WO2008034191A1 - Signal detection for optical touch input devices - Google Patents

Abstract

The present invention relates to methods for detecting optical signals in an array of optical waveguides, using a two-dimensional photo detector array. It also relates to methods for aligning one or more optical waveguides with a plurality of detectors, and to an apparatus for detecting light from one or more optical waveguides. The methods and apparatus of the invention have particular application to detecting optical signals from a waveguide array in an optical touch input device.

Description

Signal Detection for Optical Touch Input Devices

TECHNICAL FIELD The present invention relates to methods for detecting optical signals from a waveguide and particularly but not only for detecting signals from a waveguide array in an optical touch input device.

BACKGROUND ART Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Touch input devices or sensors for computers and other consumer electronics devices such as mobile phones, personal digital assistants (PDAs) and hand-held games are highly desirable due to their extreme ease of use. Touch input devices may include a display underlying the input area, in which case they are commonly known as 'touch screens'. However other touch input devices, often known as 'touch panels', do not have a display. The present invention applies to both types of input device. In the past, a variety of approaches have been used to provide touch input devices. The most common approach uses a flexible resistive overlay, although the overlay is easily damaged, can cause glare problems, and tends to dim an underlying display, requiring excess power usage to compensate for such dimming. Resistive devices can also be sensitive to humidity, and the cost and drive power consumption of the resistive overlay scale quadratically with perimeter. Another approach is capacitive touch, which also requires an overlay. In this case the overlay is generally more durable, but the glare and dimming problems remain.

In yet another common approach, a matrix of light beams (usually infrared) is established in front of a display, with a touch event detected by the interruption of one or more of the beams. Such 'optical' touch input devices have long been known (US 3,478,220; US 3,673,327), with the beams generated by arrays of optical sources such as light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) and detected by corresponding arrays of detectors (such as phototransistors or photodiodes). This type of touch input device has the advantage of being overlay-free, but has a major cost problem in that it requires a large number of source and detector components, as well as supporting electronics. Since the spatial resolution of such a system depends on the number of sources and detectors, this component cost increases with display size and resolution.

US Patent Nos. 5,914,709, 6,181,842 and 6,351,260, and US Patent Application Nos. 2002/0088930 Al and 2004/0201579 Al , disclose an improved type of optical touch input device, where waveguides are used to distribute and collect the matrix of light beams. As discussed below with reference to Figure 1, this approach requires a single optical source and a single multi-element detector, representing a substantial cost reduction, but still requiring careful alignment with the detector elements. Figure 1 illustrates the operation of an optical touch input device similar to that described in US Patent Nos. 5,914,709, 6,181,842 and 6,351,260, and US Patent Application Nos. 2002/0088930 Al and 2004/0201579 Al. In this device, an array of integrated optical waveguides ('transmit' waveguides) 10 conduct light from a single optical source 11 to integrated in-plane lenses 16 that collimate the light in the plane of a display and/or input area 13 and launch an array of light beams 12 across that display and/or input area 13. The light is collected by a second array of integrated in-plane lenses 16 and integrated optical waveguides ('receive' waveguides) 14 at the other side of the screen and/or input area, and conducted to a position-sensitive detector 15 with a plurality of photo-detector elements 20. A touch event (e.g. by a finger or stylus) cuts one or more of the beams of light and is detected as a shadow, with position determined from the particular beam(s) blocked by the touching object. That is, the position of any physical blockage can be identified in each dimension, enabling user feedback to be entered into the device. Preferably, the device also includes external vertical collimating lenses (VCLs) 17 adjacent to the integrated in-plane lenses 16 on both sides of the input area 13, to collimate the light beams 12 in the direction perpendicular to the plane of the input area.

The touch input devices are usually two dimensional and rectangular, with two arrays (X, Y) of transmit waveguides 10 along adjacent sides of the input area, and two corresponding arrays of receive waveguides 14 along the other two sides. As part of the transmit side, in one embodiment a single optical source 11 (such as an LED or a vertical cavity surface emitting laser (VCSEL)) launches light via some form of optical power splitter 18 into a set of waveguides that form both the X and Y transmit arrays. The X and Y transmit waveguides are usually fabricated on an L shaped substrate 19, and likewise for the X and Y receive waveguides, so that a single source and a single position-sensitive detector can be used to cover both X and Y dimensions. However in alternative embodiments, a separate source and/or detector may be used for each of the X and Y dimensions. For simplicity, Figure 1 only shows four waveguides per side of the input area 13; in actual touch input devices there will generally be sufficient waveguides for substantial coverage of the input area.

As is usual with integrated optical waveguides, the 'transmit' optical waveguides 10 and 'receive' optical waveguides 14 each consist of patterned, light guiding cores (of refractive index ni) surrounded by a cladding (of refractive index n₂, where n₂ < n^ and mounted on a mechanically robust substrate. Frequently, the portion of cladding between the light guiding cores and the substrate is referred to as the 'lower cladding' or 'bottom cladding', with the remainder of the cladding referred to as the 'upper cladding' or 'top cladding'.

As shown in Figure 1, each 'receive' waveguide 14 is in optical communication with one or more individual photo-detector elements 20 of the position-sensitive detector 15. It will be appreciated that for this system to accurately determine the position of a touch event, it is crucial that the signal in each of the beams 12 be faithfully guided by the receive waveguides 14 to the respective elements 20 of the detector 15. This requires precise alignment of the receive waveguides 14 to the elements 20.

The performance of optical touch input devices can also be compromised in high ambient light conditions - if too much ambient light is captured by the receive waveguides and reaches the detector pixels, the system will be unable to resolve the reduction in signal intensity required to register a touch event.

It is an object of the present invention to ameliorate some of the disadvantages of or at least provide a commercial alternative to the prior art and that, at least in the preferred embodiments, improves performance of optical touch input devices.

DISCLOSURE OF THE INVENTION

In a first aspect the present invention provides a method for detecting optical signals in an array of optical waveguides, said method comprising: optically coupling said optical waveguides with a two-dimensional photo-detector array; determining a subset of said photo-detectors that are in an alignment window for each respective waveguide; and analysing the light fields captured by said subset of photo-detectors.

In a second aspect the present invention provides a method of aligning a plurality of optical detectors with at least one optical waveguide, said method comprising: optically coupling said at least one optical waveguide with a two-dimensional photo- detector array; providing an optical signal through said at least one optical waveguide; determining a subset of said photo-detectors that receive an optical signal from said at least one waveguide, and analysing the light fields captured by said subset of photo- detectors.

In a particularly preferred embodiment the two-dimensional photo detector array comprises a digital camera detector device (hereinafter called a 'camera chip'), most preferably comprising a complementary metal oxide semiconductor (CMOS) and/or a charge-coupled device (CCD). Typically the optical waveguides will comprise a plurality of channel optical waveguides integrated onto a common substrate, preferably in the form of a linear array. Alternatively they may comprise a two-dimensional array of channel optical waveguides or an optical fibre bundle. The waveguides are optically coupled to the array of detectors. In a most preferred embodiment, each optical waveguide will be mounted such that its end face overlaps with a plurality of individual photo detectors in the two- dimensional photo detector array.

Alignment of detectors with waveguides has caused significant problems in the past. Generally a detector is attached to the end of a waveguide by a UV cured glue or similar light transmissive adhesive. One detector is normally aligned to each waveguide, or a linear array of interconnected detectors is aligned to a respective linear series of waveguides.

By providing a two-dimensional array of photo detectors, alignment of each waveguide to within a particular detector or pixel is not critical. Rather the waveguide array is positioned adjacent to the two-dimensional photo detector array such that the optical output signal from each waveguide strikes the photo detector array, and only those photo detectors or pixels that are in a predetermined alignment window i.e. those pixels/detectors that receive a minimum threshold signal from the waveguides, need be used for analysis of the signals, i.e. activated. Further, pixels that are not in a position to receive signals from the waveguides may be turned off. In preferred embodiments each optical waveguide is aligned with at least four photo -detectors of the photo-detector array.

Clearly this provides significant advantages over the prior art, as there are savings both in time and cost in alignment of the waveguides to the photo detectors. Further, the improved alignment provides better capture of the signals from the waveguides to the detector device.

Secondly, use of the selective interrogation and in some embodiments deactivation of the pixels in the detector device significantly improves clarity of the signal, reduces noise in the detector (e.g. from stray light), reduces the electrical power requirement, and increases the update speed.

To explain, only those pixels that are aligned with the waveguide end faces need to be interrogated. Unwanted signal (Le. noise) by way of stray signal light or ambient light in the waveguide cladding or substrate etc. can interfere with a receive signal at a particular detector. By only interrogating certain areas of pixels in the two-dimensional photo detector array, the update speed is improved, and stray light impinging on the photo detector array can be blocked from reaching the data processing circuitry. Further, in specific embodiments the unwanted pixels may be deactivated, reducing the electrical drive power. Such a two-dimensional photo detector array clearly has significant advantages over conventional linear detector systems. Alignment of a linear array of waveguides to a two-dimensional array of detector pixels is clearly easier than aligning a linear array of waveguides to a linear array of detector pixels.

It should also be understood that while most configurations of waveguide-based optical touch input device systems use such linear arrays of 'receive' waveguides, the present invention is also suitable for multi-layer stacks of waveguides, as shown for example in Figure 6c of US Patent No. 5,914,709 and Figure 3a of US 2006/002655 Al.

Another advantage over the prior art is that two-dimensional photo detector arrays such as camera chips are simple consumer electronic components that are widely available and inexpensive. For instance complementary metal oxide semiconductor (CMOS) camera chips are found in many mobile phones. The use of such camera chips avoids the need for custom-built detectors and therefore eliminates the engineering costs associated with design and fabrication of such customised devices.

In another aspect the present invention provides an apparatus for detecting an optical signal in at least one optical waveguide, said apparatus comprising a two- dimensional photo-detector array positioned to receive an optical signal from said at least one optical waveguide, wherein said photo -detector array is selectively interrogated such that only those photo -detectors that are in an alignment window with each at least one waveguide are interrogated. As mentioned above the alignment window can be determined by simply transmitting an optical signal through the at least one waveguide and determining those detectors that receive a minimum threshold quantity of said signal.

As mentioned above, preferably the said two-dimensional array of photo detectors is in the form of a digital camera detector chip such as a CMOS or charge- coupled device.

Preferably the photo -detector array further comprises an optical filter, which may be chosen from a long pass filter, a short pass filter or a notch filter. When the optical signals are in the infrared region of the spectrum, the optical filter is preferably chosen to block visible light. Alternatively, when the optical signals are in the visible region of the spectrum the optical filter is a Bayer filter.

As discussed previously, in preferred embodiments the gain may be independently adjusted for each detector in the photo -detector array for obtaining a minimum threshold signal value. In related embodiments, each photo -detector may comprise a plurality of sub-pixels, which themselves may be selectively interrogated. To explain, each photo -detector may comprise red, green and blue sub-pixels, thereby defining separate red, green and blue sub-pixel arrays. In the case where a plurality of optical waveguide arrays are optically coupled to the two-dimensional photo detector array, each optical waveguide array may be configured for optical communication with a corresponding sub-pixel array. In this example the gain may be independently adjusted for each sub-pixel array for obtaining a minimum threshold signal value. Furthermore, the gain may be independently adjusted for each sub-pixel in a sub-pixel array for obtaining a minimum threshold signal value. In one embodiment the optical signals are in the infrared region of the spectrum, and the blue sub-pixel array is interrogated. In this embodiment the wavelength of the optical signals are in the range from 800nm to lOOOnm.

BRIEF DESCRIPTION OF THE INVENTION

The present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 illustrates the operation of a prior art waveguide-based optical touch input device incorporating lenses to provide in-plane focusing of the light beams;

Figure 2 is a diagrammatic view of a detector side of a touch input device in accordance with a first embodiment of the present invention; Figure 3 is a diagrammatic view of a detector side of another touch input device in accordance with a first embodiment of the present invention;

Figures 4A-B show the breakdown of a digital camera pixel into four sub-pixels designated for sensitivity to different colours; Figure 5 shows the spectral sensitivity of red, green and blue sub-pixels;

Figures 6A-6B are diagrammatic views of a light field output from an array of receive waveguides in an optical touch input device in the absence and presence of a touch event respectively;

Figure 7 is a diagrammatic view of an output light field illuminating the surface of a two-dimensional photo detector array;

Figure 8 is a diagrammatic view of an output light field, captured as a still image;

Figure 9 is a diagrammatic view of illumination intensity versus position across a two-dimensional photo detector array;

Figures 10A- 1OB are graphical representations of the signal processing during the absence and presence of a touch event respectively; and

Figures 11-13 are replications of Figures 7-9 with less than optimal alignment of an array of receive waveguides to a two-dimensional photo detector array.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments below are in relation to optical touch input devices in which the invention provides particular advantages over the prior art. However, it should be understood that the method and apparatus of the present invention may be used in other forms and for other purposes. Referring to Figure 2, this diagram shows a linear array 21 of receive waveguides

14 formed on a substrate 19, and positioned to abut a detector in the form of a digital camera chip 22 including a two-dimensional array of pixels 23. In a preferred form, the detector is a CMOS (Complementary Metal Oxide Semiconductor) camera chip, although other detector technologies such as CCD (charge-coupled device) devices could also be used. Those skilled in the art will be aware that CMOS camera chips are found in many mobile phones; the use of such commonly available camera chips may avoid the need for an application specific integrated circuit (ASIC) detector thereby eliminating the engineering costs associated with design and fabrication of a customised device. Additionally, mass production costs may be further reduced through the use of such readily available and inexpensive mobile phone camera chips.

Figure 3 shows an alternative embodiment with two linear arrays 21 of receive waveguides 14 positioned to abut a detector in the form of a digital camera chip 22 including a two-dimensional array of pixels 23. In this case, each linear array 21 is on a separate substrate 19, with one array carrying signals from the X dimension of a touch input area and the other array carrying signals from the Y dimension. For compactness the two arrays are stacked on top of each other, a configuration that is not possible with linear detector arrays. It will be appreciated by those skilled in the art that each pixel of a digital camera chip is generally (but not necessarily) comprised of four sub-pixels designated, with the aid of colour filters, for red, green or blue sensitivity. For the purposes of this specification, the term 'pixel' is used to refer to each discrete grouping of colour- sensitive sub-pixels, if such groupings are in fact present, or to each and every photosensitive element if such groupings are not present. A typical situation is shown in Figure 4A, where each pixel 23 comprises one 'red' sub-pixel 24, two 'green' sub-pixels 25, and one 'blue' sub-pixel 26, with the green sub-pixels being more numerous to allow for the fact that the human eye is more sensitive in the green than in the red or blue regions of the visible spectrum. It should be noted that the sub-pixels are generally identical (typically they are silicon-based detectors), with the spectral selectivity provided by an appropriately patterned red/green/blue colour filter 27, commonly known as a Bayer filter, placed in front of each set of four sub-pixels as shown in Figure 4B. Figure 5 shows typical spectral sensitivity curves 28, 29 and 30 of the respective red, green and blue sub-pixels, determined by the convolution of the responsivity of the (silicon) sub-pixels with the transmission of the appropriate portions of the Bayer filter. Frequently, CMOS detector chips are provided with circuitry that enable each of the red, green and blue sets of sub-pixels to be interrogated independently. This feature, whereby for example only the red sub-pixels are interrogated, may be highly advantageous for certain devices incorporating CMOS detector chips, for example optical touch input devices. If an optical touch input device (as shown in Figure 1 for example) were designed for operation with green light, i.e. the optical source 11 emits in the green, the sensitivity spectrum 29 in Figure 5 shows, unsurprisingly, that it would be advantageous to interrogate only the green pixels, to enhance the rejection of ambient light of other wavelengths. However optical touch screen sensors typically operate with infrared light, to avoid disturbing a user, and in such cases the selection of an appropriate set of sub-pixels is not so obvious. A particularly favourable spectral window is around 850nm, well within the silicon detector cut-off around lOOOnm and for which inexpensive optical sources are commercially available. One might think that the red sub-pixels would be the most appropriate for 850nm operation, but the sensitivity spectra of Figure 5 show that all three sets of sub-pixels are approximately equally sensitive around 850nm (or for that matter at any wavelength longer than 800nm). Further, the green sub-pixels and especially the blue sub-pixels are both less sensitive than the red sub-pixels at most wavelengths below 800nm, and would therefore be less affected by noise from visible ambient white light, say sunlight. It appears therefore that for operation of an optical touch input device with near infrared light, in the presence of ambient white light, it may be best to interrogate only the blue sub-pixels. Of course if the ambient light were of a particular colour, say blue, it may be better to interrogate the red sub-pixels. It is also possible to change the set of sub-pixels being interrogated, which may be advantageous for example if the ambient light alters colour; this change may be automatic (for example the control algorithms may seek the sub-pixel set that maximises the signal to noise ratio) or it may be made manually. It is further possible to interrogate two sets of sub-pixels or even all three sets, or to interrogate one set of sub- pixels in one window and a different set in another window. Further variations will present themselves to someone skilled in the art.

In another variation, an optical touch screen system could be made particularly robust to ambient light by having two or more layers of transmit waveguides, each connected to an optical source emitting a different colour. Signal light from the multiple layers of transmit waveguides could be collected by a single layer of receive waveguides or by multiple layers of receive waveguides (provided the vertical collimation is sufficiently tight), and guided to the two-dimensional detector array. The optical sources could be activated singly or in combination, selected automatically or manually depending on the ambient light conditions, to maximise the signal to noise ratio at the detector array. We note that while CMOS detector chips used in digital cameras generally contain the red/green/blue Bayer filters described above, other possibilities become available if customised CMOS detector chips were to be used. In particular, customised chips could be manufactured with a filter (such as a short pass, long pass or notch filter) appropriate for the wavelength of the optical source. In general, the use of wavelength- selective filters either between the receive waveguides 14 and the detector 15 or at the input end of the receive waveguides is known in the art of optical touch input devices, see for example US Patent No 6,181,842. In particular, a visible light-blocking filter can be used in optical touch devices using infrared sensing light, to prevent ambient visible light from reaching the detector elements. Inspection of the sub-pixel response curves in Figure 5 shows that if a filter blocking light of wavelengths below 800nm is included on the receive side of an optical touch system operating at 850nm, from a sensitivity perspective there is essentially no advantage to be gained in choosing to interrogate the red, green or blue sub-pixels — as noted previously, all have essentially the same response around 850nm.

However in this circumstance there is still an advantage to be gained in being able to interrogate the red, green or blue sub-pixels independently, namely the ability to apply different amounts of gain to different sets of pixels. For example, it will be appreciated from Figures 1 and 2 (or Figure 3) that those receive waveguides 14 arrayed along one receive side of the input area 13 (the 'X receive array') and those receive waveguides arrayed along the other receive side (the 'Y receive array') will abut different portions of a detector 15 in the form of a digital camera chip 22. One can therefore use the red sub-pixels (say) to detect signals from the X receive array, and the green sub-pixels (say) to detect signals from the Y receive array, with different amounts of gain applied to the red and green sub-pixels. One situation where this is advantageous is if the signal levels in one receive array are weaker than those in the other receive array; this is likely to occur with rectangular input areas where, because of imperfect collimation of the signal beams 12, less signal power will be received from those beams propagating across the longer axis. Preferably, there will be dynamic control of the gain applied to each set of sub-pixels, to allow for changes in the signal intensity in the X and Y receive arrays.

For maximum flexibility, it would be beneficial to have independent gain control for every pixel, to allow for the signal level in each individual touch channel to be boosted (e.g. if the transmit waveguide or receive waveguide of that channel has a defect) or lowered as required. At present it is possible to obtain linear detector arrays with this level of gain control, but not two-dimensional arrays.

Referring again to Figure 2, a digital camera chip 22 comprises a two- dimensional array of photo-detectors or pixels 23 with a lateral dimension 31 and a longitudinal dimension 32, preferably chosen such that the lateral dimension 31 is at least as large as the lateral dimension 33 of the array 21 of receive waveguides, more preferably, at least as large as the lateral dimension 35 of the receive waveguide substrate 19. The longitudinal dimension 32 is at least 2 pixels high, preferably substantially larger than the longitudinal dimension 34 of the receive waveguide array 21. More preferably, the longitudinal dimension 32 is at least ten times larger than the longitudinal dimension 34 of the receive waveguide array.

Referring to Figure 3, similar principles apply to the situation where there are two or more stacked arrays of waveguides abutting a digital camera chip. In this situation the longitudinal dimension 32 is preferably at least three times larger, more preferably at least five times larger, than the longitudinal dimension 42 of the stack of waveguide arrays 21. By way of specific example, two waveguide arrays 21 (one with 'X' receive waveguides, the other with 'Y' receive waveguides) are stacked one on top of the other and abutted to a digital camera chip 22 with a lateral dimension 31 equal to 3.84mm and a longitudinal dimension 32 equal to 2.88mm. The wider array has 122 receive waveguides 14 on a 27.5μm pitch for a total lateral dimension 33 equal to

3.33mm (Le. less than the lateral dimension 31), while the narrower array has 93 receive waveguides on a 27.5μm pitch. Each array has a longitudinal dimension 34 of approximately 225 μm (comprising 175μm from the substrate and 50μm from the combined core and cladding layers) so that the total longitudinal dimension 42 of the stack is approximately 450μm (i.e. over six times less than the longitudinal dimension 32).

Whatever the configuration of the waveguide arrays, the important point is that by appropriate choice of the dimensions 31 and 32, a two-dimensional photo detector array has a much greater area (as compared to linear detector arrays) for alignment with the one or more waveguide arrays, to receive output from them.

Generally it is alignment in the direction of the longitudinal dimension 32 that causes most difficulty with linear detector arrays. Lateral misalignment of the waveguide array is generally not as serious, because the worst that can happen (given the accuracy of most 'pick and place' routines) is that light from one or two of the outermost waveguides 14 will not be received. Provided the lateral dimension 31 is at least as large as the lateral dimension 35 of the substrate 19, all of the receive waveguides should remain within the area defined by the two-dimensional camera chip 22. However in conventional systems where the detector array comprises a linear array, a longitudinal or angular misalignment may cause all or most of the receive waveguides to 'miss' the individual photo detectors. By providing a two-dimensional photo detector array with a longitudinal dimension 32 much greater than the longitudinal dimension 34 of each waveguide array, the longitudinal positioning and twisting/tilting of the waveguide array relative to the detector array becomes non-critical so that, as discussed further below, the detector array can still operate reliably to inform of a touch event on a touch input apparatus. In particular, this relaxed alignment tolerance reduces the cost and complexity of the assembly process, and effectively nullifies the effect of any creep of an adhesive used to bond a waveguide array to a detector. As mentioned above, one of the advantages of a two-dimensional photo detector array is the ability to interrogate or activate only those pixels that are aligned with the waveguides. To explain, the present invention will now be discussed by way of reference to an embodiment in which the method and apparatus are used in an optical touch input device to determine a touch event. Referring firstly to Figure 6 A, a receive waveguide array 21 is shown with a light field 36 emanating from the end face 37 of each receive waveguide. This case illustrates an absence of a touch event, where each signal beam is uninterrupted such that all signals emanating from the transmit waveguides reach the receive waveguides and subsequently the detector pixels. We note that this situation could be used during a power-up or calibration routine to identify those pixels that need to be activated during device operation. Figure 6B illustrates the situation where one or more signal beams have been blocked i.e. a touch event has occurred causing one or more receive waveguides 38 to receive no or significantly less light.

Figure 7 gives a representation of an output light field from a receive waveguide array illuminating the surface of the two-dimensional camera chip 22 with individual pixels 23 delineated by grid lines. In the case shown in Figure 7, the illuminated area 39 from each receive waveguide covers more than one pixel. When the illuminated areas 39 shown in Figure 7 are captured in a still image, they appear as the pixelated areas 40 in Figure 8. It can be seen that although various pixels receive different signal strengths, there is at least one strongly illuminated pixel in the middle of the light field emanating from each particular waveguide, and less illumination on other pixels. This situation certainly conveys no less information than would an array of perfectly aligned single pixels, which is of course much more difficult to achieve in practical applications. For averaging purposes it is preferable to have as many pixels per waveguide as possible. More preferably there should be at least four pixels (or part thereof) per waveguide.

This of course depends on the waveguide cross-sectional area and is limited by the available pixel size, however by way of example, for a typical 8μm x 1 lμm size channel waveguide, a camera chip with 4μm x 4μm pixels (routinely available) will clearly provide at least four pixels per waveguide. As discussed further below, the end faces of the waveguides may not be in intimate contact with the detector pixels, in which case beam divergence will increase the number of pixels illuminated by each waveguide.

When the pixelated image data shown in Figure 8 is integrated vertically, Le. the received intensity is summed for each column of pixels 41, a plot of illumination intensity versus position across the detector array is obtained as shown in Figure 9. The illumination peaks occurring at regular intervals across Figure 9 correspond to the positions closest to perfect or precise alignment of individual detector pixels with the outputs of the receive waveguides.

Figure 1OA is a graphical representation of Figure 9 showing a situation where no touch event is present Le. there is no blocking of any signal beam, a condition similar to that shown in Figure 6A. If a touch event occurs, the illumination intensity versus position may appear as shown in Figure 1OB, where for instance the touch event is blocking two channels i.e. preventing light from reaching at least two groups of detector pixels. Of course to register a touch event it is not necessary for all light in one or more channels to be blocked. Rather, it is sufficient for a touch object to reduce the received optical power to below a threshold level.

One significant advantage of using a two-dimensional photo detector array such as a CMOS camera chip is shown in relation to Figures 11-13. Using a two-dimensional photo detector array as opposed to a linear photo-detector array allows for significant angular misalignment of the waveguide array relative to the axis of the detector chip. This in turn significantly reduces the alignment tolerances required during the attachment process. As discussed above, the necessity for precise alignment between a waveguide array and a linear detector array results in considerable expense in the manufacture and assembly of touch input devices. Turning to Figure 11 , this diagram shows a similar output light field illumination to that displayed in Figure 7, however in this case the receive waveguide array is substantially misaligned in angle relative to the detector chip, so that the waveguides on the left hand side are positioned substantially lower than the waveguides on the right hand side. When this output light field is captured as a still image, it will appear as shown in Figure 12.

When the illumination intensity of the various pixel columns 41 is then integrated vertically and once again plotted against position along the detector array we arrive at Figure 13 that, it will be noted, is virtually identical to Figure 9. In other words, the angular alignment of the receive waveguide relative to the two dimensional detector array is, within limits, irrelevant for determining illumination versus position across the detector array and therefore a touch event.

Through the use of modern two-dimensional photo detector array technology e.g. CMOS camera chips, "windowing" of the signal may be achieved. That is, the relevant information on the detector can be extracted easily and the remainder discarded simply by interrogating only those pixels in the narrow band about the receive signal field(s). This provides an increased tolerance to stray light that may fall on pixels outside the desired band and improves processing speed since the processor is only dealing with the most relevant data. Further, electrical power consumption can be reduced by turning off all unnecessary detector pixels. In particular, windowing increases the tolerance to stray signal and ambient light that may be guided in the waveguide cladding or substrate, a problem discussed more thoroughly in US patent application No 11/552380, entitled 'Improved optical elements for waveguide-based optical touch screens'. Further it should be understood that the present invention is suitable for various waveguide arrays. The receive waveguide array 21 shown in Figure 2 is a linear waveguide array. However with an appropriately dimensioned two-dimensional photo detector array, the present invention is also suitable for multi-layer stacks of waveguides (e.g. as shown in Figure 3), optical fibre bundles, or waveguides grouped in other configurations. As disclosed in US 5,914,709 (Figure 6c) and US 2006/002655 (Figure 3a), multi-layer stacks of waveguides may for example be desirable to enhance the spatial resolution of an optical touch input device.

It will be appreciated by those skilled in the art that an optical beam guided within an optical waveguide begins to diverge as soon as it exits that waveguide. In the context of the present invention, with reference to Figure 2, it is therefore important for the waveguide array 21 to be positioned as close as possible to the camera chip 22. It will be appreciated from Figure 7 that if the gap between the two is too large, beam divergence will cause the illuminated areas 39 to overlap, potentially degrading the performance of the touch input device. This problem may arise with CMOS digital camera chips, which are often supplied with a protective glass cover slip in front of the pixels. One (time consuming) solution is to reduce the cover slip thickness by polishing. Another solution is to source detector chips without cover slips or with particularly thin cover slips, however this may be expensive (such detector chips may not be standard) and there is a risk of damaging the pixels (with adverse impact on manufacturing and assembly yields). A third solution is to provide a focusing system such as micro-optic lenslets between the waveguide array and the detector surface. Focusing could be done either in the vertical or horizontal directions or both. Individual channel horizontal focusing (with reference to the above images) would allow for more discrete spots within a captured image and thus ease of further processing, hence a more precise calculation of the touch position. A fourth, preferred solution is use suitable signal processing: even if the output signals from two adjacent waveguides were to overlap partially on the detector array, a fall in signal level below a 'touch' threshold will still be discernible by integrating the response of several detector pixels within the illuminated areas.

It should be noted that this 'divergence problem' may also occur with the linear detector arrays of the prior art; irrespective of the method used to overcome the resulting signal overlap, the aforementioned advantages in terms of alignment, windowing and colour sub-pixel selection are still gained. While either a CCD detector or a CMOS detector could be used, CMOS is preferred since they are becoming increasingly inexpensive, and typically have extraordinary sensitivity (down to the nanowatt level), lower power consumption and higher dynamic range, along with several other attributes. For instance, a single CMOS chip may have separate areas devoted to optical signal detection (a pixel array) and electrical signal processing, obviating the requirement for a separate signal processing chip.

CMOS detectors also have the possibility of individual pixel readouts, higher speeds, larger array sizes, radiation hardened capabilities, random access and the possibility to integrate 'intelligence' at the sensor level, the so called SoC 'system on chip'.

As would be clear to a person skilled in the art, there are a number of significant advantages over the prior art in using such CMOS chips including ease of alignment, elimination or reduction of noise effects caused by ambient light, the ability to interrogate particular sets of colour sub-pixels, the possibility of complex on-chip signal processing, and reduced manufacturing costs.

Although the invention has been described with reference to certain specific examples, it would be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

CLAIMS:

1) A method for detecting optical signals in an array of optical waveguides, said method comprising: optically coupling said optical waveguides with a two- dimensional photo-detector array; determining a subset of said photo-detectors that are in an alignment window for each respective waveguide; and analysing the light fields captured by said subset of photo-detectors.

2) A method of aligning a plurality of optical detectors with at least one optical waveguide, said method comprising: optically coupling said at least one optical waveguide with a two-dimensional photo -detector array; providing an optical signal through said at least one optical waveguide; determining a subset of said photo -detectors that receive an optical signal from said at least one waveguide, and analysing the light fields captured by said subset of photo-detectors.

3) A method as claimed in claim 1 or 2 wherein only those photo -detectors in said subset are activated. 4) A method as claimed in any one of the preceding claims wherein said subset of photo -detectors is defined by those detectors in said photo -detector array that receive a minimum threshold signal value from the respective waveguide. 5) A method as claimed in any one of the preceding claims when used in an optical touch input device. 6) A method as claimed in any one of the preceding claims wherein said photo- detector array comprises a digital camera detector chip.

7) A method as claimed in any one of the preceding claims wherein said photo- detector array comprises a complementary metal oxide semiconductor.

8) A method as claimed in any one of claims 1 to 6 wherein said photo -detector array comprises a charge-coupled device.

9) A method as claimed in any one of the preceding claims wherein each optical waveguide is aligned with at least four photo -detectors of said photo-detector array.

10) A method as claimed in any one of the preceding claims wherein said optical waveguides comprise channel optical waveguides.

11) A method as claimed in any one of claims 1 to 9 wherein said optical waveguides comprise optical fibres.

12) A method as claimed in any one of the previous claims wherein said photo- detector array further comprises an optical filter. 13) A method as claimed in claim 12 wherein said optical filter comprises a long pass filter, a short pass filter or a notch filter.

14) A method as claimed in claim 12 or 13 wherein said optical signals are in the infrared region of the spectrum, and said optical filter blocks visible light. 15) A method as claimed in claim 12 or 13 wherein said optical signals are in the visible region of the spectrum, and said optical filter is a Bayer filter. 16) A method as claimed in any one of the preceding claims including the step of independently adjusting the gain for each said detector in said photo-detector array for obtaining a minimum threshold signal value. 17) A method as claimed in any one of the preceding claims wherein each said photo-detector comprises a plurality of sub-pixels.

18) A method as claimed in claim 17 wherein said plurality of sub-pixels comprises red, green and blue sub-pixels such that said photo-detector array has separate red, green and blue sub-pixel arrays. 19) A method as claimed in claim 18 wherein one or more of said separate sub-pixel arrays are interrogated.

20) A method as claimed in claim 18 or 19 comprising a plurality of optical waveguide arrays optically coupled to said two-dimensional photo detector array.

21) A method as claimed in claim 20 wherein each said optical waveguide array is configured for optical communication with a corresponding said sub-pixel array.

22) A method as claimed in any one of claims 18 to 21 including the step of independently adjusting the gain for each sub-pixel array for obtaining a minimum threshold signal value.

23) A method as claimed in any one of claims 18 to 22 including the step of independently adjusting the gain for each said sub-pixel in a sub-pixel array for obtaining a minimum threshold signal value.

24) A method as claimed in any one of claims 18 to 23 wherein said optical signals are in the infrared region of the spectrum, and the blue sub-pixel array is interrogated. 25) A method as claimed in claim 24 wherein the wavelength of said optical signals is in the range from 800nm to lOOOnni.

26) An apparatus for detecting an optical signal in at least one optical waveguide, said apparatus comprising a two-dimensional photo -detector array positioned to receive an optical signal from said at least one optical waveguide, wherein said photo-detector array is selectively interrogated such that only those photo- detectors that are in an alignment window with each at least one waveguide are interrogated.

27) An apparatus as claimed in claim 26 wherein only those photo-detectors that are in an alignment window with each at least one waveguide are activated.

28) An apparatus as claimed in claim 26 or 27 wherein the alignment window includes at least four photo-detectors.

29) An apparatus as claimed in any one of claims 26 to 28 wherein the at least one optical waveguide comprises a waveguide array having a lateral dimension and a longitudinal dimension, said lateral dimension being larger than said longitudinal dimension, and wherein said photo -detector array has a corresponding lateral dimension at least as large as said lateral dimension of said waveguide array, and a corresponding longitudinal dimension at least three times larger than said longitudinal dimension of said waveguide array. 30) An apparatus as claimed in claim 29 wherein said longitudinal dimension of said photo -detector array is at least five times larger than the corresponding longitudinal dimension of said waveguide array.

31) An apparatus as claimed in any one of claims 26 to 30 wherein said photo- detector array comprises a digital camera detector chip. 32) An apparatus as claimed in any one of claims 26 to 31 wherein said photo- detector array comprises a complementary metal oxide semiconductor.

33) An apparatus as claimed in any one of claims 26 to 31 wherein said photo- detector array comprises a charge-coupled device.

34) An apparatus as claimed in any one of claims 26 to 33 wherein said photo- detector array is adapted to receive optical signals from a linear array of channel optical waveguides.

35) An apparatus as claimed in any one of claims 26 to 33 wherein said photo- detector array is adapted to receive optical signals from a two-dimensional array of channel optical waveguides. 36) An apparatus as claimed in claim 35 wherein said two-dimensional array of channel optical waveguides comprises a plurality of stacked linear arrays. 37) An apparatus as claimed in any one of claims 26 to 33 wherein said photo- detector array is adapted to receive optical signals from an optical fibre bundle. 38) An apparatus as claimed in any one of claims 26 to 37 wherein said photo - detector array further comprises an optical filter.

39) An apparatus as claimed in claim 38 wherein said optical filter comprises a long pass filter, a short pass filter or a notch filter. 40) An apparatus as claimed in claim 38 or 39 wherein said optical signals are in the infrared region of the spectrum and said optical filter blocks visible light.

41) An apparatus as claimed in claim 38 or 39 wherein said optical signals are in the visible region of the spectrum and said optical filter is a Bayer filter.

42) An apparatus as claimed in any one of claims 26 to 41 wherein the gain for each said detector in said photo-detector array is independently adjustable for obtaining a minimum threshold signal value.

43) An apparatus as claimed in any one of claims 26 to 42 wherein each said photo- detector comprises a plurality of sub-pixels.

44) An apparatus as claimed in claim 43 wherein said plurality of sub-pixels comprises red, green and blue sub-pixels such that said photo -detector array has separate red, green and blue sub-pixel arrays.

45) An apparatus as claimed in claim 44 wherein one or more of said separate sub- pixel arrays are interrogated.

46) An apparatus as claimed in claims 44 or 45 comprising a plurality of optical waveguide arrays optically coupled to said two-dimensional photo detector array.

47) An apparatus as claimed in claim 46 wherein each said optical waveguide array is configured for optical communication with a corresponding said sub-pixel array.

48) An apparatus as claimed in any one of claims 44 to 47 wherein the gain for each sub-pixel array is independently adjustable for obtaining a minimum threshold signal value.

49) An apparatus as claimed in any one of claims 44 to 48 wherein the gain for each said sub-pixel in a sub-pixel array is independently adjustable for obtaining a minimum threshold signal value. 50) An apparatus as claimed in any one of claims 44 to 49 wherein said optical signals are in the infrared region of the spectrum, and the blue sub-pixel array is interrogated.

51) An apparatus as claimed in claim 50 wherein the wavelength of said optical signals is in the range from 800nm to lOOOnm.