The present invention relates to a method of making a display device, such as a cathode ray tube, a liquid crystal display device and a solid-state electroluminescent display device, and to a display device made by the method. More particularly the invention is concerned with reducing the reflectance of a viewing screen.
Display devices have to be capable of being viewed under varying lighting conditions. However since the brightness of an image being displayed on a device is limited, the contrast must be as high as possible, so that even under conditions of high-intensity ambient-light levels a clearly visible picture is presented to the viewer. Consequently display device manufacturers endeavour to increase the contrast of such devices and one technique is to reduce the amount of ambient light reflected by the glass faceplate, glass having a reflectance of 4.9% .
As the sensitivity of the human eye has a peak sensitivity at about 550 nm (nanometers) and decreases to zero below 400 nm and above 700 nm, then this is the part of the spectrum which is of interest.
There have been prior proposals relating to reducing the reflectance from cathode ray tube faceplates. European Patent Publication No. 0 131 341, to which U.S. Application Ser. No. 926,820 corresponds, discloses mechanically roughening the outer surface of a cathode ray tube faceplate and vapour depositing a single layer of .lambda./4 thick magnesium fluoride (MgF.sub.2), where .lambda.is the peak sensitivity wavelength, that is about 550 nm. The refractive indices of air, glass and MgF.sub.2 are respectively 1.0, 1.57 and 1.39. Although such an arrangement reduces the reflectively at the faceplate, it does have some disadvantages including (1) the reduction in reflectivity is a maxima at the peak sensitivity wavelength and is progressively less on either side of this peak sensitivity wavelength and (2) in order to obtain a layer having the required degree of hardness the MgF.sub.2 has to be deposited onto a heated substrate. Typically the minimum temperature for the substrate, that is the faceplate, is 200.degree. C. but a temperature of 250.degree. C. is preferable. As the heating of the faceplate takes place under vacuum, the heat transfer is by radiation which is slow and takes about one hour. It is possible to vapour deposit layers of MgF.sub.2 at lower temperatures but these layers do not have the required degree of hardness to be of use as anti-reflection coatings. Such layers cannot be hardened further by annealing after deposition.
In the description and claims reference is made to materials having low, medium and high refractive indices (n). These relative terms are related to the refractive index of the material, for example glass, forming an optically transparent faceplate panel. For a glass having n=1.5 then a low value for n may be equal to or less than 1.5, a medium value for n may lie in the range of greater than 1.50 and less than 1.80, and a high value for n may be equal to or greater than 1.80.
The reflectivity characteristic of the anti-reflection coating should be such that in the visible part of the spectrum it has a substantially constant value and ideally be zero. In the case of a laser it is known to use an interference filter comprising a first layer having a relatively high refractive index applied to the faceplate and a contiguous second layer having a relatively low refractive index. Such an arrangement is known as a V-coating because the reflection characteristic is of generally V-shape with the vertex or minimum reflectance at the wavelength of the laser light. The flanks of the V-characteristic can be modified using a three layer interference filter comprising a first layer of a material, such as Al.sub.2 O.sub.3, having a medium refractive index, a second thicker contiguous layer of a material, such as TiO.sub.2, having a relatively high refractive index and a third contiguous layer of a lesser thickness than the second layer and being of a material, such as MgF.sub.2, having a relatively low refractive index. This coating is known as a W-coating because it has a wider characteristic with two minima, one on either side of the centre wavelength. As far as is known such anti-reflection coatings are vapour deposited onto substrates heated to about 300.degree. C. in order to obtain the required degree of hardness and scratch resistance. The materials used in making these layers cannot be hardened by subsequent annealing.