on the outer surface of the faceplate with a size of 25 inches by a spin coating method after assembling the cathode ray tube. After coating, a light filtering layer, which contained the light filtering substance acting as the stabilizing substance for maintaining antistatic characteristics, was formed by drying. In the case of the embodiment, the amount of Sulpho Rhodamine B contained in the filtering layer was 4.0g, 2.0g, 1.5g, 1.0g, 0.5g, 0.3g, 0.1g, 0.05g, and 0.02g. Transmissivity curves of the light filtering layer, which contained 4.0g, 2.0g, 1.0g, 0.5g and 0.3g of Sulpho Rhodamine B, were shown by the curves (A), (B), (C), (D), and (E) in Figure 7, respectively. In table 1, evaluations of reproduced images obtained from the cathode ray tubes with the light filtering layers and results of the heat-resistance test carried out under the same conditions mentioned above are shown. As a comparison, a 25-inch-size cathode ray tube, which has a glass plate containg Nd₂O₃ as the light filter, was evaluated. In Table 1, the body colour was evaluated whether, when black images were reproduced by these colour cathode ray tubes, the images were recognised by human sight as natural black without the black being tinged with any other colour. In practice, a black pattern of 50mm × 50mm was reproduced in the centre of the phosphor screen, and the periphery of the pattern was made white. The shade of the black pattern (reddish, bluish, green, etc.) was evaluated while illuminating the faceplate with an incandescent lamp from an angle of 45° with respect to the outer surface of the faceplate so that the illumination on the outer surface of the faceplate was 500 lux. Evaluation standards are specified thus: Recognition as natural black without being tinged by any colour was indicated as o ○, slight colouration noticed but hardly any problem was indicated as ○, colouration being rather strong and tending to cause problems was indicated as Δ, and colouration being so strong that the pattern was not as black was indicated as x. As seen from Table 1, if the amount of the dye was increased, the BCP increased and the contrast was improved. However, the body colour gradually became more strongly tinged. When the amount of the dye was 4.0g, T₄₅₀/T₅₃₀ and T₆₃₀/T₅₃₀ were 3.57 and exceed 2, respectively, and it could not be used, practically. In connection with the body colour evaluation, the dye could be present up to 3.0g. And, in these cases, T₄₅₀/T₅₃₀ and T₆₃₀/T₅₃₀ was 1.9∼2.0. Also, the BCP was 1.47 in these cases, and a great improvement in contrast was observed. As also seen from Table 1, if the amount of the dye was between 0.3g and 4.0g, the contrast was improved, and if the amount of the dye was between 0.02g and 1.5g, antistatic characteristics of the filtering layer were stabilized. Further, if the amount was between 0.3g and 1.5g, a filtering layer which had no problem in respect of body colour, improved contrast, and stable antistatic characteristics was obtained. Embodiment 5 The filtering layer of this embodiment further contained 1 wt% of LiCl as moisture absorbent for improving antistatic characteristics, compared to the filtering layer of Embodiment 4. Table 2 shows heat-resistance test results carried out under the same conditions mentioned above. As seen from Table 2, the filtering layer had stabilized antistatic characteristics. Embodiment 6 The light filtering layer of this embodiment further contained dye Kayaset Blue K-FL, which had a maximum absorption wavelength near 675 nm for correcting the body colour. The filtering layers were the same as the filtering layers which contained 4.0 g, 2.0 g and 1.0 g of Sulpho Rodamine B and had colour tones in Embodiment 5, except that the filtering layers of Embodiment 6 contained 0.2 g of Kayaset Blue K-FL. Transmissivity curves of the filtering layer are shown as curves (F), (G), and (H) in Figure 8. Table 3 shows evaluation results for cathode ray tubes with these filtering layers of the embodiment. As seen from Table 3, the BCP was slightly smaller than that of Embodiment 5 because the transmissivity near 630 nm, which was emission energy of the red phosphor, slightly reduced. However, the body colour clearly was improved, so that these filtering layers could be used practically. Embodiment 7 Filter plates of acrylic resins were produced by mixing the same amounts of Sulpho Rhodamine B as in Embodiment 5 into acrylic resins. The filter plates were attached to the outer surface of the faceplate, respectively. These cathode ray tubes with the filter plates had the same transmissivity curves as shown in Figure 7. Also, the same results as in Embodiment 5 were obtained. The filter plates did not have antistatic characteristics.

Claims: A cathode ray tube (1) comprising an envelope (2) including a faceplate (5) with inner and outer surfaces and a sidewall portion (7), a neck (3), and, a cone (4) connecting the faceplate to the neck, an electron gun (8) provided inside the neck for emitting at least one electron beam, a phosphor screen (9) which includes red, green and blue phosphors provided on the inner surface of the faceplate for emitting red, green and blue light by bombardment of the electron beam, and, light filtering means (10) provided on or in front of the faceplate for selectively transmitting light, characterised in that the light filtering means includes at least one light filtering substance comprising pigment(s) and/or dye(s) and has a maximum absorption wavelength in the wavelength range 575±20nm within the wavelength range of 400nm to 650nm and which satisfies the following relationships: T min ≦ T₅₅₀ < T₅₃₀, 1 ≦ T₄₅₀/T₅₃₀ ≦ 2, 1 ≦ T₆₃₀/T₅₃₀ ≦ 2, 0.7 ≦ T₄₅₀/T₆₃₀ ≦ 1.43 wherein T₄₅₀, T₅₃₀, T₅₅₀, T₆₃₀ and T min represent the transmissivities for lights of wavelength of 450nm, 530nm, 550nm, 630nm and of the maximum absorption wavelength, respectively. A cathode ray tube according to claim 1 wherein the light filtering means satisfies the relationship: T 650∼700 < T₆₃₀; wherein T 650∼700 represents the transmissivity for light of the maximum absorption wavelength in the wavelength range of 650nm to 700nm. A cathode ray tube according to claim 1 or 2 wherein the filtering means comprises a transparent substrate and a filtering layer which covers the substrate which layer is derived from a solution containing alcoholate of silicon as a main constituent and at least one light filtering substance present in a concentration effective to filter light. A cathode ray tube according to any preceding claim wherein the filtering means comprises a substrate which is the faceplate. A cathode ray tube according to any one of claims 1 to 3, wherein the filtering means comprises a substrate which is a plate provided in front of the faceplate. A cathode ray tube according to any of claims 1 to 3, wherein the filtering means comprises a substrate which is an adhesive resin layer containing said at least one light filtering substance in a concentration effective to filter light, and which adhesive resin layer is provided on the outer surface of the faceplate. A cathode ray tube according to any preceding claim wherein the filtering means has antistatic characteristics. A cathode ray tube according to claim 7, wherein the light filtering substance comprises at least one of the following: anthraquinone group dyes composed of anthraquinone or its derivatives, azo group dyes, carbonium dyestuffs, xanthene dyes, and phthalein dyes. A cathode ray tube according to claim 8 wherein the light filtering substance comprises one or more xanthene dyes. A cathode ray tube according to claim 8 wherein said light filtering substance comprises one or more of: rhodamine B, sulpho rhodamine B, Kayanol milling red, acid violet, methyl violet, violet dye and Kayaset Blue K-FL. A cathode ray tube according to claim 10, wherein said light filtering substance comprises both sulpho rhodamine B and Kayaset Blue K-FL.

Inventors: ITOU TAKEO C O PATENT DIVISION, MATSUDA HIDEMI C O PATENT DIVI, TANAKA HAJIME C O PATENT DIVIS, ITOU, TAKEO C/O PATENT DIVISION, MATSUDA, HIDEMI C/O PATENT DIVISION, TANAKA, HAJIME C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01J29
IPC codes: H01J29