Abstract:
A circuit for detecting ambient light on a display includes a light integrating photosensor circuit having a photosensor and being responsive to ambient light for periodically producing successive photo signals representing the intensity of the ambient light; and an averaging circuit for receiving the successive photo signals and producing an average ambient light signal representing a continuous running average of the successive photo signals.

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 fixed ratio between the ambient and displayed light even if the ambient light changes. This can, in turn, increase display brightness to improve visibility in a bright environment and increase display device lifetime and reduce power usage by reducing unnecessary display brightness in a dark environment. 
     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. 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; reduces the sensitivity of the sensor; 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. However, the arrangement of the sensor coupled with a light emitter limits the size of the photosensor and its ability to sense ambient light. Moreover, such photosensors constructed on flat panel displays do not have the efficiency of those 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. Hence, alternative circuits and designs are necessary. 
     When providing ambient compensation to a display, it is important that the light sensing device provide a continuously valid output that is always representative of the ambient illumination. If, instead, the output is periodically invalid, any compensation will be periodically incorrect and may cause flicker in the display. Alternatively, additional circuitry must be added to sample and hold the output of the light sensing device. Moreover, it is advantageous to provide a signal output that is representative of the ambient illumination over a range of light levels. 
     There is a need therefore for an improved photosensor for the detection of ambient light 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 ambient light on a display that includes a light integrating photosensor circuit having a photosensor and being responsive to ambient light for periodically producing successive photo signals representing the intensity of the ambient light; and an averaging circuit for receiving the successive photo signals and producing an average ambient light signal representing a continuous running average of the successive photo signals. 
     Advantages 
     The advantages of this invention are an improved photosensor circuit for ambient compensation in low light conditions that can be readily integrated within an active matrix flat panel display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a photosensor circuit according to the present invention; 
         FIG. 2  is a schematic circuit diagram of one embodiment of the photosensor circuit of  FIG. 1 ; 
         FIG. 3  is a timing diagram useful in describing the operation of the circuit of  FIG. 2 ; and 
         FIG. 4  is a schematic diagram of an embodiment of a photosensor circuit and display device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the present invention includes a circuit  8  having a photosensor  10  connected to a detection circuit  11 . The detection circuit  11  and photosensor  10  form a photosensor circuit  12  that is connected to an averaging circuit  14  comprising a storage circuit  13  and an output circuit  16 . The photosensor  10  may be any light-sensitive device suitable for use within a display system. For example, silicon or organic photodiodes or transistors may be employed. These photosensors and circuit elements may be discrete or, preferably, are integrated with a display to provide an integrated solution. When integrated with a display, any portion of, or all of, the circuit  8  may be constructed using thin film transistors and electrical components as are known in the flat panel display art. Display substrates may be made of rigid or flexible glass or plastic. 
     Referring to  FIG. 2 , a more detailed circuit diagram is shown. The photosensor  10  has two terminals, one of which is connected to a given voltage, for example ground, the other of which is connected to the drain of an isolation transistor  30 . The gate of the isolation transistor  30  is connected to an isolation signal Visolate and the transistor source is connected to a capacitor  20  (Csensor) for storing a charge representative of the light incident on the photosensor  10 . The capacitor  20  has one terminal connected to the given voltage terminal of the photosensor  10  and another terminal that is connected to the drain of a reset transistor  32 . The reset transistor  32 , capacitor  20 , and isolation transistor  30  comprise the detection circuit  11 . An external periodic reset signal drives the gate of the reset transistor  32  and the source of the reset transistor  32  is fixed to some known voltage (shown as Vdd) capable of charging the capacitor  20  when the reset signal turns on the reset transistor  32 . The detection circuit  11  and the photosensor  10  form the photosensor circuit  12 . 
     The periodic reset signal will periodically cause the capacitor  20  to charge to a known voltage, specified by Vdd and the reset transistor  32  characteristics. When the reset signal is charging the capacitor  20 , the isolation transistor  30  is also turned on, thereby charging the photosensor  10  as well. While the capacitor  20  and photosensor  10  is charged, the output of the detection circuit is invalid, that is, it is not representative of the light incident on the phototransistor. After the reset signal is turned off, the photosensor  10  and the capacitor  20  are connected in parallel and, as light impinges on the photosensor  10 , the capacitor  20  and the photosensor  10  discharge together through the isolation transistor  30  over time to produce a photo signal representative of the total flux of light incident on the photosensor  10  during an integration period between reset signals. After the integration period, the capacitor  20  and photosensor  10  will have a charge representative of the cumulative light incident on the photosensor during the integration period. The charge is inversely proportional to the ambient light incident on the photosensor  10 , thus if more light is present, the charge will be smaller; if less light is present, the charge will be greater. The integrated light signal is measured because a periodic, integrated light signal collected over time is much more sensitive than a design that simply measures the instantaneous current from a photosensor. 
     The averaging circuit  14  includes a transfer transistor  34  whose gate is connected to a periodic transfer signal. A source of the transfer transistor  34  is connected to the sensor capacitor  20  and receives the photo signal. The drain of the transfer transistor  34  is connected to one terminal of an averaging capacitor  22 . The other terminal of the averaging capacitor  22  is connected to the given voltage (e.g. ground). The transfer transistor  34  and the averaging capacitor  22  comprise a storage circuit  13  for an average photo signal. 
     At the end of the light integration time period, the isolation transistor  30  is turned off and the transfer transistor is turned on. The charge on the sensor capacitor  20  is then combined with the charge on the averaging capacitor  22  to form a charge representing an average signal. If the capacitors are equal in value, the charge will be the average of the charge on the two capacitors. If not, the average charge will be the ratio of the relative capacitor sizes and charges. When the capacitor charges are redistributed and the voltage across both capacitors is equal, the transfer transistor  34  is turned off and the reset and isolation transistors ( 32  and  30 ) are turned on and the cycle begins again. 
     The output circuit  16  includes an output transistor  36  whose gate is connected to the averaging capacitor  22 . The source is connected to a resistive load that is connected to a power signal such as Vdd to form an output signal  40  representative of the ambient illumination incident on the photosensor  10 . The drain can be connected to a given voltage. As configured, the output circuit provides an inverting amplifier whose input is the average signal representing a continuous running average of the successive photo signals and that produces an average ambient light signal output. When the charge stored in the capacitor  22  is large enough to cause the output transistor  36  to turn on, the output signal will be connected to the given voltage. When the charge stored in the capacitor  22  is smaller, the output transistor  36  will have an increasing impedance and the average ambient light signal  40  will increase up to the limit of the power signal, e.g. Vdd. 
     When the photosensor circuit is first powered up, the ambient light falling on the photosensor  10  is unknown, as is the charge in the capacitors and the value of the output signal. After an initial cycle, the charge in the sensor capacitor  20  will correctly represent the ambient illumination incident on the photosensor  10  and will be transferred to the averaging capacitor  22 . At this point, the voltage across the averaging capacitor  22  will not necessarily be equivalent to the voltage across the sensor capacitor  20  at the end of the light integration cycle but will be closer than before the charge transfer from the sensor capacitor  20 . The voltage across the averaging capacitor  22  will, in fact, represent the average charge in the sensor capacitor  20  and the averaging capacitor  22  weighted by the relative sizes of the capacitors  20  and  22  and the charge originally stored in them. At each subsequent cycle, the voltage across the averaging capacitor  22  will come closer to the voltage across the sensor capacitor  20  as charge is transferred to or from the averaging capacitor  22 . Eventually, the voltage across both capacitors will be the same. After each cycle, the averaging capacitor  22  will store the average of the charge in the sensor capacitor  20  and the previous charge in the averaging capacitor  22  (weighted by capacitor size). Thus, the charge in the averaging capacitor  22  represents a continuous running average of the charges stored in successive cycles within the sensor capacitor  20 . 
     If the ambient light on the photosensor  10  changes, the charge in the sensor capacitor  20  will change and the voltage across the averaging capacitor will also change to match. Note that the averaging capacitor  22  does not need an explicit reset or charge deposition into a known state. Instead, the charge on the averaging capacitor  22  gradually assumes the correct value as charge is transferred from the sensor capacitor  20 . Hence, the output from the output circuit  16  is always valid and gradually assumes the correct value without creating abrupt discontinuous changes. Moreover, the output signal provides a continuous, analog signal that is representative of the ambient illumination over a wide range, limited by the saturation of the sensor capacitor  20  in bright conditions, and by the minimum output transistor  36  threshold voltage. By modifying the sizes of the capacitors  20  and  22 , the sensitivity range of the circuit to ambient light may be modified and by changing the ratio of the capacitor sizes, the extent of averaging can be controlled. 
     The timing signals for this circuit are illustrated in  FIG. 3 , where T represents the length of time the signals are applied in the states indicated. 
     The transfer and isolate signals driving the transfer transistor  34  and isolate transistor  30  respectively, are inverses of each other; that is, one signal is the inverse of the other. Therefore, the signals can be derived from a single signal, preferably the isolation signal. An inverse signal is readily created using a circuit like the output circuit  16  having a transistor whose gate is connected to the signal, drain is connected to a known voltage such as a ground, and whose source is connected through a load to a power signal. 
     It is also possible to simplify the circuit of  FIG. 2  by eliminating the isolation transistor  30  and isolate signal. In this case, while the transfer transistor  34  is on, the sensor capacitor  20  and averaging capacitor  22  will continue to discharge, depending on the ambient light incident on the photosensor  10 . Thus, the voltage will vary more and the output will not be as stable. 
     Alternative photosensor circuits may be employed and are included in the present invention. For example, photo capacitors that charge in the presence of light may be employed to provide a photo signal. In this embodiment, a reset signal must be employed to periodically discharge the photo capacitor. Photo resistors, photodiodes, and phototransistors may also be employed to discharge a sensor capacitor. 
     The photosensor circuit of  FIGS. 1 and 2  may be employed in a display system as shown in  FIG. 4 . Referring to  FIG. 4 , a substrate  50  has an array of light emitting elements, for example OLEDs, in a display area  52  and a photosensor circuit  8  integrated on the substrate  50 . The photosensor circuit  8  provides an output signal  40  to a controller  44 . The controller  44  responds to the output signal  40  and an input signal  46  to produce a display signal  42  that drives the display. 
     The signal from a thin film photosensor  10  is directly related to the area that it covers and the ambient radiation incident upon it. By increasing the area of an integrated photosensor  10 , the output signal  40  from the circuit  8  may be increased without significantly increasing the size of the display. 
     A plurality of photosensors  10  can be electrically connected in common to provide one integrated photo signal or, alternatively, they can be separately addressed or combine their output. A greater number or size of integrated photosensors  10  can increase the signal, thereby improving the responsiveness of the ambient light detection. Moreover, the signal 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. Indeed, the location and shape of any shadows falling upon the display area  52  may be determined to a limited extent, thereby providing further information that can be used to optimize the performance of the display. 
     The photosensor(s)  10  of the present invention are sensitive to the frequency distribution of the light incident upon the photosensor. This sensitivity is due to the absorption spectrum of the materials and to the structure of the layers used to construct the photosensor. The frequency sensitivity of the device may be modified by providing color filters between the photosensor and the ambient radiation. Such filters can be used to customize the ambient light response of the photosensor(s)  10 . 
     The present invention may be used in both top and bottom emitting OLED display structures. Thin film structures used for active matrix OLED displays may be employed to form the photosensors  10  and to provide circuitry  11  and  14  to generate and process suitable control signals for the photosensors  10 . The same power and control signal methods may be used to operate the display. There are also 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. 
     The photosensor elements may be selected individually (as are the display pixel elements) or in groups. Existing address and signal lines may be used to select or reset elements using existing electronic control methods. Groups of photosensor elements can be joined either physically or logically to provide a measure of incident light over larger areas thus reducing both the specificity of the information and the need for supporting logic and interconnects. 
     It is also possible to use the present invention to obtain information concerning the color of the ambient illumination. By utilizing color filters located between the photosensor and the ambient light, the ambient light may be filtered. Color filter deposition techniques are well known in the art and have been publicly demonstrated for displays. If a plurality of photosensors are provided with different filters, the signals from the photosensors can be used to optimize the display, for example by adjusting the color or white point of the display. In this case only, photosensors with color filters of the same color would be connected in parallel. 
     The light emitting display 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. The photosensor circuit  8  may be deposited in a common step with active matrix display circuitry and may include identical materials to simplify processing and manufacturing. 
     Any or all of the photosensor circuit  10 , the detector circuit  11 , and the averaging circuit  14  can be integrated directly onto the same substrate as the display device or it can be implemented externally to the display. 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 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 
     
         
           8  circuit 
           10  photosensor 
           11  detection circuit 
           12  photosensor circuit 
           13  storage circuit 
           14  averaging circuit 
           16  output circuit 
           20  capacitor 
           22  averaging capacitor 
           30  isolation transistor 
           32  reset transistor 
           34  transfer transistor 
           36  output transistor 
           40  output signal 
           42  display signal 
           44  controller 
           46  input signal 
           50  substrate 
           52  display area