Light guide lamps are known, and they provide for the light generated by one or more sources to be directed into a light guide, that is to say, a closed structure having a surface at least partly transparent, and at least partly reflecting on the internal side. The light rays propagate within the structure through a series of successive reflections, and exit therefrom in a more or less gradual manner. The light guides obtained through the use of total internal reflection films (or TIR films) are especially effective.
Said TIR films are known, for example, from European Patent No. EP 0 225 123, to which reference shall be made for a detailed description of their features; an example of said TIR films are those produced and marketed by Minnesota Mining and Manufacturing Company under the brand name of OLF—Optical Lighting Film. They are shaped as flexible sheets or tapes, exhibiting a surface with a series of parallel micro-relieves with a substantially triangular section; said films can be applied onto the surface of a transparent carrier material, with the microrelieves oriented in the propagation direction and usually facing outwards, thus creating an effective light guide. In fact, thanks to the optical characteristics of said films and as described, for example, in document U.S. Pat. No. 4,805,984—to which reference shall be made for further details—the light forming with the main propagation direction a smaller angle than a critical angle θmax is always internally reflected, whereas the light forming with the main propagation direction a greater angle θ than angle θmax is internally reflected if it is incident on the TIR film with an angle, with respect to the normal, that is smaller than a certain angle, depending on angle θ. Thus, contrary to the current definition, in the following description and in the attached claims, the incidence angle of a light ray on a surface shall indicate that formed by the ray with the plane tangent to the surface.
Angle θmax depends on the characteristics of the TIR film and, for the OLF film mentioned above, it is of 27.6°.
Another material used in light guide lamps is the so-called multilayer optical film, described for example in document U.S. Pat. No. 5,882,774.
Special elements called light extractors are inserted into the light guide to cause a controlled diffusion of a portion of the light travelling within the light guide, thus making some of the light rays deviate and be incident onto the TIR film with such angles as to exit from the light guide.
Known light extractors comprise, for this purpose, a typically white diffusing surface, and they consist of tridimensional bodies arranged in axial position in the light guide (reference can be made, for example, to Italian patent application TO98A000513), or of bidimensional bodies applied to the internal surface of the light guide (reference can be made, for example, to European patent application EP-A-1006312).
The technical problem at the basis of the present invention is that of providing a light extractor for a light guide lamp which should allow obtaining an illumination with preselected features, for example a highly uniform illumination.