Optical coupling of light into touch-sensing systems

Abstract

A touch-sensitive apparatus operates by light frustration (FTIR) and comprises a light transmissive panel ( 1 ) with a front surface ( 5 ) and a rear surface ( 6 ). Light emitters ( 2 ) and light detectors are optically coupled to the panel ( 1 ) to define a grid of light propagation paths inside the panel ( 1 ) between pairs of light emitters ( 2 ) and light detectors. A light in coupling structure comprises a diffusively reflective element ( 20 ) on the rear surface ( 6 ) and a specularly reflective element ( 22 ) on the front surface ( 5 ). Each light emitter ( 2 ) is arranged to project a beam of light onto a transmissive surface portion ( 24 ) on the rear surface ( 6 ), such that at least a portion of the beam of light enters the light transmissive panel ( 1 ) through the transmissive surface portion ( 24 ), is specularly reflected against the specularly reflective element ( 22 ) and impinges on the diffusively reflective element ( 20 ) from inside the light transmissive panel ( 1 ).

1 . A touch-sensitive apparatus, comprising:
 a light transmissive panel that defines a front surface and an opposite, rear surface;   a plurality of light emitters optically coupled to the light transmissive panel and a plurality of light detectors optically coupled to the light transmissive panel, so as to define a grid of light propagation paths inside the light transmissive panel between pairs of light emitters and light detectors;   wherein a structure for optically coupling one of the light emitters to the light transmissive panel comprises a first reflective element on the rear surface and a second reflective element on the front surface, the first reflective element being configured to be diffusively reflective to impinging light from inside the light transmissive panel, and the second reflective element being configured to be specularly reflective to impinging light from inside the light transmissive panel, and wherein said one light emitter is arranged to project a beam of light onto a transmissive surface portion on the rear surface, such that at least a portion of the beam of light enters the light transmissive panel through the transmissive surface portion, is specularly reflected against the second reflective element and impinges on the first reflective element from inside the light transmissive panel.   
     
     
         2 . The touch-sensitive apparatus of  claim 1 , wherein the transmissive surface portion is defined within the extent of the first reflective element. 
     
     
         3 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is shaped as a ring of diffusively reflective material surrounding the transmissive surface portion. 
     
     
         4 . The touch-sensitive apparatus of  claim 1 , wherein the transmissive surface portion is formed by an uncovered portion of the rear surface between the first reflective element and a peripheral edge of the panel. 
     
     
         5 . The touch-sensitive apparatus of  claim 1 , wherein the structure is configured to define an origin for a subset of the light propagation paths (D), said subset of light propagation paths (D) extending in different directions from the structure across the light transmissive panel, and wherein the first reflective element is configured to have different extent in at least part of said different directions, and wherein the extent of the first reflective element in a subset of the different directions exceeds a limit extent W Um=2·t·tan(θ min ), with t being a thickness of the light transmissive panel and θ min  min being a minimum angle of incidence for light to propagate by internal reflections inside the light transmissive panel. 
     
     
         6 . The touch-sensitive apparatus of  claim 5 , wherein the first reflective element is elliptical. 
     
     
         7 . The touch-sensitive apparatus of  claim 1 , wherein the light transmissive panel comprises a peripheral edge surface that connects the front and rear surfaces, wherein a third reflective element is arranged on the peripheral edge surface adjacent to the first and second reflective elements, and wherein the third reflective element is configured to be diffusively reflective to impinging light from inside the light transmissive panel. 
     
     
         8 . The touch-sensitive apparatus of  claim 7 , wherein the first, second and third reflective elements define at least part of a reflective enclosure that extends from the first reflective element across the peripheral edge surface to the second reflective element. 
     
     
         9 . The touch-sensitive apparatus of  claim 7 , wherein said one light emitter is arranged to project the beam of light onto the transmissive surface portion such that a portion of the beam of light impinges on the third reflective element. 
     
     
         10 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is a sheet-like element applied to or integrated in the rear surface of the light transmissive panel. 
     
     
         11 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is non-transmissive to the beam of light. 
     
     
         12 . The touch-sensitive apparatus of  claim 1 , wherein the second reflective element is arranged opposite to the first reflective element. 
     
     
         13 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is configured to exhibit at least 50% diffuse reflection, and preferably at least 90% diffuse reflection. 
     
     
         14 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is a near-Lambertian diffuser. 
     
     
         15 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is configured to promote emission of diffusively reflected light at angles that sustain light propagation by total internal reflection inside the light transmissive panel. 
     
     
         16 . The touch-sensitive apparatus of  claim 1 , wherein the light emitter is optically coupled to the transmissive surface portion by a light transmissive material with a refractive index that is adapted to the refractive index of the light transmissive panel at the transmissive surface portion. 
     
     
         17 . The touch-sensitive apparatus of  claim 1 , wherein the first reflective element is configured as an elongate strip, and the second reflective element is configured as an elongate strip which is arranged on the front surface to co-extend with the first reflective element, wherein the first and second reflective elements define a structure for coupling light from a number of light emitters into the light transmissive panel, the number of light emitters being located beneath the light transmissive panel dispersed along the first reflective element. 
     
     
         18 . The touch-sensitive apparatus of  claim 17 , wherein the first and second reflective elements define a respective frame structure around a center portion of the light transmissive panel. 
     
     
         19 . The touch-sensitive apparatus of  claim 1 , comprising a plurality of first reflective elements which are spatially separated and arranged on the rear surface along a rim of the light transmissive panel, wherein each of the first reflective elements is included in a structure for optically coupling a number of light emitters to the light transmissive panel, said number of light emitters being located beneath the light transmissive panel dispersed along the rim of the light transmissive panel. 
     
     
         20 . The touch-sensitive apparatus of  claim 1 , wherein the second reflective element is configured to block light that is visible to the human eye.