Abstract:
A novel light scattering reflector and methods of making and using it are disclosed. The reflector, which can advantageously be used in conjunction with liquid crystal displays, includes a layer carrier formed of glass or rigid PVC foil. The layer carrier is roughened by sandblasting, impressing with a grooved die, or by other techniques to provide an irregular surface. A reflective metal coating is subsequently evaporated onto the roughened surface to complete the reflector structure. Auxiliary materials may also be coated onto the layer carrier to improve the characteristics of the device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a reflector with a light-scattering surface for liquid-crystal displays operated in reflection, as well as to a method for the manufacture of such reflectors. 
     2. Description of the Prior Art 
     Reflectors of the type described are known for example from DT-OS 2,310,219. They consist of glass or plastics and are ground or sand-blasted on their front surface, while their other surface is provided with a highly reflective metal layer (possibly: aluminium, nickel or chromium). 
     From now abandoned U.S. patent application Ser. No. 610,753 filed Sept. 5, 1975 which is assigned to the assignee of this application there is also known a diffusely-scattering reflector, in which bronze paint is applied either to the surface of the polarizer turned away from the liquid crystal cell or upon a separate carrier. 
     In addition to these diffusely-reflecting reflectors there are also known reflectors for liquid crystal display devices which employ specular (i.e. mirror-like) reflection. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a light scattering reflector for a liquid crystal display which is simple and economical to manufacture and which exhibits an optimal light diffusion characteristic which may be matched to a particular display element. 
     Another object of this invention is the provision of a novel method of producing such reflectors. 
     Briefly, these and other objects are achieved in accordance with the invention wherein a light scattering surface consisting of a highly reflective metal layer is produced and is applied to a layer carrier provided with depressions. 
     Thus, for example, the intensity of scattered light effective for the display with the novel reflectors is considerably greater than with diffusion-scattering reflectors with cosine characteristics, since in these the light is scattered through too great an angle and is strongly absorbed by the polarizers. As a result, with the employment of diffusely-scattering reflectors a considerable part of the incident light is absorbed in the display. Accordingly, display devices constructed with the new reflectors are characterized by high brightness and good readability. In addition the background reflection appearing with mirror-like reflection in planar reflectors is absent, as well as the limitation to very narrow viewing angles associated with these reflectors. 
     The manufacture of the novel reflectors is effected in accordance with the invention such that depressions are produced by roughening and subsequent etching of the surface of the layer carrier, or that they are impressed into the layer carrier with the help of a stamp or a roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the novel reflectors as well as of the method for their manufacture are explained in the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIGS. 1a to 1f are schematic illustrations of the different steps in the method for the manufacture of the novel reflectors of the invention; 
     FIG. 2 is a photomicrograph of the surface of the novel reflector provided with depressions; 
     FIGS. 3 and 4 are graphical illustrations showing two diffusion characteristics of the reflectors in accordance with FIG. 1f for light incident at different angles. 
     FIG. 5 is an illustration of a liquid crystal display device with a reflector of the type shown in FIG. 1f; 
     FIG. 6 is a schematic illustration of a first apparatus and technique for producing the desired surface depressions; 
     FIGS. 7a, 7b and 7c are schematic illustrations of a second apparatus and technique for producing the desired surface depressions; and 
     FIG. 8 is a schematic illustration of a third apparatus and technique for producing the desired surface depressions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIGS. 1a to 1f, there are represented the different steps of the method for the manufacture of a novel reflector, especially suitable for use with liquid-crystal display devices, which is designated by 1 in FIG. 1f. For this purpose a major surface 2 of a substrate 3 (FIG. 1a), which in the present case may consist of a small glass plate of dimensions 20.0 × 9.4 × 0.4 mm. is subjected to a grinding operation until the glass plate has the desired thickness and its surface 2&#39; has a uniformly roughened structure (FIG. 1b) caused by the grinding medium (a corresponding structure may be obtained by sand blasting the glass surface). Silicon-carbide powder of grain sizes 400, 800, 1000, or 1200 (such as is supplied by the Struers firm in Denmark) has proved particularly suitable. The grinding process itself especially in the mass-production of reflectors is preferably effected with the help of a lapping machine such as is employed in semiconductor technology for the lapping of silicon wafers. Less than 5 minutes are required in order to obtain a substantially uniformly roughened surface. 
     Following the grinding or sand-blasting operation the small glass plate is cleaned several times in distilled water, for example in an ultrasonic bath and finally immersed for 3 to 10 minutes in an etching bath, that preferably consists of a 1:1 mixture of H 2  O and HF, and moved to and fro in the acid. 
     Finally the thus treated glass plate is again rinsed with distilled water and then dried. 
     FIG. 2 shows a photomicrograph of such a surface after an etching time of 15 minutes. The bubble-like structure is characteristic of the etching process. Shorter etching times result in bubbles (depressions) of smaller diameters. 
     In FIG. 1c there is schematically represented a section of such a glass plate. The depressions 4 are here shown in an idealised form. 
     The highly reflective metal layer is applied upon the surface which has been etched and cleaned with water. In order to achieve a better attachment of this metal layer to the glass plate, it has proved to be advantageous first to evaporate upon the glass surface a chromium layer 5 (FIG. 1d) some 10 to 50A thick and then to deposit upon this chromium layer 5 the reflective metal layer 6, which may for example consist of silver (FIG. 1e). For protection of the silver layer 6 against ambient influences, there is finally applied upon this layer a layer 7 of silicon dioxide about 250A thick (FIG. 1f). 
     FIG. 3 shows how the intensity of the reflected and scattered light (normalized to a maximum value of 1) varies in accordance with the angle of reflection, when measured in the plane of incidence of a parallel light bundle incident at 45° to the normal upon a glass plate ground with silicon carbide of grain No. 800. The curves a, b and c correspond to reflectors which were respectively etched for 0, 5, and 10 minutes. The longer the etching time, the narrower becomes the lobe characterizing the diffusion. 
     FIG. 4 shows corresponding test results for the same reflectors for a light beam incident at 30° to the normal. 
     By using the fine grained grinding powder of grade 1200, reflectors are obtained of which the dispersion-representing lobes for the longer etching times likewise become narrower. 
     FIG. 5 shows schematically a liquid-crystal display device, in which the new reflector 1 is employed. The actual liquid-crystal cell 8 (which may in practice be a twisted cell) is situated between two polarizers 9, 10. The reflector 1, which is situated behind the polarizer 10 on the side remote from the liquid crystal cell 8, is either stuck directly to the polarizer 10 or is positioned behind the polarizer. Any epoxy cement of high quality (e.g. Lens Bond M 62) may be employed for adhesion. 
     The following TABLE gives in column 4 the total reflected radiant power in front of the display device for seven liquid crystal display devices, which differ only in the reflectors employed, of which the characteristic values are reproduced in columns 2 and 3. A disk smoked with MgO serves as a reference value. The measurements are effected with the help of a monochromatic light beam (λ=6328 A) incident on the polarizer 9 at 18.5° to the normal. The regular reflection at the surface of the polarizer 9 is already taken into account in the values given in Column 4 of TABLE 1. 
     
                       TABLE 1______________________________________                            TotallyDisplay                 Etching  ReflectedNo.    Grain No.        Time     Radiant Power______________________________________1      (layer of aluminium bronze  applied by screen printing) 19.3%2      1200             3&#39;    20&#34;  30.8%3      1200             4&#39;    30&#34;  32.5%4      1200             6&#39;         33.3%5      1200             7&#39;    30&#34;  35.2%6      1200             10&#39;        35.2%7       800             15&#39;        34.6%MgO    --               --         100.0%______________________________________ 
    
     As may be taken from the TABLE, the effective radiant power of, for example, displays 5 and 6, is greater by 85.4% than that of the known display 1, that employs a reflector with a layer of aluminium bronze applied by screen printing. 
     In FIG. 6 there is represented a method for the production of the depressions of the novel reflector that is particularly suitable for mass production. A glass plate 11, which in accordance with the present embodiment has been provided with a reflector structure, serves as a stamping die. Under pressure and heat (about 120° C) the reflector structure is impressed into a hard PVC foil 12 about 0.2 mm thick. After evaporation with a thin metal layer there is thus obtained a thin, flexible reflector which may very readily be formed by cutting or stamping into any desired shape, and which exhibits the same dispersion characteristics as the glass plate provided with the reflector structure. 
     FIG. 7 shows a method for the manufacture of reflectors that exhibit the same (positive) structure as the glass plate 11 (FIG. 6 and FIG. 7a). For this purpose the glass plate is made conductive by chemical silver flashing and thereafter thickened by electrotyping (FIG. 7b). There thus results what is called a master matrix (negative) 13 of high mechanical strength, which is readily released from the glass plate 11. Afterwards its structure is impressed with this matrix 13 into the hard PVC foil 12 at a raised temperature (some 120° C) and under pressure (FIG. 7c). The foil to be impressed is, after pressing, advanced by a stamp width and rolled up again. The whole roll of foil, thus impressed, is then, in passing through a high vacuum evaporating apparatus, evaporated with aluminium, for example. These reflector foils can afterwards be laminated with a polarizing foil coated with an adhesive, or else be divided by stamping or cutting into pieces of appropriate size. 
     In addition to the stamping of a separate reflector foil 12 the rear side of the polarizer 10 (FIG. 5) can also be provided with the desired reflector structure by the described method. Additionally it may be advantageous for mass-production to employ instead of an impressing stamp 11 (FIG. 6) or 13 (FIG. 7c) a roller 14 (FIG. 8), which exhibits the corresponding reflector structure. 
     For liquid-crystal displays, which are to be employed both in daytime and also in the night (wrist-watches) the use of semi-transparent, diffuse metal reflectors has proved especially advantageous. In night-time operation, a light source that is situated at the rear of the reflector, supplies the necessary illumination. 
     Such reflectors are obtained if the reflecting metal layers (Al, Ag, Au or Cu) on the foils or glass plates provided with the reflector structure are evaporated sufficiently thinly. Such layers then have exactly the required characteristics: diffuse, polarization-retaining reflection in the reflection mode and transmission with rear illumination. The magnitude of the transmission and the reduction of the reflection as compared with a nontransmissive reflector depends upon the layer thickenss of the evaporated metal layer. For a silver layer of 200 A the transmission amounts, for example, to some 20%. To still obtain sufficient reflection, the transmission of the reflectors should lie between 20 and 50% and correspondingly the layer thickness of the reflective silver of aluminium layer between 100 and 400 A. Instead of on glass plates, the reflective metal layers may also be disposed on the roughened rear side of the back polarizer. 
     In addition to the foregoing the following preferred dimensional ranges are noted: 
     Chromium Layer--between 10 and 100 A thick 
     Reflective Layer--between 400 and 1000 A thick 
     SiO 2  Protective Layer--between 100 and 1000 A thick. 
     Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.