It is well known that ambient illumination, that is light originating from sources external to the display device, is reflected to the observer from various optical interfaces of the device and thus reduces the image contrast by increasing the apparent brightness of the dark image areas. Under conditions of high ambient illumination, the image contrast is severely degraded. In addition, a part of the light emitted by the luminescent material of the device also undergoes undesired reflections, producing a further degradation of contrast and of resolution. When the luminescent material consists of a layer of phosphor material in the form of small powder particles, scattering of the emitted light also occurs, further degrading contrast and resolution.
Various means for overcoming these problems have been proposed. These include the use of various filters including polarizing neutral density and restricted angle or multi-apertured opaque filters. Other methods include the incorporation of a dark material into the glass of the tube face, or a black dye in the phosphor binder of the luminescent layer of the display device. All of the methods have the common disadvantage that the emitted light as well as the reflected ambient light intensity is reduced, with the result that the improvement in contrast ratio is less than desired because the emitted light intensity is a factor upon which the contrast ratio depends.
Recently, U.S. Pat. No. 4,132,919 has disclosed a light absorbing inhomogeneous film having a tapered composition varying continuously from a metal oxide at the phosphor film interface to the metal constituent of the oxide at a point remote from the interface, the film exhibiting a continuous gradient of refractive index from a index approximating the index of the phosphor, at the phosphor-film interface to an index approximating that of the metal. The metal oxide is selected from the group of oxides consisting of tantalum oxide and vanadium oxide. The film is conveniently made by reactive RF sputtering in an argon atmosphere containing an initial small partial pressure of oxygen which is gradually reduced to zero oxygen during the course of the run. The resulting film is capable of absorbing more than ninety-nine percent of initial ambient light and provides excellent contrast even when the display device is viewed in direct sunlight.
It has been found, however, that when a film of U.S. Pat. No. 4,132,919 based on tantalum oxide is utilized in a cathode ray tube device, a considerable portion of the incident electron beam energy is absorbed in penetrating the light absorbing film. The loss of energy of the electron beam appears to result from the large backscattering of electrons associated with the relatively high periodic number of tantalum (73). Thus, in a bilayer CRT phosphor screen having a first phosphor layer of 4000A of europium-activated lanthanum oxysulfide, the red, europium activated layer is well excited whereas the terbium activated layer is insufficiently excited to produce a good green color, when the incident electron beam has an initial energy of 20 Kv. Under the same conditions, a light absorbing layer of the same thickness made of vanadium oxide, permits good excitation of the green phosphor. The periodic number of vanadium (23) is less tantalum, so that the electron beam undergoes less energy loss in penetrating the light absorbing layer. Unfortunately, the pentoxide of vanadium has a melting point of only 690 degrees C., as compared with the 1872 degrees C. melting point of tantalum oxide. Great care must be exercised, therefore, in sealing a faceplate having a vanadium based absorbing film to the glass funnel of the CRT. In this sealing process it is necessary to insure an absence of moisture and oxygen as well as insuring that temperature during sealing does not exceed about 500 degrees C., otherwise serious degradation of the absorbing film will occur.