PATENT DOCUMENT

Publication Number: US-8872093-B2
Application Number: US-201213449842-A
Country: US
Kind Code: B2

Title: Calibrated image-sensor-based ambient light sensor

Abstract:
An electronic device may be provided with an image sensor for capturing digital images. The image sensor may be used as part of image-sensor-based ambient light sensing circuitry for producing ambient light sensor readings. The image-sensor-based ambient light sensing circuitry may include a reference array. The reference array may be formed from an array of light sensor elements that are matched to elements in the image sensor but that are covered with a light blocking material. Control circuitry can measure current flow into the reference array and the image sensor array and can use current measurements from these arrays in producing a calibrated ambient light sensor reading. The control circuitry may make current measurements by measuring a decay time associated with the voltage of a discharging capacitor. A comparator, pulse generator, and switch may be used in periodically recharging the capacitor. The capacitor may be adjusted to ensure accurate readings.

Claims:
What is claimed is: 
     
       1. Image-sensor-based ambient light sensing circuitry, comprising:
 an image sensor array; 
 an optically-black reference sensor array; and 
 control circuitry that produces an ambient light sensor reading based on data from the image sensor array and the reference sensor array, wherein the control circuitry is configured to measure current flow into the image sensor array and the reference light sensor array and is configured to produce the ambient light sensor reading based at least partly on how much current flows into the reference sensor array and how much current flows into the image sensor array. 
 
     
     
       2. The image-sensor-based ambient light sensing circuitry defined in  claim 1  wherein the control circuitry includes a capacitor and wherein the control circuitry is configured to discharge the capacitor into the image sensor array to measure current flow into the image sensor array, and is configured to discharge the capacitor into the reference sensor array to measure current flow into the reference sensor array. 
     
     
       3. The image-sensor-based ambient light sensing circuitry defined in  claim 2  wherein the reference sensor array comprises a plurality of light sensor elements that are covered with a light blocking material to prevent ambient light from reaching the light sensor elements. 
     
     
       4. The image-sensor-based ambient light sensing circuitry defined in  claim 3  wherein the image sensor array has M light sensing elements, wherein the reference light sensor array has N light sensing elements that are covered with the light blocking material, and wherein the control circuitry is configured to produce the ambient light sensor reading based at least partly on the ratio of M to N. 
     
     
       5. The image-sensor-based ambient light sensing circuitry defined in  claim 2  wherein the capacitor comprises a variable capacitor that is adjusted by the control circuitry. 
     
     
       6. The image-sensor-based ambient light sensing circuitry defined in  claim 5  wherein the control circuitry is configured to adjust the variable capacitor to ensure accuracy in performing current flow measurements. 
     
     
       7. The image-sensor-based ambient light sensing circuitry defined in  claim 2  wherein the control circuitry comprises a counter for measuring a voltage decay time associated with discharge of the capacitor during current flow measurements. 
     
     
       8. The image-sensor-based ambient light sensing circuitry defined in  claim 2  further comprising at least a first switch and a second switch, wherein the control circuitry is configured to discharge the capacitor into the image sensor array during measurement of current flow into the image sensor array by closing the first switch and opening the second switch and is configured to discharge the capacitor into the reference light sensor array during measurement of current flow into the reference light sensor array by closing the second switch and opening the first switch. 
     
     
       9. The image-sensor-based ambient light sensing circuitry defined in  claim 2  wherein the control circuitry comprises a switch that bridges the capacitor. 
     
     
       10. The image-sensor-based ambient light sensing circuitry defined in  claim 9  wherein the control circuitry is configured to close the switch to charge the capacitor. 
     
     
       11. The image-sensor-based ambient light sensing circuitry defined in  claim 9  wherein the control circuitry includes a comparator that compares a capacitor voltage associated with the capacitor to a threshold voltage. 
     
     
       12. The image-sensor-based ambient light sensing circuitry defined in  claim 11  wherein the control circuitry includes a pulse generator. 
     
     
       13. The image-sensor-based ambient light sensing circuitry defined in  claim 2  wherein the control circuitry comprises:
 a switch that bridges the capacitor; and 
 a pulse generator having an output that supplies a control signal to the switch. 
 
     
     
       14. A method of calibrating image-sensor-based ambient light sensor circuitry, comprising:
 closing a second switch to supply current to an image sensor; 
 while the second switch is closed, measuring an image sensor current for the image sensor while the image sensor is exposed to ambient light at an ambient light level; 
 opening the second switch and closing a first switch to supply current to a reference array of light sensor elements that are covered with light blocking material; 
 while the first switch is closed, measuring a reference current for the reference array; and 
 producing an ambient light reading representative of the ambient light level based at least partly on the measured image sensor current and the measured reference current. 
 
     
     
       15. The method defined in  claim 14  wherein the image-sensor-based ambient light sensor circuitry comprises a capacitor and wherein measuring the image sensor current and the reference current comprises monitoring a capacitor voltage associated with the capacitor as the capacitor is discharged. 
     
     
       16. The method defined in  claim 15  wherein monitoring the capacitor voltage comprises using a counter to measure a voltage decay time associated with discharging the capacitor from a first predetermined voltage to a second predetermined voltage. 
     
     
       17. The method defined in  claim 15  wherein the image-sensor-based ambient light sensor circuitry comprises a third switch, the method further comprising:
 charging the capacitor to a predetermined voltage by closing the third switch. 
 
     
     
       18. The method defined in  claim 17  wherein the capacitor comprises a variable capacitor, the method further comprising adjusting the variable capacitor using the control circuitry.

Description:
BACKGROUND 
     This relates generally to electronic devices, and more particularly, to image-sensor-based ambient light detection circuitry for electronic devices. 
     Electronic devices such as portable computers and cellular telephones are often provided with ambient light sensors. Ambient light sensors can be used to measure the amount of ambient light in the immediate vicinity of an electronic device. When ambient light conditions change, an electronic device may take appropriate actions. For example, display brightness levels may be adjusted to accommodate bright or dim lighting conditions. 
     Ambient light sensors are typically mounted under dedicated ambient light sensor windows in electronic device housings. The need for individual ambient light sensors and mounting arrangements can add undesired cost and complexity to an electronic device. Ambient light sensor windows may also detract from the appearance of an electronic device. 
     It would therefore be desirable to be able to provide improved ambient light detection circuitry for electronic devices. 
     SUMMARY 
     An electronic device may be provided with an image sensor for capturing digital images. The image sensor may be used as part of image-sensor-based ambient light sensing circuitry for producing ambient light sensor readings. Ambient light data from the image-sensor-based ambient light sensing circuitry may be used to make adjustments to display brightness or other adjustments during operation of the electronic device. 
     The image-sensor-based ambient light sensing circuitry may include a optically-black reference array. The reference array may be formed from an array of light sensing elements that are matched to active light-sensing elements in the image sensor but that are covered with a light blocking material. 
     Control circuitry can open and close switches to measure current flow into the reference array and the image sensor array. The current measurements from these arrays can be used in producing a calibrated ambient light sensor reading. 
     The control circuitry may make current measurements by measuring a decay time associated with the voltage of a capacitor that is discharged using current flowing into the reference array or image sensor array. A comparator, pulse generator, and switch may be used in periodically recharging the capacitor. The capacitor may be adjusted by the control circuitry. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device with image-sensor-based ambient light sensing circuitry in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of an illustrative electronic device with ambient light sensor circuitry in accordance with an embodiment of the present invention. 
         FIG. 3A  is a cross-sectional side view of a semiconductor substrate showing how some light sensing devices may have a light-blocking layer for supporting dark current measurements and how some light sensing devices may be free of light-blocking material to allow light to be sensed in accordance with an embodiment of the present invention. 
         FIG. 3B  is a diagram of an image sensor array formed from unblocked light sensor pixels and an associated set of blocked light sensor pixels for dark current measurements in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of image-sensor-based ambient light sensor circuitry of the type that may be used in accordance with an embodiment of the present invention. 
         FIG. 5  is a signal trace associated with the measurement of sensor current during operation of the image-sensor-based ambient light sensor circuitry of  FIG. 4  in accordance with an embodiment of the present invention. 
         FIG. 6  is a flow chart of illustrative steps involved in gathering a calibrated ambient light sensor reading representative of an ambient light level using image-sensor-based ambient light sensor circuitry in accordance with an embodiment of the present invention. 
         FIG. 7  is a flow chart of illustrative steps involved in gathering and using ambient light sensor data and other sensor data during operation of an electronic device of the type shown in  FIG. 1  in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as electronic device  10  of  FIG. 1  may be provided with ambient light sensor circuitry. The ambient light sensor circuitry may be used to support operations such as display brightness level adjustments and other operations in an electronic device. The ambient light sensor circuitry may use data that is gathered from an image sensor array and may therefore sometimes be referred to as image-sensor-based ambient light sensor circuitry. 
     Image-sensor-based ambient light sensor circuitry may be used in an electronic device such as electronic device  10  of  FIG. 1 . Electronic device  10  may be a portable electronic device or other suitable electronic device. For example, electronic device  10  may be a laptop computer, a tablet computer, a somewhat smaller device such as a wrist-watch device, pendant device, headphone device, earpiece device, or other wearable or miniature device, a cellular telephone, or a media player. Device  10  may also be a television, a set-top box, a desktop computer, a computer monitor into which a computer has been integrated, or other suitable electronic equipment. 
     Device  10  may include a housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of these materials. In some situations, parts of housing  12  may be formed from dielectric or other low-conductivity material. For example, glass structures, plastic structures, or other dielectric structures may be used to form exterior and interior portions of housing  12 . In other situations, housing  12  or at least some of the structures that make up housing  12  may be formed from metal elements. 
     Device  10  may, if desired, have a display such as display  14 . Display  14  may be a touch screen that incorporates a touch sensor array or may be insensitive to touch. A touch sensor array for display  14  may be formed from capacitive touch sensor electrodes or touch sensors based on other touch technologies (e.g., acoustic touch, light-based touch sensor configurations, force sensor arrangements, etc.). Display  14  may include image pixels formed from light-emitting diodes (LEDs), organic LEDs (OLEDs), plasma cells, electrowetting pixels, electrophoretic pixels, liquid crystal display (LCD) components, or other suitable image pixel structures. A display cover layer such as a clear plastic layer or transparent glass layer may cover the surface of display  14 . Peripheral regions of the display cover layer may be provided with an internal opaque masking layer to help hide internal components from view by a user. Buttons such as button  16  may pass through openings in the cover layer. The cover layer may also have other openings such as an opening for speaker port  18 . 
     One or more optically clear windows such as window  20  may be provided in device  10 . Windows such as window  20  may be formed from glass or plastic elements that are mounted within an opening in an opaque housing structure or may be formed by creating an opening in an opaque masking layer on the underside of a display cover layer or other transparent member. In the example of  FIG. 1 , window  20  has been formed in a portion of the display cover layer for display  14  (e.g., by creating an opening in a peripheral opaque masking layer that overlaps inactive portions of display  14 ). If desired, windows such as window  20  may be formed on the rear surface of device  10 , on sidewall surfaces of device  10 , or elsewhere in device  10 . 
     Optical components may be mounted within housing  12  in alignment with optical windows such a window  20 . For example, discrete ambient light sensor devices, light-based proximity sensors, status indicator lights, digital image sensors, and other optical devices may be mounted under windows such as window  20 . To enhance the compactness of device  10  and to enhance device aesthetics by reducing unnecessary additional optical windows, one or more ambient light sensors for device  10  may be formed using image sensor circuitry. With this type of approach, a digital image sensor may be mounted under window  20 . When operated in digital image sensor mode, device  10  may obtain digital image sensor data from the image sensor. The digital image sensor data may be processed to form digital images and video clips. When operated as an ambient light detector, the same digital image sensor may be used in collecting ambient light sensor readings. Because a single image sensor can be used for both digital imaging operations and ambient light measurements, space within device  10  may be conserved and device aesthetics may be enhanced. Device cost and complexity may also be reduced. 
       FIG. 2  is a schematic diagram of device  10  showing how device  10  may have control circuitry  22  and input-output circuitry such as sensor circuitry  24 . Control circuitry  22  may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Control circuitry  22  may also include processing circuitry based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio codec chips, application specific integrated circuits, etc. 
     The storage and processing circuitry of control circuitry  22  may be used to run software on device  10 , such as internet browsing applications, voice-over-internet-protocol (VoIP) telephone call applications, email applications, media playback applications, operating system functions, software for supporting display brightness adjustment functions and other functions associated with gathering and using data from ambient light sensor circuitry and other sensors, etc. To support interactions with external equipment, storage and processing circuitry  28  may be used in implementing communications protocols. Communications protocols that may be implemented using storage and processing circuitry  28  include internet protocols, wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as WiFi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, cellular telephone protocols, etc. 
     Input-output circuitry  24  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output circuitry  24  may include input-output devices such as touch screens, buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through these input-output devices and may receive status information and other output from device  10  using the output resources of these input-output devices. Wireless communications circuitry in circuitry  24  may include radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     The sensors in circuitry  24  may include discrete ambient light sensors, proximity sensors (e.g., light-based proximity sensors that include an infrared transmitter and an infrared receiver or other light-based proximity sensor circuitry), capacitance sensors (e.g., for touch sensor and/or proximity sensing applications), force sensors (e.g., for touch sensing devices), accelerometers (e.g., an accelerometer for measuring the force of gravity and thereby determining the orientation of device  10  relative to the surface of the Earth), digital image sensors (e.g., image sensor arrays) and other sensors. 
     The sensors in circuitry  24  may include one more image-sensor-based ambient light detectors. Image-sensor-based ambient light detectors may be based on digital image sensor integrated circuits. Digital image sensor circuits can be used for capturing digital images or can be used to produce ambient light measurements. 
     The digital image sensor for an image-sensor-based ambient light sensor may be based on an image sensor array that contains rows and columns of image sensor pixels. Each image sensor pixel may contain a light sensitive element (e.g., a semiconductor-based device such as a photodiode or phototransistor) and associated support circuitry (e.g., transistors for transferring charge from the light sensitive element to a charge storage node, transistors for forming pixel-based amplifier circuitry, readout transistors for controlling the transfer of signals from the image sensor pixels to column-based readout circuitry, etc.). 
     To ensure accurate operation, it may be desirable to provide image-sensor-based ambient light sensor circuitry in device  10  with circuitry for calibrating the response of the light sensitive elements in the image sensor pixels. The response of the semiconductor light sensitive elements may fluctuate as a function of PVT variations (i.e., variations in fabrication process parameters (P), fluctuations in power supply voltage (V), and temperature (T) changes during operation). To prevent errors related to PVT variations, the performance of the image-sensor-based ambient light sensor circuitry may be calibrated using a set of reference light sensor elements. The reference light sensor circuitry may, as an example, be formed from the same type of light sensitive elements that are used in forming the light sensor elements in the image sensor pixels of the digital image sensor array, but may be coated with a light-blocking coating. This reference circuitry, which may sometimes be referred to as dark current reference circuitry, optically-black pixels, reference pixels, reference light sensitive elements, or reference array, may be used to make reference current measurements (i.e., dark current measurements). The amount of current that flows into the reference light sensor circuitry is affected by process, voltage, and temperature conditions, but is not influenced by the amount of ambient light that is present. Accordingly, the amount of current that is measured as flowing into the reference light sensor circuitry may be subtracted from the amount of current that is measured as flowing into the image sensor to calibrate the image sensor ambient light data. 
       FIG. 3A  is a cross-sectional side view of a portion of an illustrative image-sensor-based ambient light sensor integrated circuit. As shown in  FIG. 3A , image-sensor-based ambient light sensor circuitry  26  may be formed from a semiconductor substrate such as substrate  28 . Substrate  28  may be formed from silicon or other semiconductor materials. Light sensitive elements may be formed in substrate  28 . The light sensitive elements may be formed from phototransistors, photodiodes, or other light-sensitive devices. 
     Some elements such as light-sensitive element  32  may be uncovered by light-blocking layers. As a result, light-sensitive elements such as element  32  may receive and measure light  36  (e.g., to form a digital image when element  32  is operated in digital image sensor mode or to gather ambient light data when element  32  is operated in ambient light sensing mode). Other elements such as reference element  30  may be covered with an opaque layer such as light-blocking structure  40 . The attributes of reference elements such as reference element  30  (e.g., the size, shape, doping concentration, and other properties of the reference elements) are preferably matched to those of elements such as element  32 , so that elements  30  may be used in accurately calibrating elements  32 . Structures such as opaque structure  40  in each reference element  30  may be formed from metal or other opaque material. During operation, the amount of reference signal that flows into reference elements  30  will be unaffected by the strength of ambient light  34 , because structure  40  will block light  34  and thereby prevent light  34  from contributing to the response of element  30 . 
     Elements  32  may be organized into rows and columns to form an image sensor array (e.g., an image sensor array having, thousands or millions of pixels). Elements  30  may be organized into groups in the vicinity of the image sensor array. For example, elements  30  may be used to form one or more arrays that are smaller in size than the image sensor array (e.g., thin rectangular arrays running along the edge of the image sensor array). To ensure that the process, voltage, and temperature variations experienced by reference elements  30  are representative of the process, voltage, and temperature variations experienced by the image sensor array formed from elements  32 , reference elements  30  may be formed adjacent to the image sensor array on substrate  28  (i.e., reference elements  30  and image sensor elements  32  may be formed as part of a common integrated circuit). 
     As shown in  FIG. 3B , reference elements  30  may be organized to form a set of reference sensing elements  30 A (sometimes referred to as dark pixels) and light sensing elements  32  may be organized to form an adjacent image sensor array  32 A. Biasing and readout circuitry  42  may be used to control the operation of reference elements  30 A and image sensor array  32 A. For example, when image sensor array  32 A is being used as an image sensor for a digital camera, circuitry  42  may be used to capture and read out digital image data from array  32 A and/or  30 A. When image-sensor-based circuitry  26  is being used to make ambient light measurements, readout and biasing circuitry  42  and other control circuitry (collectively control circuitry  22 ) may be used to measure the amount of current that collectively flows into the light sensing elements of image sensor array  32 A and, by subtracting the amount of measured dark current flowing into reference elements  30 A, may calibrate the data from image sensor array  32 A to produce calibrated ambient light sensor readings. Some or all of light sensing elements  32  may be used during ambient light sensing operations using array  32 A (e.g., all elements in array  32 A, a subset of elements in array  32 A, elements in array  32 A of a particular color or colors, etc.). 
       FIG. 4  is a diagram of illustrative image-sensor-based ambient light sensing circuitry  26 . As shown in  FIG. 4 , circuitry  26  may include an image sensor array such as image sensor array  32 A formed from rows and columns of light sensitive elements (image sensor pixels)  32  and a set of reference pixels  30 A (e.g., an array of reference elements  30 ). Circuitry such as current controlled oscillator circuitry  60  may be used in measuring signals from reference pixels  30 A and image sensor pixels  32 A. Circuitry  60  may include switches such as switches S 0 , S 1 , and S 2  (e.g., transistor-based switches or other suitable switches) or other switching circuitry for controlling current flow to reference pixels  30 A and image sensor pixels  32 A during use of image-sensor-based ambient light sensor circuitry  26  to collect ambient light sensor data. 
     When switch S 1  is closed and switch S 2  is open, circuitry  60  may measure the magnitude of current Iarray. Current Iarray corresponds to the cumulative current drawn by a set of the image sensor pixels  32  in image sensor pixel array  32 A in parallel (e.g., all of the image sensor pixels in array  32 A) when image sensor array  32 A is being used as an ambient light sensor. When switch S 2  is closed and switch S 1  is open, circuitry  60  may measure current Iref. Current Iref corresponds to the reference current (dark current) drawn by reference pixels  30 A during operation of circuitry  26  as an ambient light sensor. By using circuitry  22  and circuitry  60 , device  10  may calibrate the current measurements associated with array  32 A using the reference current measurements associated with reference pixel array  30 A. 
     Power supply circuitry and voltage reference circuitry (e.g., a band gap voltage reference circuit or other voltage reference circuitry that is part of control circuitry  22 ) may be used to supply a power supply voltage such as voltage Vaapix to power supply node  44 , bias voltage Vbias for transistor  48 , and threshold voltage Vth for comparator  50 . 
     Capacitor  46  may be charged to voltage Vaapix by momentarily closing switch S 0  to short node  74  to terminal  4 ). Capacitor  46  may then be discharged through switch S 1  or switch S 2 . By measuring the rate at which capacitor  46  discharges when switch S 1  or S 2  is closed, control circuitry  22  can measure the amount of current flow through array  32 A or the amount of current flow through array  30 A, respectively. 
     The states of switches S 1  and S 2  may be controlled by control circuitry  22  using control signals on control paths  70  and  78 , respectively. Variable capacitor  46  may exhibit a capacitance that is controlled by control signals supplied to input  62  from control circuitry  22 . The capacitance Cosc of capacitor  46  may be given by equation 1.
 
Cosc= k*C 0  (1)
 
In equation 1, capacitance C 0  is a fixed capacitance value and k is a variable scaling factor that is controlled by control circuitry  22  by application of corresponding control signals to input  62  of variable capacitor  46 . Capacitance Cosc is the corresponding capacitance for capacitor  46 . Control circuitry  22  can adjust the capacitance value exhibited by capacitor  46  to ensure sufficient accuracy when making current measurements (i.e., to prevent capacitor  46  from being too large and thereby discharging too slowly to effectively measure or from being too small and thereby discharging too quickly to effectively measure).
 
     Comparator  50  may be used in monitoring the discharge of capacitor  46 . As shown in  FIG. 4 , comparator  50  may have inputs  52  and  54 . Input  54  may receive threshold voltage (reference voltage) Vth. Input  52  may receive the voltage Vx from capacitor  46  while switch S 0  is open. During capacitor discharge operations, comparator  50  may compare the voltage on input  52  with the voltage on input  54  and may produce a corresponding output voltage on output  56 . So long as voltage Vx is greater than voltage Vth, the signal on output  56  of comparator  50  may be deasserted (i.e., held at a logic low value). When voltage Vx falls to Vth, comparator  50  may assert its output (i.e., comparator  50  may take the signal on output line  56  high). In response, one-shot pulse generator  72  will generate an output pulse of a fixed duration. The output of pulse generator  72  (voltage Vcnt) may be provided to the control input of switch S 0  using path  58 . So long as output  56  is deasserted and no pulse is being generated by pulse generator  72 , the value of Vcnt will be low and switch S 0  will be open. In this situation, capacitor  46  may discharge and voltage Vx may fall towards Vth. Once voltage Vx reaches Vth, comparator  50  will assert the output signal on output  56 , pulse generator  72  will generate an output pulse (e.g., a pulse having a positive voltage), and switch S 0  will be closed (i.e., for the duration of the output pulse from pulse generator  72 ). 
     When S 0  is momentarily closed in this way, voltage supply terminal  44  at voltage Vaapix will be shorted to node  74 , capacitor  46  will be charged, and capacitor voltage Vx will be raised to Vaapix. After recharging capacitor  46  in this way, capacitor  46  can be discharged through array  32 A using closed switch S 1  and open switch S 2  or can be discharged through array  30 A using closed switch S 2  and open switch S 1 , as appropriate. 
     When switch S 2  is closed and switch S 1  is open, the amount of current Iref that is drawn from capacitor  46  will be equal to cumulative dark current of all of the dark (optically black) pixels  30  in reference pixels  30 A. When switch S 1  is closed and switch S 2  is open, the amount of current Iarray that is drawn from capacitor  46  will be equal to the cumulative current of the pixels in array  32 A (or a selected subset of these pixels). The value of Iarray includes a current contribution due to the detection of ambient light and includes a dark current contribution. To calibrate image-sensor-based ambient light sensor circuitry  26 , control circuitry  22  may subtract a scaled version of the reference current measured by reference pixels  30 A (Iref) from the measured value of Iarray. 
     Control circuitry  22  may be used in measuring the values of currents Iref and Iarray. To make a current measurement, control circuitry  22  can receive a clock signal such as clock CLK. Using a counter such as counter  21  or other suitable circuit that is driven by clock signal CLK, control circuitry  22  may measure the amount of time between successive changes in the state of signal Vcnt. These measurements are indicative of the decay time associated with discharging capacitor  46  from Vaapix to Vth and are therefore indicative of the magnitudes of currents such as currents Iref and Iarray. 
     Consider, as an example, the trace of voltage Vx that is shown in the graph of  FIG. 5 . In the example of  FIG. 5 , curve  76  corresponds to the discharge of capacitor  46  when a current of a first magnitude is being drawn. Curve  78  corresponds to the voltage on discharging capacitor  46  when a current of a second magnitude that is smaller than the first magnitude is being drawn. 
     As shown in  FIG. 5 , the voltage Vx on capacitor  46  is reset to Vaapix at time t 1 , following application of a pulse of duration TD from pulse generator  72  to switch S 0 . Starting at time t 1 , current is drawn from capacitor  46  by pixels in array  32 A or by pixels in array  30 A, depending on whether control circuitry  22  has closed switch S 1  and opened switch S 2  or vice versa. 
     As shown by curve  76 , when the current of the first magnitude is drawn from capacitor  46 , voltage Vx will fall to voltage Vth at time t 2 . When comparator  50  senses that Vx has fallen below Vth, comparator  50  will take its output on line  56  high and pulse generator  72  will again generate a momentary pulse of length TD for switch S 0  to reset the voltage Vx on capacitor  46  to Vaapix. This process may repeat, so that control circuitry  22  may make a series of measurements (e.g., to average data or otherwise process data to enhance accuracy). 
     If the current being drawn from the capacitor has the second magnitude, the voltage Vx will decrease slower, as indicated by curve  78 . As a result, voltage Vx will not reach voltage Vth until time t 3 . 
     Control circuitry  22  may use a counter or other circuitry for measuring the duration of the Vaapix to Vth decay time. In the example of  FIG. 5 , the current that resulted in voltage decay curve  76  is associated with a time period TP 0 , whereas the current that resulted in voltage decay curve  78  is associated with a time period TP 1 . Control circuitry  22  may measure decay periods such as TP 0  and TP 1  (e.g., using a counter or other timing circuitry) and can convert the measured decay time (i.e., the time between successive pulses output from pulse generator  72 ) to measured current values. Using this type of technique, control circuitry  22  may measure array current Iarray and reference current Iref. The values of these measured currents may, in turn, be processed (e.g., using digital processing techniques implemented using control circuitry  22 ), to produce calibrated ambient light data. 
     A flow chart of illustrative steps involved in using image-sensor-based ambient light sensor circuitry  26  to make an ambient light sensor reading representative of an ambient light level to which the sensor circuitry is exposed is shown in  FIG. 6 . At step  80 , control circuitry  22  may measure dark current Iref associated with reference pixels  30  in reference pixel array  30 A. During these measurements, control circuitry  22  may supply switches S 2  and S 1  with control signals to close switch S 2  and open switch S 1 . 
     Initially, the value of k (i.e., the value of capacitor Cosc) may be set to a default value. If the value of capacitance Cosc is too low, the decay period TP of voltage Vx will be too short to effectively measure using control circuitry  22 . If the value of capacitance Cosc is too high, the decay period TP of voltage Vx will be too long to effectively measure using control circuitry  22 . Accordingly, control circuitry  22  may, during the operations of step  80 , ascertain whether decay period TP is greater to a predetermined satisfactory lower decay time threshold TPLOWER and less than a predetermined satisfactory upper decay time threshold TPUPPER. If the measured decay time TP is not within this range, control circuitry  22  may adjust k to increase or decrease Cosc as appropriate. The measurement operations of step  80  may then be repeated with the newly selected value of k (and the corresponding newly selected value of Cosc). 
     Once a satisfactory k value and Cosc value have been selected, the resulting measured value of TP will fall within an acceptable range that allows control circuitry  22  to make accurate measurements. Accordingly, the value of TP associated with reference current Iref (and the corresponding value of k) may be retained by control circuitry  22  for use in calibration operations. Processing may then proceed to step  84 , where control circuitry  22  may measure the current Iarray that is associated with pixels  32  of image sensor array  32 A (e.g., using a value of k=1 or using a default value of Cosc that is appropriate for making Iarray measurements). 
     The value of Iarray is equal to the sum of an ambient light contribution (i.e., a current component due to the magnitude of ambient light received by pixels  32 ) and a dark current contribution. The dark current contribution may be affected by PVT variations. For example, changes in operating temperature for image sensor array  32 A may have a significant impact on the magnitude of Iarray. The pixels in array  30 A are likewise affected by PVT variations, but because the reference pixels in array  30 A are optically blocked and do not receive light, the reference pixels in array  30 A are not affected by changes in the amount of ambient light that is present. To calibrate Iarray and thereby produce an accurate ambient light reading, control circuitry  22  may, at step  86 , compute the value of calibrated current Iambient using equation 2.
 
 I ambient= I array−( N*k/M )* I ref  (2)
 
In equation 2, Iambient corresponds to a calibrated current value that is proportional to the amount of ambient light detected by image-sensor-based ambient light sensor  26 . Current Iarray is the measured current drawn by N light sensor elements (i.e., N pixels) in array  32 A as measured using circuitry  60  and circuitry  22 . Current Iref is the measured current drawn by M reference pixels  30 A in reference pixel array  30 A as measured using circuitry  60  and circuitry  22 . The scaling factor k corresponds to the size of capacitor  46  that was selected during the operations of step  82 , assuming that k was 1 during the operations of step  84 . If k has a value other than 1 during the operations of step  84 , k in equation 2 may be set to the ratio of Cosc during step  80  to Cosc during step  84 .
 
     By scaling the value of Iref using equation 2, control circuitry  22  can compensate for the smaller size of array  30 A relative to array  32 A and the settings used for adjustable capacitor  46  when computing calibrated ambient light current Iambient. Calibrated current Iambient serves as an accurate ambient light reading for device  10  and may be used in taking any suitable action in device  10 . For example, device  10  may use the value of Iambient in determining whether to increase and/or decrease the brightness of display  14 . The measurement process of  FIG. 6  may be performed multiple times to produce averaged ambient light data (e.g., data that is processed using a digital low-pass filter and/or other signal processing techniques). 
     If desired, device  10  may use ambient light data from one or more image-sensor-based ambient light sensors in conjunction with additional sensor data in determining how to control the operation of device  10 . A flow chart of illustrative operations associated with controlling device  10  using multiple sensor inputs of this type is shown in  FIG. 6 . 
     At step  88 , device  10  may use image-sensor-based ambient light sensor circuitry  26  to make ambient light sensor measurements. One or more image-sensor-based ambient light detectors may be used in gathering ambient light readings. For example, in an electronic device that has both front-facing and rear-facing digital cameras, a front-mounted sensor circuit may be used to gather front-facing ambient light data and/or a rear-mounted sensor circuit may be used to gather rear-facing ambient light data. 
     At step  90 , device  10  may use additional sensors to gather sensor readings such as accelerometer readings to determine whether device  10  is face up or face down or has another orientation relative to the surface of the Earth, proximity sensor readings to determine whether ambient light sensor data may have been corrupted by shadows due to external objects in the vicinity of the image-sensor-based ambient light detector(s), data from touch sensors indicating whether or not a user is using a touch screen on the front (or rear) of a device, and data from other sensors. 
     At step  92 , control circuitry  22  may take appropriate action based on one or more of the sensor readings gathered during the operations of steps  88  and  90 . If, for example, the rear ambient light sensor is darker than the front ambient light sensor reading and accelerometer data and touch sensor data indicate that device  10  is facing upwards and is being used by a user, the ambient light sensor data from the rear ambient light sensor can be considered to be inaccurate (because the rear camera is being shadowed by being placed on a table or in a user&#39;s hand). Control circuitry  22  may therefore discard the rear ambient light sensor data (in this example). 
     Following adjustment of display brightness or other suitable actions, processing may return to step  88 , as indicated by line  94 . 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.

Metadata:
Filing Date: 20120418
Publication Date: 20141028
Grant Date: 20141028
Priority Date: 20120418
Inventors: LEE CHIAJEN
SHARMA ANUP
FAN XIAOFENG
Assignee: APPLE INC
CPC Classifications: [{"code": "H10F39/8057", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10F39/8057", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2360/144", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/0626", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/20", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/20", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 49379662