GB1587375A

GB1587375A - Partly transparent reflector for liquid crystal displays and a method of producing it

Info

Publication number: GB1587375A
Application number: GB525178A
Authority: GB
Original assignee: BBC BROWN BOVERI and CIE; BBC Brown Boveri AG Switzerland
Current assignee: BBC BROWN BOVERI and CIE; BBC Brown Boveri AG Switzerland
Priority date: 1977-02-11
Filing date: 1978-02-09
Publication date: 1981-04-01
Also published as: JPS53100265A; DE2709731A1; HK48981A; FR2380558A1; CH610668A5

Abstract

Liquid-crystal display (1) having an illuminating element (2) and a partially transparent reflector arranged between the liquid-crystal cell and the illuminating element (2). The reflector consists of a translucent aluminium foil (3) whose surface has a scattering effect and which has holes or pores (4) in such an extent that the transmittance is 10 to 30%. This results in achievement of effective scattering in daylight and in effective illumination during night time operation, and at the same time a depolarization of the incident light is avoided. <IMAGE>

Description

(54) PARTLY TRANSPARENT REFLECTOR FOR LIQUID CRYSTAL DISPLAYS, AND A METHOD OF PRODUCING IT (71) We, BBC BROWN, BOVERI AND COMPANY LIMITED, a body corporate organised and existing under the laws of Switzerland, of ClI-5401 Baden, Switzerland, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a partly transparent reflector for liquid crystal displays, which has a translucent film between the cell which contains the liquid crystal and a lighting element.

According to the technique hitherto customary a translucent film is used which has a depolarising action; when used in daylight the display is often too dark. The lighting element is either a thin sheet of plexiglass (trade mark) with a microlamp or a betalight bulb.

The problem underlying the invention is that of finding a reflector which has the following properties: 1) good scatter characteristics for incident light for the use of the display in daylight; 2) transmission of about 10 to 30% for the use of the display at night with integrated rear illumination; 3) It must not depolarise incident light.

According to the present invention there is provided a liquid crystal display comprising a cell which contains the liquid crystal and a lighting element, and a partly translucent film, disposed between the liquid crystal and lighting element, in the form of a thin aluminium foil whose surface has a scattering action and which contains a number of holes or pores which are of such a nature that the transmission amounts selectively to from 10 to 30%.

The invention also provides a method of producing the above liquid crystal display comprising forming the partly translucent film by etching a piece of thin aluminium foil in an etching bath for from 2 to 3 minutes, and disposing the translucent film between the liquid crystal and the lighting element.

In this connection it may be observed that from German Offenlegungsschrift 2 264 702 a partly transparent reflector in the interior of a liquid crystal cell is known which is either in the form of a semi-transparent mirror or of a vapour-deposited metal coating provided with holes by an etching process.

The invention will now be explained more fully with reference to Figures 1 to 3. Figure 1 shows the usual arrangement with the translucent film 3 which generally depolarises incident light; the film 3 is situated between the cell which contains the liquid crystal 1 and the lighting element 2.

In Figure 2 the translucent film 3 is of the type according to the invention. 3 is a thin foil of aluminium whose surface has a scattering action and which contains a number of holes or pores (4), which are of such a nature that transmission amounts selectively to from 10 to 30%. This aluminium foil may be adhesively bonded directly to the bottom polariser.

In order to improve the lighting action, the lighting element may also have on its lower face a reflecting layer 5, for example aluminium applied by vapour deposition.

Figure 3 shows a method of producing the semi-transparent reflector proposed in accordance with the invention. A piece of thin aluminium foil is used which has a thickness of about 15 to 20 jXum and which is etched for from 2 to 3 minutes, for example in a mixture of one part H20 and one part of 38-40% HF. It is then rinsed with distilled water. Because of the presence of foreign elements in the aluminium, or because of other inhomogeneities in the surface layer, the etching action is not uniform, so that a multiplicity of small holes with random distribution are formed.

After this chemical treatment the thickness of the aluminium foil is about 10cm. The size of the etched holes is from 5 to 200,um, and their density ranges between 10 and l,000/mm2.

The transmission achieved is determined in practice by the treatment time.

In the detailed example of embodiment illustrated in Figure 3 the process starts with a roll 6 of thin aluminium foil. The aluminium passes through the bath 15, in which the chemical treatment takes place. With the aid of the guide rollers 7,8, and 8' the foil passes through a bath 16 which is filled with continuously renewed distilled water. Between the bath 15 and the bath 16 the aluminium foil must not come into contact with air. With the aid of the guide roller 9 the aluminium foil passes out of the bath, is dried, and then passes between the lamp 13 and the photocell 14, where the transmission can be verified and the speed of passage of the film through the etching bath can be controlled.

With the aid of the guide roller 10 the aluminium foil is brought into contact with the polariser coming from the roll 11 and adhesively bonded to it, whereupon the laminate is wound onto the reel 12.

Instead of a single aluminium foil which is etched and then laminated with the polariser, it is also possible to etch by the method described above a polariser foil already laminated with an aluminium foil (the polariser-reflector combination already provided with its adhesive coating is not attacked by the etching solution). The critical lamination of the aluminium foil which has been mechanically weakened by the etching process is thus eliminated. Moreover, the etching process cannot give rise to crumpling or creasing. In order to increase the resistance to wear of the etched side of the foil, it may be protected by a thin coating of transparent varnish.

The process is relatively inexpensive; since it is a continuous process, it can be applied on a large industrial scale. The reflector produced in this manner has metallic brilliance.

The degree of transmission can be selected within a wide range (10--30%).

Finally, no depolarisation occurs on the reflector.

WHAT WE CLAIM IS: 1. A liquid crystal display comprising a cell which contains the liquid crystal and a lighting element, and a partly translucent film, disposed between the liquid crystal and lighting element, in the form of a thin aluminium foil whose surface has a scattering action and which contains a number of holes or pores which are of such a nature that the transmission amounts selectively to from 10 to 30%.

2. A liquid crystal display according to claim 1, wherein the aluminium foil is adhesively bonded directly to a bottom polariser of the liquid crystal cell.

3. A liquid crystal display according to claim 1 or 2, wherein the lighting element also has on its lower face a reflecting layer.

4. A liquid crystal display according to claim 3, wherein the reflecting layer consists of vapour deposited aluminium.

5. A method of producing a liquid crystal display according to claim 1 comprising forming the partly translucent film by etching a piece of thin aluminium foil in an etching bath for from 2 to 3 minutes, and disposing the translucent film between the liquid crystal and the lighting element.

6. A method according to claim 5, wherein the thickness of the aluminium foil amounts to about 15 to 20,us.

7. A method according to claim 5, wherein the etching bath consists of one part of H2O and one part of 3840% HF.

8. A method according to claim 5, 6 or 7, wherein the thin aluminium foil wound off a roll, passes through the etching bath, and is then passed with the aid of guide rollers through a bath which is filled with continuously renewed distilled water, and wherein the aluminium foil is passed out of the bath with the aid of another guide roller and thereupon subjected to a drving process.

9. A method according to claim 8, wherein the dried aluminium foil passes through between a lamp and a photocell, where its transmission can be verified and the speed of passage of the foil through the etching bath can be controlled.

10. A method according to claim 9, wherein with the aid of another guide roller the aluminium foil is brought into contact with a polariser wound off a roll and adhesively bonded thereto, and that the laminate is then wound onto a reel provided for the purpose.

11. A method according to claim 5, wherein the thin aluminium foil is laminated to a polariser foil prior to the etching process.

12. A liquid crystal display, substantially as hereinbefore described with reference to and as shown in Figure 2 of the accompanying drawings.

13. A method of producing a liquid crystal display according to claim 1 substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In the detailed example of embodiment illustrated in Figure 3 the process starts with a roll 6 of thin aluminium foil. The aluminium passes through the bath 15, in which the chemical treatment takes place. With the aid of the guide rollers 7,8, and 8' the foil passes through a bath 16 which is filled with continuously renewed distilled water. Between the bath 15 and the bath 16 the aluminium foil must not come into contact with air. With the aid of the guide roller 9 the aluminium foil passes out of the bath, is dried, and then passes between the lamp 13 and the photocell 14, where the transmission can be verified and the speed of passage of the film through the etching bath can be controlled. With the aid of the guide roller 10 the aluminium foil is brought into contact with the polariser coming from the roll 11 and adhesively bonded to it, whereupon the laminate is wound onto the reel 12. Instead of a single aluminium foil which is etched and then laminated with the polariser, it is also possible to etch by the method described above a polariser foil already laminated with an aluminium foil (the polariser-reflector combination already provided with its adhesive coating is not attacked by the etching solution). The critical lamination of the aluminium foil which has been mechanically weakened by the etching process is thus eliminated. Moreover, the etching process cannot give rise to crumpling or creasing. In order to increase the resistance to wear of the etched side of the foil, it may be protected by a thin coating of transparent varnish. The process is relatively inexpensive; since it is a continuous process, it can be applied on a large industrial scale. The reflector produced in this manner has metallic brilliance. The degree of transmission can be selected within a wide range (10--30%). Finally, no depolarisation occurs on the reflector. WHAT WE CLAIM IS:

1. A liquid crystal display comprising a cell which contains the liquid crystal and a lighting element, and a partly translucent film, disposed between the liquid crystal and lighting element, in the form of a thin aluminium foil whose surface has a scattering action and which contains a number of holes or pores which are of such a nature that the transmission amounts selectively to from 10 to 30%.