Light sensitive display

[0001] This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/383,040 filed on May 23, 2002.

[0002] The present invention relates to touch sensitive displays.

[0003] Touch sensitive screens (“touch screens”) are devices that typically mount over a display such as a cathode ray tube. With a touch screen, a user can select from options displayed on the display's viewing surface by touching the surface adjacent to the desired option, or, in some designs, touching the option directly. Common techniques employed in these devices for detecting the location of a touch include mechanical buttons, crossed beams of infrared light, acoustic surface waves, capacitance sensing, and resistive materials.

[0004] For example, Kasday, U.S. Pat. No. 4,484,179 discloses an optically-based touch screen comprising a flexible clear membrane supported above a glass screen whose edges are fitted with photodiodes. When the membrane is flexed into contact with the screen by a touch, light which previously would have passed through the membrane and glass screen is trapped between the screen surfaces by total internal reflection. This trapped light travels to the edge of the glass screen where it is detected by the photodiodes which produce a corresponding output signal. The touch position is determined by coordinating the position of the CRT raster beam with the timing of the output signals from the several photodiodes. The optically-based touch screen increases the expense of the display, and increases the complexity of the display.

[0005] Denlinger, U.S. Pat. No. 4,782,328 on the other hand, relies on reflection of ambient light from the actual touch source, such as a finger or pointer, into a pair of photosensors mounted at corners of the touch screen. By measuring the intensity of the reflected light received by each photosensor, a computer calculates the location of the touch source with reference to the light received by each photosensor, a computer calculates the location of the touch source with reference to the screen. The inclusion of the photosensors and associated computer increases the expense of the display, and increases the complexity of the display.

[0006] May, U.S. Pat. No. 5,105,186, discloses a liquid crystal touch screen that includes an upper glass sheet and a lower glass sheet separated by spacers. Sandwiched between the glass sheets is a thin layer of liquid crystal material. The inner surface of each piece of glass is coated with a transparent, conductive layer of metal oxide. Affixed to the outer surface of the upper glass sheet is an upper polarizer which comprises the display's viewing surface. Affixed to the outer surface of glass sheet is a lower polarizer. Forming the back surface of the liquid crystal display is a transflector adjacent to the lower polarizer. A transflector transmits some of the light striking its surface and reflects some light. Adjacent to transflector is a light detecting array of light dependent resistors whose resistance varies with the intensity of light detected. The resistance increases as the light intensity decreases, such as occurs when a shadow is cast on the viewing surface. The light detecting array detect a change in the light transmitted through the transflector caused by a touching of viewing surface. Similar to touch sensitive structures affixed to the front of the liquid crystal stack, the light sensitive material affixed to the rear of the liquid crystal stack similarly pose potential problems limiting contrast of the display, increasing the expense of the display, and increasing the complexity of the display.

[0007] Touch screens that have a transparent surface which mounts between the user and the display's viewing surface have several drawbacks. For example, the transparent surface, and other layers between the liquid crystal material and the transparent surface may result in multiple reflections which decreases the display's contrast and produces glare. Moreover, adding an additional touch panel to the display increases the manufacturing expense of the display and increases the complexity of the display. Also, the incorporation of the touch screen reduces the overall manufacturing yield of the display.

[0008] Accordingly, what is desired is a touch screen that does not significantly decrease the contrast ratio, does not significantly increase the glare, does not significantly increase the expense of the display, and does not significantly increase the complexity of the display.

[0030] Referring to FIG. 1, a liquid crystal display (LCD) 50 (indicated by a bracket) comprises generally, a backlight 52 and a light valve 54 (indicated by a bracket). Since liquid crystals do not emit light, most LCD panels are backlit with fluorescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel. To disperse the light and obtain a more uniform intensity over the surface of the display, light from the backlight 52 typically passes through a diffuser 56 before impinging on the light valve 54.

[0031] The transmittance of light from the backlight 52 to the eye of a viewer 58, observing an image displayed on the front of the panel, is controlled by the light valve 54. The light valve 54 normally includes a pair of polarizers 60 and 62 separated by a layer of liquid crystals 64 contained in a cell gap between glass or plastic plates, and the polarizers. Light from the backlight 52 impinging on the first polarizer 62 comprises electromagnetic waves vibrating in a plurality of planes. Only that portion of the light vibrating in the plane of the optical axis of the polarizer passes through the polarizer. In an LCD light valve, the optical axes of the first 62 and second 60 polarizer are typically arranged at an angle so that light passing through the first polarizer would normally be blocked from passing through the second polarizer in the series. However, the orientation of the translucent crystals in the layer of liquid crystals 64 can be locally controlled to either “twist” the vibratory plane of the light into alignment with the optical axes of the polarizer, permitting light to pass through the light valve creating a bright picture element or pixel, or out of alignment with the optical axis of one of the polarizes, attenuating the light and creating a darker area of the screen or pixel.

[0032] The surfaces of the a first glass substrate 61 and a second glass substrate 63 form the walls of the cell gap are buffed to produce microscopic grooves to physically align the molecules of liquid crystal 64 immediately adjacent to the walls. Molecular forces cause adjacent liquid crystal molecules to attempt to align with their neighbors with the result that the orientation of the molecules in the column of molecules spanning the cell gap twist over the length of the column. Likewise, the plane of vibration of light transiting the column of molecules will be “twisted” from the optical axis of the first polarizer 62 to a plane determined by the orientation of the liquid crystals at the opposite wall of the cell gap. If the wall of the cell gap is buffed to align adjacent crystals with the optical axis of the second polarizer, light from the backlight 52 can pass through the series of polarizers 60 and 62 to produce a lighted area of the display when viewed from the front of the panel (a “normally white” LCD).

[0033] To darken a pixel and create an image, a voltage, typically controlled by a thin film transistor, is applied to an electrode in an array of transparent electrodes deposited on the walls of the cell gap. The liquid crystal molecules adjacent to the electrode are attracted by the field produced by the voltage and rotate to align with the field. As the molecules of liquid crystal are rotated by the electric field, the column of crystals is “untwisted,” and the optical axes of the crystals adjacent to the cell wall are rotated progressively out of alignment with the optical axis of the corresponding polarizer progressively reducing the local transmittance of the light valve 54 and attenuating the luminance of the corresponding pixel. In other words, in a normally white twisted nematic device there are generally two modes of operation, one without a voltage applied to the molecules and one with a voltage applied to the molecules. With a voltage applied (e.g., driven mode) to the molecules the molecules rotate their polarization axis which results in inhibiting the passage of light to the viewer. Similarly, without a voltage applied (e.g., non-driven mode) the polarization axis is not rotated so that the passage of light is not inhibited to the viewer.¹

[0034] Conversely, the polarizers and buffing of the light valve can be arranged to produce a “normally black” LCD having pixels that are dark (light is blocked) when the electrodes are not energized and light when the electrodes are energized. Color LCD displays are created by varying the intensity of transmitted light for each of a plurality of primary color (typically, red, green, and blue) sub-pixels that make up a displayed pixel.

[0035] The aforementioned example was described with respect to a twisted nematic device. However, this description is only an example and other devices may likewise be used, including but not limited to, multi-domain vertical alignment, patterned vertical alignment, in-plane switching, and super-twisted nematic type LCDs. In addition other devices, such as for example, plasma displays, organic displays, active matrix organic light emitting display, electroluminescent displays, liquid crystal on silicon displays, reflective liquid crystal devices may likewise be used. For such displays the light emitting portion of the display, or portion of the display that permits the display of selected portions of light may be considered to selectively cause the pixels to provide light.

[0036] For an active matrix LCD (AMLCD) the inner surface of the second glass substrate 63 is normally coated with a continuous electrode while the first glass substrate 61 is patterned into individual pixel electrodes. The continuous electrode may be constructed using a transparent electrode, such as indium tin oxide. The first glass substrate 61 may include thin film transistors (TFTs) which act as individual switches for each pixel electrode (or group of pixel electrodes) corresponding to a pixel (or group of pixels). The TFTs are addressed by a set of multiplexed electrodes running along the gaps between the pixel electrodes. Alternatively the pixel electrodes may be on a different layer from the TFTs. A pixel is addressed by applying voltage (or current) to a selected line, which switches the TFT on and allows charge from the data line to flow onto the rear pixel electrodes. The combination of voltages between the front electrode and the pixel electrodes sets up a voltage across the pixels and turns the respective pixels on. The thin-film transistors are typically constructed from amorphous silicon, while other types of switching devices may likewise be used, such as for example, metal-insulator-metal diode and polysilicon thin-film transistors. The TFT array and pixel electrodes may alternatively be on the top of the liquid crystal material. Also, the continuous electrode may be patterned or portions selectively selected, as desired. Also the light sensitive elements may likewise be located on the top, or otherwise above, of the liquid crystal material, if desired.²

[0037] Referring to FIG. 2, the active matrix layer may include a set of data lines and a set of select lines. Normally one data line is included for each column of pixels across the display and one select line is included for each row of pixels down the display, thereby creating an array of conductive lines. To load the data to the respective pixels indicating which pixels should be illuminated, normally in a row-by-row manner, a set of voltages are imposed on the respective data lines 204 which imposes a voltage on the sources 202 of latching transistors 200. The selection of a respective select line 210, interconnected to the gates 212 of the respective latching transistors 200, permits the voltage imposed on the sources 202 to be passed to the drain 214 of the latching transistors 200. The drains 214 of the latching transistors 200 are electrically connected to respective pixel electrodes and are capacitively coupled to a respective common line 221 through a respective Cst capacitor 218. In addition, a respective capacitance exists between the pixel electrodes enclosing the liquid crystal material, noted as capacitances Clc 222 (between the pixel electrodes and the common electrode on the color plate). The common line 221 provides a voltage reference. In other words, the voltage data (representative of the image to be displayed) is loaded into the data lines for a row of latching transistors 200 and imposing a voltage on the select line 210 latches that data into the holding capacitors and hence the pixel electrodes.

[0038] Referring to FIG. 3, a schematic layout is shown of the active matrix layer. The pixel electrodes 230 are generally grouped into a “single” effective pixel so that a corresponding set of pixel electrodes 230 may be associated with respective color filters (e.g., red, green, blue). The latching transistors 200 interconnect the respective pixel electrodes 230 with the data lines and the select line. The pixel electrodes 230 may be interconnected to the common line 221 by the capacitors Cst 218.

[0039] Referring to FIG. 4, the active matrix layer may be constructed using an amorphous silicon thin-film transistor fabrication process. The steps may include gate metal deposition (FIG. 4A), a photolithography/etch (FIG. 4B), a gate insulator and amorphous silicon deposition (FIG. 4C), a photolithography/etch (FIG. 4D), a source/drain metal deposition (FIG. 4E), a photolithography/etch (FIG. 4F), an ITO deposition (FIG. 4G), and a photolithography/etch (FIG. 4H). Other processes may likewise be used, as desired.

[0040] The present inventors considered different potential architectural touch panel schemes to incorporate additional optical layers between the polarizer on the front of the liquid crystal display and the front of the display. These additional layers include, for example, glass plates, wire grids, transparent electrodes, plastic plates, spacers, and other materials. In addition, the present inventors considered the additional layers with different optical characteristics, such as for example, birefringence, non-birefringence, narrow range of wavelengths, wide range of wavelengths, etc. After an extensive analysis of different potential configurations of the touch screen portion added to the display together with materials having different optical properties and further being applied to the different types of technologies (e.g., mechanical switches, crossed beams of infrared light, acoustic surface waves, capacitance sensing, and resistive membranes), the present inventors determined that an optimized touch screen is merely a tradeoff between different undesirable properties. Accordingly, the design of an optimized touch screen is an ultimately unsolvable task. In contrast to designing an improved touch screen, the present inventors came to the realization that modification of the structure of the active matrix liquid crystal device itself could provide an improved touch screen capability without all of the attendant drawbacks to the touch screen configuration located on the front of the display.

[0041] Referring to FIG. 5, with particular attention to the latching transistors of the pixel electrodes, a black matrix 240 is overlying the latching transistors so that significant ambient light does not strike the transistors. Color filters 242 may be located above the pixel electrodes. Ambient light striking the latching transistors results in draining the charge imposed on the pixel electrodes through the transistor. The discharge of the charge imposed on the pixel electrodes results in a decrease in the operational characteristics of the display, frequently to the extent that the display is rendered effectively inoperative. With the realization that amorphous silicon transistors are sensitive to light incident thereon, the present inventors determined that such transistors within the active matrix layer may be used as a basis upon which to detect the existence of or non-existence of ambient light incident thereon (e.g., relative values thereto).

[0042] Referring to FIG. 6, a modified active matrix layer may include a photo-sensitive structure or elements. The preferred photo-sensitive structure includes a photo-sensitive thin film transistor (photo TFT) interconnected to a readout thin film transistor (readout TFT). A capacitor Cst2 may interconnect the common line to the transistors. Referring to FIG. 7, a black matrix may be in an overlying relationship to the readout TFT. The black matrix is preferably an opaque material or otherwise the structure of the display selectively inhibiting the transmission of light to selective portions of the active matrix layer. Preferably the black matrix is completely overlying the amorphous silicon portion of the readout TFT, and at least partially overlying the amorphous silicon portion of the readout TFT. Preferably the black matrix is completely non-overlying the amorphous silicon portion of the photo TFT, and at least partially non-overlying the amorphous silicon portion of the photo TFT. Overlying does not necessarily denote direct contact between the layers, but is intended to denote in the general sense the stacked structure of materials. In some embodiments, the black matrix inhibits ambient light from impacting the amorphous silicon portion of the readout TFT to an extent greater than inhibiting ambient light from impacting the amorphous silicon portion of the photo TFT.

[0043] As an example, the common line may be set at a negative voltage potential, such as −10 volts. During the previous readout cycle, a voltage is imposed on the select line which causes the voltage on the readout line to be coupled to the drain of the photo TFT and the drain of the readout TFT, which results in a voltage potential across Cst2. The voltage coupled to the drain of the photo TFT and the drain of the readout TFT is approximately ground (e.g., zero volts) with the non-inverting input of the operational amplifier connected to ground. The voltage imposed on the select line is removed so that the readout TFT will turn “off”.

[0044] Under normal operational conditions ambient light from the front of the display passes through the black matrix and strikes the amorphous silicon of the photo TFT. However, if a person touches the front of the display in a region over the opening in the black matrix or otherwise inhibits the passage of light through the front of the display in a region over the opening in the black matrix, then the photo TFT transistor will be in an “off” state. If the photo TFT is “off” then the voltage across the capacitor Cst2 will not significantly discharge through the photo TFT. Accordingly, the charge imposed across Cst2 will be substantially unchanged. In essence, the voltage imposed across Cst2 will remain substantially unchanged if the ambient light is inhibited from striking the photo TFT.

[0045] To determine the voltage across the capacitor Cst2, a voltage is imposed on the select line which causes the gate of the readout TFT to interconnect the imposed voltage on Cst2 to the readout line. If the voltage imposed on the readout line as a result of activating the readout TFT is substantially unchanged, then the output of the operational amplifier will be substantially unchanged (e.g., zero). In this manner, the system is able to determine whether the light to the device has been inhibited, in which case the system will determine that the screen has been touched at the corresponding portion of the display with the photo TFT.

[0046] During the readout cycle, the voltage imposed on the select line causes the voltage on the respective drain of the photo TFT and the drain of the readout TFT to be coupled to the respective readout line, which results in resetting the voltage potential across Cst2. The voltage coupled to the drain of the photo TFT and the drain of the readout TFT is approximately ground (e.g., zero volts) with the non-inverting input of the operational amplifier connected to ground. The voltage imposed on the select line is removed so that the readout TFT will turn “off”. In this manner, the act of reading the voltage simultaneously acts to reset the voltage potential for the next cycle.

[0047] Under normal operational conditions ambient light from the front of the display passes through the black matrix and strikes the amorphous silicon of the photo TFT. If a person does not touch the front of the display in a region over the opening in the black matrix or otherwise inhibits the passage of light through the front of the display in a region over the opening in the black matrix, then the photo TFT transistor will be in an “on” state. If the photo TFT is “on” then the voltage across the capacitor Cst2 will significantly discharge through the photo TFT, which is coupled to the common line. In essence the voltage imposed across Cst2 will decrease toward the common voltage. Accordingly, the charge imposed across Cst2 will be substantially changed in the presence of ambient light. Moreover, there is a substantial difference in the voltage potential across the hold capacitor when the light is not inhibited versus when the light is inhibited.

[0048] Similarly, to determine the voltage across the capacitor Cst2, a voltage is imposed on the select line which causes the gate of the readout TFT to interconnect the imposed voltage to the readout line. If the voltage imposed on the readout line as a result of activating the readout TFT is substantially changed or otherwise results in an injection of current, then the output of the operational amplifier will be substantially non-zero. The output voltage of the operational amplifier is proportional or otherwise associated with the charge on the capacitor Cst2. In this manner, the system is able to determine whether the light to the device has been uninhibited, in which case the system will determine that the screen has not been touched at the corresponding portion of the display with the photo TFT.

[0049] Referring to FIG. 8, a layout of the active matrix layer may include the photo TFT, the capacitor Cst2, the readout TFT in a region between the pixel electrodes. Light sensitive elements are preferably included at selected intervals within the active matrix layer. In this manner, the device may include touch panel sensitivity without the need for additional touch panel layers attached to the front of the display. In addition, the additional photo TFT, readout TFT, and capacitor may be fabricated together with the remainder of the active matrix layer, without the need for specialized processing. Moreover, the complexity of the fabrication process is only slightly increased so that the resulting manufacturing yield will remain substantially unchanged. It is to be understood that other light sensitive elements may likewise be used. In addition, it is to be understood that other light sensitive electrical architectures may likewise be used.

[0050] Referring to FIG. 11, a graph of the photo-currents within amorphous silicon TFTs is illustrated. Line 300 illustrates a dark ambient environment with the gate connected to the source of the photo TFT. It will be noted that the leakage currents are low and relatively stable over a range of voltages. Line 302 illustrates a dark ambient environment with a floating gate of the photo TFT. It will be noted that the leakage currents are generally low and relatively unstable over a range of voltages (significant slope). Line 304 illustrates a low ambient environment with the gate connected to the source of the photo TFT. It will be noted that the leakage currents are three orders of magnitude higher than the corresponding dark ambient conditions and relatively stable over a range of voltages. Line 306 illustrates a low ambient environment with a floating gate of the photo TFT. It will be noted that the leakage currents are generally three orders of magnitude higher and relatively unstable over a range of voltages (significant slope). Line 308 illustrates a high ambient environment with the gate connected to the source of the photo TFT. It will be noted that the leakage currents are 4.5 orders of magnitude higher than the corresponding dark ambient conditions and relatively stable over a range of voltages. Line 310 illustrates a high ambient environment with a floating gate of the photo TFT. It will be noted that the leakage currents are generally 4.5 orders of magnitude higher and relatively unstable over a range of voltages (significant slope). With the significant difference between the dark state, the low ambient state, and the high ambient state, together with the substantially flat responses over a voltage range (source-drain voltage), the system may readily process the data in a confident manner, especially with the gate connected to the source.

[0051] Referring to FIG. 9, under high ambient lighting conditions the photo TFT will tend to completely discharge the Cst2 capacitor to the common voltage, perhaps with an offset voltage because of the photo TFT. In this manner, all of the photo TFTs across the display will tend to discharge to the same voltage level. Those regions with reduced ambient lighting conditions or otherwise where the user blocks ambient light from reaching the display, the Cst2 capacitor will not fully discharge, as illustrated by the downward spike in the graph. The downward spike in the graph provides location information related to the region of the display that has been touched.

[0052] Referring to FIG. 10, under lower ambient lighting conditions the photo TFT will tend to partially discharge the Cst2 capacitor to the common voltage. In this manner, all of the photo TFTs across the display will tend to discharge to some intermediate voltage levels. Those regions with further reduced ambient lighting conditions or otherwise where the user blocks ambient light from reaching the display, the Cst2 capacitor will discharge to a significantly less extent, as illustrated by the downward spike in the graph. The downward spike in the graph provides location information related to the region of the display that has been touched. As shown in FIGS. 9 and 10, the region or regions where the user inhibits light from reaching the display may be determined as localized minimums. In other embodiments, depending on the circuit topology, the location(s) where the user inhibits light from reaching the display may be determined as localized maximums or otherwise some measure from the additional components.

[0053] In the circuit topology illustrated, the value of the capacitor Cst2 may be selected such that it is suitable for high ambient lighting conditions or low ambient lighting conditions. For low ambient lighting conditions, a smaller capacitance may be selected so that the device is more sensitive to changes in light. For high ambient lighting conditions, a larger capacitance may be selected so that the device is less sensitive to changes in light. In addition, the dimensions of the phototransistor may be selected to change the photo-leakage current. Also, one set of light sensitive elements (e.g., the photo TFT and the capacitance) within the display may be optimized for low ambient lighting conditions while another set of light sensitive elements (e.g., the photo TFT and the capacitance) within the display may be optimized for high ambient lighting conditions. Typically, the data from light sensitive elements for low ambient conditions and the data from light sensitive elements for high ambient conditions are separately processed, and the suitable set of data is selected. In this manner, the same display device may be used for high and low ambient lighting conditions. In addition, multiple levels of sensitivity may be provided. It is to be understood that a single architecture may be provided with a wide range of sensitivity from low to high ambient lighting conditions.

[0054] Another structure that may be included is selecting the value of the capacitance so that under normal ambient lighting conditions the charge on the capacitor only partially discharges. With a structure where the capacitive charge only partially discharges, the present inventors determined that an optical pointing device, such as a light wand or laser pointer, might be used to point at the display to further discharge particular regions of the display. In this manner, the region of the display that the optical pointing device remains pointed at may be detected as local maximums (or otherwise). In addition, those regions of the display where light is inhibited will appear as local minimums (or otherwise). This provides the capability of detecting not only the absence of light (e.g., touching the panel) but likewise those regions of the display that have increased light incident thereon. Referring to FIG. 12, a graph illustrates local minimums (upward peaks) from added light and local maximums (downward peaks) from a lack of light. In addition, one set of light sensitive elements (e.g., the photo TFT and the capacitance) within the display may be optimized for ambient lighting conditions to detect the absence of light while another set of light sensitive elements (e.g., the photo TFT and the capacitance) within the display may be optimized for ambient lighting conditions to detect the additional light imposed thereon.

[0055] A switch associated with the display may be provided to select among a plurality of different sets of light sensitive elements. For example, one of the switches may select between low, medium, and high ambient lighting conditions. For example, another switch may select between a touch sensitive operation (absence of light) and an optical pointing device (addition of light). In addition, the optical pointing device may communicate to the display, such as through a wire or wireless connection, to automatically change to the optical sensing mode.

[0056] It is noted that the teachings herein are likewise applicable to transmissive active matrix liquid crystal devices, reflective active matrix liquid crystal devices, transflective active matrix liquid crystal devices, etc. In addition, the light sensitive elements may likewise be provided within a passive liquid crystal display. The sensing devices may be, for example, photo resistors and photo diodes.

[0057] Alternatively, light sensitive elements may be provided between the rear polarizing element and the active matrix layer. In this arrangement, the light sensitive elements are preferably fabricated on the polarizer, or otherwise a film attached to the polarizer. In addition, the light sensitive elements may be provided on a thin glass plate between the polarizer and the liquid crystal material. In addition, the black matrix or otherwise light inhibiting material is preferably arranged so as to inhibit ambient light from striking the readout TFT while free from inhibiting light from striking the photo TFT. Moreover, preferably a light blocking material is provided between the photo TFT and/or the readout TFT and the backlight, such as gate metal, if provided, to inhibit the light from the backlight from reaching the photo TFT and/or the readout TFT.

[0058] Alternatively, light sensitive elements may be provided between the front polarizing element and the liquid crystal material. In this arrangement, the light sensitive elements are preferably fabricated on the polarizer, or otherwise a film attached to the polarizer. In addition, the light sensitive elements may be provided on a thin glass plate between the polarizer and the liquid crystal material. The light sensitive elements may likewise be fabricated within the front electrode layer by patterning the front electrode layer and including suitable fabrication techniques. In addition, a black matrix or otherwise light inhibiting material is preferably arranged so as to inhibit ambient light from striking the readout TFT while free from inhibiting light from striking the photo TFT. Moreover, preferably a light blocking material is provided between the photo TFT and/or the readout TFT and the backlight, if provided, to inhibit the light from the backlight from reaching the photo TFT and/or the readout TFT.

[0059] Alternatively, light sensitive elements may be provided between the front of the display and the rear of the display, normally fabricated on one of the layers therein or fabricated on a separate layer provided within the stack of layers within the display. In the case of a liquid crystal device with a backlight the light sensitive elements are preferably provided between the front of the display and the backlight material. The position of the light sensitive elements are preferably between (or at least partially) the pixel electrodes, when viewed from a plan view of the display. This may be particularly useful for reflective displays where the pixel electrodes are opaque. In addition for reflective displays, any reflective conductive electrodes should be arranged so that they do not significantly inhibit light from reaching the light sensitive elements.³ In this arrangement, the light sensitive elements are preferably fabricated on one or more of the layers, or otherwise a plate attached to one or more of the layers. In addition, a black matrix or otherwise light inhibiting material is preferably arranged so as to inhibit ambient light from striking the readout TFT while free from inhibiting light from striking the photo TFT. Moreover, preferably a light blocking material is provided between the photo TFT and/or the readout TFT and the backlight, if provided, to inhibit the light from the backlight from reaching the photo TFT and/or the readout TFT.

[0060] In many applications it is desirable to modify the intensity of the backlight for different lighting conditions. For example, in dark ambient lighting conditions it may be beneficial to have a dim backlight. In contrast, in bright ambient lighting conditions it may be beneficial to have a bright backlight. The integrated light sensitive elements within the display stack may be used as a measure of the ambient lighting conditions to control the intensity of the backlight without the need for an additional external photo-sensor. One light sensitive element may be used, or a plurality of light sensitive element may be used together with additional processing, such as averaging.

[0061] In one embodiment, the readout line may be included in a periodic manner within the display sufficient to generally identify the location of the “touch”. For example the readout line may be periodically added at each 30^th column. Spacing the readout lines at a significant number of pixels apart result in a display that nearly maintains its previous brightness because most of the pixel electrodes have an unchanged size. However, after considerable testing it was determined that such periodic spacing results in a noticeable non-uniform gray scale because of differences in the size of the active region of the pixel electrodes. One potential resolution of the non-uniform gray scale is to modify the frame data in a manner consistent with the non-uniformity, such as increasing the gray level of the pixel electrodes with a reduced size or otherwise reducing the gray levels of the non-reduced size pixel electrodes, or a combination thereof. While a potential resolution, this requires additional data processing which increases the computational complexity of the system.

[0062] A more desirable resolution of the non-uniform gray scale is to modify the display to include a readout line at every third pixel, or otherwise in a manner consistent with the pixel electrode pattern of the display (red pixel, green pixel, blue pixel). Alternatively, a readout line is included at least every 12^th pixel (36 pixel electrodes of a red, blue, green arrangement), more preferably at least every 9^th pixel (27 pixel electrodes of a red, blue, green arrangement), even more preferably at least every 6^th pixel (18 pixel electrodes of a red, blue, green arrangement or 24 pixel electrodes of a red, blue, blue green arrangement), and most preferably at least every 3^rd pixel (3 pixel electrodes of a red, blue, green arrangement). The readout lines are preferably included for at least a pattern of four times the spacing between readout lines (e.g., 12^th pixel times 4 equals 48 pixels, 9^th pixel times 4 equals 36 pixels). More preferably the patten of readout lines is included over a majority of the display. The resulting display may include more readout lines than are necessary to accurately determine the location of the “touch”. To reduce the computational complexity of the display, a selection of the readout lines may be free from interconnection or otherwise not operationally interconnected with readout electronics. In addition, to further reduce the computational complexity of the display and to increase the size of the pixel electrodes, the readout lines not operationally interconnected with readout electronics may likewise be free from an associated light sensitive element. In other words, additional non-operational readout lines may be included within the display to provide a gray scale display with increased uniformity. In an alternative embodiment, one or more of the non-operational readout lines may be replaced with spaces. In this manner, the gray scale display may include increased uniformity, albeit with additional spaces within the pixel electrode matrix.

[0063] The present inventors considered the selection of potential pixel electrodes and came to the realization that the electrode corresponding to “blue” light does not contribute to the overall white transmission to the extent that the “green” or “red” electrodes. Accordingly, the system may be designed in such a manner that the light sensitive elements are associated with the “blue” electrodes to an extent greater than their association with the “green” or “red” electrodes. In this manner, the “blue” pixel electrodes may be decreased in size to accommodate the light sensitive elements while the white transmission remains substantially unchanged. Experiments have shown that reducing the size of the “blue” electrodes to approximately 85% of their original size, with the “green” and “red” electrodes remaining unchanged, results in a reduction in the white transmission by only about 3 percent.⁴

[0064] While such an additional set of non-operational readout lines provides for increased uniform gray levels, the reduction of pixel apertures results in a reduction of brightness normally by at least 5 percent and possibly as much as 15 percent depending on the resolution and layout design rules employed. In addition, the manufacturing yield is decreased because the readout line has a tendency to short to its neighboring data line if the processing characteristics are not accurately controlled. For example, the data line and readout line may be approximately 6–10 microns apart along a majority of their length.

[0065] Referring to FIG. 13, to increase the potential manufacturing yield and the brightness of the display, the present inventors came to the realization that the readout of the photo-sensitive circuit and the writing of data to the pixels may be combined on the same bus line, or otherwise a set of lines that are electrically interconnected to one another. To facilitate the use of the same bus line, a switch 418 may select between providing new data 420 to the selected pixels and reading data 414 from the selected pixels. With the switch 418 set to interconnect the new data 420 with the selected pixels, the data from a frame buffer or otherwise the video data stream may be provided to the pixels associated with one of the select lines. Multiple readout circuits may be used, or one or more multiplexed readout circuits maybe used. For example, the new data 420 provided on data line 400 may be 4.5 volts which is latched to the pixel electrode 402 and the photo TFT 404 by imposing a suitable voltage on the select line 406. In this manner, the data voltage is latched to both the pixel electrode and a corresponding photo-sensitive circuit.

[0066] The display is illuminated in a traditional manner and the voltage imposed on the photo TFT 404 may be modified in accordance with the light incident on the photo-sensitive circuit, as previously described. In the topology illustrated, the photo TFT 404 is normally a N-type transistor which is reverse biased by setting the voltage on the common line 408 to a voltage lower than an anticipated voltage on the photo TFT 404, such as −10 or −15 volts. The data for the current frame may be stored in a frame buffer for later usage. Prior to writing the data for another frame, such as the next frame, the data (e.g., voltage) on the readout TFT 410 is read out. The switch 418 changes between the new data 420 to the readout line 414 interconnected to the charge readout amplifier 412. The select line 406 is again selected to couple the remaining voltage on the photo TFT 404 through the readout TFT 410 to the data line 400. The coupled voltage (or current) to the data line 400 is provided as an input to the charge readout amplifier 412 which is compared against the corresponding data from the previous frame 422, namely, the voltage originally imposed on the photo TFT 404. The difference between the readout line 414 and the data from the previous frame 422 provides an output to the amplifier 412. The output of the amplifier 412 is provided to the processor. The greater the drain of the photo TFT 404, normally as a result of sensing light, results in a greater output of the amplifier 412. Referring to FIG. 14, an exemplary timing for the writing and readout on the shared data line 400 is illustrated.

[0067] At low ambient lighting conditions and at dark lighting conditions, the integrated optical touch panel is not expected to operate well to the touch of the finger because there will be an insufficient (or none) difference between the signals from the surrounding area and the touched area. To alleviate the inability to effectively sense at the low and dark ambient lighting conditions a light pen or laser pointer may be used (e.g., light source), as previously described. The light source may be operably interconnected to the display such as by a wire or wireless communication link. With the light source operably interconnected to the display the intensity of the light source may be controlled, at least in part, by feedback from the photo-sensitive elements or otherwise the display, as illustrated in FIG. 15. When the display determines that sufficient ambient light exists, such as ambient light exceeding a threshold value, the light source is turned “off”. In this manner, touching the light source against the display results in the same effect as touching a finger against the display, namely, impeding ambient light from striking the display. When the display determines that insufficient ambient light exists, such as ambient light failing to exceed a threshold value, the light source is turned “on”. In this manner, touching or otherwise directing the light from the light source against the display results in a localized increase in the received light relative to the ambient light level. This permits the display to be operated in dark ambient lighting conditions or by feedback from the display. In addition, the intensity of the light from the light source may be varied, such as step-wise, linearly, non-linearly, or continuously, depending upon the ambient lighting conditions. Alternatively, the light source may include its own ambient light detector so that feedback from the display is unnecessary and likewise communication between the light source and the display may be unnecessary.

[0068] While using light from an external light source while beneficial it may still be difficult to accurately detect the location of the additional light because of background noise within the system and variable lighting conditions. The present inventors considered this situation and determined that by providing light during different frames, such as odd frames or even frames, or odd fields or even fields, or every third frame, or during selected frames, a more defined differential signal between the frames indicates the “touch” location. In essence, the light may be turned on and off in some manner, such as blinking at a rate synchronized with the display line scanning or frames. An exemplary timing for an odd/even frame arrangement is shown in FIG. 16. In addition, the illumination of some types of displays involves scanning the display in a row-by-row manner. In such a case, the differential signal may be improved by modifying the timing of the light pulses in accordance with the timing of the gate pulse (e.g., scanning) for the respective pixel electrodes. For example, in a top-down scanning display the light pulse should be earlier when the light source is directed toward the top of the display as opposed to the bottom of the display. The synchronization may be based upon feedback from the display, if desired.

[0069] In one embodiment, the light source may blink at a rate synchronized with the display line scanning. For example, the light source may use the same driver source as the image pixel electrodes. In another embodiment the use of sequential (or otherwise) frames may be subtracted from one another which results in significant different between signal and ambient conditions. Preferably, the light sensitive elements have a dynamic range greater than 2 decades, and more preferably a dynamic range greater than 4 decades. If desired, the system may use two sequential fields of scanning (all lines) subtracted from the next two fields of scanning (all lines) so that all the lines of the display are used.

[0070] Another technique for effective operation of the display in dark or low level ambient conditions is using a pen or other device with a light reflecting surface that is proximate (touching or near touching) the display when interacting with the display. The light from the backlight transmitted through the panel is then reflected back into the photo-sensitive element and the readout signal will be greater at the touch location than the surrounding area.

[0071] Referring to FIG. 17, another type of reflective liquid crystal display, typically used on handheld computing devices, involves incorporating a light guide in front of the liquid crystal material, which is normally a glass plate or clear plastic material. Normally, the light guide is constructed from an opaque material having an index of refraction between 1.4 and 1.6, more typically between 1.45 and 1.50, and sometimes of materials having an index of refraction of 1.46. The light guide is frequently illuminated with a light source, frequently disposed to the side of the light guide. The light source may be any suitable device, such as for example, a cold cathode fluorescent lamp, an incandescent lamp, and a light emitting diode. To improve the light collection a reflector may be included behind the lamp to reflect light that is emitted away from the light guide, and to re-direct the light into the light guide. The light propagating within the light guide bounces between the two surfaces by total internal reflections. The total internal reflections will occur for angles that are above the critical angle, measured relative to the normal to the surfaces, as illustrated in FIG. 18. To a first order approximation, the critical angle β maybe defined by Sin(β)=1/n where n is the index of refraction of the light guide. Since the surfaces of the light guide are not perfectly smooth there will be some dispersion of the light, which causes some illumination of the display, as shown in FIG. 19.

[0072] The present inventors came to the realization that the critical angle and the disruption of the total internal reflections may be modified in such a manner as to provide a localized increase in the diffusion of light. Referring to FIG. 20, one suitable technique for the localized diffusion of light involves using a plastic pen to touch the front of the display. The internally reflected light coincident with the location that the pen touches the display will significantly diffuse and be directed toward the photo sensitive elements within the display. The plastic pen, or other object including the finger or the eraser of a pencil, preferably has an index of refraction within 0.5, more preferably within 0.25, of the index of refraction of the light guide. For example, the index of refraction of the light guide may be between 1.2 and 1.9, and more preferably between 1.4 and 1.6. With the two indexes of refraction sufficiently close to one another the disruption of the internal reflections, and hence amount of light directed toward the photo-sensitive elements, is increased. In addition, the plastic pen preferably has sufficient reflectivity of light as opposed to being non-reflective material, such as for example, black felt.

[0073] Referring to FIG. 21, after further consideration the present inventors were surprised to note that a white eraser a few millimeters away from the light guide results in a darkened region with generally consistent optical properties while a white eraser in contact with the light guide results in a darkened region with generally consistent optical properties together with a smaller illuminated region. In the preferred embodiment, the light sensitive elements are positioned toward the front of the display in relation to the liquid crystal material (or otherwise the light valve or electroluminescent material) so that a clearer image may be obtained.⁵ It is to be understood that any suitable pointing device may be used. The illuminated region has an illumination brighter in relation to the remainder of the darkened region. The illuminated region may be located by any suitable technique, such as for example, a center of gravity technique.

[0074] After further consideration of the illuminated region the present inventors came to the realization that when users use a “touch panel” display, there is a likelihood that the pointing device (or finger) may “hover” at a location above the display. Normally, during this hovering the user is not actually selecting any portion of the display, but rather still deciding where to select. In this manner, the illuminated region is beneficial because it provides a technique for the determination between when the user is simply “hovering” and the user has actually touched (e.g., “touching”) the display.

[0075] Another potential technique for the determination between “hovering” and “touching” is to temporally model the “shadow” region (e.g., light impeded region of the display). In one embodiment, when the user is typically touching the display then the end of the shadow will typically remain stationary for a period of time, which may be used as a basis, at least in part, of “touching”. In another embodiment, the shadow will typically enlarge as the pointing device approaches the display and shrinks as the pointing device recedes from the display, where the general time between enlarging and receding may be used as a basis, at least in part, of “touching”. In another embodiment, the shadow will typically enlarge as the pointing device approaches the display and maintain the same general size when the pointing device is touching the display, where the general time where the shadow maintains the same size may be used as a basis, at least in part, of “touching”. In another embodiment, the shadow will typically darken as the pointing device approaches the display and maintain the same shade when the pointing device is touching the display, where the general time where the shadow maintains the same general shade may be used as a basis, at least in part, of “touching”.

[0076] While attempting to consider implementation of such techniques on a handheld device it came to the inventor's surprise that the display portion of a handheld device has a refresh rate generally less than the refresh rate of the portion of the handwriting recognition portion of the display. The handheld portion of the display may use any recognition technique, such as Palm OS™ based devices. The refresh rate of the display is typically generally 60 hertz while the refresh rate of the handwriting portion of the display is typically generally 100 hertz. Accordingly, the light-sensitive elements should be sampled at a sampling rate corresponding with the refresh rate of the respective portion of the display.

[0077] The technique described with respect to FIG. 20 operates reasonably well in dark ambient lighting conditions, low ambient lighting conditions, regular ambient lighting conditions, and high ambient lighting conditions. During regular and high ambient lighting conditions, the display is alleviated of a dependency on the ambient lighting conditions. In addition, with such lighting the illumination point is more pronounced and thus easier to extract. Unfortunately, during the daytime the ambient light may be sufficiently high causing the detection of the pointing device difficult. In addition, shades of the ambient light may also interfere with the detection techniques.

[0078] The present inventors considered improving the robustness of the detection techniques but came to the realization that with sufficient “noise” in the system the creation of such sufficiently robust techniques would be difficult. As opposed to the traditional approach of improving the detection techniques, the present inventors came to the realization that by providing light to the light guide of a limited selection of wavelengths and selectively filtering the wavelengths of light within the display the difference between touched and un-touched may be increased. As an initial matter the light from the light source provided to the light guide is modified, or otherwise filtered, to provide a single color. Alternatively, the light source may provide light of a range of wavelengths, such as 600–700 nm, or 400–500 and 530–580, or 630. Typically, the light provided to the light guide has a range of wavelengths (in any significant amount) less than white light or otherwise the range of wavelengths of ambient light. Accordingly, with the light provided to the light guide having a limited color gamut (or reduced color spectrum) the touching of the pointing device on the display results in the limited color gamut light being locally directed toward the light-sensitive elements. With a limited color gamut light being directed toward the display as a result of touching the light guide (or otherwise touching the front of the display), a color filter may be included between the light guide and the light-sensitive elements to filter out at least a portion of the light not included within the limited color gamut. In other words, the color filter reduces the transmission of ambient light to an extent greater than the transmission of light from the light source or otherwise within the light guide. For example, the ambient light may be considered as “white” light while the light guide has primarily “red” light therein. A typical transmission of a red color filter for ambient white light may be around 20%, while the same color filter will transmit about 85% of the red light. Preferably the transmission of ambient light through the color filter is less than 75% (greater than 25% attenuation) (or 60%, 50%, 40%, 30%) while the transmission of the respective light within the light guide is greater than 25% (less than 25% attenuation) (or 40%, 50%, 60%, 70%), so that in this manner there is sufficient attenuation of selected wavelengths of the ambient light with respect to the wavelengths of light within the light guide to increase the ability to accurately detect the touching.

[0079] In another embodiment, the light source to the light guide may include a switch or otherwise automatic modification to “white” light when operated in low ambient lighting conditions. In this manner, the display may be more effective viewed at low ambient lighting conditions.

[0080] In another embodiment, the present inventors determined that if the light source providing light to the display was modulated in some fashion an improvement in signal detection may be achieved. For example, a pointing device with a light source associated therewith may modulate the light source in accordance with the frame rate of the display. With a frame rate of 60 hertz the pointing device may for example modulate the light source at a rate of 30 hertz, 20 hertz, 10 hertz, etc. which results in additional light periodically being sensed by the light sensitive elements. Preferably, the light source is modulated (“blinked”) at a rate synchronized with the display line scanning, and uses the same raw drivers as the image thin-film transistors. The resulting data may be processed in a variety of different ways.

[0081] In one embodiment, the signals from the light sensitive elements are used, as captured. The resulting improvement in signal to background ratio is related to the pulse length of the light relative to the frame time. This provides some additional improvement in signal detection between the light generated by the pointing device relative to the ambient light (which is constant in time).

[0082] In another embodiment, multiple frames are compared against one another to detect the presence and absence of the additional light resulting from the modulation. In the case of subsequent frames (sequential or non-sequential), one without additional light and one with additional light, the data from the light sensitive elements may be subtracted from one another. The improvement in signal to background ratio is related to the periodic absence of the additional light. In addition, this processing technique is especially suitable for low ambient lighting and high ambient lighting conditions. Preferably the dynamic range of the sensors is at least 4 decades, and two sequential frames with additional light and two sequential frames without additional light are used so that all of the scanning lines are encompassed. When the system charges a sensor it takes a whole frame for it to discharge by the light. Since the first line will start at time zero and take a frame time, the last line will be charged after almost a frame and will take another frame time to discharge. Therefore, the system should preferably use two frames with additional illumination and then two frames without additional illumination.⁶

[0083] All references cited herein are hereby incorporated by reference.

[0084] The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.