Ambient light sensing system

Abstract

A method of sensing an ambient light level in an electronic device includes sensing a combined light level of ambient light and light from a display, integrating it to determine an integrated light level, determining an integrated display light level, and using the integrated display light level to compensate the integrated light level to determine the ambient light level. The device modulates the display between first and second brightness levels, and determining the integrated display light level includes sensing a combination of light from the display and ambient light when at each of the first and second brightness levels, determining a difference, and applying a calibration value to the difference to determine the integrated display light level.

Description

TECHNICAL FIELD

[0001] This specification relates to systems for sensing ambient light levels. BACKGROUND

[0002] It is useful for electronic devices such as mobile phones to be able to sense ambient light levels, for example so that display illumination can be controlled to reduce power consumption. A technique for ambient light sensing is described in EP3370226A. Ambient light sensing techniques involving turning off a display are described in WO2014077950. An optical sensor arrangement is described in EP3401701A. It is desirable to be able to sense ambient light levels without turning off the display. SUMMARY

[0003] This specification relates generally to techniques for sensing ambient light levels in a portable electronic device having an emissive display screen. The sensed light level is affected by the display screen, particularly if the light sensor is in close proximity to the display screen, for example behind the display screen. Techniques are described which can compensate for this.

[0004] In one aspect, a method of sensing an ambient light level in an electronic device having an emissive display and a sensor is described. The method can comprise sensing, using the sensor, a combined light level of ambient light and light from the emissive display. The method can further comprise integrating the combined light level over an ambient light sensing time to determine an integrated light level. The method can further comprise determining an integrated display light level of the emissive display over the ambient light sensing time. The method can further comprise using the integrated display light level to compensate (e.g. subtract) the integrated light level to determine the ambient light level;

[0005] The electronic device can control a display luminance of the emissive display by modulating the emissive display between a first luminance level and a second luminance level, for example using PWM (pulse width modulation). Determining the integrated display light level can comprise sensing, using the sensor, a combination of light from the emissive display when at the first luminance level and a light level of the ambient light to determine a first sensed luminance level; and a combination of light from the emissive display when at the second luminance level and the light level of the ambient light to determine a second sensed luminance level. Determining the integrated display light level can further comprise determining a difference between the first sensed luminance level and the second sensed luminance level to suppress a contribution of the light level of the ambient light. Determining the integrated display light level can further comprise applying a calibration value to the difference to determine the integrated display light level.

[0006] Light sensors can be used to measure ambient light levels, but often space in electronic devices is limited and depending on its location, the sensor can also pick up light from the display. The methods described herein can be used in such cases. Some embodiments of the method are particularly useful in cases where the sensor is located behind the display, but the technique can be used in cases where the sensor is located elsewhere.

[0007] The ambient light level can include a component that varies at twice the mains frequency, and it is therefore desirable to integrate this out, or to sample quickly so that the ambient light level does not change significantly between samples. In cases where the sensed light includes a component from the display, this component can be subtracted, but the display content can change within the integration period. It can therefore be desirable to sample the light from the display one or more times during the integration period. This sample can then be amplified to determine which component of the integrated sensed light is from the display, and the integrated sensed light can then be compensated for light from the display by subtracting the amplified sample.

[0008] As described later, the integrated sensed light (display + ambient) can be designated as intS. The corresponding (amplified) component of the sensed light from the display can be designated as intD. The integrated ambient light intA can then be given by intA = intS - intD. The ambient light level depends on the integrated ambient light intA (proportional to it), so intA can be used directly as a measure of the ambient light level, or scaled by a factor depending on the unit of measurement.

[0009] In electronic devices with PWM displays, there are periods where part or all of the display is turned off. For example, in some devices the entire display can be PWM modulated, e.g. by controlling the power supply to one or more emissive elements of the display, i.e. all display areas of the display can be turned on / off simultaneously. In some devices only a part of the display can be PWM modulated at any one time. For example, such a display can have a dark band across the display that travels down the display; some devices can have more than one such dark band. The width of the band(s) can depend on the (overall) display brightness, i.e. larger for darker displays and smaller for brighter displays. In some devices, both approaches can be combined. Additionally or alternatively, the size of the power supply to the display can be controlled, e.g. by controlling the current or voltage supplied to the display, to adjust the display brightness, i.e. the overall brightness of the display.

[0010] Such techniques can be used with any type of emissive display, e.g. OLED (organic light emitting diode) displays or LCDs (liquid crystal displays). With LCD displays, the (overall) display brightness can be controlled by controlling the brightness of the backlight of the display.

[0011] In one approach, light from the display can be sampled by briefly measuring the combined display + ambient light, and then, in the portion of the PWM cycle where the display is off (or is off at the sensor), similarly sampling only the ambient light. The ambient light sample can then be subtracted from the display + ambient light sample to obtain a sample of the display light output (as a single ambient sample is not suitable for use alone because the ambient light can flicker). However, this approach has difficulties, for example where only part of the display is blanked during the PWM cycle, and can be inaccurate.

[0012] For example, the display brightness can not decrease to zero during the PWM cycle, or in the case of a PWM involving a moving dark band, the sensor can see light from either side of the band, and thus can not see the light level drop to zero. In principle, if the change in display light level is known, the two measurements can be subtracted to remove the ambient light component. For example, if the overall display brightness changes from, say, 80% to 20%, the sensed light level can change from (80% + ambient) to (20% + ambient), and subtraction will remove the ambient component. However, in practice, such percentages can be unknown.

[0013] Implementations of the described technology address these problems. Thus, a light sensor is used to sense each of the emitted display brightness levels, the method then determines the difference (to compensate for the ambient light level), and applies a calibration value, e.g., retrieved from a storage device, to the difference to amplify the difference for subtraction from the integrated (i.e., total) light level. For example, the calibration value can be multiplied by (or divided into) the difference between the first sensed brightness level and the second sensed brightness level. In this way, multiple problematical effects can be compensated for simultaneously, such as the sensor capturing light from pixels on either side of a dark band, effects due to fading of the display or a portion of the display rather than instantaneously turning off, and effects on the sensed light level due to the aperture of the sensor, its placement, and the timing of the sensing. The technique can be used with PWM-based display brightness control, whether or not the display brightness becomes zero during the "off" period of the PWM control signal.

[0014] In some implementations, the output of the method is a value representing the sensed level of light from the display. That is, the value can be used independently, without sensing the ambient light level, e.g., as part of a display brightness control feedback loop.

[0015] Additionally or alternatively, the integrated display light level can be used to determine the ambient light level, as previously described.

[0016] In some implementations, the calibration value depends on the display's brightness. For example, display brightness can be controlled by a combination of PWM and current modulation. Additionally or alternatively, the sensor can see light from multiple lines (e.g., rows) of the display: in this case, when the display is bright, the sensor can have an aperture that sees several bright lines to either side of the dark line when the dark band is positioned centrally above the sensor, but when the display is less bright, the sensor can only see the dark line when the dark band is positioned centrally above the sensor, and only the bright lines when the leading and trailing edges of the dark band move past the sensor.

[0017] Thus, in some implementations, the method determines the (overall) display brightness, e.g., by obtaining a display brightness control value from the electronic device. For example, the method can sense a signal on one or more internal control lines of the display, read a value from memory or a register, or determine the display brightness in some other way. The display brightness (e.g., display brightness control value) can then be used to determine the calibration value, e.g., by reading a calibration value stored in memory such as a lookup table, or by reading a plurality of calibration values and interpolating or extrapolating between them. The lookup table can store a set of calibration values, one for each of a corresponding set of display brightness values or ranges. Then, applying the calibration value to the difference between the first sensed brightness level and the second sensed brightness level can include scaling (e.g., multiplying or dividing) the difference by the calibration value.

[0018] In some implementations, the sensor is configured to view only a portion of the display, e.g., only one or a few rows of the display. The display can be PWM modulated so as to have one or more dark (reduced brightness) bands that move across the display. The first and second (e.g., bright and dark) brightness levels of the display can then be sensed by controlling the timing of the sensing to measure the brightness when the dark band is present above the sensor (the second brightness level) and when it is not (the first brightness level). The timing can be predetermined, e.g., fixed based on knowledge of how the display / device operates; or it can be determined dynamically, i.e., by the sensing system as the display / device is being used.

[0019] Thus, in some embodiments, the electronic device controls the display brightness by modulating the emissive display such that different regions of the emissive display are simultaneously at each of the first and second brightness levels and move across the emissive display. The method can then further comprise using the sensor to sense light from the sensed portion of the emissive display, and controlling the timing of the sensing such that the sensor senses the first and second brightness levels at different times as the different regions move over the sensed portion of the emissive display. For example, the sensing can be at times corresponding to (sensed by the sensor) the first and second brightness levels, e.g. a first time and a second time. In embodiments where the timing of the sensing is variable (whether or not at predetermined points), the method can further comprise determining the calibration value dependent on the timing. That is, the determination of the calibration value, e.g. using a look-up table, can also be dependent on the (sample) timing of the sensing.

[0020] The display can refresh each display (frame) period; this can be determined using a synchronisation signal. The ambient light sensing time over which the combined light level is integrated can encompass one or more complete display (frame) periods (the ambient light sensing time need not be an exact multiple of the display period).

[0021] In some embodiments, the first and second brightness levels of the display can be sensed in each of the (n) multiple display periods encompassed by the ambient light sensing time. The method can then further comprise determining the calibration value dependent on the number (n) of times the brightness levels of the display are sensed during the ambient light sensing time. That is, the determination of the calibration value, e.g. using a look-up table, can also be dependent on the number of times the display brightness is sensed.

[0022] In embodiments, the same sensor measurement can be used to sense the combined light level of ambient light and light from the emissive display, and to sense the first and second sensed brightness levels of the display. That is, the sensor can collect measurements of the combined light level, and some of these measurements (i.e. those representing the first and second brightness levels of the display) can be differenced to suppress the contribution of the light level from the ambient light.

[0023] In some embodiments, the method can further comprise determining a ratio of the integrated display light level to the ambient light level. This ratio can then be used as an indication of the reliability of the determination of the ambient light level. For example, the way in which the total display brightness is controlled can change dependent on the absolute brightness of the display, e.g. switching to a different PWM control technique or current control technique at high brightness. This can make the determination of the integrated display light level (and hence also the ambient light level) unreliable. This can be flagged to, for example, the software controlling the electronic device / display, e.g. the display brightness control software, to avoid unwanted brightness control behaviour.

[0024] The above techniques are particularly advantageous in electronic devices in which the emissive display has rows of pixels and the sensor is located behind the emissive display such that the sensor simultaneously senses light from multiple rows of pixels. For example, the sensor, i.e. the light sensing portion of the sensor, can be entirely behind the display, e.g. behind the emissive area of the display.

[0025] The method can further comprise determining a calibration value. This can comprise sensing (using the sensor) a combined calibration light level of ambient light and light from the emissive display in the absence of any (external) ambient lighting, i.e. with zero ambient lighting. Determining the calibration value can further comprise integrating the combined calibration light level over an ambient light sensing time to determine an integrated calibration light level. The method can further comprise determining an integrated display light level of the emissive display over the ambient light sensing time. The method can further comprise determining a ratio of the integrated calibration light level to the integrated display light level to determine the calibration value.

[0026] In another aspect, an electronic device having an emissive display, e.g. a PWM emissive display, and a sensor is described. The device can be a portable and / or battery powered electronic device, e.g. a smartphone or tablet. The device can be configured to modulate the emissive display between a first brightness level and a second brightness level to control a display brightness of the emissive display. The device can be further configured to use the sensor to sense a combined light level of ambient light and light from the emissive display. The device can be further configured to integrate the combined light level over an ambient light sensing time to determine an integrated light level. The device can be further configured to use the sensor to sense a combination of light from the emissive display when at the first brightness level and a light level of the ambient light to determine a first sensed brightness level, and to sense a combination of light from the emissive display when at the second brightness level and the light level of the ambient light to determine a second sensed brightness level. The device can be further configured to determine a difference between the first sensed brightness level and the second sensed brightness level. The device can be further configured to apply a calibration value to the difference to determine an integrated display light level. The device can be further configured to use the integrated display light level to compensate the integrated light level to determine an ambient light level.

[0027] The device can be further configured to control the display brightness in response to the display brightness control value. The device can comprise a memory storing a look-up table comprising a set of calibration values, e.g. one calibration value for each of a corresponding set of display brightness control values.

[0028] The device can also be configured to control the display brightness by controlling one or both of a power supply (e.g. current) to the emissive display and a ratio of time the emissive display is at the first and second brightness levels (e.g. using PWM techniques). The sensor can be configured to sense light from the sensed portion of the emissive display, e.g. from a group or subset of lines of the display. The electronic device can be configured to control the display brightness by modulating the emissive display such that different areas of the emissive display are simultaneously at each of the first and second brightness levels and move across the emissive display. The electronic device can control the timing of the sensing such that the sensor senses the first and second brightness levels at different times as the different areas move over the sensed portion of the emissive display, e.g. during the bright and dark intervals of the PWM brightness control.

[0029] In another aspect, there is provided a method of using an electronic device having a display and a sensor, comprising sensing a combined light level of ambient light and light from the display, determining a level of light from the display, adjusting the level of light from the display using a calibration value to determine an adjusted light level, and compensating the combined light level using the adjusted light level.

[0030] In another aspect, there is provided an electronic device, comprising a display, a sensor for sensing a combined light level of ambient light and light from the display, and a processing system configured to determine a level of light from the display, adjust the level of light from the display using a calibration value to determine an adjusted light level, and compensate the combined light level using the adjusted light level.

[0031] The electronic device can be configured to implement the above-mentioned features and aspects by controlling software of one or more processors of the device, or by dedicated hardware (e.g. electronic circuitry) which can be on one or more integrated circuits, or a combination of e.g. software and hardware.

[0032] Thus, there is also provided (dedicated) hardware, e.g. electronic circuitry, configured to implement the methods as described above.

[0033] Processor control code for implementing the systems and methods described above is also provided; that is, processor control code that, when executed by a processor (computer), causes the processor to implement the systems or perform the methods described above. The code may be provided as a signal transmitted over a network or on one or more computer-readable media, such as one or more physical data carriers, such as disks or programmable memory (e.g., non-volatile memory, such as flash memory, or read-only memory (firmware)). The code and / or data used to implement the system / method may include source, object, or executable code in a conventional programming language (interpreted or compiled) (such as C), or assembly code, or code for a hardware description language. The code and / or data used to implement the system may be distributed among multiple coupled components that communicate with each other.

[0034] The following examples illustrate these and other details of the system. Attached Figure Description

[0035] Figure 1 shows an electronic device including an ambient light sensing system.

[0036] Figure 2 The procedure for determining ambient light levels is shown.

[0037] Figure 3 The graph shows dSratio relative to display brightness.

[0038] Figure 4 The graph shows the integral ambient light level relative to the display brightness.

[0039] Figure 5 The graph shows (display light level / ambient light level) relative to display brightness.

[0040] Figure 6 The procedure for determining ambient light levels is shown.

[0041] Figure 7 The process for determining the calibration value of the calibration table is shown.

[0042] In the accompanying drawings, the same reference numerals denote the same elements. Detailed Implementation

[0043] This specification describes a system, which can be implemented by an electronic device (such as a mobile phone) with a display, to sense the brightness of ambient light. The system can be implemented by the device's processor, in dedicated hardware, or both.

[0044] In general, embodiments of the system use a sensor to measure ambient light level without the need to turn off the display, even if the sensor picks up light from the display. For example, in a BOLED (behind OLED) sensor configuration, the sensor can be located under the display.

[0045] The sensor senses the display when it is at two different brightness levels, for example when the display is PWM modulated between these brightness levels to control the average display brightness. The sensor also senses ambient light, but the difference between these two brightness levels is not sensitive to the ambient light level, which cancels out.

[0046] The actual time-integrated light level (brightness) is determined by time integrating a signal or value derived from the sensor. This signal or value depends on the combined light level of ambient light and light from the display. The integration can be over an ambient light sensing time, for example one or more display refresh periods.

[0047] The ambient light insensitive difference can be amplified by a calibration value to determine what the time-integrated display light level (brightness) would be if integrated over the same ambient light sensing time in the case of zero ambient light. The calibration value can depend on the display brightness, i.e. on the control value that controls the display brightness.

[0048] Subtracting the time-integrated display light level from the actual time-integrated light level leaves the time-integrated ambient light level, which is a measure of the ambient light level.

[0049] Using a calibration value that depends on the control value that controls the display brightness automatically takes into account many unknowns, such as timing and sensor aperture errors that affect the brightness level seen by the sensor, and details of how the device uses, for example, PWM to control the display brightness; all of which can be display brightness dependent.

[0050] Figure 1 shows an electronic device 102, such as a mobile phone or tablet, with an emissive display 104, including an ambient light sensing system 100. The emissive display 104 can comprise an OLED display screen. The device has a light sensor 106 coupled to the ambient light sensing system 100. For convenience, the ambient light sensing system 100 is shown as separate from the electronic device 102, but would typically be incorporated into the electronic device.

[0051] The light sensor 106 typically senses visible light. It can be mounted behind the display 104, as shown in Figure 1B. In this position, light blocking parts of the display stack, such as a protective barrier or metallization, can be removed locally. In some other configurations, the light sensor 106 can be located behind the bezel between the display stack and the device frame.

[0052] Ambient light sensing system 100 includes a sensing engine 110, implemented in hardware and / or software, and configured to implement the ambient light sensing functionality described later. Sensing engine 110 receives light sensor signals 120 from light sensor 106, and in embodiments obtains a display brightness control value 124 from electronic device 102, e.g., from a display drive portion of the device. Display brightness control value 124 can be provided, e.g., by a physical connection, or can be obtained from a register. The display brightness control value can be a signal defining the brightness of display 104.

[0053] Sensing engine 110 can also receive a synchronization signal 122 from electronic device 102, e.g., from a display drive portion of the device. In general, synchronization signal 122 is a signal that allows ambient light sensing system 100 to sample the signal from the light sensor when the display is at two different brightness levels, e.g., due to PWM modulation. For example, synchronization signal 122 can be a signal defining the display refresh timing, e.g., a vertical or frame synchronization signal, or a signal representing a trigger point in time for display blanking, i.e., a signal defining the time at which the display brightness is reduced from a first level to a second level.

[0054] Sensing engine 110 is coupled to a calibration value store 112, which stores calibration value data for one or more calibration values, as described later. Store 112 can be a non-volatile memory. For example, the calibration value data can be written once, e.g., into a read-only memory, when the electronic device is manufactured, or store 112 can include a flash memory or other non-volatile memory into which the calibration value data is written when the ambient light sensing system 100 is calibrated.

[0055] Ambient light sensing system 100 produces data including one or more of: ambient light level data 130, display light level data 132, and measurement reliability data 134. These data can be available on electrical connections, e.g., of an integrated circuit, and / or as readable data values in registers or memory locations.

[0056] Ambient light level data 130 can include a value indicative of the ambient light level determined by the system. Display light level data 130 can include a value indicative of the brightness level of the display determined by the system. Measurement reliability data 134 can include a value indicative of the reliability of the determined ambient light level.

[0057] Figure 2 is a graph schematically illustrating a change in brightness level on an example pulse width modulated display. In the graph, time is on the x-axis, and brightness at the location of the light sensor is on the y-axis; both are arbitrary units.

[0058] The pulse 210 is a frame sync signal, and the time period 202 represents the refresh rate of the display. The levels 204 and 208 represent the maximum and minimum levels of display brightness, respectively, and the level 206 represents the currently selected brightness level of the display. In Figure 2 the example shown, the display brightness is PWM modulated with a dark band travelling across the display. This is shown by the dip 220 in the brightness level 206.

[0059] The period 200 represents the ambient light sensing time. The signal from the light sensor representing the combined light level of ambient light and light from the display is integrated over this period. In the example shown, the period 200 extends over two display (frame) periods. The integrated combined light level is designated IntS.

[0060] The display brightness is sampled outside the dip 220 to provide a first sensed brightness level dS1, and inside the dip 220 to provide a second, reduced sensed brightness level dS2. This can be done by using the sync signal 122 to control the timing of the sampling of the signal from the light sensor. In some implementations, the timing of the samples is fixed; in other implementations, it can be controlled, for example, by an external signal of the value in a register (e.g. the memory 112).

[0061] In some implementations, each of the first sensed brightness level dS1 and the second sensed brightness level dS2 can be integrated over the sampling time (periods 212, 214, respectively, in the example shown). Figure 2 In implementations, the first and second sensed brightness levels are sensed at closely spaced short time intervals (e.g. within substantially adjacent sampling times) so that the indoor ambient light level does not change significantly due to the AC mains.

[0062] As shown, the sensor first senses the bright display during the dS1 period 212, and then the dark display during the dS2 period 214. As shown, the shape and duration (length) of the dip 220, which typically does not have vertical edges, and the physical extent of the sensor aperture mean that the light level seen when sensing the dark display depends on the display brightness, i.e. the brightness when the display is at its high PWM level rather than its low PWM level.

[0063] A delta sample value dS can be defined as the difference between the first and second brightness levels, dS = dS1 - dS2. The subtraction can be performed in the analogue or digital domain. Optionally, the delta sample value can be formed from the sum or average of multiple pairs of first and second brightness levels, e.g. dS = (dS1 1 - dS2 1) +... + (dS1 N - dS2 N) / N, where N is the number of pairs. n dS2 n). Each of the first sensed luminance level dS1 and the second sensed luminance level dS2 is a combination of the ambient light level Ambient and some percentage (n% and m%, respectively) of the maximum display luminance Display. Thus:

[0064] dS1 ∝ n% · Display + Ambient

[0065] dS2 ∝ m% · Display + Ambient

[0066] and

[0067] dS = Display. (n% - m%)

[0068] This is independent of the ambient light level.

[0069] A ratio dSratio between the integrated combined light level and the delta sample value can be defined: dSratio = IntS / dS. This depends on the ambient light level because IntS depends on the ambient light level. However, the delta sample value can also be calculated for a zero ambient light level. In this case, the light sensor sees only light from the display, and determines an integrated display light level IntD, and dSratio = IntD / dS. The value of dSratio can be determined by a calibration process.

[0070] When ambient light is present, the integrated combined light level comprises the sum of the integrated display light level and an integrated ambient light level IntA (i.e. the ambient light level integrated over the ambient light sensing time). Thus, IntS = IntD + IntA.

[0071] The IntD component of IntS can be determined from the delta sample value dS and dSratio. In principle, a single value of dSratio can be sufficient to determine IntD. However, in practice, the display luminance can be controlled by a combination of the control luminance level 206 and the duration of the fall 220. As mentioned before, there are also other factors, which means that the light level seen when sensing a dark display can depend on the display luminance, such as timing errors, sensor aperture effects, illumination from side pixels, effects due to the fact that OLEDs can take a finite time to decay, etc.

[0072] Thus, in embodiments, a dSratio calibration value is determined for each of a set of display luminance levels defined by, for example, the display luminance control values 124. For example, the memory 112 can store a calibration table defining a dSratio calibration value for each of a range of display luminance control values (e.g. spanning the controllable luminance range of the display 104).

[0073] Optionally, in cases where the ambient light sensing system has variable or configurable sampling timing of the first and second sensed luminance levels or a variable or configurable number of first and second sensed luminance levels to sample, the calibration table can also include different dSratio calibration values for each of these variable or configurable parameters.

[0074] The dSratio calibration values in the calibration table can be determined using a calibration procedure. This calibration is performed at zero ambient light level, in which case the signal from the light sensor integrated over the ambient light sensing time can be taken as IntD. In embodiments, the calibration is performed for each of the display luminance control values in the range of display luminance control values to generate a calibration table of dSratio calibration values versus display luminance control values.

[0075] Thus, to determine the integrated ambient light level IntA, the ambient light sensing system 100 measures the integrated combined light level IntS as well as the first and second luminance levels dS1 and dS2. The system then determines the delta sample value dS = dS1 - dS2 and uses the current display luminance control value to look up and retrieve dSratio from the calibration table. This is used to determine the value of IntD from IntD = dS.dSratio and the value of IntA from:

[0076] IntA = IntS - IntD

[0077] Looking up dSratio from the calibration table compensates for various previously mentioned disturbances in the sensed luminance levels.

[0078] Figure 3 A plot of dSratio on the y-axis versus display luminance on the x-axis is shown; arbitrary units. In this example, the value of dSratio is not constant but increases with increasing display luminance. At high luminance, the curve is not monotonic; this can indicate that the electronic device uses a different display luminance control technique at high display luminance.

[0079] Figure 4 A plot of the integrated ambient light level IntA on the y-axis versus display luminance on the x-axis is shown; arbitrary units. The dots indicate a series of different display colors; the ambient light level was constant at 50 Lux. It can be seen that the ambient light measurement is accurate except at the maximum display luminance.

[0080] Figure 5A plot is shown of the ratio on the y-axis (display light level / ambient light level) against display brightness (in arbitrary units) on the x-axis. The display light level can be determined from the IntD value or from the display brightness control value. When the ratio is large (e.g. greater than a threshold, in this example greater than about 3), i.e. when the display light level is much higher than the ambient light level, the ambient light measurement is inaccurate.

[0081] Thus, the ratio of the display light level to the ambient light level can be used as an indicator of the (in)accuracy of the ambient light measurement, and can provide measurement reliability data 134. The measurement reliability data can be used by the display brightness control software of the electronic device to disable brightness control when the ambient light level measurement is inaccurate, or to provide a weighting for the ambient light measurement in general.

[0082] Figure 6 A process is shown for determining the ambient light level according to the method described above. Thus, the process uses the signal from the light sensor to determine the integrated light level IntS (step 602), and potentially in parallel determines the first and sensed brightness levels dS1, dS2 (step 604). The process then obtains the display brightness control value 124, and uses it to retrieve the corresponding dSratio calibration value from the memory 112 (step 606). The process then calculates the value of IntD as described above (step 608), and from this calculates the value of IntA (step 610), which is used as a measure of the ambient light intensity. Additionally or alternatively, the value of IntD can be the output of the process. Optionally, the process can also provide the ratio of IntD to IntA as measurement reliability data 134.

[0083] Figure 7 A process is shown that can be performed at zero ambient light and for each of a range of display brightness levels to determine the dSratio calibration values for the calibration table. This process can be performed only once for a particular type or configuration of electronic device 102, or for each particular electronic device 102.

[0084] Figure 7 The process of Figure 7 first sets the display brightness control value and determines the integrated combined brightness level IntS as well as the first brightness level dS1 and the second brightness level dS2, thus determining dS = dS1 - dS2 (step 702). The process then calculates dSratio = IntS / dS (step 704), and stores this in the calibration value memory 112 in association with the set display brightness control value (step 706).

[0085] Optionally, the table can also be time data indicating when one or both of the first and second luminance levels are measured (e.g., time offset from a synchronization signal 122), and / or data indicating the number of pairs of first and second luminance levels used to determine dS. These can be used by the system to find the appropriate dSratio in systems where the timing offset and / or the number of pairs can vary.

[0086] The process then sets another display brightness control value and loops back to step 702 until the calibration table is complete. When complete, the table can store, for example, 10-100 pairs of values in the calibration value memory 112.

[0087] Embodiments of the above-described systems and methods allow for ambient light sensing from behind a display (e.g., an OLED display) and work well, providing high accuracy even in cases where the PWM display is operating at very high duty cycles (i.e., high brightness); and can also tolerate short display blanking times.

[0088] When implemented in hardware, the above-described systems and methods require less die area and can be simpler than some prior art.

[0089] System embodiments and the described calibration process together can reduce ambient light sensing errors caused by misalignment of the light sensor, device manufacturing tolerances, display degradation, and other effects.

[0090] List of Reference Characters:

[0091] 100 ambient light sensing system

[0092] 102 electronic device

[0093] 104 emissive display

[0094] 106 light sensor

[0095] 110 sensing engine

[0096] 112 calibration value memory

[0097] 120 light sensor signal

[0098] 122 synchronization signal

[0099] 124 display brightness control value

[0100] 130 ambient light level data

[0101] 132 display light level data

[0102] 134 measurement reliability data

[0103] 200 ambient light sensing time

[0104] 202 refresh rate of the display

[0105] 204 maximum level of display brightness

[0106] 206 currently selected level of brightness of the display

[0107] 208 minimum level of display brightness

[0108] 210 frame sync signal

[0109] 212 first sensed brightness level sample time

[0110] 214 second sensed brightness level sample time

[0111] 220 drop in brightness level

[0112] 602 determine integral light level IntS

[0113] 604 determine first and sensed brightness levels dS1, dS2

[0114] 606 retrieve dSratio calibration value from memory

[0115] 608 calculate value of IntD

[0116] 610 calculate value of IntA

[0117] 702 set display brightness control value and determine IntA, dS1, dS2, dS

[0118] 704 calculate dSratio

[0119] 706 store dSratio in calibration value memory with display brightness control value

[0120] Features of the methods and systems described or depicted in the examples, for example, can be implemented alone or in sub-combinations. Features from different embodiments can be combined. Thus, each feature disclosed or shown in the specification can be incorporated in the invention, alone or in any appropriate combination with any other feature disclosed or shown in the specification. Unless specifically stated otherwise, method steps should not be viewed as requiring a particular order, e.g., the order in which they are described or depicted. Systems can be configured to perform tasks by providing processor control code and / or dedicated or programmed hardware (e.g., electronic circuitry) to perform the tasks to achieve the tasks.

[0121] Aspects of the method and system have been described in terms of embodiments, but these embodiments are only illustrative of the present disclosure and not restrictive of the claims. Those skilled in the art will be able to modify and alter the described embodiments without departing from the scope of the claims.

Claims

1. A method of sensing ambient light level in an electronic device having an emissive display and a sensor, comprising: sensing a combined light level of ambient light and light from the emissive display using the sensor; integrating the combined light level over an ambient light sensing time to determine an integrated light level (IntS); determining an integrated display light level (IntD) of the emissive display over the ambient light sensing time; and compensating the integrated light level using the integrated display light level to determine an ambient light level; wherein the electronic device controls a display luminance of the emissive display by modulating the emissive display between a first luminance level and a second luminance level; and wherein determining the integrated display light level comprises: sensing a combination of light from the emissive display when at the first luminance level and a light level of the ambient light using the sensor to determine a first sensed luminance level (dS1), and sensing a combination of light from the emissive display when at the second luminance level and the light level of the ambient light to determine a second sensed luminance level (dS2); determining a difference (dS) between the first sensed luminance level and the second sensed luminance level to suppress a contribution of the light level from the ambient light; and applying a calibration value (dSratio) to the difference to determine the integrated display light level, wherein the method further comprises determining the calibration value by: sensing a combined calibration light level of ambient light and light from the emissive display using the sensor, wherein the sensing is performed in the absence of any ambient illumination; integrating the combined calibration light level over an ambient light sensing time to determine an integrated calibration light level; determining the integrated display light level of the emissive display over the ambient light sensing time; and determining a ratio of the integrated calibration light level to the integrated display light level to determine the calibration value.

2. The method of claim 1, wherein applying the calibration value comprises obtaining a display luminance control value from the electronic device, wherein the display luminance control value defines the display luminance, and determining the calibration value in dependence on the display luminance control value.

3. The method of claim 2, wherein determining the calibration value comprises reading one or more calibration values from a lookup table indexed by the display luminance control value to determine the calibration value; and wherein applying the calibration value to the difference comprises scaling the difference by the calibration value.

4. The method of claim 1, 2 or 3, wherein the electronic device controls the display luminance by modulating the emissive display such that different regions of the emissive display are simultaneously at each of the first and second luminance levels and move across the emissive display, the method further comprising using the sensor to sense light from a sensed portion of the emissive display, and controlling timing of the sensing such that the sensor senses the first and second luminance levels at different times as the different regions move over the sensed portion of the emissive display.

5. The method of claim 4, wherein the timing of the sensing is variable, the method further comprising determining the calibration value in dependence on the timing.

6. The method of claim 1, 2 or 3, wherein the electronic device is configured to refresh the emissive display at each display period, and the ambient light sensing time comprises a set of n display periods, where n > 1; wherein determining the integrated display light level comprises sensing each of the first and second luminance levels in each of a plurality of display periods; and wherein the calibration value is dependent on n.

7. The method of claim 1, 2 or 3, wherein using the sensor to sense a combined light level of ambient light and light from the emissive display comprises using the sensor to sense a combination of each of the first and second luminance levels with a light level of the ambient light to determine respective first and second sensed luminance levels.

8. The method of claim 1, 2 or 3, further comprising determining a ratio of the integrated display light level to the ambient light level, and using the ratio as an indication of a reliability of the determination of the ambient light level.

9. The method of claim 1, 2 or 3, wherein the emissive display has rows of pixels, the method further comprising positioning the sensor behind the emissive display such that the sensor senses multiple rows of pixels.

10. A computer readable medium storing processor control code for implementing the method of any of claims 1-9.

11. An electronic device having an emissive display and a sensor, wherein the electronic device is configured to: modulate the emissive display between a first luminance level and a second luminance level to control a display luminance of the emissive display; use the sensor to sense a combined light level of ambient light and light from the emissive display; integrate the combined light level over an ambient light sensing time to determine an integrated light level; use the sensor to sense a combination of light from the emissive display when at the first luminance level with a light level of the ambient light to determine a first sensed luminance level, and to sense a combination of light from the emissive display when at the second luminance level with a light level of the ambient light to determine a second sensed luminance level; determining a difference between the first sensed luminance level and the second sensed luminance level; and applying a calibration value to the difference to determine an integrated display light level; and compensating the integrated light level using the integrated display light level to determine an ambient light level, wherein the calibration value is determined by: sensing, using the sensor, a combined calibration light level of ambient light and light from the emissive display, wherein the sensing is performed in the absence of any ambient illumination; integrating the combined calibration light level over an ambient light sensing time to determine an integrated calibration light level; determining the integrated display light level of the emissive display over the ambient light sensing time; and determining a ratio of the integrated calibration light level to the integrated display light level to determine the calibration value.

12. The electronic device of claim 11, configured to control the display luminance in response to a display luminance control value; the electronic device further comprising a memory storing a look-up table comprising a set of the calibration values, one for each of a corresponding set of the display luminance control values.

13. The electronic device of claim 12, configured to control the display luminance by controlling one or both of a power supply of the emissive display and a ratio of times the emissive display is at the first luminance level and the second luminance level.

14. The electronic device of claim 11, 12 or 13, wherein the sensor is configured to sense light from a sensed portion of the emissive display; wherein the electronic device is configured to control the display luminance by modulating the emissive display such that different areas of the emissive display are simultaneously at each of the first luminance level and the second luminance level and move across the emissive display, and is further configured to control the timing of the sensing such that the sensor senses the first luminance level and the second luminance level at different times as the different areas move over the sensed portion of the emissive display.

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