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A light-guide light is described that is capable of providing a substantially monochromatic beam of light, and is suitable for use as a signal lamp in an automotive vehicle. The light (1) comprises a housing defining a light-guiding optical cavity (4) having first and second major faces (5, 6), and

A light-guide light is described that is capable of providing a substantially monochromatic beam of light, and is suitable for use as a signal lamp in an automotive vehicle. The light (1) comprises a housing defining a light-guiding optical cavity (4) having first and second major faces (5, 6), and a light source (11) arranged to direct light into the cavity from one side. The light source may be an array of light emitting diodes providing substantially monochromatic light. The first major face (5) comprises an interference film (for example, a multilayer polymeric film) that, for light at the design wavelength, substantially transmits light that is incident on the film over a limited angular range and substantially reflects light that is incident at other angles. The second major face (6) comprises a scattering reflective material having a reflectance of at least 85%, for example a highly-efficient reflective material provided with a suitable textured pattern.

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1. A light-guide comprising a housing defining a light-guiding optical cavity having first and second major faces, and a light source of substantially monochromatic visible light at a selected wavelength arranged to direct light into the cavity, to be guided between the major faces; wherein (a) the

1. A light-guide comprising a housing defining a light-guiding optical cavity having first and second major faces, and a light source of substantially monochromatic visible light at a selected wavelength arranged to direct light into the cavity, to be guided between the major faces; wherein (a) the first major face comprises an interference film which, for light at the selected wavelength, substantially transmits such light that is incident on the film over a limited angular range and substantially reflects such light that is incident at other angles; and(b) the second major face comprises a scattering reflective material having a total reflectance of at least 85% for light at the selected wavelength incident on the surface at any angle;wherein light is emitted from the optical cavity substantially uniformly across the first major face in the form of a substantially monochromatic beam of limited angular spread and said scattering reflective material defines either a wide scattering reflective surface having a dispersion angle between about 15 degrees and about 45 degrees or a narrow scattering reflective surface having a dispersion angle of less than about 15 degrees. 2. A light-guide light as claimed in claim 1, in which, at the said selected wavelength, the interference film transmits substantially only light that is incident on the film within a restricted range of angles centered on normal incidence. 3. A light-guide light as claimed in claim 1. in watch the second major face comprises a specularly-reflecting material having a light-spreading structure formed thereon. 4. A light as claimed in claim 1, in which the light source comprises at least one substantially point source, arranged to direct light into the optical cavity. 5. A light-guide light as claimed in claim 1, further including a light diverter positioned to receive, and change the direction of, the beam of light emitted from the first major face of the optical cavity. 6. A light-guide light as claimed in claim 1, in which a graphic symbol is formed on one of the major faces of the optical cavity. 7. A signal lamp for an automotive vehicle, incorporating a light-guide light as claimed in claim 1. 8. A light-guide light as claimed in claim 1, wherein said scattering reflective material defines a wide scattering reflective surface having a dispersion angle of between about 15 degrees and about 45 degrees. 9. A light-guide light as claimed in claim 1, wherein said scattering reflective material defines a narrow scattering reflective surface having a dispersion angle of less than about 15 degrees. 10. A light-guide light as claimed in claim 1, wherein said scattering reflective material can reflect an incident collimated beam into a broadened beam having a dispersion angle of less than about 45 degrees. 11. A light-guide light comprising a housing defining a light-guiding optical cavity having first and second major faces, and a light source of substantially monochromatic visible light at a selected wavelength arranged direct light into the cavity, to be guided between the major faces; wherein (a) the first major face comprises an interference film which, for light at the selected wavelength, substantially transmits such light that is incident on the film over a limited angular range and substantially reflects such light that is incident at other angles; and(b) the second major face comprises a scattering reflective material having a total reflectance of at least 85% for light at the selected wavelength incident on the surface at any angle,wherein light is emitted from the optical cavity substantially uniformly across the first major face in the form of a substantially monochromatic beam of limited angular spread, and said light-guide light further comprises an absorption filter arranged to receive light from the interference film, in which the absorption filter transmits substantially only light at the said selected wavelength and appears a different color when unlit. 12. A light-guide light comprising a housing defining light-guiding optical cavity having first and second major faces, and a light source arranged to direct visible light into the cavity, to be guided between the major faces; wherein (a) the first major face comprises a multi-layer polyimeric film which, at least for light at a selected wavelength, substantially transmits such light that is incident on the film over a limited angular range and substantially reflects such light that is incident at other angles, and an absorption filter arranged to receive light from the multi-layer film, the absorption filter transmitting light at the said selected wavelength; and(b) the second major face comprises a scattering reflective material having a reflectance of at least 85% for visible light incident on the surface at any angle;whereby light from within the optical cavity is emitted substantially uniformly across the first major face in the form of a substantially monochromatic beam of limited angular spread and said scattering reflective material defines either a wide scattering reflective surface having a dispersion angle between about 15 degrees and about 45 degrees or a narrow scattering reflective surface having a dispersion angle of loss than about 15 degrees. 13. A light-guide light as claimed in claim 12, in which the optical cavity is generally wedge-shaped, the said first and second major faces being inclined to one another and meeting at one side of the optical cavity, and the light source being arranged to direct light into the cavity from the opposite side. 14. A light-guide light as claimed in claim 12, in which the second major face has a conical form, the light source being located at the apex of the second major face to direct light into the optical cavity. 15. A light-guide light as claimed in claim 12, in which, at the said selected wavelength, the interference film transmits substantially only light that is incident on the film within a restricted range of angles centered on normal incidence. 16. A light-guide light as claimed in claim 12, in which the second major face has a total reflectance of at least 98% for visible light incident on the surface at any angle. 17. A light-guide light as claimed in claim 12, in which the second major face comprises a specularly-reflecting material having a light-spreading structure formed thereon. 18. A light-guide light as claimed in claim 17, in which the second major face comprises an optical film or sheet metal. 19. A light-guide light as claimed in claim 17, in which a graphic symbol is formed in the light spreading structure on the second major face of the optical cavity. 20. A light-guide light as claimed in claim 12, in which the light source comprises at least one substantially point source, arranged to direct light into the optical cavity. 21. A light-guide light as claimed in claim 12, further including a light diverter positioned to receive, and change the direction of, the beam of light emitted from the first major face of the optical cavity. 22. A light-guide light as claimed in claim 12, in which a graphic symbol is formed on one of the major faces of the optical cavity. 23. A signal lamp for an automotive vehicle, incorporating a light-guide light as claimed in claim 12.