Patent Publication Number: US-11029783-B2

Title: Optical touch system comprising means for projecting and detecting light beams above and inside a transmissive panel

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to improved touch determination on touch surfaces of optical touch-sensing systems, and in particular in relation to FTIR-based (frustrated total internal reflection) touch systems. 
     Description of the Related Art 
     In one category of touch-sensitive panels known as ‘above surface optical touch systems’ and known from e.g. U.S. Pat. No. 4,459,476, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid of intersecting light paths above the touch surface. Each light path extends between a respective emitter/receiver pair. An object that touches the touch surface will block certain ones of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the location of the intercept between the blocked light paths. This type of system is only capable of detecting the location of one object (single-touch detection). Further, the required number of emitters and receivers, and thus cost and complexity, increases rapidly with increasing surface area and/or spatial resolution of the touch panel. 
     In a variant, e.g. shown in WO2006/095320, each optical emitter emits a beam of light that diverges across the touch surface, and each beam is detected by more than one optical receiver positioned around the periphery of the touch surface. Thus, each emitter creates more than one light path across the touch surface. A large number of light paths are created by sequentially activating different emitters around the periphery of the touch surface, and detecting the light received from each emitter by a plurality of optical receivers. Thereby, it is possible to reduce the number of emitters and receivers for a given surface area or spatial resolution, or to enable simultaneous location detection of more than one touching object (multi-touch detection). 
     If the display screen is contaminated by e.g. fingerprints, the optical transmission path may become unintentionally interrupted and the information retrieved from the system erroneous or incomplete as the contaminated surface becomes insensitive to touches. If contaminants are collected in front of one of the emitters or detectors there will always be blocked or occluded light paths. 
     Another category of touch-sensitive panels known as ‘in-glass optical systems’ is now described and is also known from e.g. U.S. Pat. No. 8,581,884. 
       FIG. 1  illustrates an example of a touch-sensitive apparatus  100  that is based on the concept of FTIR (Frustrated Total Internal Reflection), also denoted “FTIR system”. The apparatus operates by transmitting light inside a transmissive panel  10 , from light emitters  30   a  to light sensors or detectors  30   b,  so as to illuminate a touch surface  20  from within the transmissive panel  10 . The transmissive panel  10  is made of solid material in one or more layers and may have any shape. The transmissive panel  10  defines an internal radiation propagation channel, in which light propagates by internal reflections. 
     In the example of  FIG. 1 , the propagation channel is defined between the touch surface  20  and bottom surface  25  of the transmissive panel  10 , where the touch surface  20  allows the propagating light to interact with touching object  60  and thereby defines the touch surface  20 . This is achieved by injecting the light into the transmissive panel  10  via coupling element  40  such that the light is reflected by total internal reflection (TIR) in the touch surface  20  as it propagates through the transmissive panel  10 . The light may be reflected by TIR on the bottom surface  25  or against a reflective coating thereon. Upon reaching coupling element  40  on a far side of the panel, the light is coupled out of transmissive panel  10  and onto detectors  30   b.  The touch-sensitive apparatus  100  may be designed to be overlaid on or integrated into a display device or monitor. 
     U.S. Pat. No. 8,553,014 describes an attempt to combine the above surface and in-glass optical systems described above. U.S. Pat. No. 8,553,014 describes an optical coupling technique for introducing light into a transmissive panel and above a transmissive panel simultaneously. However, the in-coupling component shown in  FIG. 126  of U.S. Pat. No. 8,553,014 is a complex prism and appears to rely on total internal reflection and diffraction to couple the light above the touch surface. Such an arrangement would be highly tolerance sensitive, making the optical signal highly sensitive to, for example, the load on the touch surface, the tolerances of process used to mount the prism to the transmissive panel, and the manufacturing of both the transmissive panel and the prism. Furthermore, the spread of the light in a plane parallel to the transmissive panel is limited to a range of less than 80 degrees as light outside this range will be diffracted up and away from the panel. Such a system is best suited to a rectangular grid of detection lines, such as described in U.S. Pat. No. 4,459,476 above. Furthermore, a complex prism as described in U.S. Pat. No. 8,553,014 would be both expensive to manufacture and bulky, taking up valuable space underneath and to the side of the transmissive panel. 
     SUMMARY OF THE INVENTION 
     It is an objective of the invention to at least partly overcome one or more of the above-identified limitations of the prior art. 
     One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by means of a method for data processing, a computer readable medium, devices for data processing, and a touch-sensing apparatus according to the independent claims, embodiments thereof being defined by the dependent claims. 
     An embodiment of the invention provides a touch sensing apparatus, comprising: a light transmissive element that defines a touch surface; a set of emitters arranged around the periphery of the touch surface to emit beams of light into the light transmissive element, wherein a first portion of the beams of light propagate inside the light transmissive element while illuminating the touch surface such that an object touching the touch surface causes an attenuation of the propagating light, and wherein a second portion of the beams of light pass out of the light transmissive element and are reflected to travel above the touch surface, a set of light detectors arranged around the periphery of the touch surface to receive light from the set of emitters from the transmissive element and from above the touch surface, wherein each light detector is arranged to receive light from more than one emitter; a processing element configured to determine, based on output signals of the light detectors, a light energy value for each light path; to generate a transmission value for each light path based on the light energy value; and to operate an image reconstruction algorithm on at least part of the thus-generated transmission values so as to determine the position of the object on the touch surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described in more detail with reference to the accompanying schematic drawings. 
         FIG. 1  shows a cross section of an FTIR-based touch-sensitive apparatus according to the prior art. 
         FIG. 2  is a top plan view of an FTIR-based touch-sensitive apparatus. 
         FIG. 3  shows a section view of an extended FTIR touch system according to an embodiment of the present invention. 
         FIG. 4  shows a top plan view of an extended FTIR touch system according to an embodiment of the present invention. 
         FIG. 5  shows the narrow detection lines within transmissive panel  10 . 
         FIG. 6  shows the broad detection lines above transmissive panel  10 . 
         FIG. 7  shows the signal profile of detection lines  95  and detection lines  96 . 
         FIG. 8  shows an embodiment of the present invention in which the touch surface is curved. 
         FIG. 9  shows an embodiment of the present invention with deflectors set back from the edge of the active area. 
         FIG. 10  shows an embodiment of the present invention having a dust shield. 
         FIG. 11  shows an embodiment of the present invention having a first set of emitters and detectors for projecting light above transmissive panel  10  and a second set of emitters and detectors for projecting light into transmissive panel  10 . 
         FIG. 12  shows a top plan view of the  FIG. 11 . 
         FIG. 13  shows a variation of  FIG. 11  wherein the emitters are configured to simultaneously project light above and into transmissive panel  10 . 
         FIG. 14  shows a variation of  FIG. 11  wherein the detectors are configured to simultaneously receive light from above and from within transmissive panel  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing embodiments of the invention, a few definitions will be given. 
     A “touch object” or “touching object” is a physical object that touches, or is brought in sufficient proximity to, a touch surface so as to be detected by one or more sensors in the touch system. The physical object may be animate or inanimate. 
     An “interaction” occurs when the touch object affects a parameter measured by the sensor. 
     A “touch” denotes a point of interaction as seen in the interaction pattern. 
     Throughout the following description, the same reference numerals are used to identify corresponding elements. 
       FIG. 2  illustrates a top plan view of  FIG. 1  in an example of a touch-sensitive apparatus  100  that is based on the concept of FTIR. Emitters  30   a  are distributed around the periphery of touch surface  20 , to project light into the transmissive panel  10  such that at least part of the light is captured inside the transmissive panel  10  for propagation by internal reflection in the propagation channel. Detectors  30   b  are distributed around the periphery of touch surface  20 , to receive part of the propagating light. The light from each of emitters  30   a  will thereby propagate inside the transmissive panel  10  to a number of different detectors  30   b  on a plurality of light paths D. 
     Even if the light paths D correspond to light that propagates by internal reflections inside the panel  1 , the light paths D may conceptually be represented as “detection lines” that extend across the touch surface  20  to the periphery of touch surface  20  between pairs of emitters  30   a  and detectors  30   b,  as shown in  FIG. 2 . Thus, the detection lines D correspond to a projection of the light paths D onto the touch surface  20 . Thereby, the emitters  30   a  and detectors  30   b  collectively define a grid of detection lines D (“detection grid”) on the touch surface  20 , as seen in a top plan view. The spacing of intersections in the detection grid defines the spatial resolution of the touch-sensitive apparatus  100 , i.e. the smallest object that can be detected on the touch surface  20 . The width of the detection line is a function of the width of the emitters and corresponding detectors. A wide detector detecting light from a wide emitter provides a wide detection line with a broader surface coverage, minimising the space in between detection lines which provide no touch coverage. A disadvantage of broad detection lines may be the reduced touch precision and lower signal to noise ratio. 
     As used herein, the emitters  30   a  may be any type of device capable of emitting radiation in a desired wavelength range, for example a diode laser, a VCSEL (vertical-cavity surface-emitting laser), an LED (light-emitting diode), an incandescent lamp, a halogen lamp, etc. The emitters  30   a  may also be formed by the end of an optical fiber. The emitters  30   a  may generate light in any wavelength range. The following examples presume that the light is generated in the infrared (IR), i.e. at wavelengths above about 750 nm. Analogously, the detectors  30   b  may be any device capable of converting light (in the same wavelength range) into an electrical signal, such as a photo-detector, a CCD device, a CMOS device, etc. 
     The detectors  30   b  collectively provide an output signal, which is received and sampled by a signal processor  130 . The output signal contains a number of sub-signals, also denoted “projection signals”, each representing the energy of light received by one of light detectors  30   b  from one of light emitters  30   a.  Depending on implementation, the signal processor  130  may need to process the output signal for separation of the individual projection signals. The projection signals represent the received energy, intensity or power of light received by the detectors  30   b  on the individual detection lines D. Whenever an object touches a detection line D, the received energy on this detection line is decreased or “attenuated”. 
     The signal processor  130  may be configured to process the projection signals so as to determine a property of the touching objects, such as a position (e.g. in a x,y coordinate system), a shape, or an area. This determination may involve a straight-forward triangulation based on the attenuated detection lines, e.g. as disclosed in U.S. Pat. No. 7,432,893 and WO2010/015408, or a more advanced processing to recreate a distribution of attenuation values (for simplicity, referred to as an “attenuation pattern”) across the touch surface  20 , where each attenuation value represents a local degree of light attenuation. The attenuation pattern may be further processed by the signal processor  130  or by a separate device (not shown) for determination of a position, shape or area of touching objects. The attenuation pattern may be generated e.g. by any available algorithm for image reconstruction based on projection signal values, including tomographic reconstruction methods such as Filtered Back Projection, FFT-based algorithms, ART (Algebraic Reconstruction Technique), SART (Simultaneous Algebraic Reconstruction Technique), etc. Alternatively, the attenuation pattern may be generated by adapting one or more basis functions and/or by statistical methods such as Bayesian inversion. Examples of such reconstruction functions designed for use in touch determination are found in WO2009/077962, WO2011/049511, WO2011/139213, WO2012/050510, and WO2013/062471, all of which are incorporated herein by reference. 
     In the illustrated example, the apparatus  100  also includes a controller  120  which is connected to selectively control the activation of the emitters  30   a  and, possibly, the readout of data from the detectors  30   b.  Depending on implementation, the emitters  30   a  and/or detectors  30   b  may be activated in sequence or concurrently, e.g. as disclosed in U.S. Pat. No. 8,581,884. The signal processor  130  and the controller  120  may be configured as separate units, or they may be incorporated in a single unit. One or both of the signal processor  130  and the controller  120  may be at least partially implemented by software executed by a processing unit  140 . 
       FIG. 3  illustrates an embodiment of the invention extending the FTIR system of  FIG. 1  to include touch detection lines above touch surface  20 . 
     As with  FIG. 1 ,  FIG. 3  shows an embodiment of the invention in which light travels inside a transmissive panel  10 , from light emitters  30   a  to detectors  30   b,  so as to illuminate a touch surface  20  from within the transmissive panel  10 . The transmissive panel  10  is made of solid material in one or more layers and may have any shape. The transmissive panel  10  defines an internal radiation propagation channel, in which light beam  50  propagates by internal reflections. In  FIG. 3 , the propagation channel is defined between the touch surface  20  and bottom surface  25  of the transmissive panel  10 , where the touch surface  20  allows the propagating light beam  50  to interact with touching object  60  and thereby defines the touch surface  20 . This is achieved by injecting the light into the transmissive panel  10  via coupling element  40  such that the light is reflected by total internal reflection (TIR) in the touch surface  20  as it propagates through the transmissive panel  10 . The light beam  50  may be reflected by TIR on the bottom surface  25  or against a reflective coating thereon. Upon reaching coupling element  40  on a far side of the panel, the light is coupled out of transmissive panel  10  and onto detectors  30   b.  The touch-sensitive apparatus  100  may be designed to be overlaid on or integrated into a display device or monitor. 
       FIG. 3  further illustrates that a portion of the light emitted by emitters  30   a  is transmitted through transmissive panel  10  in a manner that does not cause the light to TIR within transmissive panel  10 . Instead, the light the light exits transmissive panel  10  through touch surface  20  and is reflected by reflector surface  80  of edge reflector  70  to travel along path  90   a  in a plane parallel with touch surface  20 . The light will then continue until deflected by reflector surface  80  of the edge reflector  70  at an opposing edge of the transmissive panel  10 , wherein the light will be deflected back down through transmissive panel  10  and onto detectors  30   b.  The feature of the transmitting the light from the emitters  30   a  to reflector surface  80  via transmissive panel  10  has a number of advantages over the solutions presented by the prior art. In particular, manufacture of touch-sensitive apparatus  100  becomes significantly less expensive. This feature allows an arrangement where nothing is in contact with the edges of the transmissive panel  10 , allowing expensive finishing (where the transmissive panel  10  is formed from glass) to regulate the edges of the glass to be avoided. Furthermore, fastening of the components to the transmissive panel  10  is simplified and optical tolerances are improved. 
       FIG. 4  shows a top plan view of the embodiment of  FIG. 3 . As viewed from above, light beam  50  travelling through transmissive panel  10  originates from where the light is coupled into the transmissive panel  10  by coupling element  40  at in-coupling point  45 . Light beam  90  travelling above touch surface  20  (along path  90   a,  path  90   b ) originates from reflector surface focal point  80   a  of reflector surface  80  where the light emitted from emitters  30   a  and having passed through transmissive panel  10  is reflected across touch surface  20 . The distance travelled by the (un-collimated) light from emitters  30   a  to reflector surface focal point  80   a  is greater than the distance travelled by the light from emitters  30   a  to in-coupling point  45 . Consequently, the spread of light reflected from the reflector surface focal point  80   a  is broader than the spread of light entering the transmissive panel  10  at in-coupling point  45 . The resulting effect is equivalent to that of using a wider emitter for emitting the above surface light beam  90  than that of light beam  50 , travelling inside the glass. A corresponding effect occurs at the detector end, wherein the light arriving at detectors  30   b  via the above surface route is reflected onto detectors  30   b  from a broader area than the area of in-coupling point  45 , providing the equivalent effect of broader detectors  30   b.    
     The result is that detection lines derived from light beam  90  are broader than detection lines derived from light beam  50 .  FIG. 5  and  FIG. 6  show the difference in detection lines  95  derived from the light travelling within the glass and detection lines  96  derived from light travelling above the touch surface. In  FIG. 5 , detection lines  95  have a width corresponding to the short distance travelled to the in-coupling point  45  from emitters  30   a.  In  FIG. 6 , detection lines  96  have a width corresponding to the extra distance travelled to reflector surface focal point  80   a  from emitters  30   a.    
     A stylus and a finger may have very large differences in size (or width as viewed from the perspective of a detection line). A stylus may typically provide a width of 2 mm to 5 mm, whereas a finger may provide a width of 5 mm to 15 mm. However, the size of a decoded touch will depend on the convolution of the detection line and the object. It is preferable to have wider detection lines above the glass, both in order to provide better cover the touch surface  20  and to get a broadened stylus interaction since this may increase the resolution. However, broadening of detection lines will reduce the ability to separate two closely spaced touch objects, potentially a key requirement for multi-touch systems. 
     Therefore, a solution with different detection line widths above and within the glass is required.  FIG. 7  shows a signal profile of a narrow detection line overlaid on a wide detection line. Narrow detection lines  95  have a signal profile corresponding to signal profile  150  and wide detection lines  96  have a signal profile corresponding to signal profile  160 . 
     Since a stylus will usually be used as a single touch object, broader detection lines are possible. Wide detection lines  96  with a width larger than 4 mm and possibly up to 20 mm are advantageous, although 4 mm-6 mm is preferred. 
     For narrow detection lines  95  designed to resolved multiple touching objects such as fingers, broadening must be kept down to a size less than or comparable to the touching objects. The width of narrow detection lines  95  is also usually limited by the width of emitters  30   a  and detectors  30   b.  Narrow detection lines  95  should be less than 5 mm in width. In a preferred embodiment, detection lines  95  are between 2 mm and 3 mm wide. 
     In one embodiment, reflector surface  80  is a diffusive reflecting surface. In a preferred embodiment, reflector surface  80  is a lambertian diffusive reflecting surface preferably providing a scattering of greater than 90%. Suitable materials for reflector surface  80  may include Titanium oxide paint or Microcellular foamed reflector MCPET. The advantage of using a diffusive reflecting surface is that it makes the optical system less sensitive to production, mounting and load tolerances than a specular reflector or lens. This allows the touch-sensitive apparatus  100  to be cheaper and simpler to produce. Furthermore, a diffusive reflector surface  80  also allows broader and overlapping detection lines. 
     The amount of light reflected by reflector surface  80  may be controlled by adjusting the size of reflector surface  80 . A reflector surface  80  having a smaller surface area will reflect a small amount of light. Alternatively, paint or spray coatings may be selected to reduce the reflection, and may be applied in a specific pattern to the surface for accurate control of reflectivity. 
     In one embodiment, the amount of light received at each of detectors  30   b  via the in-glass route (shown in the figures as light beam  50 ) is greater than or equal to the amount of light received at each of detectors  30   b  via the above surface route (shown in the figures as light beam  90 ). In a preferred embodiment, the ratio of light received at each of detectors  30   b  via the in-glass route is ten times greater than the amount of light received at each of detectors  30   b  via the above surface route. This feature is advantageous as it allows the attenuation of the optical signal resulting from FTIR to be easily compared at the reconstruction phase to the attenuation of the optical signal resulting from occlusion of the above surface light, even though the latter is usually significantly larger than the former. 
     In the embodiment shown in  FIG. 3 , reflector surface  80  is configured to reflect a portion of light beam  90  to travel along path  90   a  and a portion of light beam  90  to travel along path  90   b,  by reflecting off touch surface  20  and coupling out to the detector. This advantageously results in a larger portion of light beam  90  being detected by detectors  30   b.  Furthermore, the use of path  90   b  allows load tolerances of the touch-sensitive apparatus  100  to be improved. A heavy load on touch surface  20  may deform the panel and bring path  90   a  out of alignment. However, path  90   b  would likely be less affected by said deformation, allowing sufficient signal to continue to be received by detectors  30   b.    
     In an embodiment of the invention shown in  FIG. 8 , transmissive panel  10  is curved to form a concave surface. In this embodiment, reflector surface  80  is configured to cause a portion of light beam  90  to reflect a plurality of times off touch surface  20  to follow a path  90   c  shown in  FIG. 8 . Similarly to the above embodiment, this feature allows a further enhancement of the signal to noise ratio, even for a curved panel. 
     In an embodiment of the invention shown in  FIG. 9 , edge reflector  70  is set further back from the periphery of touch surface  20  than in the previous embodiments. The positioning of the edge reflector  70  further back from the periphery of touch surface  20  provides a longer distance from emitters  30   a  to reflector surface  80 , allowing the above surface detection lines to be broader. Furthermore, the extra distance that the edge reflector  70  is set back provides larger overlap between wide detection lines  96  in the peripheral regions of touch surface  20  resulting in improved accuracy in areas. This is especially advantageous where narrow detection lines  95  provide limited coverage. In a preferred embodiment, the edge reflector  70  is positioned so that reflector surface  80  is set  10  mm back from in-coupling point  45 . 
       FIG. 10  shows an embodiment of the invention featuring dust shield  110 . A known problem with above-surface touch systems is the accumulation of dust and contamination around the sensor area or the area in which the light signal is emitted to travel across the touch panel. Dust or other contamination accumulating at this point will block the light signal and seriously degrade the ability of the touch system to determine a touch. For a system such as the embodiment presented in  FIG. 3 , the accumulation of contamination may be increased where reflector surface  80  is angled to form an overhang. This overhang forms a natural shelter for accumulating contamination, resulting in further touch signal degradation. A solution presented in  FIG. 10  is that of a dust shield  110  forming a physical barrier preventing the dust from reaching reflector surface  80  and comprising transparent window  115  through which the light signal may pass unhindered. Preferably, dust shield  110  forms a sloping edge, sloping from the inside edge in contact with touch surface  20  outwardly to the top surface of edge reflector  70 . This allows dust shield  110  to be effectively wiped clean. 
     In a preferred embodiment, transparent window  115  comprises a material of coating configured to allow only IR or Near-IR light to pass through. This feature provides improved ambient light noise reduction as light from artificial lighting or sun light is filtered before reaching detectors  30   b.    
     In a preferred embodiment, dust shield  110  is configured with a longer dimension extending from edge reflector  70  towards touch surface  20  and with an internal top surface providing a light baffle effect so as to provide an angular filter for light entering through transparent window  115 . This is advantageous for reducing ambient noise as light entering at the wrong angle is absorbed into the roof of the dust shield  110 . Furthermore, when combined with the embodiment from  FIG. 8 , the angle of light paths travelling above the panel may be limited so that that detection lines very high above the glass  90   a  may be suppressed. 
       FIG. 11  shows an alternative embodiment to the embodiment shown in  FIG. 3 . In  FIG. 11 , apparatus  100  is configured to transmit light from a first set of emitters  31   a  to a first set of detectors  31   b  inside a transmissive panel  10  so as to illuminate a touch surface  20  from within the transmissive panel  10 . Apparatus  100  is also configured to transmit light from a second set of emitters  32   a  to a second set of detectors  32   b  such that the light is emitted by emitters  32   a,  exits transmissive panel  10  through touch surface  20  and is reflected by reflector surface  80  of edge reflector  70  to travel along path  90   a  in a plane parallel with touch surface  20 . The light will then continue until deflected by reflector surface  80  of the edge reflector  70  at an opposing edge of the transmissive panel  10 , wherein the light will be deflected back down through transmissive panel  10  and onto detectors  32   b.  Significant advantages may be obtained from using two separate emitting and detecting systems rather than a single set of emitters and detectors for both the above-surface and FTIR light paths. A significant problem with trying to differentiate between the attenuation of the light travelling along a path above the touch surface from the attenuation of light travelling along a path within the panel via FTIR is that a typical finger touch is likely to produce an attenuation of the light above the panel is greater than the attenuation of the light travelling within the panel via FTIR by as much as a factor of  50 . This results in an attenuation signal of the light travelling in the panel which is difficult to differentiate from noise relative to the attenuation signal of the light travelling above the panel. For objects such as stylus tips, this relative difference in signal strength can be even greater. Therefore, the use of separate emitting and detecting systems for light paths above (above-surface system) and within the panel via FTIR (FTIR system) allows each system to be configured appropriately for the respective signal-to-noise ratios. The separate resulting signals can then be combined to provide a system that provides the following features:
         Oil or water contamination on the touch surface may appear to the FTIR system as an attenuation surface area and generate a false touch. However, in the above embodiment, the touch output of the FTIR system may be compared to the touch output of the above-surface system to identify touches of the FTIR system which do not appear in the touch output of the above-surface system. This would indicate that the identified touches do not correspond to actual objects above the touch surface but mere contamination on the surface. The output of the identified touches can then be suppressed.   Similarly to the above, when a user raises their finger from the touch surface, a previously identified touch should be removed from the touch output. However, on occasion, finger grease from the skin is left on the touch surface and an FTIR system continues to detect and report a touch. In the above embodiment, the output of the above-surface system may be used to identify touches of the FTIR system where the touching object has now been removed. The output of the identified touches can then be suppressed.   Certain object types produce very little attenuation of the FTIR light when in contact with the touch surface e.g. Hard objects such as stylus tips. Where the above-surface system registers an object but the FTIR system does not, it can be determined that the object is likely to be a ‘hard object’ as opposed to a normal touch from a finger. Differentiation between hard and soft surfaced objects may allow differentiation between e.g. a pen and a finger. A touch system configured to differentiate between a stylus and a finger tip may generate a different UI output in dependence on the identified object touching the touch surface.   One problem with above-surface systems is that the object touching the touch surface may completely occlude one or more light paths of the above-surface system. Where a large number of touches are simultaneously applied to the touch surface, portions of the touch surface may become significantly shielded from the light paths of the above-surface system, resulting in little or no touch signal in the shielded portion. In the above embodiment, the FTIR may continue to provide a touch signal within the occluded areas, as the attenuation of the FTIR light paths resulting from a touch is relatively small and non-occluding.       

     In the embodiment of  FIG. 11 , a first wavelength of light emitted by first set of emitters  31   a  and detected by first set of detectors  31   b  may be different to a second wavelength of light emitted by second set of emitters  32   a  and detected by second set of detectors  32   b.  This allows light to be emitted from one of the first set of emitters  31   a  and one of the second set of emitters  32   a  simultaneously and detected by the first set of detectors  31  band second set of detectors  32   b  without co-interference. This may also allow improved ambient light noise reduction in environments where ambient light comprises more light with a first wavelength than light with a second wavelength or vice versa. E.g. Wherein the first and second wavelengths are both near IR wavelengths. 
     In the embodiment of  FIG. 11 , the timing sequence used to activate emitters of the first and second set of emitters may be chosen to ensure that activation of the emitters of the first set of emitters does not chronologically overlap with activation of emitters of the second set of emitters. This allows potential co-interference to be minimized. 
       FIG. 12  shows a top plan view of the  FIG. 11 . In this embodiment, emitters  31   a  are spatially interlaced with emitters  32   a  around the peripheral edge of transmissive panel  10  so that emitters of first set  31   a  are positioned between adjacent emitters of second set  32   a.  Similarly, detectors  31   b  are spatially interlaced with detectors  32   b  around the peripheral edge of transmissive panel  10  so that detectors of first set  31   b  are positioned between adjacent emitters of second set  32   b.  This has the advantage of improving coverage of the touch surface where detection paths of the above-surface system cover gaps between detection paths of the FTIR system and vice versa. 
     In one embodiment, emitters  31   a  and detectors  31   b  are only positioned along sub-portions of the periphery of the touch surface. In this embodiment, the portion of the periphery of the touch surface along which emitters  31   a  and detectors  31   b  are positioned is smaller than the portion of the periphery of the touch surface along which emitters  31   a  and detectors  31   b  are positioned. In one example, emitters  31   a  and detectors  31   b  are only located along two opposing edges of a rectangular touch surface. In an alternative embodiment, emitters  31   a  are placed along one edge of the rectangular touch surface and detectors  31   b  are positioned along an opposing edge of the touch surface. Alternatively, emitters  31   a  and detectors  31   b  may be positioned along L-shaped portions of the periphery of the rectangular touch surface at the corners. In one embodiment, the number of emitters  31   a  and detectors  31   b  are fewer than the number of emitters  32   a  and detectors  32   b  respectively. This may result in an FTIR system with a lower resolution than the above-surface system. Alternatively, for all of the above arrangements, emitters  31   a  and detectors  31   b  may be swapped for emitters  32   a  and detectors  32   b  so that the FTIR system has a higher resolution and/or coverage than the above-surface system. These arrangements allow the advantages of a complete above-surface system or FTIR system to be supplemented with the advantages of a limited FTIR system or limited above-surface system respectively without the need for a complete version of both systems. This would allow a significant reduction in manufacturing cost, power usage, and even physical size of the touch frame. E.g. Where high accuracy pressure detection needed to be added to an above-surface system, a limited FTIR type system configured to detect pressure (as is known in the art) may be added to the above-surface system with only as many emitters and detectors needed to accurately detect pressure. In one example, the limited system comprises only 25% of the number of emitters and detectors of the complete system. 
     In one embodiment, a low-power mode is provided wherein only the above-surface system is powered. When a touch is detected by the above-surface system, a full-power mode is activated and power is provided to the FTIR system. This has the advantage of preserving energy during periods that the above-surface system detects no touches whilst enabling the features of the FTIR system once it is required. Alternatively, an embodiment is provided wherein only the FTIR system is powered in a low-power mode and the above-surface system is only powered on when required. This may include a system wherein the above-surface system is only activated periodically or in response to a determination that a touch detected by the FTIR system is possibly a false touch caused by contamination. 
       FIG. 13  shows a variation of  FIG. 11  wherein emitters  31   a  are configured to simultaneously project light above and into transmissive panel  10  but wherein separate detectors  31  band  32   b  are used to provide separate above-surface and FTIR type touch signals. This advantageously allows simultaneous emission of both signals using a single emitter, allowing low energy consumption and cheaper manufacturing costs, whilst the above features. 
       FIG. 14  shows an alternative to  FIG. 13  wherein the detectors  31   b  are configured to simultaneously receive light from above and from within transmissive panel  10  but wherein separate emitters  31   a  and  32   a  are used to provide separate above-surface and FTIR type touch signals. This configuration advantageously allows separate control of the light intensity of the light emitted from emitters  31   a  and  32   a  to account for environmental light noise or other situations in which light levels need to be separately altered, whilst also allowing low energy consumption and cheaper manufacturing costs. 
     For all of the above embodiments, alternative in-coupling and out-coupling solutions used for coupling the light into and out of transmissive panel  10  may be employed according to techniques known in the prior art. E.g. Coupling the light into the edge of the panel rather than from below. 
     Furthermore, alternative waveguide, lens, and reflective surface configurations to convey light from emitters  32   a  to a plane parallel with touch surface  20  and back to detectors  32   b  may be employed according to techniques known in the prior art. E.g. Configurations for conveying the light around the edge of the panel rather than through it. 
     REFERENCE SIGNS LIST 
     
         
         A. touch-sensitive apparatus  100   
         B. transmissive panel  10   
         C. touch surface  20   
         D. bottom surface  25   
         E. emitters  30   a    
         F. detectors  30   b    
         G. coupling element  40   
         H. in-coupling point  45   
         I. light beam  50   
         J. touching object  60   
         K. edge reflector  70   
         L. reflector surface  80   
         M. reflector surface focal point  80   a    
         N. light beam  90   
         O. path  90   a    
         P. path  90   b    
         Q. path  90   c    
         R. detection lines  95   
         S. detection lines  96   
         T. dust shield  110   
         U. transparent window  115   
         V. light paths D 
         W. controller  120   
         X. signal processor  130   
         Y. processing unit  140   
         Z. signal profile  150   
         AA. signal profile  160