Light modulator including a collimator comprising an interference filter

• US 6,078,421 A
• Filed: 10/25/1999
• Issued: 06/20/2000
• Est. Priority Date: 04/25/1997
• Status: Expired due to Fees

• Current Assignee: DEFENCE, SECRETARY OF STATE FOR, THE
• Original Assignee: The secretary of state for defence in her Britannic Majesty's Government of the United Kingdom of Great Britain And Northern Ireland
• Inventors: Davey, Anthony B., Coker, Timothy M., Crossland, William A., Bayley, Paul A.
• Primary Examiner(s): Ben, Loha
• Application Number:
  US09/402,613
• Time in Patent Office: 239 Days
• Field of Search: 359/237, 359/247, 359/350, 359/359, 359/360, 359/361, 359/586, 359/588, 359/589, 359/707, 359/722, 362/31, 349/64, 349/65, 428/214, 428/216
• US Class Current: 359/237
• CPC Class Codes: G02B 27/30 Collimators
• CPC Class Codes: G02B 5/285 comprising deposited thin solid films G02B5/281 - G02B5/289 take precedence; multilayered film filters for fibre optic multiplexing G02B6/29361
• CPC Class Codes: Y10T 428/24975 No layer or component greater than 5 mils thick

Abstract

A collimator is formed of a stack of layers (11) of different thicknesses in a way analogous to a low-pass interference filter. When illuminated by narrow-band light of a predetermined wavelength just within the pass band of the filter, the collimator preferentially transmits light incident within a predetermined angular range, usually near-normal. The collimator is especially useful with photoluminescent liquid-crystal display, having phosphor emitters (17), because the collimator layers can simply be deposited on one face of the liquid-crystal modulator (1).

Claims

1. A collimator for light in a narrow band around a predetermined wavelength, comprising an interference filter (11) having a stack of dielectric layers with optical thicknesses adapted to transmit light in the band incident at a predetermined range of angles of incidence centered at normal incidence more efficiently than light in the band incident at other angles, in which the interference filter has the form of a low-pass filter with a cut-off wavelength longer than the predetermined wavelength.

2. A collimator according to claim 1, in which the pass band of the filter is optimized so as to be substantially flat.

3. A collimator according to claim 1, in which the response of the filter has a first peak of transmission at the short-wavelength side of the cut-off at a wavelength which is less than or equal to the predetermined wavelength.

4. A collimator according to claim 1, wherein the predetermined wavelength falls in the visible region.

5. A collimator according to claim 1, wherein the predetermined wavelength falls in the ultra-violet.

6. A directional light source including a collimator according to claim 1 and a diffuse lighting arrangement adapted to produce light in a narrow band around the predetermined wavelength.

7. A light source according to claim 6, further having a means to return light rejected by the collimator to the collimator to improve the light throughput.

8. A light source according to claim 7, in which the light return means includes a highly reflecting diffuse surface.

9. A light modulator comprising an optical modulating device for light in a narrow band around a predetermined wavelength and a collimator according to claim 1.

10. A liquid-crystal display, including

a collimator (11) according to claim 1 and

a liquid-crystal modulator (1) for modulating light.

11. A liquid-crystal display according to claim 10, further including a photoluminescent layer (17) for receiving the modulated light and producing a display.

12. A liquid-crystal display according to claim 11, wherein the interference filter (11) is formed directly on one face (5) of the liquid-crystal modulator and the photoluminescent layer is formed on the opposite face (3).

13. A collimator for light in a narrow band around a predetermined wavelength, comprising an interference filter (11) having a stack of dielectric layers with optical thicknesses adapted to transmit light in the band incident at a predetermined range of angles of incidence excluding a range centered at normal incidence more efficiently than light in the band incident at angles around normal incidence, in which the interference filter has the form of a high-pass filter with a cut-off wavelength longer than the predetermined wavelength.

14. A directional light source including:

a collimator for light in a narrow band around a predetermined wavelength, comprising an interference filter (11) having a stack of dielectric layers with optical thicknesses adapted to transmit light in the band incident at a predetermined range of angles of incidence centered at normal incidence more efficiently than light in the band incident at other angles; and

a diffuse lighting arrangement adapted to produce light in a narrow band around the predetermined wavelength, for passage through the collimator.

15. A light modulator comprising an optical modulating device for light in a narrow band around a predetermined wavelength and a collimator for light in a narrow band around a predetermined wavelength, the collimator comprising an interference filter (11) having a stack of dielectric layers with optical thicknesses adapted to transmit light in the band incident at a predetermined range of angles of incidence centered at normal incidence more efficiently than light in the band incident at other angles;

in which the pre-determined range of angles is such as to improve the performance of the light modulator.

16. A light modulator according to claim 15 in which the modulating device is a liquid-crystal device.

17. A display device including a light modulator according to claim 15.

18. A method for collimating light from a diffuse extended light source which emits in a narrow band around a pre-determined wavelength, in which the said light is incident upon and is transmitted through a collimator comprising an interference filter (11) having a stack of dielectric layers with optical thicknesses adapted to transmit light in the said narrow band incident at a predetermined range of angles of incidence centered at normal incidence more efficiently than light in the band incident at other angles.