Surface coating with enhanced colour contrast for video display

Abstract: For use in a self-emitting colour display device such as a cathode ray tube (CRT) having a display screen with a phosphor coating on its inner surface, a surface coating disposed on the outer surface of the display screen affords improved colour purity, or contrast, by reducing overlap in the colour spectrum between two adjacent primary colours, i.e. between red and green and/or between green and blue. An organic dye is added to either an inner antistatic layer or an outer antireflective layer of an antireflective/antistatic outer coating on the display screen, where both of the aforementioned layers contain silane. The dye functions as a colour filter absorbing only light within the frequency range between adjacent primary colours, such as between the red and green colour phosphors, i.e. in the range of 460-500 nm, and/or between the green and blue colour phosphors, i.e. in the range of 560-600 nm. A silane coupling agent is also added to serve as a binding agent bridging the organic dye and silane together. In this manner, the organic dye is bonded to the silane in either the antistatic layer or the antireflective layer to prevent separation and escape of the organic dye filter from either of these layers of the outer coating on the display screen.

Description: This invention relates generally to the provision of coatings on video screens or panels, in particular , but not exclusively, of self-emitting colour video display devices, such as of the cathode ray tube (CRT) type. More particularly, the invention is directed to the provision of coatings on the outer surface of such screens which provide improved colour separation and contrast. Self-emitting display devices, such as of the CRT type, provide a video image by bombarding high energy electrons onto phosphor elements disposed on the inner surface of the display of the device. The phosphor elements are separated into three groups, with each group emitting one of the primary colours of red, green or blue when impinged upon by the energetic electrons. For optimum colour purity, the three groups of colour phosphors should emit light characterised by three discrete spectra, with no overlap between adjacent spectral components. In practice, however, there is always some colour spectrum overlap between adjacent primary colour phosphors, i.e. between red and green as well as between green and blue. The spectral regions of overlap between red and green typically covers wavelengths in the range of from 460-500 nm, while spectral overlap between the colours green and blue typically includes wavelengths in the range of from 560-600 nm. Any spectral overlap between the primary colours degrades video image colour purity and colour contrast. US-A-4,987,338 disclosed an antistatic/antiglare coating in the form of a single layer to which an organic dye is added. This approach is not particularly desirable because the organic dye is subject to bleaching out of the coating when the faceplate is wiped with a wet cloth containing either water or alcohol. EP-A-0 603 941 discloses a filtering layer for the display screen of a CRT consisting of silicon dioxide, an oxide of Ge, Zr, Al or Ti and a black dye which operates on the visible spectrum of 410-680 nm. This dye is intended to eliminate the purplish-violet appearance characteristic of other prior art filtering dyes. More recently, US-A-5,291,097 teaches the addition of an organic dye to the inner antistatic layer of a double-layer antireflective/antistatic coating on the outer surface of the faceplate of a CRT. The inner antistatic layer containing the organic dye is then covered with the outer antireflective layer. Even in this approach where the inner antistatic layer is covered by a protective outer antireflective layer, the organic dye has been observed to diffuse outwardly from the inner antistatic layer and through the outer antireflective layer, with the organic dye eventually bleaching out of the antireflective/antistatic coating on the faceplate through repeated wiping of the faceplate with either a dry or wet cloth. The present invention addresses the aforementioned limitations of the prior art by permanently bonding an organic dye to a silane in either the outer antireflective layer or the inner antistatic layer to prevent bleaching of the dye from the surface coating. The organic dye functions as a colour spectrum filter between two adjacent primary colour phosphors so as to substantially eliminate primary colour overlap resulting in improved colour contrast and purity. Accordingly, it is an object of the present invention to provide a surface coating for the outer surface of a screen, e.g. a screen of a self-emitting display device such as of the CRT type, which provides improved colour contrast for a video image. It is another object of the present invention to provide an antistatic or antireflective coating for the outer surface of a CRT faceplate which provides improved colour separation and thus enhanced colour contrast in a video image. Yet another object of the present invention is to introduce a colour absorbing organic dye in the antistatic or antireflective coating of a video screen, e.g. of a CRT, in a manner which permanently links the dye to the coating materials preventing bleaching of the dye from the coating and allowing the coating to also function as a colour filter without degradation over time and with extensive use. A further object of the present invention is to improve colour contrast in the video image of a screen, e.g. a screen of a self-emitting display device, by permanently affixing a colour filter on the outer surface of the screen which substantially reduces spectral overlap of the three primary colours. According to various aspects of the present invention there is provided a light-transmitting screen as claimed in the ensuing claim 1 and a method of providing a coating on an outer surface of a video display panel according to the ensuing claim 12. Embodiments of the invention will now be described, by way of example only, with particular reference to the accompanying drawings, where like reference characters identify like elements throughout the various figures and in which: Figure 1 is a longitudinal sectional view of a self-emitting display device according to the present invention and in the form of a CRT incorporating an antireflective or antistatic coating; Figure 2 is a partial sectional view of a self-emitting display device according to the present invention in the form of a flat display screen having an outer surface coating comprised of an inner antistatic layer and an outer antireflective layer; Figure 3 is a flow chart illustrating the steps involved in preparing and applying the combination of an antistatic and antireflective coating to the outer surface of the faceplate of a self-emitting display device in accordance with the present invention; Figure 4 is a CIE chromaticity diagram for the range of colours obtainable on a typical CRT display screen illustrating the filtering out of light in the wavelength range between 560-600 nm; and Figure 5 is a CIE chromaticity diagram illustrating the range of colours obtainable in a typically CRT display screen illustrating the filtering out of light in the wavelength range between 460-500 nm. Referring to Figure 1, there is shown a sectional view of a colour CRT 10 incorporating an antistatic/antireflective coating 32 in accordance with the principles of the present invention. In the following discussion the terms "display screen", "display panel" and "faceplate" are used interchangeably. The CRT 10 includes a sealed glass envelope 12 having a forward display screen or faceplate 14, an aft neck portion 18, and an intermediate funnel portion 16. A phospher screen 24 is disposed on the inner surface of glass faceplate 14 and includes a plurality of discrete phosphor deposits, or elements, which emit light when an electron beam is incident thereon to produce a video image on the faceplate. The colour CRT 10 includes three electron beams 22 directed onto, and focused upon, the glass faceplate 14. Disposed in the neck portion 18 of the glass envelope 1 are a plurality of electron guns 20 typically arranged in an inline array for directing the electron beams 22 onto the phosphor screen 24. The electron beams 22 are deflected vertically and horizontally in unison across the phosphor screen 24 by a magnetic deflection yoke, which for simplicity, is not shown in the figure. A shadow mask 26 having a plurality of spaced electron beam passing apertures 26a and a skirt portion 28 around the periphery thereof is spaced from the screen 24. The shadow mask skirt portion 28 is securely attached to a shadow mask mounting fixture 30 around the periphery of the shadow mask. The shadow mask mounting fixture 30 is attached to an inner surface of the glass envelope 12 and may include conventional attachment and positioning structures such as a mask attachment frame and a mounting spring which, for simplicity, also are not shown in the figure. The shadow mask mounting fixture 30 may be attached to the inner surface of the CRT's glass envelope 12 and the shadow mask 26 may be attached to the mounting fixture by conventional means such as weldments or a glass-based frit. Referring to Figure 2, there is shown a portion of a glass display panel, or faceplate, 40 having a phosphor layer 42 on the inner surface thereof and an outer coating 44 on the outer surface thereof. The glass display panel 40 is shown in Figure 2 as being flat although the present invention is applicable to both curved and flat display screens. In addition, while the present invention has been described thus far in terms of use on the outer surface of the display panel of a CRT, the present invention is not limited to use with this type of display device. For example, the outer coating of the present invention may be used equally as well on the outer surface of the display panel of virtually any type of self-emitting colour display device, i.e. where the video image is produced by phosphor activated by energetic electrons incident thereon. The phosphor layer 42 on the inner surface of the glass display screen 40 may be in the form of a large number of discrete dots or stripes. The outer coating 44 typically includes an inner antistatic layer 46 and an outer antireflective layer 48 which are described in more detail below. A conductor 50 may be attached to the inner antistatic layer 46 or to the outer surface portion of the display screen 40 for electrically coupling the display screen to neutral ground potential. Referring to Figure 3, there is shown a flow chart of the steps involved in preparing and applying the combination inner antistatic layer 46 and outer antireflective layer 48 forming the outer coating 44 on the display screen of a colour CRT in accordance with the principles of the present invention. The process starts in the addition of an organic colour dye and a silane coupling or binding agent either to an antistatic solution (see step 52) or to an antireflective solution (see step 54). The organic colour dye and silane coupling agent combination may thus be added either to the inner antistatic layer or to the outer antireflective layer applied to the outer surface of the faceplate of the CRT. The antistatic solution typically includes conductive metal oxides, such as, for example, Sb-doped SnO 2 , In-doped SnO 2 , etc., and silane such as TCS (Tetrachloro Silane) or TES (Tetraethoxy Silane) and water. The antireflective solution applied as the outer layer typically includes silanes such as TCS or TES, water, etc. The coupling agent added to either the antistatic or antireflective solution serves as a binding agent bridging the organic colour dye and silane together. An azo, triarylmethane, or anthraquinone dye, with acid substituents such as nitro, carboxy or sulphonic acid may be employed as the organic dye while the silane coupling agent suitably has the general formula where M is a functional group, e.g., -NH 2 amino group or epoxy group, and R,R' and R" are alkyl groups. When the organic dye and silane coupling agent are added to either the antistatic solution or to the antireflective solution containing silane (either halogenated silane or alkoxyl silane), the following reaction occurs By means of this reaction, the organic dye becomes bonded to the silane compound through the bridging of the silane coupling agent. The organic dye thus becomes permanently linked to the other coating materials in either the antistatic or antireflective layer. With the organic dye permanently bonded to the silane compound in the antistatic or antireflective layer, bleaching of the dye from the outer coating of the CRT is thus prevented whether the dye is added to the inner or outer layer of the coating on the surface of the glass faceplate. The organic dye is added to either the antistatic or antireflective solution in the range of 0.1-25 wt%, with the silane coupling solvent added in the range of 0.1-20 wt%. As indicated above, the typical antistatic solution includes a conductive metal oxide, silane and water, while the typical antireflective solution also includes a silane, water and other alcohol-based solvent mixtures. At step 56, the faceplate is preheated to a temperature in the range of 40-45° C. The outer surface of the faceplate is then covered with the antistatic solution at step 58 to form an antistatic layer. The coated outer surface of the faceplate is then further covered at step 60 with the antireflective solution over the inner antistatic layer to form an outer antireflective layer. The antistatic and antireflective layers may be applied by either a conventional spin or spray method. The faceplate with the inner antistatic layer and the outer antireflective layer is then baked at step 62 at a temperature of 110-180° C for 20-40 minutes and then air cooled to room temperature at step 64. As an example of the present invention, an organic dye as described above was added to an antireflective solution in the amount of 0.5 wt%. The organic dye functioned as a light absorber in the range between 580-600 nm so as to remove overlap between the red and green spectra. 0.1 wt% of a silane coupling agent was added to 1 litre of antireflective solution containing 1.0 wt% tetraethoxy silane and other solvents. The faceplate of the CRT was then heated to a temperature of 40° C and was spin coated with a layer of antistatic solution containing 1.0 wt% of antimony doped tin oxide and other solvent. The antistatic coated faceplate was then dried and again preheated to 40° C followed by spin coating of the antireflective layer over the inner antistatic layer using the solution described above. The faceplate was then baked at 180° C for twenty minutes and then air cooled. A colour spectrum of the CRT faceplate coated with an inner antistatic layer and an outer antireflective layer having a light absorbing dye is shown in Figure 4 which is a CIE chromaticity diagram representing the spectrum of colours displayed on a typical colour CRT faceplate. The shift in the apices of the triangle shown in dashed line form illustrates that the red and green colours become noticeably purer as compared with a CRT faceplate without the light absorbing organic dye and silane coupling agent combination. After the data shown in Figure 4 was taken, the coated faceplate was then dipped into boiling water for five minutes, with no evidence of dye bleaching occurring as observed by both the lack of colour change of the water and a conventional cloth wiping test using both water and ethanol as a cleaning agent. The results shown in Figure 4 are for a litre antireflective solution containing tetraethoxy silane and other solvents to which has been added naphthol red dark (CI-12355) dye at 0.5 wt% and 0.1% γ-amino propyl triethoxy silane. A colour spectrum of a CRT faceplate coated with an inner antistatic layer and an outer antireflective layer having a light absorbing dye in accordance with another aspect of the present invention is shown in Figure 5 which is a CIE chromaticity diagram representing the spectrum of colours displayed on a typical colour CRT faceplate. The results shown in Figure 5 are for a 1 litre antireflective solution containing tetraethoxy silane and other solvents to which has been added naphthol Yellow S (CI-10316) dye at 0.5 wt% and 0.1% γ-amino propyl triethoxy silane. The shift in the apices of the triangle shown in dashed line form in Figure 5 illustrates that the green and blue colours become noticeably purer as compared with the CRT faceplate without the light absorbing organic dye and silane coupling agent combination as shown by the triangle in solid lines in the figure. As in the case of the organic colour dye for filtering light in the range between 580-600 nm as described above, there was no evidence of bleaching of the organic colour dye from the antireflective layer in the coating used for absorbing light in the wavelength range of 480-550 nm as illustrated in Figure 5 when the faceplate with its coating was dipped into boiling water and subjected to a wiping test with a cleaning agent. It should be noted that the two aforementioned organic colour dyes may both be added either to the antireflective coating or to the antistatic coating to provide light absorption between the red and green colour phosphors as well as between the green and blue colour phosphors. There has thus been shown a surface coating with enhanced colour contrast for a self-emitting colour display such as a CRT employing a light emitting phosphor coating responsive to energetic electrons incident thereon. The coating may be either in the form of an antireflective layer or an antistatic layer or a combination thereof, on the outer surface of the CRT for filtering out light between the primary colours produced on the display screen, i.e. between the colours red and green and between the colours green and blue. An organic colour dye is added to the coating layer in combination with a silane binding agent which couples the organic colour dye to the silane in either the antireflective or antistatic layer. The dye functions as a colour filter absorbing only light within the frequency range between adjacent primary colours, such as in the frequency range of 460-500 nm between the blue and green colour phosphors, as well as in the range of 560-600 nm between the green and red colour phosphors. The silane coupling agent serves as a binding agent bridging the organic colour dye and silane together and preventing separation and escape of the organic colour dye from either of these layers of the outer coating on the display screen. Although the invention has been described with particular reference to coatings applied to screens or faceplates of self-emitting display devices, e.g. CRTs, it will be appreciated that the application also finds application in the coating of removable screens, e.g. antireflective screen filters or the like, for such self-emitting display devices. While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims.

Claims: A light-transmitting screen (14), suitable for a glass faceplate of a self-emitting display device, comprising an antireflective/antistatic layer (44) containing a silane disposed on the outer surface of said screen, said layer containing at least one organic colour dye for absorbing light between two adjacent primary colours, characterised in that said antireflective/antistatic layer (44) contains a silane binding agent, the silane binding agent coupling said at least one organic colour dye to the silane in said layer (44). A screen according to claim 1, characterised in that said at least one organic colour dye comprises an azo, triarylmethane, or anthraquinone dye having at least one acid substituent. A screen according to claim 2, characterised in that said acid substituent is a nitro, carboxy or sulphonic acid substituent. A screen according to any one of claims 1 to 3, characterised in that said silane binding agent has the general formula where M is a functional group and R, R' and R" are alkyl groups. A screen according to claim 4, characterised in that said functional group is a -NH 2 amino group or a epoxy group. A screen according to any preceding claim, characterised in that said at least one organic colour dye comprises from 0.1 to 25 wt% of said antireflective/antistatic layer. A screen according to any preceding claim, characterised in that said silane binding agent comprises from 0.1 to 20 wt% of said antireflective/antistatic layer. A screen according to any preceding claim, characterised in that said at least one organic colour dye comprises naphthol yellow S (CI-10316) dye for absorbing light in the wavelength range of 480-500 nm and/or naphthol red dark (CI-12355) dye for absorbing light in the wavelength range of 580-600 nm, and in that said silane binding agent comprises γ-amino propyl triethoxy silane. A screen according to any preceding claim, characterised in that said at least one organic colour dye includes first and second organic colour dyes disposed in said antireflective/antistatic coating for absorbing light between the primary colours of red and green and between the primary colours of green and blue. A screen according to any preceding claim, characterised in that said antireflective/antistatic coating includes an inner antistatic layer and an outer antireflective layer, each containing a silane, and wherein said at least one organic colour dye and said silane binding agent are disposed in either said inner antistatic layer or said outer antireflective layer. A self-emitting display device (10) comprising a light-transmitting screen according to any preceding claim, the screen comprising a glass faceplate (14) having disposed on its inner surface a phosphor coating (42) responsive to energetic electrons incident thereon, for providing three primary colours of red, green and blue. A method of providing a coating with enhanced colour contrast on an outer surface of a video display panel whereon are presented the primary colours red, green and blue, said method comprising the steps of: adding a combination of at least one organic colour dye and a silane binding agent to a coating solution containing a silane to form a bond between said organic colour dye(s) and said silane and to prevent removal of said organic colour dye(s) from said coating solution, wherein said at least one organic colour dye absorbs light between and enhances spectral separation between two adjacent primary colours; preheating the video display panel to a first temperature in excess of room temperature; applying said coating solution containing said organic colour dye(s) and silane binding agent combination to an outer surface of the video display panel; heating the video display panel with said applied coating solution to a second temperature in excess of said first temperature for a predetermined period of time; and air cooling the video display panel to room temperature. A method according to claim 12, characterised in that said organic colour dye is an azo, triarylmethane, or anthraquinone dye having at least one acid substituent. A method according to claim 13, characterised in that said acid substituent is a nitro, carboxy or sulphonic acid substituent. A method according to any one of claims 12 to 14, characterised in that said silane binding agent has the general formula where M is a functional group and R, R' and R" are alkyl groups. A method according to claim 15, characterised in that said functional group is a -HN 2 amino group or a epoxy group. A method according to any one of claims 12 to 16, characterised in that said at least one organic colour dye comprises naphthol yellow S (CI-10316) dye for absorbing light in the wavelength range of 480-500 nm and/or naphthol red dark (CI-12355) dye for absorbing light in the wavelength range of 580-600 nm, and in that said silane coupling agent is γ-amino propyl triethoxy silane. A method according to any one of claims 12 to 17, characterised in that the combination of said at least one organic colour dye and silane binding agent is added either to a first antistatic coating solution or a second antireflective coating solution, and in that, following said preheating of the video display panel, the first antistatic coating solution is applied to the outer surface of the video display panel and the second antireflective coating solution is applied to the first antistatic coating solution. A method according to any one of claims 12 to 18, characterised in that said first temperature is in the range 40° to 45°C. A method according to any one of claims 12 to 19, characterised in that said second temperature is in the range 110° to 180°C and said predetermined time period is in the range 20 to 40 minutes.

Inventors: HSU MING-YU, HU CHUN-MIN, TONG HUA-SOU, WANG KUO-CHU, YU YU-CHUNG, HSU,MING-YU, HU,CHUN-MIN, TONG, HUA-SOU, WANG, KUO-CHU, YU, YU-CHUNG
Applicants: CHUNGHWA PICTURE TUBES LTD, CHUNGHWA PICTURE TUBES, LTD.
Application year: 1995
Main classification: H01J29
IPC codes: H01J29, C09D183, H01J9