Patent Publication Number: US-2023140368-A1

Title: Method and device for ambient light measurement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/144,357, filed Jan. 8, 2021, which application claims the benefit of European Application No. 20153603.4, filed on Jan. 24, 2020, which applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to electronics systems and methods, and more specifically to electronic systems that comprise an ambient light sensor and to methods of measuring ambient light with such systems. 
     BACKGROUND 
     Electronic systems such as mobile telephones or tablets comprising screens displaying information and/or images destined for a user of the system are known. 
     In such systems, the light power emitted by the screen can be at least partly adapted as a function of the level of ambient light, this level of ambient light being measured by means of an ambient light sensor (ALS). For example, this ambient light measurement is used to adjust the light power emitted by the screen as a function of the level of ambient light for a better perception of image displayed on the screen by the human eye, as well as to save energy, and thus extend the life of a battery supplying the screen. 
     In known electronic systems comprising a screen and an ambient light sensor for measuring the intensity of the surrounding or ambient light, the sensor is disposed under a protective glass covering the screen, under a dedicated opening in the screen. It would be desirable to position the sensor under the screen, without a dedicated opening in the screen, the sensor capturing the weak transmission of the light through the screen. However, it is then difficult for the sensor to distinguish with precision the light emitted by the screen in the direction of the sensor from the ambient light passing through the screen from the exterior to the sensor. 
     SUMMARY 
     There is a need to address all or some of the drawbacks of the known electronic systems comprising a screen and a light sensor for measuring the level of ambient light surrounding the system. 
     Thus, one embodiment addresses all or some of the drawbacks of the known electronic systems comprising a screen and a light sensor for measuring the level of ambient light surrounding the system. 
     In particular, one embodiment makes it possible to avoid that the level of ambient light measured by the light sensor is distorted by the light emitted by the screen. 
     One embodiment provides a method of measuring ambient light comprising:
         generating, by an ambient light sensor associated with a screen which alternates between first phases in which light is emitted by the screen, a part of which is received by the ambient light sensor, and second phases in which no light is emitted by the screen, a first signal representative of an intensity of light received by the ambient light sensor during the first and second phases;   comparing the first signal with a threshold intensity value; and controlling a timing of an ambient light measurement by the light sensor based on the comparison.       

     According to an embodiment, comparing the first signal with the threshold intensity value comprises generating a second signal having a first state when the intensity of the light received by the ambient light sensor is below the threshold intensity value, and a second state when the intensity of light received by the ambient light sensor is above the threshold intensity value, and wherein a start of the measurement is triggered by at least one transition of the second signal to the first state such that the measurement starts when the second signal is in the first state, and a duration of the measurement is controlled based on at least one duration of the first state of the second signal. 
     According to an embodiment, the measurement starts following a first transition of the second signal to the first state. 
     According to an embodiment, the measurement starts a time delay between 0.1 μs and 100 μs after the first transition. 
     According to an embodiment, the first transition triggers the start of the measurement, and a next transition of the second signal to the second state following the first transition ends the measurement. 
     According to an embodiment, the duration of the measurement is controlled based on at least one previous duration of the first state of the second signal. 
     According to an embodiment, the duration of the measurement is controlled to be shorter than the at least one previous duration of the first state of the second signal. 
     According to an embodiment, the duration of the measurement is controlled based on an average value of at least two successive previous durations of the first state of the second signal. 
     According to an embodiment, the at least one previous duration of the first state of the second signal comprises the previous duration of the first state of the second signal immediately preceding the first state of the second signal during which the measurement starts. 
     According to an embodiment, the method comprises counting a number of clock cycles with a counter during the at least one previous duration of the first state of the second signal, the duration of the measurement being controlled based on the counted number of clock cycles. 
     According to an embodiment, the duration of the measurement is equal to a duration of a first number of successive clock cycles, the first number being lower than the counted number of clock cycles. 
     According to an embodiment, the first signal is generated based on an output signal of at least one pixel of a plurality of pixels of the ambient light sensor. 
     According to an embodiment, the threshold intensity value is determined such that the second signal is in the first state during each second phase, and in the second state during each first phase. 
     One embodiment provides an ambient light sensor configured to perform the described method. 
     One embodiment provides an electronic device comprising: a screen; a screen driver configured to control the screen to alternate between first phases in which light is emitted by the screen and second phases in which no light is emitted by the screen; and the ambient light sensor defined above disposed such that a part of the light emitted by the screen is received by the ambient light sensor. 
     According to an embodiment, the ambient light sensor is disposed at a first side of the screen opposite a second side of the screen from which the light is emitted, and wherein, preferably, a transmittance of the ambient light through the screen is in a range from 0.5% to 5% when no light is emitted by the screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG.  1    illustrates, in front and cross-section views, an embodiment of an electronic device; 
         FIG.  2    illustrates, in front and cross-section views, a further embodiment of an electronic device; 
         FIG.  3    schematically illustrates, in the form of blocks, a light sensor of the device of  FIG.  1  or  2   ; 
         FIG.  4    is a timing diagram illustrating a mode of operation of the sensor of  FIG.  3    according to an embodiment; 
         FIG.  5    is a timing diagram illustrating a mode of operation of the sensor of  FIG.  3    according to a further embodiment; 
         FIG.  6    is a timing diagram illustrating a mode of operation of the sensor of  FIG.  3    according to yet a further embodiment; 
         FIG.  7    schematically illustrates, in the form of blocks, a circuit according to an example embodiment; and 
         FIG.  8    is a timing diagram illustrating a mode of operation of the circuit of  FIG.  7   . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties. 
     For the sake of clarity, only the operations and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. 
     Unless indicated otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements. 
     In the following disclosure, unless indicated otherwise, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “higher”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures. 
     Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%. 
     In the following disclosure, electronic systems are considered in which the screen operates by alternating phases in which the screen emits light and phases in which the screen is turned off, i.e. the screen emits no light. In such systems, the average light power emitted by the screen and perceived by a user is adapted by modifying the duty cycle and/or the frequency of the screen activation, for example by adjusting the duration of the phases of light emission and/or the duration of the phases in which no light is emitted. With adequate switching frequencies between the phases in which the screen emits light and the phases in which the screen is turned off, the user of the screen does not perceive the transitions between these phases, due to the persistence of vision of the human eye. For example, the switching frequency is at least 25 Hz. 
     For instance, the screen is controlled by a binary control signal, a first state of which causes a phase of light emission by the screen, and a second state of which causes a phase in which no light is emitted by the screen. This control signal generally undergoes pulse-width modulation (PWM) or pulse-frequency modulation (PFM). The type of screen, for example LCD (Liquid Crystal Display) or OLED (Organic Light Emitting Diode), to which such control modes apply and the manner of implementation of these control modes are not described in detail. The described embodiments are compatible with these known control modes and the known screens to which these control modes apply. 
       FIG.  1    illustrates two views A and B of an embodiment of an electronic device  2000 , in this example a mobile telephone  2000 , the view A being a front view of the telephone  2000  and the view B being a cross-section view along the plane BB indicated in view A. 
     The device  2000  comprises an electronic system or circuit  1000 . The electronic circuit  1000  comprises a screen  100  configured to display images and/or information destined for a user. The display screen, or panel,  100  comprises a matrix of light emitting pixels (not illustrated). 
     The system  1000  further comprises various electronic circuits including an ambient light sensor  104 . In the example shown in  FIG.  1   , in the view B, two further electronic circuits, namely a processing unit  106  and a driver or control circuit  108  of the screen  100 , are illustrated. 
     The various electronic circuits of the system  1000  are, for example, mounted on a printed circuit board (PCB) no, preferably a flexible printed circuit board, in order to be electrically coupled with one another via the board  110 . Although a single board  110  is illustrated in the view B shown in  FIG.  1   , the system  1000  can comprise a plurality of boards  110 , possibly electrically coupled with one another via ribbon cables. 
     For instance, the display screen  100  can be of the OLED type (Organic Light Emitting Diode). The screen  100  is thus, for example, controlled by a binary control signal, for example generated by the driver  108 . This control signal is, for example, provided selectively to each diode of the screen, so as to cause alternate phases in which at least certain diodes of the screen  100  emit light and phases in which no diode of the screen  100  emits light. The selection of the diodes of the screen  100  receiving or not receiving the control signal is, for example, implemented by the driver  108 . In certain cases, the driver  108  can further adapt, for each diode, the voltage level of the binary signal corresponding to a phase of light emission so as to adapt the light power emitted by the diode. Each pixel of the screen can consist of one or more diodes, possibly covered by an RGB (Red, Green, Blue) color filter. 
     For instance, the display screen  100  can also be of the LCD type (Liquid Crystal Display). The screen  100  thus comprises, for example, a matrix of pixels each comprising polarizing liquid crystal filters, and an illuminating plate or panel disposed under the matrix of pixels. The plate is, for example, controlled by a binary control signal, for example generated by the driver  108 , so that the plate operates by alternating phases of light emission and phases in which the plate does not emit any light. In certain cases, the driver  108  can further adapt the voltage level of the binary signal corresponding to a phase of light emission so as to adapt the light power emitted by the plate. The polarizing filters of each pixel are controlled, for example by the driver  108  of the screen  100 , to allow or prevent the light emitted by the plate to pass through the polarizing filters, towards a user. Each pixel of the screen can be covered by one or more RGB color filters. 
     In the illustrated example, the system  1000  further comprises, above the display screen  100 , a touch screen or touch plate  112 . The touch screen  112  entirely covers the display screen  100 , the screens  100  and  112  having substantially the same surface areas, preferably the same surface areas. 
     Typically, the device  2000  comprises a protective glass pane  114  covering the screen  100 , and, more specifically in this example, the assembly comprising the two screens  100  and  112 . The glass pane  114  entirely covers the screen  100 , the surface area of the glass pane  114  being substantially equal to that of the screen  100 , preferably equal to that of the screen  100 . 
     The device  2000  comprises a housing, or shell,  116 , in which the system  1000  is disposed, i.e. in which the electronic circuits  104 ,  106  and  108  and the one or more boards  110  are disposed. The assembly of the screen  100 , the optional touch screen  112  and the glass pane  114  closes the housing  116  on the side of a face of the system, the upper face in the view B of  FIG.  1   , and the face that is visible in the view A of  FIG.  1   . 
     In this embodiment, the telephone  2000  is called “borderless”, i.e. the screen  100 , and more specifically the assembly of the screen  100 , the optional touch screen  112  and the glass pane  114 , occupies substantially the entire face, preferably the entire face, of the device intended to be viewed by the user of the system, i.e. the upper face of the device  2000  in the view B of  FIG.  1   . The ambient light sensor  104  is thus disposed under the screen  100 , i.e. on the side of the screen  100  opposite the face of the screen  100  from which light is emitted by the screen. The display screen  100 , the optional touch screen  112  and the glass pane  114  are thus at least partially transparent to the ambient light, the ambient light for example corresponding here to the visible light and possibly to infra-red and/or ultra-violet light. Thus, ambient light can pass through the assembly of the glass pane  114 , the optional touch screen  112  and the display screen  100 , and reach the sensor  104 . 
       FIG.  2    illustrates two views A and B of a further embodiment of an electronic device  3000 , in this example a mobile telephone  3000 , the view A being a front view of the telephone and the view B being a cross-section view along the plane BB indicated in view A. 
     The device  3000  of  FIG.  2    differs from the device  2000  of  FIG.  1    in that the display screen  100  and the optional touch screen  112  are interrupted above the sensor  104  in order to allow the ambient light to reach the sensor  104 . More specifically, a window, or a notch,  118  is provided through the screen  100  and the optional touch screen  112 , above the sensor  104 . The glass pane  114  covers the window  118  so as to protect the electronic circuits disposed in the housing  116 , and in particular the sensor  104 . 
     For example, in the devices  2000  and  3000 , during a phase in which the screen is turned off, and, thus, no light is emitted by the screen, the transmittance of the light through the assembly of the glass pane  114 , the optional touch screen  114  and the display screen  112  is in the range from 0.5% to 5%. 
     It should be noted that the devices  2000  and  3000  are illustrated in a very schematic fashion, and that not all details of these devices have been illustrated. The embodiments that will be described in the following are not limited to the example devices shown in  FIGS.  1  and  2   , but apply to all electronic devices comprising an electronic system  1000 , for example tablet computers, connected watches, computer screens, mobile telephones, multimedia apparatus equipped with, for example, a flexible or foldable screen, etc. More specifically, the described embodiments apply to electronic systems  1000  comprising a display screen  100  and an ambient light sensor  104  disposed under the screen  100  as illustrated in  FIG.  1   , or under a window, or opening,  118  of the screen  100  as illustrated in  FIG.  2   , in which the screen  100  operates by alternating phases of light emission and phases in which no light is emitted. 
       FIG.  3    schematically illustrates, in the form of blocks, the light sensor  104  of the device of  FIG.  1  or  2   . 
     The ambient light sensor  104  for example comprises a light sensitive area  1041  comprising at least one pixel (not shown) receiving light. Preferably, the area  1041  comprises more than one pixel. Each pixel of the area  1041  provides an output signal  1045 . 
     For instance, the output signal  1045  of a pixel is an analog signal, the value of which being for example representative of a value of a photocurrent generated by a photodiode of the pixel. Although this is not shown, the output signal of the pixel is for example provided by a transimpedance amplifier; a source follower transistor having its gate coupled to a buffering capacitor in which a voltage representative of the charges photogenerated in the photodiode are stored; or a charge integrator converting the photogenerated charges in the photodiode into a rising voltage. The output signal could also be provided by a readout circuit of the pixel as for example described in the application U.S. Pat. No. 9,927,291, the content of which being hereby incorporated by reference in its entirety. 
     The output signal  1045  of a pixel could also be a binary signal having a pulse each time a photon received by a single-photon avalanche diode (SPAD) of the pixel triggers an avalanche phenomenon, or a voltage having a value representative of the number of pulses generated for a given time period. 
     The light sensor  104  comprises a read circuit  1043 . The read circuit  1043  is configured to receive the output signals  1045  of the pixels of the area  1041 . In this embodiment, the read circuit  1043  is further configured to provide, or generate, an output signal OUT of the sensor  104 , based on the signals  1045 . The signal OUT, which is for example a digital signal comprising a plurality of bits, is representative of the amount of light received by the area  1041  during a given measurement phase, and thus of the amount of light received by the sensor  104  during this given measurement phase. 
     For instance, all the pixels of the sensor  104  are configured to receive light in a same and single wavelength range, and the signal OUT is representative of the amount of light received during a measurement phase in this single wavelength range. 
     The pixels of the sensor  104  could also be separated in a plurality of sets of pixels such that, in each set of pixels, the pixels of the set are configured to receive light only in a given corresponding range of wavelengths, different from the ranges of wavelength of the other sets of pixels, for example by associating each pixel of each set of pixels with corresponding filters adapted to the range of wavelengths of this set of pixels. The signal OUT can thus be representative of the amount of light received during a measurement phase, in each of the plurality of wavelength ranges corresponding to the plurality of sets of pixels. With such a signal OUT, the system  1000  ( FIG.  1  or  2   ) can be configured to determine the type of ambient light, for example if the light is natural, from a filament light bulb, from a fluorescent light bulb, if the light is a cold or warm light, etc., based on the spectral repartition of the light between each of the plurality of wavelength ranges. Furthermore, in the case where the screen  100  is a colour screen of the OLED type, the processing unit  106  and/or the driver  108  ( FIG.  1  or  2   ) can thus be configured to control the screen  100  such that, for each wavelength range that the screen  100  can emit, the screen  100  receives an indication of the average target power that the screen  100  needs to emit for this wavelength range. Indeed, in the case of an OLED colour screen, the circuit  108  is generally configured to control each pixel of the screen individually. As a result, the system  1000  can thus adapt the type of light emitted by its screen  100  to the type of ambient light which surrounds the system  1000 . 
     The read circuit  1043  is further configured to provide, or generate, a signal L_int based on the signals  1045  coming from at least certain pixels of the area  1041 , the signal L_int being preferably an analog signal. The signal L_int is representative of the intensity of light received by the sensor  104 , and, more precisely, by the area  1041  of the sensor  104 , during the sensor  104  operation. In other words, the value of the signal L_int changes when the intensity of the light received by the sensor  104  changes. For example, the value of the signal L_int is updated such that a time period between each two successive values of the signal L_int is at least 10 times, preferably 100 times, shorter than the minimal time duration of the phases in which no light is emitted by the screen  100 . 
     For instance, in the case where each signal  1045  is a photocurrent generated by one of the photodiodes of the pixels of the area  1041 , the value of signal L_int can correspond to a mean value of the values of the signals  1045 . 
     For instance, in the case where each signal  1045  presents a pulse for each avalanche phenomenon occurring in a SPAD of the pixel corresponding to the signal  1045 , the value of the signal L_int can be representative of the number of pulses received by the read circuit  1043  for a given time duration among successive periodic time durations. In such a case, the value of the signals  1045 , and thus the value of the signal L_int, is updated at the end of each of these successive periodic time durations. 
     The read circuit  1043  is under the control of a binary control signal MES, such that, for each ambient light measurement phase, a start and an end of the measurement phase, and thus its duration, are determined based on the signal MES. The circuit  1043  comprises an input configured to receive the signal MES. Each time the signal MES is in a first state, for example a high state, an ambient light measurement phase is performed by the sensor  104 . The start of each ambient light measurement is triggered by a transition of the signal MES to its first state, and the end of the ambient light measurement is triggered by a next transition of the signal MES to its second state, for example a low state. 
     The sensor  104  further comprises a control circuit  1047 . The control circuit  1047  comprises an input configured to receive the signal L_int, and an output configured to provide the signal MES. The control circuit  1047  is configured to generate, or provide, the signal MES based on the signal L_int. Thus, the circuit  1047  controls, by mean of signal MES, timings of each ambient light measurement phase, that is to say the start and the end of the ambient light measurement phase. The circuit  1047  is configured such that the ambient light measurement occurs during a phase in which no light is emitted by the screen  100 . 
     For example, the circuit  1047  is configured to compare the value of the signal L_int with a threshold intensity value th. The circuit  1047  is, for example, further configured to generate a binary signal COMP representative of the result of the comparison of the signal L_int with the threshold intensity value th. The threshold intensity value th is chosen such that the state of the signal COMP indicates whether the screen  100  is emitting light or not. 
     A first state, for example a high state, of the signal COMP indicates that the value of intensity of the light received by the sensor  104  is below a threshold determined based on the threshold intensity value th, the signal COMP being in its first state when, for example, the signal L_int is below the threshold th. A second state, for example a low state, of the signal COMP indicates that the value of intensity of the light received by the sensor  104  is above the threshold determined based on the threshold intensity value th, the signal COMP being in its second state when, for example, the signal L_int is above the threshold th. The threshold intensity value th is chosen such that the first state of the signal COMP indicates that no light is emitted by screen  100 , and the second state of the signal COMP indicates that screen  100  is emitting light and that a part of this emitted light is received by the sensor  104 . 
     For example, the circuit  1047  comprises a circuit, or comparator,  1049  configured to compare the signal L_int with the threshold intensity value th, and generate the COMP signal accordingly. For example, the comparator  1049  comprises a first input, for example a negative input (−), configured to receive the signal L_int, a second input, for example a positive input (+), configured to receive the threshold intensity value th, and an output configured to generate the COMP signal. 
     The circuit  1047  is then configured to generate the signal MES based on the signal COMP, such that an ambient light measurement is performed during a corresponding phase in which no light is emitted by the screen  100 . By doing this, value of the measured ambient light is not distorted by the light emitted by the screen  100  to the sensor  104  during the phases in which light is emitted by the screen  100 . 
       FIG.  4    is a timing diagram illustrating a mode of operation of the sensor  104  of  FIG.  3    according to an embodiment. In the embodiment of  FIG.  4   , the signal MES is identical to the signal COMP. 
     At an instant to, the screen  100  is controlled to be in a phase ON in which light is emitted by screen  100 . As a part of the emitted light is received by the sensor  104 , the signal L_int has a value superior to the threshold intensity value th. Thus, the signal COMP and the signal MES, which are here identical to each other, are in their second states, in this example low states. 
     The screen  100  is in the ON-phase until an instant t 1  posterior to the instant to. At the instant t 1 , the screen  100  is switched to a phase OFF in which no light is emitted by the screen  100 . As no light emitted by the screen can be received by the sensor  104 , the value of the signal L_int decreases and becomes inferior to the threshold intensity value th. Thus, the signals COMP and MES transition to their first states, in this example high states. 
     The screen  100  is in the OFF-phase until an instant t 2  posterior to the instant t 1 . At the instant t 2 , the screen is switched to an ON-phase. The value of the signal L_int increases and becomes superior to the threshold intensity value th. Thus, the signals COMP and MES transition to their second states. 
     The screen  100  is in the ON-phase until an instant t 3  posterior to the instant t 2 . At the instant t 3 , the screen is switched to an OFF-phase, the signal L_int drops below the threshold intensity value th, and the signals COMP and MES transition to their first states. 
     The screen  100  is in the OFF-phase until an instant t 4  posterior to the instant t 3  At the instant t 4 , the screen is switched to an ON-phase, the signal L_int goes above the threshold intensity value th, and the signals COMP and MES transition to their second states. 
     The screen  100  is in the ON-phase until an instant t 5  posterior to the instant t 4 . At the instant t 5 , the screen is switched to an OFF-phase, the signal L_int drops below the threshold intensity value th, and the signals COMP and MES transition to their first states. 
     In the embodiment of  FIG.  4   , each transition of the signal MES to its first state triggers a start of a corresponding ambient light measurement phase (instants t 1 , t 3  and t 5 ), and each transition of the signal MES to its second state ends a corresponding ambient light measurement phase (instants t 2  and t 4 ). In other words, each ambient light measurement is performed from a transition of the signal COMP to its first state, until the immediately following or next transition of the signal COMP to its second state. 
     Thus, in the embodiment of  FIG.  4   , the duration of an ambient light measurement phase triggered by a given transition of the signal MES to its first state is determined by a corresponding duration of the first state of the signal MES, that is to say the duration of the first state of the signal MES following this given transition. In this way, each ambient light measurement phase is performed during a corresponding OFF-phase. 
     Although three successive ON-phases, which alternate with three successive OFF-phases, have been represented in  FIG.  4   , the sensor  104  is configured to operate in the manner described above whatever the number of alternating ON-phases and OFF-phases. 
       FIG.  5    is a timing diagram illustrating a mode of operation of the sensor  104  of  FIG.  3    according to a further embodiment. 
     More particularly, in this embodiment, it is considered that, when the screen  100  switches from an ON-phase to an OFF-phase, due to the afterglow of the screen  100 , the intensity of the light emitted by the screen decreases progressively between a high value at the end of the ON-phase and a null value, which is reached during the OFF-phase. Thus, the transition of the signal COMP may occur even when the intensity of the light emitted by the screen is not yet null. 
     In this embodiment, an ambient light measurement phase starts a given time delay Ts after a transition of the signal COMP to its first state, the time delay Ts being chosen such that, at the start of the ambient light measurement, the intensity of light emitted by the screen  100  is null. Thus, the time delay is determined based on the duration of the afterglow of the screen. For example, the time delay Ts is in the range 0.1 μs to 100 μs. 
     In  FIG.  5   , as in  FIG.  4   , at the instant t 0 , the screen  100  is in an ON-phase, the signal L_int being thus above the threshold intensity value th, and the signals COMP and MES being in their second states. 
     In  FIG.  5   , as in  FIG.  4   , at the instant t 1 , the screen  100  is switched to an OFF-phase. The intensity of the light emitted by the screen  100  then decreases progressively to reach a null value at an instant t 1 _ 2  posterior to the instant t 1 . As a consequence, the signal L_int decreases from the instant t 1  to the instant t 1 _ 2 , and drops below the threshold intensity value th at an instant t 1 _ 1  between the instants t 1  and t 1 _ 2 . It results that signal COMP transitions to its first state at the instant t 1 _ 1 , whereas the intensity of the light emitted by the screen  100  is not yet null. However, the signal MES is transitioned to its first state, by circuit  1047 , the time delay Ts after the instant t 1 _ 1 , at an instant t 1 _ 3  posterior to the instant t 1 _ 2  and equal to t 1 _ 1 +Ts. Thus, when an ambient light measurement phase starts at the instant t 1 _ 3  because of the transition of the signal MES to its first state, the intensity of the light emitted by the screen is null. 
     At the instant t 2  posterior to the instant t 1 _ 3 , the screen  100  is switched to an ON-phase, the signal L_int goes above the threshold intensity value th, and the signals COMP and MES transition to their second states. 
     At the instant t 3  posterior to the instant t 2 , the screen  100  is switched to an OFF-phase. The sensor  104  then operates at successive instants t 3 _ 1 , t 3 _ 2  and t 3 _ 3  in a manner identical to that described in relation with the respective instants t 1 _ 1 , t 1 _ 2  and t 1 _ 3 , the instant t 3 _ 3  being thus equal to t 3 _ 1 +Ts. 
     At the instant t 4  posterior to the instant t 3 _ 3 , the screen  100  is switched to an ON-phase, the signal L_int goes above the threshold intensity value th, and the signals COMP and MES transition to their second states. 
     At the instant t 5  posterior to the instant t 4 , the screen  100  is switched to an OFF-phase. The sensor  104  then operates at successive instants t 5 _ 1 , t 5 _ 2  and t 5 _ 3  in a manner identical to that described in relation with the respective instants t 1 _ 1 , t 1 _ 2  and t 1 _ 3 , the instant t 5 _ 3  being thus equal to t 5 _ 1 +Ts. 
     Although three successive ON-phases, which alternate with three successive OFF-phases, have been represented in  FIG.  5   , the sensor  104  is configured to operate in the manner described above whatever the number of alternating ON-phases and OFF-phases. 
     In the embodiments of  FIGS.  4  and  5   , each transition of the signal COMP to its first state triggers, possibly after the time delay Ts, a corresponding ambient light measurement, and the next transition of the signal COMP to its second state ends the measurement phase. Thus, the duration of the measurement phase is determined by the duration of the first state of the signal COMP following the transition of the signal COMP which triggers the measurement phase. 
     In alternative embodiments, each transition of the signal COMP to its first state triggers the start of a corresponding ambient light measurement phase, but the duration of this measurement phase is controlled based on at least one duration of the first state of the signal COMP occurred before this measurement phase. In other words, the duration of this measurement phase is controlled based on at least one duration of the first state of the signal COMP that occurred before the transition of the signal COMP that triggered this measurement phase. 
       FIG.  6    is a timing diagram illustrating a mode of operation of the sensor  104  of  FIG.  3    according to a further embodiment, in which the duration of an ambient light measurement phase is determined based on at least one previous duration of the first state of signal COMP, in this example by the immediately preceding duration of the first state of signal COMP. 
     In  FIG.  6   , as in  FIGS.  4  and  5   , at the instant to, the screen  100  is in an ON-phase, the signal L_int being thus above the threshold intensity value th, and the signals COMP and MES being in their second states. Furthermore, an information representative of the duration T 0  (not shown on  FIG.  6   ) of the last first state of the signal COMP before the instant to has been stored by the circuit  1047 , for example in a memory of the circuit  1047 . 
     At the instant t 1 , the screen  100  is switched to an OFF-phase, and the signal L_int decreases progressively to reach a null value at the instant t 1 _ 2 , the signal L_int dropping below the threshold intensity value th at the instant t 1 _ 1  between the instants t 1  and t 1 _ 2 . 
     Because the signal L_int drops below the threshold th at the instant t 1 _ 1 , the signal COMP transitions to its first state at the instant t 1 _ 1 . The transition of the signal COMP to its first state causes the signal MES to transition to its first state, in this example at the instant t 1 _ 3  separated from the instant t 1 _ 1  by the time delay Ts. Furthermore, the signal MES is maintained at its first state for a duration To′ inferior to the duration To, such that the signal MES transitions to its second state, while the screen  100  is still in the OFF-phase. Thus, during the OFF-phase between the instants t 1  and t 2 , the duration of the measurement phase performed is thus T 0 ′. 
     In this example, the duration To′ is equal to To−Ts−Te, Te being a given time duration. The time duration Te is chosen such that, at the end of the time duration To′, when the signal MES is transitioned to its second state at an instant t 1 _ 4  equal to t 1 _ 3 +To′, the screen  100  is still in the OFF-phase. 
     The subtraction of the duration Te from the previous duration To allows to ensure that the ambient light measurement phase ends before the next ON-phase in which light is emitted by screen, even in case where the signal COMP transition to its second state with a delay compared to the instant at which the signal L_int goes above the threshold intensity value th. 
     Moreover, between the instants t 1  and t 2 , the circuit  1047  obtains or generates an information representative of a duration T 1  of the first state of the signal COMP between these instants, and stores this information. 
     At the instant t 3 , the screen  100  is switched to an OFF-phase, and the signal L_int decreases progressively to reach a null value at the instant t 3 _ 2 , the signal L_int dropping below the threshold intensity value th at the instant t 3 _ 1  between the instants t 3  and t 3 _ 2 . Thus, the signal COMP transitions to its first state at the instant t 3 _ 1 , and the signal MES is transitioned to its first state, in this example at the instant t 3 _ 3  equal to t 3 _ 1 +Ts. The signal MES is then maintained at its first state for a duration T 1 ′ inferior to the duration T 1 , at least by the time duration Te, and, more precisely in this example, for a time duration T 1 ′ equal to T 1 −Ts−Te. The signal MES is transitioned to its second state at an instant t 3 _ 4  equal to t 3 _ 3 +T 1 ′, while the screen  100  is still in the OFF-phase, which ends at the instant t 4  posterior to the instant t 3 _ 4 . Thus, during the OFF-phase between the instants t 3  and t 4 , the duration of the measurement phase performed is T 1 ′. 
     Moreover, information representative of a duration T 2  of the first state of signal COMP between the instants t 3 _ 1  and t 4  is stored by the circuit  1047 . 
     At the instant t 5 , the screen  100  is switched to an OFF-phase, and the signal L_int decreases progressively to reach a null value at the instant t 5 _ 2 , the signal L_int dropping below the threshold intensity value th at the instant t 5 _ 1  between the instants t 5  and t 5 _ 2 . Thus, the signal COMP transitions to its first state at the instant t 5 _ 1 , and the signal MES is transitioned to its first state, in this example at the instant t 5 _ 3  equal to t 5 _ 1 +Ts. The signal MES is then maintained at its first state for a duration T 2 ′ inferior to the duration T 2 , at least by the time duration Te, and, more precisely in this example, for a time duration T 2 ′ equal to T 2 −Ts−Te. The signal MES is transitioned to its second state at an instant (not shown) equal to t 5 _ 3 +T 2 ′, while the screen  100  is still in the OFF-phase. Thus, the duration of the measurement phase performed during the OFF-phase starting at the instant t 5  is T 2 ′. 
     Moreover, information representative of a duration T 3  of the first state of signal COMP starting at the instant t 5 _ 1  is stored by the circuit  1047 . 
     Although three successive ON-phases, which alternate with three successive OFF-phases, have been represented in  FIG.  6   , the sensor  104  is configured to operate in the manner described above whatever the number of alternating ON-phases and OFF-phases. 
     In an alternative embodiment, the duration of an ambient light measurement phase following a given transition of the signal COMP to its first state is determined based on several previous durations of the first state of the signal COMP. For example, the duration T 2 ′ of the ambient light measurement phase following the transition of signal COMP to its first state at the instant t 5  is determined based on the durations T 0 , T 1  and T 2 , the duration T 2 ′ being for example equal to Tmean−Te−Ts, with Tmean the mean value of durations T 0 , T 1 , and T 2 . 
     In the embodiment of  FIG.  6   , the information representative of a duration of the first state of the signal COMP is for example output by a counter, which is configured to count a number of cycles of a periodic signal, for example, a clock signal, while the signal COMP is in its first state. The period of the periodic signal is, for example, at least 10 times shorter than the minimal duration of an OFF-phase, preferably at least 100 times shorter than the minimal duration of an OFF-phase. The duration of a measurement phase could then correspond to the duration of a given number of successive cycles of the periodic signal, for example a number of successive cycles equal to the number of cycles counted during the previous first state of signal COMP, from which a number of cycles corresponding to the time duration Te, and, possibly, a number of cycles corresponding to the time duration Ts have been subtracted. Thus, when signal COMP switches to its first state, the signal MES is switched to its first state after a number of cycles corresponding to the time duration Te, the signal MES being then switched to its second state at the end of the duration of the measurement phase, which was determined as described above. 
     In the embodiment of  FIG.  6   , the signal MES is for example generated by using at last one phase-locked loop of the circuit  1047 . 
     For example, one phase-locked loop is locked on the transitions of the signal COMP to its first state, and one phase-locked loop is locked on the transitions of the COMP signal to its second state, the output signal of the two phase-locked loops being used to generate a corresponding signal MES. In this case, the information representative of the duration of the first state of the signal COMP corresponds to a phase difference between the two outputs of the phase-locked loops, and is stored inside the circuit  1047  by the phase-locked loops themselves. 
     As another example, a single phase-locked loop is locked on transitions of the signal COMP to its second state. For example, the phase locked loop comprises a voltage-controlled oscillator, the output signal of which being delayed by the time duration Te, and a phase detector outputting a signal representative of the phase difference between transitions of the delayed signal to its second state and transitions of the COMP signal to its second state. The output signal of the phase detector is used, possibly after being filtered by a low-pass filter, to control the voltage-controlled oscillator, thus to control the phase and/or the frequency of the output signal of the voltage-controlled oscillator. The transitions of the signal COMP to its first state are used, preferably after being delayed by the time duration Ts, to switch the signal MES to its first state, and the transitions of the output signal of the voltage-controlled oscillator to its second state are used to switch the signal MES to its second state. For example, the delayed signal COMP is provided to a set input of a RS type latch, the output signal of the voltage-controlled oscillator is provided to a reset input of the latch, and the signal MES is provided by an output of the latch. 
       FIG.  7    illustrates, in the form of blocks, a circuit configured to generate the signal MES based on the COMP signal using a single phase-locked loop, according to an example embodiment of the type described above. 
     The phase-locked loop PLL comprises a voltage-controlled oscillator VCO, a delay circuit eD, a phase shift detector circuit PSD, and, in this example, a low-pass filter LPF. 
     The oscillator VCO provides a MESe signal to the delay circuit eD. 
     The delay circuit eD applies a delay equal to the time duration Te to the signal MESe, the resulting delayed signal MESed being provided by the circuit eD. 
     The circuit PSD receives the MESed signal and the COMP signal and provides a signal PSDo representative of the phase shift between signals MESed and COMP. 
     The circuit VCO is controlled based on the signal PSDo such that, in stationary operation, the switching of the signal MESed to its second state are synchronized with the switching of the COMP signal to its second state. More particularly, in this example, the signal PSDo is provided to the filter LPF, and the resulting filtered signal fPSDo is the control signal of the circuit VCO. 
     Moreover, the COMP signal is delayed by the time duration Ts by a delay circuit sD which provides a delayed signal MESs. 
     The signal MES is then generated based on the MESe and MESs signals. In this example, this is done using a RS type latch RS, an output of which providing the MESe signal. A reset input of the latch RS receives the signal MESe inverted by an inverter circuit INV, a set input of the latch RS receiving the signal MESs. 
       FIG.  8    is a timing diagram illustrating a mode of operation of the circuit of  FIG.  7   .  FIG.  8    shows the evolution of signals COMP, MESs, MESe, MESed and MES. 
     As it can be seen, the signal MES switches to its first state when the signal MESs, thus the signal COMP delayed by the time duration Ts, switches to its first state, and switches to its second state when the signal MESe switches to its second state. Thus, the signal MES switches to its first state with a delay equal to the duration Te after a corresponding switching of the COMP signal to its first state, and to its second state with a time advance equal to the time duration Te before a corresponding switching of the signal COMP to its second state. 
     In another alternative embodiment, a filtering function is applied to the value of each duration of the first state of the signal COMP to remove values that are relatively far from the previous values, for example to remove a value that differs from at least one previous value by more than 10%. This alternative embodiment is compatible with all the embodiments described in relation with  FIG.  6   . 
     In another alternative embodiment, each transition of the signal COMP to its first state is not delayed by the time duration Ts after a corresponding transition of the signal MES to its first state. Thus, in this case, the time duration Ts is not subtracted from the previous duration of the first state of signal COMP. This alternative embodiment is compatible with all the embodiments described in relation with  FIG.  6   . 
     In another alternative embodiment, a value representative of the amount of light received by the sensor  104  during a given ambient light measurement phase is weighted by the duration of this measurement phase, for example, by dividing this value by the duration of the measurement phase or by a number of cycles of a periodic signal representative of this duration. The signal OUT provided at the end of the measurement is then representative of the weighted value. This allows the ambient light to be measured without being sensitive to the fact that different ambient light measurement phases could have different durations. This alternative embodiment is compatible with all the embodiments described in relation with  FIGS.  4 ,  5  and  6   . 
     In another alternative embodiment, the circuit  1047  is further configured to detect when a current ambient light measurement phase ends during an ON-phase of the screen  100 , for example by comparing the states of signals COMP and MES and detecting when the signal MES is at its first state while the signal COMP is at its second state. The result of this detection can be output by the sensor  104 , for example in order to validate or invalidate an output value of the signal OUT. The generation of an output value of the signal OUT could also be conditioned by the results of this detection, in order to ensure that only values of the signal OUT corresponding to ambient light measurements integrally performed during corresponding OFF-phases are output. This alternative embodiment is compatible with all the embodiments described in relation with  FIG.  6   . 
     As an example, in the embodiments described above, each ON-phase has a duration comprised between 1 ms and 10 ms, and each OFF-phase has a duration comprised between 50 μs and 500 μs. 
     The embodiments described above in relation with  FIGS.  3 ,  4 ,  5 ,  6 ,  7  and  8    allow to synchronize an ambient light measurement phase with a corresponding OFF-phase of a screen alternating between OFF-phases and ON-phases. This synchronization is done without the use of a signal controlling the switching of the screen between the OFF-phases and the ON-phases, or a signal derived from this control signal. Compared to the embodiments described above, the use of such a signal would require an additional input pin for the sensor  104 , and would lead to issues because of the unknown and uncontrolled time delay occurring in these signals. 
     It is particularly interesting to implement the above-described embodiments in electronic devices comprising an ambient light sensor  104  disposed below a screen  100  and without a notch or window above the light sensor  104 . However, the embodiments described here could also be implemented in the case where the screen  100  comprises such a notch or window above the sensor  104 . Indeed, even with such a notch or window, an ambient light measurement at least partly performed during an ON-phase of the screen will be distorted by the light emitted by the screen  100 . 
     Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art. In particular, when it has been indicated that the first state, respectively the second state, of a binary signal corresponds to a high state, respectively to a low state, of this signal, those skilled in the art are capable of adapting the described embodiments to the case where the first state, respectively the second state, of this signal corresponds to the low state, respectively to the high state, of this signal. Furthermore, although the value of the signal L_int described above increases, respectively decreases, when the intensity of the light received by the sensor  104  increases, respectively decreases, those skilled in the art are capable of adapting the described embodiments to the case where the value of the signal L_int described above increases, respectively decreases, when the intensity of the light received by the sensor  104  decreases, respectively increases. 
     Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional description provided hereinabove. In particular, the implementation of the read circuit  1043  and/or the implementation the control circuit  1047  and/or the choice of the value Ts and/or the choice of the value Te are within the capabilities of those skilled in the art based on the functional description provided hereinabove.