Cathode ray tube

Abstract

 A cathode ray tube faceplate arrangement including a halo suppression layer (32) disposed between a faceplate (14) and a thin film luminescent screen (16) having a light scattering surface (20). The halo suppression layer has a refractive index which is smaller than that of the faceplate and the screen and which reflects toward the scattering surface part of the rays of luminescent radiation which would otherwise contribute to halo.

Description

[0001] The invention relates to a cathode ray tube having a faceplate arrangement for suppressing halo, said arrangement comprising a faceplate consisting essentially of a transparent material and an internally disposed thin film luminescent screen having an index of refraction larger than that of the faceplate and having opposing surfaces, one of said surfaces being a light scattering surface disposed further from the faceplate than the other.

[0002] Cathode ray tubes can be operated at higher electron beam currents, and thus at higher brightness levels, if the conventional powdered layer luminescent screen is replaced with a thin film luminescent screen capable of operating at higher temperatures. This improvement in brightness is offset, however, by the adverse effects of multiple reflections within the thin film screen. Thin film screen cathode ray tubes are especially useful in projection systems because of the high brightness required in these systems.

[0003] Such a cathode ray tube with a thin film screen is known from GB-patent 20 00 173 and also from GB-patent 20 24 842.

[0004] U.S. Patent 4,310,783 discloses a cathode ray tube faceplate construction including a multilayer absorbing filter disposed between a faceplate and a luminescent screen for reducing halo by attenuating light rays multiply-reflected within the filter, which would otherwise contribute to halo. This absorbing filter not only reduces halo, but also useable light. In an alternative embodiment disclosed in the patent, the absorption filter is combined with a multilayer layer halo suppressing interference filter disclosed in U.S. Patent 4,310,784. This interference filter is angle sensitive to provide low observer side reflectances and high screen side reflectance. Such a combination of a multilayer interference filter on a multilayer absorption filter is aerly complicated.

[0005] It is an object of the invention to provide a simple cathode ray tube faceplate arrangement which effective suppresses halo.

[0006] It is another object of the invention to provide such a faceplate arrangement which suppresses halo without substantially reducing luminescent light which does not contribute to halo.

[0007] According to the invention the cathode ray tube comprises an intermediate thin film layer disposed between the screen and the faceplate, said intermediate layer having a refractive index smaller than that of the faceplate.

[0008] Said face plate arrangement may include according to the invention a multilayer interference filter disposed between the screen and the faceplate, the layers of said interference filter having alternating lower and higher refractive indices, one of said layers being said intermediate thin film layer.

[0009] The objects of the invention are accomplished by providing a faceplate arrangement which not only substantially prevents transmission of light rays that would ordinarily contribute to halo, but which also partially converts these rays to useable light which does not contribute to halo, thereby increasing image brightness and improving contrast. The manner in which this is accomplished can be best understood by referring to Figure 2 which graphically depicts as a function of emission angle the distribution of light rays emitted from any excited point on the luminescent screen. This figure illustrates only the principal sources of light transmits ted through the faceplate-air interface 28 and ignores the relatively weak rays I'_H which are derived from light rays that have been largely transmitted through interface 28. Further, the rays I'_H do not derive from rays originally emitted at any particular band of angles, but from rays distributed over the entire range of angles outside ^θ_CFA -θ_CPF and are thus dispersed over a large area of the face-plate arrangement, thereby preventing their collective contribution to any localized halo effect. This is not true of the rays I_H' however, which are high intensity rays deriving from fully reflected rays emitted in the screen at angles within the well defined band of angles θ_CFA-θ_CPF. In accordance with the invention, the light rays emitted from the screen within this band of angles are largely converted to rays I_B which are reflected back toward the scattering surface, which redirects part of the rays toward the interface at angles within the useful band of angles 0° - Q_COL. This conversion is effected by disposing between the faceplate and the screen a thin film intermediate layer of a material having an index of refraction which is sufficiently smaller than that of the faceplate to decrease the angle θ_CPF to a value near that of θ_CFA thereby causing reflection of rays within a band of angles which would otherwise have contributed to halo. The refractive index of the intermediate layer should be smaller than that of the screen material.

[0010] The intermediate layer may be provided as the sole layer between the faceplate and the screen or in combination with other layers disposed between the face- plate and the screen. In one embodiment the intermediate layer is incorporated as one of the layers of an interference filter, which futher improves performance of the faceplate arrangement for a narrow band of wavelengths near the primary emission wavelength of the luminescent screen, by converting a large part of both the rays I_H and I_M to rays I_B which are reflected toward the scattering surface. This arrangement has the advantage that it can be designed to convert spurious rays having wavelengths outside the narrow band to rays I_M which totally miss the lens in a projection system, thereby reducing chromatic aberration.

[0011] The present invention will now be described by way of example with reference to the accompanying drawings, in which :

Figure 1 is a sectional view of one end of a cathode ray tube faceplate and a lens in a projection system employing a proior art cathode ray tube;

Figure 2 is a schematic diagram showing the angular distribution of light rays emitted from the cathode ray tube screen in the system of Figure 1;

Figure 3 is a sectional view of one end of a cathode ray tube faceplate and a lens in a projection system employing a first embodiment of a cathode ray tube in accordance with the invention;

and Figure 4 is a sectional view of one end of a cathode ray tube faceplate and a lens in a projection system employing a second embodiment of a cathode ray tube in accordance with the invention. The multiple reflections are illustrated in Figure 1, which depicts part of a cathode ray tube projection system including the right end of a cathode ray tube face plate arrangement 10 spaced from a focusing lens 12, both shown in cross-section. The lens 12 magnifies an image formed by light rays received from the faceplate arrangement 10 and projects the image onto a relatively large reflective or transmissive screen (not shown). The arrangement 10 includes a faceplate 14 made of a material having good thermal conductivity, such as sapphire, and a thin film luminescent screen 16 deposited onto the faceplate. Typical thicknesses for the face plate and the screen, which are not drawn to scale, are 2-5 millimeters and 1-3 microns, respectively.

[0012] Although Figure 1 is not drawn to scale, it demonstrates conceptually the effects of multiple reflections within the thin film luminescent screen. Because the refractive indicies of luminescent screen materials are higher than those of conventionally used faceplate materials, a very small percentage of light emitted by the excited screen succeeds in reaching the lens 12. For example, in a projection cathode ray tube having a sapphire faceplate 14 with a refractive index n_f= 1.8 and a thin film luminescent screen 16 with a refractive index n p = 2.3, the amount of emitted light actually leaving the faceplate was determined to be less than 5%. This amount can be doubled by covering the inner surface of the screen with a highly reflective layer 18 of a material such as aluminum, thereby reflecting light directed toward the vacuum of the tube back toward the faceplate. A further increase in the amount of light reaching the lens can be achieved by roughening the inner surface of the screen 16, such as by chemically etching this surface before applying the reflective layer 18. The roughened surface 20 serves to scatter light emitted within the screen and reflected from a faceplate-screen interface 21 such that some of this light is redirected toward the interface at angles for which there is less reflection and more light directed toward the lens.

[0013] The reflective layer 18 and the scattering surface 20 not only increase the amount of useful light reaching the lens 12, however, they also increase light contributing to halo surrounding the image of the electron beam spot focussed by the lens.

[0014] The manner in which light rays emitted by the luminescent screen are transmitted through the faceplate arrangement 10 can be best understood by referring to Figure 1 which shows a plurality of light rays emitted at different angles from a point 22 in a spot excited by an electron beam 24. All angles are measured relative to a line 26 originating at point 22 and passing perpendicularly through the faceplate-screen interface 21 and a faceplate-air interface 28. All light rays emitted toward the interface 21 are at least partly reflected back toward the scattering surface 20 as rays I_B9 where they are scattered and redirected toward the interface. Light rays emitted at angles equal to or greater than the critical angle θ_CPF for total internal reflection from the interface 21 are totally reflected to the scattering surface 20. Part of this light is redirected toward the interface 21 at an angle less than θ_CPF and passes through the interface. The lateral shift between point 22 and the point at which the reflected rays impinge on the scattering surface 20 are typically on the order of the Sickness of the thin film screen 16 (e.g. 1-3 microns) and thus does not substantially increase the diameter of the luminescent electron beam spot, which is typically about 100 microns.

[0015] The light rays emitted from point 22 which pass through the faceplate-screen interface 21 reach the face- plate-air interface 28. Portions I_L of these rays, emitted from point 22 at angles between 0 and θ_COL pass through interface 28 and are collected by lens 12. Portions I_M emitted from point 22 at angles between θ_COL and Q_CFA (the critical angle for the face-plate-air interface) totally miss the lens and are lost within the system. A portion I_H or I'_H of each ray reaching the interface 28 is reflected, passes through or is reflected by interface 21, and eventually returns to and passes through interface 28. The lateral shifts between the point 22 and the points at which the rays I_H and I'_H eventually pass through the interface 21 are on the order of the faceplate thickness (e.g. 2-5 millimeters). These laterally-shifted rays form a number of concentric ring-shaped halos around the image of the electron beam spot, causing a decrease in image contrast.

[0016] Figure 3 illustrates a first embodiment of a cathode ray tube faceplate arrangement including a thin film halo suppression layer in accordance with the invention. The face plate arrangement 30 includes the same face- plate 14, thin film screen 16, reflective layer 18 and scattering surface 20 as the arrangement in Figure 1, but further includes a thin film layer 32 disposed between the faceplate and the screen. The layer 32 consists essentially of MgF₂ having a refractive index n_M = 1.38, which substantially decreases the angle θ_CPF from that of the prior art Figure 1 embodiment. This is demonstrated by Table 1 which lists the angles θ_CFA and Q_CPF for the Figure 1 and Figure 3 embodiments. The smaller band of angles lying between θ_CPF and 9_CFA is also apparent from the rays shown in Figure 3.

[0017] The thickness of the intermediate layer 32 is not critical, but should be greater than one-half the wavelength of the light emitted by the screen to prevent interference effects and should be substantially smaller than the diameter of the luminescent spot produced by the electron beam. For an intermediate layer thickness of 0.8 microns it has been determined that the exemplary arrangement shown in Figure 3 will reduce halo intensity by a factor of three and increase image brightness by a factor of two.

[0018] Figure 4 illustrates a second embodiment of a cathode ray tube faceplate arrangement in which a thin film halo suppression layer in accordance with the invention is incorporated into an interference filter. The faceplate arrangement 40 includes the same faceplate 14, thin film screen 16, reflective layer 18 and scattering surface 20 as the arrangement in Figure 3, but the halo suppression layer 42 also serves as a low refractive index layer in the multilayer interference filter 44 which has alternating low and high refractive indices. The halo suppression layer 42 need not be disposed on the thin film screen 16 itself, as is shown, but may serve as any one of the low refractive index layers in the filter 44. A ray diagram is not presented in Figure 4 because of the iifficulty in illustrating the operation of the interference filter, but the angles illustrated in Figure 3 would be identical in the Figure 4 embodiment.

[0019] Both the thickness and the refractive index of the halo suppression layer 42 will be determined by the same criteria as for the Figure 3 embodiment. The refractive indices of the remaining layers in the interference filter 44 are not critical, but the difference between the refractive indices of any two adjacent Layers should be as large as possible to maximize the reflection of rays originating from point 22 at angles between θ_COL and θ_CPF. The thicknesses of the layers in the filter 44 are very important and are determined by use of conventional techniques such as those described in Born and Wolf, Principles of Optics, Pergaman Press, 6th edition, 1980. The thicknesses of the layers are selected to provide a pass band centered around the primary waveLength of luminescent light emitted by the screen 16.

[0020] An exemplary, 8 layer interference filter has been designed for use in a face plate arrangement, such as that of Figure 4, having a primary emission wavelength of 5440Å and refractive indices and thickness as listed in Table 2. The layers are listed in order of successive distance from the screen 16, with layer A corresponding to the halo suppression layer 42. The materials used for this filter are M _gF₂(n_M = 1.38) and Z_n S (n = 2.3).

[0021] It has been determined that in comparison with the prior art faceplate arrangement of Figure 1, the above described filter will reduce halo by a factor of 5, increase image brightness by a factor of 5, increase image brightness by a factor of 3, and reduce spurious wavelength emissions by a factor of 3.

Claims

1. A cathode ray tube having a faceplate arrangement for suppressing halo, said arrangement comprising a face- plate consisting essentially of a transparent material and an internally disposed thin film luminescent screen having an index of refraction larger than that of the faceplate and having opposing surfaces, one of said surfaces being a light scattering surface disposed further from the face- plate than the other, characterized in that an intermediate thin film layer is disposed between the screen and the faceplate, said intermediate layer having a refractive index smaller than that of the faceplate.

2. Cathode ray tube according to Claim 1, characterized in that said faceplate arrangement includes a multilayer interference filter disposed between the screen and the faceplate, the layers of said interference filter having alternating lower and higher refractive indices, one of said layers being said intermediate thin film layer.

Drawing