Liquid crystal display devices

• US 3,910,681 A
• Filed: 01/11/1974
• Issued: 10/07/1975
• Est. Priority Date: 01/11/1973
• Status: Expired due to Term

• Current Assignee: Brian William Ely, George Elliott
• Original Assignee: Brian William Ely, George Elliott
• Inventors: Elliott, George, Ely, Brian William
• Primary Examiner(s): Bauer, Edward S.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US05/432,583
• Time in Patent Office: 634 Days
• Field of Search: 350/16LC,164,166
• US Class Current: 349/114
• CPC Class Codes: G02F 1/1335 Structural association of cells with optical devices, e.g. polarisers or reflectors
• CPC Class Codes: G02F 1/133553 Reflecting elements associated to illuminating devices G02F1/133605

Abstract

A liquid crystal display arrangement is described in which the liquid crystal cell includes a reflective mirror in the form of a number of layers of dielectric material, the refractive indices and thicknesses of the various layers being chosen to allow reflection of light scattered from the liquid crystal material at large angles to the normal whilst light from external light sources incident normally on the cell is not reflected thereby improving the contrast of the display.

Claims

1. A liquid crystal display device operating by reflection for radiant energy within a given spectral range and including a front plate and a reflective mirror with a layer of liquid crystal material therebetween, wherein the reflective mirror is a dielectric mirror consisting of layers of material of different refractive indices the thicknesses of which are such that the mirror is substantially transparent for said radiant energy which is incident at angles close to normal incidence and is substantially reflective for said radiant energy which is incident at other angles.

2. A device as claimed in claim 1 and wherein said given spectral range is the visible part of the spectrum.

3. A device as claimed in claim 1 and wherein said layers are alternately of high and low refractive index.

4. A device as claimed in claim 1 and wherein said dielectric mirror has three to seven layers.

5. A device as claimed in claim 1 and wherein each of said layers has a thickness which is equal to a quarter wavelength at a frequency in the ultraviolet portion of the spectrum.

6. A device as claimed in claim 1 and wherein said dielectric mirror comprises alternate layers of zinc sulphide and magnesium fluoride, both extreme layers being of zinc sulphide.

7. A device as claimed in claim 1 and further comprising two electrodes to activate the device, one of which is provided as a thin light transmissive conductive coating on the surface of the dielectric mirror adjacent the liquid crystal material.

8. A device as claimed in claim 1 and further comprising two electrodes to activate the device, one of which is provided as an electrode on the extreme surface of the dielectric mirror remote from the liquid crystals material.

9. In a liquid crystal display device of the reflective type including first and second insulating substrates disposed one parallel to the other, a first electrode comprising a coating of light transmissive metal formed adjacent to the face of said first substrate between said first and second substrates, a second electrode comprising a coating of light transmissive metal formed on the face of said second substrate between said first electrode and said second substrate, and a layer of nematic liquid crystal formed between said aforementioned electrodes, and means connected to said electrodes for applying a voltage across said liquid crystal so as to cause light incident on said liquid crystal to be obliquely scattered therein, the improvement wherein there is provided between said liquid crystal and said first substrate and adjacent to said first electrode a dielectric mirror comprising a plurality of layers of material of different refractive indices, the thickness of which are such that light within the visible spectrum impinging at an approximate normal angle of incidence and not scattered within said liquid crystal is substantially transmitted through said mirror, while visible light impinging on said device and obliquely scattered within said liquid crystal is substantially reflected by said mirror due to the greater effective depth of said dielectric mirror experienced by the obliquely scattered light.

10. A device as recited in claim 9 wherein said dielectric mirror comprises a plurality of layers so arranged as to provide alternately high and low refractive indices.

11. A device as recited in claim 9 wherein said dielectric mirror comprises a plurality of layers three to seven in number.

12. A device as recited in claim 9 wherein each layer within said dielectric mirror has a thickness equal to one-quarter of a wavelength of radiant energy having a frequency in the ultraviolet.

13. A device as recited in claim 9 wherein said dielectric mirror comprises alternate layers of zinc sulphide and magnesium fluoride, both outer layers being zinc sulphide.

14. A device as recited in claim 9 wherein said dielectric mirror is positioned between said first electrode and said first substrate so that the coating of light transmissive metal comprising said first electrode is formed on the surface of said dielectric mirror adjacent to said liquid crystal.

15. A device as recited in claim 9 wherein said dielectric mirror is positioned between said liquid crystal and said first electrode so that the coating of light transmissIve metal comprising said first electrode is formed on the surface of said dielectric mirror remote from said liquid crystal.

16. In a liquid crystal display device operating by reflection of radiant energy having a given spectral range and of the type having a substantially transparent front plate, a substantially transparent back plate provided with reflective means on that side thereof facing said front plate, a layer of liquid crystal material between said plates, and means for applying voltage across said layer of liquid crystal material to produce light scattering centers therein, the improvement wherein:

said back plate has a selected index of refraction n2 and said reflective means includes at least two layers of substantially transparent material, one layer being in contact with said back plate and having an index of refraction n1 and the other layer being in contact with said one layer and having an index of refraction no, the thickness of said reflective means being an odd multiple of a quarter wavelength for radiant energy having a selected frequency higher than frequencies within said given spectral range and the aforesaid indices of refraction being such that n0<

n1>

n2 whereby (1) the reflectance for radiant energy of said selected frequency higher than the frequencies within said given spectral range which enters said front plate at angles near to normal incidence and passing through said liquid crystal material in the absence of voltage thereacross is a maximum while the reflectance for radiant energy within said given spectral range which enters said front plate at angles near to normal incidence and passing through said liquid crystal material in the absence of voltage thereacross is greatly reduced in magnitude from said maximum reflectance and (2) the reflectance for radiant energy within said given spectral range entering said front plate at angles near to normal incidence and passing through said liquid crystal material in the presence of voltage thereacross is substantially said maximum reflectance.