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 severly 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 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 dielectric 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 is contrast ratio is less than desired because the emitted light intensity is a factor upon which the contrast ratio depends.
The remarkable reflection-reducing properties of inhomogeneous films were recognized as early as 1880 by Lord Rayleigh (Proc. Lond. Math. Soc. 11, 51, 1880); the properties of such films have been extensively reviewed in a recent series of articles by Jacobsson (Progr. in Optics 5, 247, 1965; Arkiv Fysik 31, 191, 1966; Physics of Thin Films 8, 51, 1975). According to Jacobsson, experimental studies to date have been mainly devoted to transparent inhomogeneous films composed of graded mixtures of two nonabsorbing materials such as ZnS--Na.sub.3 AlF.sub.6, ZnS--CeF.sub.3, CeO.sub.2 --CeF.sub.3, and CeO.sub.2 --MgF.sub.2. These films were found to be durable and of good optical quality. A high index mixture of Ge--ZnS has been produced for application in the infrared wavelength region but were found to be relatively soft and sensitive to moisture and inferior to Ge--MgF.sub.2 films. KBr--Au films were found to have a very low absorption index, with k = 0.01 even at a concentration of gold of 0.16 parts by volume of gold. By contrast, an absorption index of 1.0 was found for a Ge--Au mixture containing 0.1 parts by volume of gold. Ge--In films were also found to have relatively high absorption. Due to the low solubility of In in Ge, the In was expected to remain a separate phase in the form of more or less spherical inclusions.
An inhomogeneous Ge--Si.sub.x O.sub.y film was shown by Jacobsson (1965) and also Olsen and Brown (Res./Develop. 16, 52, 1965) to lower the reflectance of a Ge surface to that of a surface of Si.sub.x O.sub.y (refractive index 1.62). Even lower reflectance was obtained with Ge--MgF.sub.2 films, although the transmittance was higher than expected (Jacobsson and Martensson, App. Optics, 5, 29, 1966). One of the first applications of inhomogeneous films as an antireflection coating was described by Nadeau and Hilburn in Canadian Pat. No. 418,289 (1944), and U.S. Pat. No. 2,331,716 (Oct. 12, 1944), in which a plastic layer of polystyrene or urea-formaldehyde resin having a high refractive index is diffused into the surface of an article and overcoated with a second plastic of low refractive index such as cellulose caproate or ethylcellulose. An important commercial application of inhomogeneous films as a low reflectance, absorbing coating on sunglasses was described by Anders in U.S. Pat. No. 3,042,542 (German Pat. No. 1,075,808; 1960). The inhomogeneous films described by Anders consisted of a mixture of low refractive index material, CeF.sub.4, ThF.sub.4, MgF.sub.2, or SiO.sub.2, and a metal, Ni, Fe, Mn, or Cr, or lower oxide of Nb, Ta, or Ti.
Recently, Steele has proposed in U.S. Pat. No. 3,560,784 the use of a dark dielectric layer consisting of SiO.sub.2 with a tapered concentration of codeposited aluminum applied to the rear side of a light transmissive phosphor layer to serve as a light absorbing layer. The tapered concentration of aluminum results in a continuous variation of the index of refraction through the layer, and such layer comprises an optically inhomogeneous film. Steele claimed novelty for a high contrast cathode ray tube utilizing this construction in which the refractive index of the silicon oxide was substantially equal to that of the phosphor. Phospors suitable for use with the inhomogeneous film of Steele were not otherwise identified. The same objective was the object of an earlier patent of Coltman (U.S. Pat. No. 2,616,057) in which the light absorbing layer was described as lampblack or the black deposits produced by evaporating metals such as aluminum or antimony under poor vacuum conditions.
Up to the present, the deposition of tapered inhomogeneous films such as in the Steele patent has required the evaporation of two different materials, with the rate of evaporation of each varied as a function of time. Also, it is usually desired that the initial portion of the deposit consist of one component only with the end portion consisting of the second different material only. Steele shows the initial and end materials to be SiO.sub.2 and aluminum, respectively. These requirements pose severe technical difficulties and to achieve reproducible results, elaborate monitoring and control equipment is required so that despite the superior performance offered by inhomogeneous films as compared to homogeneous films, very limited commerical application has been made of inhomogeneous films.
Osterberg (J. Opt. Soc. Am. 48, 513, 1958) has shown that transmitted waves cannot suffer loss of energy by reflection as they traverse nonabsorbing, inhomogeneous media in which the optical properites have no discontinuities. This result is strictly true only when the medium is infinite in extent. For practical applications, film thicknesses used are of the order of the wavelength of light so that interference due to reflection at the boundaries occurs. The width of the reflectance minimum has been found, however, to be greater than can be achieved with homogeneous films. It also has been shown by Osterberg that inhomogeneous absorbing media similarly cannot exhibit reflectance when the optical properties are continuous. In this case, the medium need not be infinite in extent. Anders (Dunne Schichten fur die Optik, Wissenschafftliche Verlagsgesellschaft mbH, Stuttgart, 1965, English translation as Thin Films in Optics, The Focal Press, London, 1967) has observed that a film thickness of only one wavelength is sufficient for essentially complete absorption in an absorbing inhomogeneous film. This property is basic to the dark dielectric layer described by Steele in U.S. Pat. No. 3,560,784 (1971) since the tapered concentration of aluminum results in an absorbing inhomogeneous film. The deposition of such film entails, however, the technical difficulties previously described, including the deposition of two different materials from two sources.