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).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]The present application claims the benefit of Swedish patent application No. 1251437-8, filed 17 Dec. 2012, and U.S. provisional application No. 61 / 738,059, filed 17 Dec. 2012, both of which are incorporated herein by reference.TECHNICAL FIELD[0002]The present invention relates to touch-sensing systems that operate by propagating light by internal reflections along well-defined light paths inside a light transmissive panel, and in particular to optical solutions for coupling light into the light transmissive panel.BACKGROUND ART[0003]This type of touch-sensing system may be implemented to operate by transmitting light inside a thin light transmissive panel of solid material, which defines two parallel boundary surfaces connected by a peripheral edge surface. Light generated by a plurality of emitters is coupled into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces to a plurality of detectors. T...

AI Technical Summary

Benefits of technology

[0013]Yet another objective is to enable a touch-sensitive apparatus that is simple to assemble and suited for mass production.

[0016]In this touch-sensitive apparatus, the first reflective element will act as a secondary light source which is located in contact with the panel to emit diffuse light into the panel. This secondary light source defines the actual origin of the propagation path(s) that are generated by the light from the light emitter. Thus, origin of the propagation path(s) is given by the first reflective element, which may have a well-defined location and extent on the rear surface. Further, the first reflective element is diffusively reflective and thereby re-distributes the incoming light more or less randomly. This means that the first reflective element has the ability to act as a secondary light source for many different types of light emitters and is relatively insensitive to manufacturing and mounting tolerances for the light emitter. The first and second reflective elements are also simple to apply to the panel. All in all, this facilitates mass production.

[0017]The combination of the first and second reflective elements also enables a compact and light-weight touch-sensitive apparatus, since one or both of the first and second reflective elements may be provided as sheet-like elements.

[0018]Further, the incoupling structure allows light to be coupled into the panel also with restricted external access to the edge surface, since the transmissive surface portion is located on the rear surface of the panel.

[0019]The touch-sensitive apparatus allows the light emitter to be arranged underneath the panel, which may reduce the footprint of the apparatus.

Problems solved by technology

An object that touches one of the boundary surfaces (“the touch surface”) will attenuate (“frustrate”) the light on one or more propagation paths and cause a change in the light received by one or more of the detectors.

This may be difficult and / or costly to achieve, especially if the panel is thin and / or manufactured of a comparatively brittle material such as glass.

Incoupling via the edge surface may also add to the footprint of the touch system.

Furthermore, it may be difficult to optically access the edge surface if the panel is attached to a mounting structure, such as a frame or bracket, and it is also likely that the mounting structure causes strain in the edge surface.

Such strain together with load variations may result in undesirable variations in incoupling efficiency.

However, the prisms or wedges may add significant thickness and weight to the system.

The present applicant has found that the difference in thermal expansion between the plastic material and the glass may cause a bulky wedge to come loose from the panel as a result of temperature variations during operation of the touch system.

Even a small or local detachment of the wedge may cause a significant decrease in the performance of the system.

This means that the first reflective element has the ability to act as a secondary light source for many different types of light emitters and is relatively insensitive to manufacturing and mounting tolerances for the light emitter.

Further, the incoupling structure allows light to be coupled into the panel also with restricted external access to the edge surface, since the transmissive surface portion is located on the rear surface of the panel.

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