Abstract:
A circuit for detecting light is disclosed comprising: a) a light-integrating photo-sensor circuit having one or more thin-film photosensors and being responsive to a variable integration period signal and to ambient light for producing a photo signal representing the intensity of the ambient light, wherein the photo signal may be in one of at least three states including a no-signal state, an in-range state, and a saturated state; and b) a control circuit for receiving the photo signal and automatically increasing the period of the integration period signal when the photo signal is in the no-signal state and decreasing the period of the integration period signal when the photo signal is in the saturated state so as to result in the photo signal being in the in-range state and producing a corresponding ambient light signal. In particular embodiments of the invention, the circuit for detecting light is employed as a component of a flat panel display, and the ambient light signal is used for adjusting the brightness of the flat-panel display. The invention enables an improved dynamic range for thin-film photosensors, particularly when used with a flat-panel display.

Description:
FIELD OF THE INVENTION 
   The present invention relates to photosensor circuits and more particularly to solid-state flat-panel displays having photosensors for sensing ambient illumination. 
   BACKGROUND OF THE INVENTION 
   Flat-panel displays such as liquid-crystal displays (LCDs) or organic light emitting diode (OLED) displays are useful in a wide variety of applications under a wide variety of environmental conditions. When viewed in a dark environment (little ambient radiation), such displays need not be as bright as when viewed in a lighter environment (more ambient radiation). If the display light output is adjusted periodically to compensate for ambient light conditions, the display can maintain a constant relative brightness with respect to the ambient illumination even if the ambient illumination changes. In a bright environment, this will increase display brightness to improve visibility. In a dark environment, this will increase display device lifetime and reduce power usage by reducing unnecessary display brightness. 
   The use of photosensors with displays to detect ambient light and adjusting the brightness of the display in response to ambient illumination is known. Efficient silicon photosensors are available and generally provide a current proportional to the light incident on the sensor. These photosensors are constructed on silicon substrates and may have a wide dynamic range. Such sensors can be combined with displays to provide ambient sensing. For example, see JP2002-297096-A, which describes a circuit for providing ambient compensation to an electroluminescent display. However, as implemented, the sensor is separate from the display and senses the light at a single point. This increases the cost, number of components, and size of the device and does not directly measure the light incident on the display itself. 
   It is known to integrate a light sensor on an active-matrix display device for the purpose of sensing light emitted from the display device itself. See, for example, U.S. Pat. No. 6,489,631 issued Dec. 3, 2002 to Young et al., which describes a display having integrated photosensors for sensing light emitted by a light emitting element of the display. There is no disclosure of the use of such photosensors for detecting ambient light, however, and the arrangement of the sensor coupled with a light emitter limits the size of the photosensor and its ability to sense ambient light. 
   When providing ambient compensation to a display, it is important that the light sensing device provide a signal having a wide dynamic range representative of the ambient illumination. The human visual system can effectively detect light from very dark ambient conditions of only a few photons to very bright outdoor conditions greater than 75,000 lux. However, tests conducted by applicant demonstrate that photosensors constructed on flat-panel displays using thin-film technology do not have the efficiency of photosensors constructed on silicon substrates and do not have the sensitivity necessary to provide a signal representative of lower light levels, for example &lt;100 cd/m2, where displays are often used. Nor do they have the dynamic range necessary to accommodate the range of the human visual system. 
   There is a need therefore for an improved photosensor circuit for the detection of ambient light, particularly within an active-matrix flat-panel display. 
   SUMMARY OF THE INVENTION 
   The need is met according to the present invention by providing a circuit for detecting light comprising: a) a light-integrating photo-sensor circuit having one or more thin-film photo sensors and being responsive to a variable integration period signal and to ambient light for producing a photo signal representing the intensity of the ambient light, wherein the photo signal may be in one of at least three states including a no-signal state, an in-range state, and a saturated state; and b) a control circuit for receiving the photo signal and automatically increasing the period of the integration period signal when the photo signal is in the no-signal state and decreasing the period of the integration period signal when the photo signal is in the saturated state so as to result in the photo signal being in the in-range state and producing a corresponding ambient light signal. 
   In particular embodiments of the invention, the circuit for detecting light is employed as a component of a flat panel display, wherein the display comprises a substrate and a plurality of light-emitting elements located thereon in a display area; and the one or more thin-film photosensors of the light-integrating photo-sensor circuit are located on the substrate, and being responsive to a variable integration period signal and to ambient light for producing a photo signal representing the intensity of the ambient light incident on the flat-panel display. 
   In a further embodiment, the invention is directed towards a method for controlling a flat-panel display, comprising: a) providing a flat-panel display comprising a substrate and a plurality of light-emitting elements located thereon in a display area; b) providing a light-integrating photo-sensor circuit having one or more thin-film photosensors located on the substrate and responding to a variable integration period signal and to ambient light for producing a photo signal representing the intensity of the ambient light incident on the flat-panel display, wherein the photo signal may be in one of at least three states including a no-signal state, an in-range state, and a saturated state; c) iteratively receiving the photo signal and automatically increasing the period of the integration signal when the photo signal is in the no-signal state and decreasing the period of the integration signal when the photo signal is in the saturated state so as to result in the photo signal being in the in-range state and producing a corresponding ambient light signal; and d) adjusting the brightness of the flat-panel display in response to the ambient light signal. 
   The invention enables an improved dynamic range for thin-film photosensors, particularly when used with a flat-panel display. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a photosensor circuit according to one embodiment of the present invention; 
       FIG. 2  is a schematic diagram of a display system utilizing the photosensor circuit of  FIG. 1 ; 
       FIG. 3   a  is a schematic diagram of a photosensor and control circuit according to an embodiment of the present invention; 
       FIG. 3   b  is a schematic diagram of a photosensor and control circuit according to another embodiment of the present invention; and 
       FIG. 4  is a schematic diagram of a control circuit according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the present invention includes a circuit  10  for detecting ambient light on a display comprising a light integrating photosensor circuit  12  having one or more thin-film photosensors  14  located on a flat-panel display substrate, connected to a detection circuit  16 , and being responsive to an integration period signal  32  and to ambient light (as detected by photosensor  14 ) for producing a photo signal  20  representing the intensity of the ambient light incident on the flat-panel display. The photo signal has at least three states including a no-signal state, an in-range state, and a saturated state. A control circuit  30  receives the photo signal  20  and automatically increases the period of the integration signal  32  when the photo signal is in the no-signal state and decreases the period of the integration signal  32  when the photo signal is in the saturated state so as to maintain the photo signal in the in-range state. 
   Referring to  FIG. 2 , a flat-panel display  40  includes a plurality of light emitters  42  in a display area  41  integrated on the flat-panel display substrate  43  and responsive to control signals  34  from the control circuit  30 . The photosensor  14  is integrated on the same substrate as the light emitters  42 . The detection circuit  16  may also be integrated with photosensor  14  in photosensor control circuit  12  on the same substrate, as shown in  FIG. 2 . The photo signal  20  produced by the photosensor circuit  12  is connected to the external control circuit  30 . Alternatively, some or all of the control circuit  30  may be integrated on the substrate. 
   In operation, the control circuit  30  is responsive to an input signal  38  and drives the display using control signals  34 . Ambient light incident on the display is also incident on the photosensor  14  and the photosensor circuit  12  produces a photo signal  20  representative of the amount of ambient light incident on the display. The photosensor circuit  12  is an integrating circuit, that is the circuit integrates a signal from the photosensor over a period of time to produce the photo signal  20 . Such integrating circuits are more sensitive than circuits that directly detect current produced by a photosensor in the presence of light. The integration signal  32  specifies the period of the integration. The more frequent the integration signal, the shorter the integration period. The less frequent the integration signal, the longer the integration period. 
   The photo signal  20  is in one of at least three states. The first state is a “no-signal” state and is seen when so little ambient light is incident on the display  40  that any decrease in the ambient light will not further reduce the value of the photo signal  20 . The second state is an “in-range” state and is seen when sufficient ambient light is incident on the display  40  to provide a photo signal  20  having a value representative of the ambient light incident on the display  40 . The third state is a “saturated” state and is seen when so much ambient light is incident on the display  40  that any increase in the ambient light incident on the display  40  will not further increase the value of the photo signal  20 . Because, as demonstrated by applicant, thin-film photo-sensors typically have a limited sensitivity and dynamic range, whenever the ambient light incident on the display  40  is out of the photosensor  14  range, the photo signal  20  will be in either a “no-signal” or “saturated” state. 
   The control circuit  30  responds to the photo signal  20  by adjusting the period of the integration signal  32 . If the photo signal  20  is in a “no-signal” state, the integration period is increased to provide more time for the photo-sensor  14  to accumulate a signal representative of the ambient light incident on the display  40 . If the photo signal  20  is in an “in-range” state, the period of the integration signal remains unchanged. If the photo signal  20  is in a “saturated” state, the period of the integration signal  32  is reduced to provide less time for the photo-sensor  14  to accumulate a signal responsive to the ambient light incident on the display  40 . This process of adjusting the integration signal period is repeated until the photo signal  20  is in the “in-range” state. The value of the integration signal  32  period and the photo signal  20  together represent the amount of ambient light incident on the display  40 . Once the photo signal is in-range, the control circuit  30  modifies the input signals  38  according to the value of the photo signal  20  to produce control signals  34  to drive the light emitters  42  and compensate for any ambient light incident on the display  40 . When a relatively brighter ambient illumination is detected, the control signals  34  drive the light emitters  42  to produce a brighter display output. When a relatively darker ambient illumination is detected, the control signals  34  drive the light emitters  42  to produce a dimmer display output. 
   A suitable photosensor circuit is disclosed in co-pending, commonly assigned U.S. application Ser. No. 10/694,560, the disclosure of which is hereby incorporated by reference. The integration signal  32  may be a digital signal that periodically restarts the photosensor signal integration. The sensitivity to ambient illumination of this photosensor circuit may be adjusted by modifying the size of the photosensor or the value of the circuit components. When in the in-range state, the photo signal  20  output from the photosensor circuit is an analog value that represents the amount of ambient light incident on the display  40 . When the output is at a ground voltage, the photo signal is in a “no-signal” state. When the output is at the voltage used to provide power to the circuit, the photo signal is in a “saturated” state. When the voltage is between zero and the power voltage, the photo signal is in the “in-range” state and represents the ambient illumination incident on the display. 
   The control circuit may be an analog control circuit and use analog techniques for controlling the period of the integration signal and modifying the input signal to produce a control signals  34  to compensate for ambient illumination. Such techniques are known in the art, for example using operational amplifiers, transistors, capacitor, and resistors. 
   Alternatively, digital means may be employed to control the period of the integration signal. Referring to  FIG. 3   a , two photosensor circuits  12   a  (for relatively low light detection) and  12   b  (for relatively higher light detection) are employed having different sensitivities to ambient illumination. Different sensitivities may be obtained, for example, by adjusting capacitance in the photosensor circuit, by adjusting the size of the photosensor  14 , by locating different filters over the photosensors, by using different periods for the integration signals, or by adjusting the aspect ratio or configuration of the photosensors. 
   The photo signals  20  may be applied to digital circuits such as an AND gate  50 , as shown, with or without inverters. When applied to a digital circuit gate, the photo signal  20  will be in a saturation state when the photo signal  20  reaches the switching voltage for the gate. When the photo signal does not reach the switching voltage for the gate, it is in a “no-signal” state. Thus, the individual photo signals  20  indicate in one state that the ambient illumination incident on the display  40  generates a signal below the switching voltage and in the other state that the ambient illumination incident on the display generates a signal above the switching voltage. Taken together, however, the binary signals  70 ,  72 ,  74  and  76  output from the AND gates  50  represent four possible photo signal states. Thus, when both photo signals  20  are HIGH, the ambient illumination is above the threshold for the less sensitive photosensor circuit  12   b , representing a saturation state with signal  70 . When both photo signals  20  are LOW, then the ambient illumination is below the threshold for the more sensitive photosensor circuit  12   a , representing a no-signal state with signal  72 . When the photo signal  20  from the more sensitive circuit is HIGH and the photo signal  20  from the less sensitive photo-sensor circuit  12   b  is LOW, the ambient illumination is between the thresholds of the two photosensor circuits  12   a ,  12   b , representing an in-range state with signal  74 . When the photo signal  20  from the less sensitive circuit  12   b  is HIGH and the photo signal  20  from the more sensitive photo-sensor circuit  12   a  is LOW, there is an error state, represented by signal  76 . 
   By adjusting the sensitivities of the two photosensor circuits  12   a ,  12   b , and the period of the integration signal  32 , any particular detection range may be obtained. For example, if one photosensor circuit is set with a switching threshold at ambient light levels of 1000 cd/m 2  and a second is set with a switching threshold at ambient light levels of 5000 cd/m 2 , the circuit will detect three light levels: 0-1000 cd/m 2 , 1000-5000 cd/m 2 , and &gt;5000 cd/M 2 , for a given integration signal period. If the integration signal period is then reduced, for example by half, the three light levels may detect signals in the range of 0-500, 500-2,500, and &gt;2,500 cd/M 2 . Alternatively, the integration signal may be doubled so that the three light levels may detect signals in the range of 0-2,000, 2,000-10,000, and &gt;10,000 cd/m 2 . 
   If the signal from the two circuits does not indicate an in-range state, the period of the integration signal  32  may be adjusted until it does. If the sensitivity of the two photosensor circuits  12   a ,  12   b  are relatively close, for example differ by only 20%, the accuracy of the ambient light detection can be very good. In this case, by adjusting the period of the integration signal until an “in-range” state is achieved, the ambient illumination may be measured to an accuracy of 20%. 
   Referring to  FIG. 3   b , an alternative arrangement may be employed having a single photo-sensor  12 . In this arrangement, external LOW signal  54  and HIGH signal  56  are compared to the photo signal  20  using comparators  52 . If the photo signal  20  is comparable to the LOW signal  54 , a no-signal state is indicated with signal  72 . If the photo signal  20  is comparable to the HIGH signal  56 , a saturated signal  70  is indicated. If neither state is indicated the photo signal  20  is in-range. In this case, the controller  30  receives three signals and responds as described above. The comparators  52  may include operational amplifiers and the controller  30  may digitize the analog photo signal  20  using analog-to-digital converters as is known in the art. 
   A suitable digital mechanism for implementing an auto-ranging capability is shown in  FIG. 4 . Referring to  FIG. 4 , the saturation-state signal  70  and no-signal-state signal  72  from the AND gates  50  are connected to an up/down digital counter  60 . The counter  60  stores a value representing the period of the integration signal  32 . A clock signal  64  increments or decrements the value stored in the counter  60  depending on the state signals. The value of the counter (shown as an 8-bit value) is loaded into a down counter  62  (e.g., by using an inverse of the clock signal). A count signal  66  then decrements the down counter until it reaches 0 at which point the output of the down counter  62  provides the integration signal  32  to reset the photosensor circuit  12 . The process is iterated until an “in-range” signal  74  is obtained. Counters, clock signals, and the digital logic necessary to implement such a circuit are well known in the art. 
   The thin-film photosensor  14  may be any thin-film light-sensitive device suitable for use within a flat-panel display system. For example, silicon or organic photodiodes, photocapacitors or phototransistors may be employed. Thin film materials may be deposited, e.g., by evaporation or photolithographic processes as known in the art (typically in layers less than 1 micrometer thick). These photosensors and circuit elements may be integrated with a flat panel display to provide an integrated solution. When integrated with a display, any portion of, or all of, the circuit  12  may be constructed using thin-film transistors and electrical components as are known in the flat-panel display art. 
   A typical flat panel display includes a rigid or flexible substrate, typically made of glass or plastic, together with a plurality of light-emitting elements, such as organic light emitting diode materials (OLEDs) or light controlling elements having polarizing layers in combination with an emissive back light, such as an LCDs. The individual light emitting elements may be controlled using thin-film transistors and capacitors together with an external controller to provide data, power, and timing signals. 
   A plurality of thin-film photosensors  14  can be electrically connected in common to provide one integrated photo signal or, alternatively, they can be separately addressed or their output combined. The plurality of photosensors  14  may be located near each other or dispersed over the flat-panel display  40 . A greater number or size of integrated photosensors  14  can increase the signal, thereby improving the responsiveness of the ambient light detection. These may, or may not, have a common detection circuit  16  but will utilize a single control circuit  30 . Moreover, the photo signals  20  will be more representative of the overall ambient illumination incident on the display since, if a portion of the display is shadowed, having several sensors can provide several signals that can be averaged to produce an overall average of the illumination incident on the display area. 
   The present invention may be used in both top- and bottom-emitting OLED flat-panel display devices. The light emitting display  40  may be an organic light emitting diode (OLED) display that includes multiple supporting layers such as light emitting layers, hole injection, hole transport, electron injection, and electron transport layers as is known in the art. Any or all portions of the photosensor circuit  12  may be deposited in a common step with active-matrix display circuitry and may include identical materials to simplify processing and manufacturing. As demonstrated by applicant, thin-film structures used for active-matrix OLED displays may be employed to form the photosensors  14  and detection circuit  16 . There are a variety of ways in which the photosensors can be connected that depend on various factors such as the layout of the display and the conductivity of the electrodes and signal lines connected to the photosensors. 
   Any or all of the detector circuit  16  or control circuit  30  can be integrated directly onto the same substrate as the display device  40  or it can be implemented externally to the display  40 . In general, higher performance and greater accuracy can be achieved by integrating the circuitry directly with the display device but this may not be desirable for all display devices. 
   In a preferred embodiment, the invention is employed in a flat-panel device that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al., and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Many combinations and variations of organic light emitting displays can be used to fabricate such a device. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
   PARTS LIST 
   
       
         10  circuit 
         12  photosensor circuit 
         12   a  photosensor circuit 
         12   b  photosensor circuit 
         14  photosensor 
         16  detection circuit 
         20  photo signal 
         30  control circuit 
         32  integration signal 
         34  control signals 
         38  input signals 
         40  flat-panel display 
         41  display area 
         42  light emitters 
         43  display substrate 
         50  AND gate 
         52  comparator 
         54  LOW signal 
         56  HIGH signal 
         60  up/down counter 
         62  down counter 
         64  clock signal 
         66  count signal 
         70  saturation-state signal 
         72  no-signal-state signal 
         74  in-range-state signal 
         76  error-state signal