WO2025190830A1

WO2025190830A1 - Method of operating an optical proximity sensor and computer device for execution of the method

Info

Publication number: WO2025190830A1
Application number: PCT/EP2025/056382
Authority: WO
Inventors: Helmut Theiler; Josef Kriebernegg; Whitney HERNANDEZ
Original assignee: Ams Osram AG
Current assignee: Ams Osram AG
Priority date: 2024-03-11
Filing date: 2025-03-10
Publication date: 2025-09-18
Anticipated expiration: 2026-09-11
Also published as: TW202603393A

Abstract

A method of operating an optical proximity sensor (6) of a computer device having a display (2), where the optical proximity sensor (6) is located beneath or otherwise adjacent to the display (2), the method comprising obtaining a vertical synchronization signal from a display driver and synchronizing periodic illumination of a light emitter (8) of the optical proximity sensor (6) with the vertical synchronization signal, wherein said synchronization comprises introducing a delay time period (54) to generate emitter drive pulses (48) to drive the light emitter (8), should be provided that allows to provide a good tradeoff for the emitted IR-power, in order to provide sufficient signal strength in the reflected signal and thereby sufficient signal-to-noise ratio for a robust and fast-enough identification of an approaching target, and on the other hand side to provide low enough IR-power to avoid display-distortion. In accordance with the invention this is achieved in that said synchronization further comprises introducing an ignore time period (62) to suppress generation of emitter drive pulses (48).

Description

METHOD OF OPERATING AN OPTICAL PROXIMITY SENSOR AND COMPUTER

DEVICE FOR EXECUTION OF THE METHOD

DESCRIPTION

Technical background of the invention

The invention relates to a method of operating an optical proximity sensor. It more particularly relates to optical proximity sensing in devices with displays, where the proximity sensors are located behind or otherwise in proximity to the displays. The invention further relates to a computer device, in particular a smartphone, comprising an optical proximity sensor.

Background

Optical sensors are increasingly being used in such diverse areas of technology as smart phones and mobile devices, smart homes and buildings, industrial automation, medical technology and connected vehicles, etc. In particular in smart phones and mobile devices, proximity sensors are of increasing importance. For example, many smartphones use optical proximity sensors to determine when the display should be turned on or off. This is typically for the purpose of saving battery power when, for example, the smartphone is placed in a user's pocket, or for deactivating the display when the smartphone is held to the user's ear. Further, they might be used for preventing unwanted onscreen display button selections when a phone call is received. This can also be the case with other computer devices having displays, such as tablets and laptop computers, where it is desirable to turn off the display when the device is closed or otherwise covered.

Optical proximity sensing typically relies on emitting near infra-red (NIR) light and measuring any light energy reflect- ed back. Reflection above a certain threshold indicates that the display is likely to be covered, and thus, appropriate action should be taken.

Previously, NIR light emitters for this purpose were located in the bezel of a smartphone or other device. In recent years however, due to demand for a high screen to body ratio, the bezel previously used to host the proximity sensor has been eliminated from many devices. Many smartphones today use Organic Light Emitting Diode (OLED) displays. OLED displays may permit some light, including NIR light, to pass through the display. Therefore, proximity sensors may be located behind the OLED display. In such setups, they may be configured to be synchronized with the OLED display such that emission of the proximity sensor signal is initiated close to the final phase of a synchronization period of the display. The proximity sensor in particular may be configured to work in a way synchronized to the display in order to use an optimized point in time for IR-emission in respect to the sensitivity of the OLED which may be time-dependent as a consequence of the regular updates of the picture on display.

As an optimized or selected point in time for initiating the OLED emission a point in time may be chosen about just bevor the capacitively stored picture content of the display is updated. Depending on the sensor position, this is at a delaytime from the trigger signal for display update, the "VSYNC- edge", equal to a fixed fraction of the SYNC-period.

The energy of the NIR light emitted through the display, however, may cause a visible distortion on the display; for example a bright spot may appear in the display above the location of the proximity sensor. These undesirable distortions may be visible under many conditions, for example even if the screen is displaying a black image.

Accordingly, it may be desirable to keep the emission power of the NIR emitters low in order to minimize the effects of such undesired distortions of the display. On the other hand, a sufficiently high emission power of the NIR emitter is needed in order to make sure that the signal strength of the reflected proximity signal is high enough for proper detection and analysis.

Summary

The object of the invention is therefore to provide an improved method for operating an optical proximity sensor that is particularly suitable for a sensor behind an OLED display and synchronized to the OLED display. In particular, it is desirable to provide a good tradeoff for the emitted IR- power, in order to provide sufficient signal strength in the reflected signal and thereby sufficient signal-to-noise ratio for a robust and fast-enough identification of an approaching target, and on the other hand side to provide low enough IR- power to avoid display-distortion. Further, an improved computer device comprising an optical proximity sensor should be provided .

With respect to the method of operating an optical proximity sensor, this object is achieved in that the method comprises: obtaining a vertical synchronization signal from a display driver; synchronizing periodic illumination of a light emitter of the optical proximity sensor with the vertical syn-chronization signal, wherein said synchronization comprises introducing a delay time period to generate emitter drive pulses to drive the light emitter; and wherein said synchronization further comprises introducing an ignore time period to suppress generation of emitter drive pulses .

Preferred embodiments are subject of the dependent claims.

The invention is based on the consideration that in order to overcome the potential deficiencies mentioned above and to provide the IR-power density at an optimized level to obtain low display-distortions for all display update-rates at a sufficiently high proximity data rate, various operational modes of the respective device should be considered and taken into account. In particular, a mobile phone or similar handheld device with a proximity sensor placed behind the display may, depending in the current mode of operation, use different update rates, potentially also depending on the application currently in use. For example, a "standard" 60Hz update rate may be used for normal and slow applications like e-mail reading or SMS-writing. When the device is used with a gaming application, however, the display update rate might be higher (e.g. 120Hz) in order to have better response. Yet further, if the device is in idle mode, with more or less constant display content, the update rate might be reduced to 30Hz or even down to 1Hz to reduce power-consumption.

In recognition of these different modes of operation, in accordance with one aspect of the invention the trigger mechanism for the emission of the OLED (proximity sensor) signal, with reference the VSYNC edge, may be adjusted with respect to potentially varying synchronization rates. In particular, and for a relatively simple and efficient adjustment, certain VSYNC edges may be ignored as a trigger for subsequent emission of proximity signals. In particular, in situations directly after the change of the functional mode to a "slow mode", an emitter drive pulse scheduled directly after the time of change of modes might be triggered in the "former mode" and thus too early.

In order to compensate for effects of such type, in one aspect the invention suggests to, in addition to the delay timer functionality, further provide an Ignore-timer- functionality . This ignore timer functionality in particular may be provided to suppress individual emitter drive pulses, until adaptation of the operation of the device to the new mode of use has been completed.

In one aspect of the invention, the use and mode of operation of the ignore timer may be adjustable. For example, in cases of low-pass filtering used for the proximity result data, such as for averaging of the latest result numbers, the ig- note-time period 62 might be lowered or set to zero when the latest unfiltered result exceeds the threshold in either direction for a number of measurements equal to the number of data used in the averaging filter. This helps to speed up the response while allowing worse display distortion just for a short time when a target approaches or leaves the threshold region .

In yet another aspect of the invention, considered independently inventive or an inventive combination with any of the aspect explained above, the method may be modified by introducing further trigger pulses initiated by a repetition timer in order to initiate the generation of additional emitter drive pulses. This aspect is considered particularly useful for appropriately addressing a situation in which the display is used with particularly low update rates. In such situations, the time distance between two subsequent display updates may extend over a number of delay time periods. The gap resulting therefrom may result in an undesired gap in the sequence of emitter drive pulses. In order to compensate for this, in accordance with this aspect of the invention, a repetition timer may be provided. This repetition timer may be used to add additional trigger pulses initiating the execution of an additional proximity measurement and, accordingly, adding a further delay time period to the regular sequence.

Similar to the options for the ignore timer mentioned above, in one aspect of the invention, the use of the repetition timer also may be adjustable. For example, in case of any low-pass filtering used for the proximity result data like averaging of the latest result numbers, the repetition-time might be lowered with division by 2 or 4 when the latest unfiltered result exceeds the threshold in either direction for a number of measurements equal to the number of data used in the averaging filter. This helps to speed up the response while allowing worse display distortion just for a short time when a target approaches or leaves the threshold region. With respect to the computer device, the object identified above in accordance with aspects of the invention is achieved in that the computer device comprises: a display, an optical proximity sensor located beneath or otherwise adjacent to the display and comprising a light emitter, a display driver for generating a vertical synchronization signal, said driver being configured to synchronize periodic illumination of the light emitter with the vertical synchronization signal, wherein said synchronization comprises introducing a delay time period to generate emitter drive pulses to drive the light emitter; and wherein said synchronization further comprises introducing an ignore time period to suppress generation of emitter drive pulses .

In a preferred embodiment, the display of the computer device may be an Organic Light Emitting Diode display. In another preferred embodiment, the optical proximity sensor is an Infra-Red optical proximity sensor, for example a Near Infrared optical proximity sensor. Further, in preferred embodiment, the device is one of a smartphone, a tabletop, or a laptop computer.

In summary, for a method of operating an optical proximity sensor, aspects of the invention suggest the combination of the synchronization of the proximity measurements to a vertical-sync signal of a display with an ignore-timer , and in further aspects with an optional repetition-timer , and in further aspects with an optional ignore-timer adaptation for confirmation of results, and in further aspects with an optional repetition- timer adaptation for confirmation of results .

By some or more of these aspects, the inventions enables the method to ne operated such that the IR-power density of the NIR emitter may be kept at an optimized level to obtain par- ticularly low display distortion with sufficiently high proximity data rate for all display update rates.

Brief Description of the Preferred Embodiments

Preferred embodiments and aspects of the invention are described further in connection with a drawing. In this drawing,

FIG. 1 shows a smartphone with a proximity sensor;

FIG. 2 is a flow chart illustrating at a high level an optical proximity sensing method;

FIG. 3 shows schematically the operational interaction of components of the smartphone of FIG. 1;

FIG. 4 illustrates a timing scheme for the smartphone of FIG. 1;

FIG. 5 illustrates a timing scheme for the smartphone of FIG. 1 incorporating aspects of the present invention; and

FIG. 6 illustrates a timing scheme for the smartphone of FIG. 1 incorporating further aspects of the present invention .

Identical parts are labelled by the same reference numerals in all Figures.

Detailed Description of the Preferred Embodiments

FIG. 1 shows a smartphone 1, depicted here in cross-section through a plane perpendicular to the plane of the display 2 (which in the embodiment shown is an OLED display) . Associated with the display 2, the smartphone 1 comprises an OLED display driver 4 and an NIR optical proximity sensor module 6 within the body of the smartphone 1, positioned behind the OLED display 2. The display driver 4 is typically implemented by way of a combination of hardware and software, where the hardware may comprise a Graphical Processing Unit (GPU) and associated memory. The proximity sensor module 6 comprises a NIR emitter 8 and, positioned adjacently, an associated detector 10. The NIR emitter 8 and the detector 10 are provided with a clear line-of-sight to the underside or backface of the display 2. Further, the proximity sensor module 6 may comprise optical components such as lenses. The proximity sensor module 6 further is provided with an integrated delay circuitry 12, and is configurable through a register to optimize the timing of light emission for reduced display pixel distortion. The exact configuration for a given display may be determined empirically, e. g. during a prototyping phase of a product.

As shown in the flow diagram in FIG. 2, the process for optical proximity sensing in the smartphone 1 is configured for reduced pixel distortion in the display 2 resulting from signals emitted by the NIR emitter 8. During operation of the smartphone 1, in step 21 a Vertical Synchronization (VSYNC) signal is generated periodically by the OLED display driver 4 for the purpose of refreshing the display. The picture refresh rate may be, e. g., 60Hz, i.e. the display 2 is updated 60 times in one second, wherein the start of a new frame is indicated by the VSYNC pulse signal. The VSYNC signal in particular synchronises the processor's frame rate and the display's refresh rate such that the frames per second (FPS) is limited by the refresh rate and no frames are skipped.

The VSYNC pulse indicates the start of a new frame (image) on the display 2. In the embodiment shown, the VSYNC pulse is also used to synchronize the emission of light from the proximity sensor module 6 to the display refresh rate. Specifically, the NIR optical emission is synchronized to start at a time point in the refresh cycle of the display 2 when resulting distortion to the image is reduced or eliminated. Depending upon the specific OLED display used in the device, the optimal time for emission may for example be immediately before or during the time when the display pixels above the proximity sensor module 6 are inactive. For this purpose, the VSYNC signal in step 22 is additionally provided to the integrated delay circuitry 12 of the optical proximity sensor module 6. When the delay circuitry 12 detects an incoming timing (refresh) pulse in the VSYNC signal ("VSYNC edge") , it initiates (in step 23) a delay timer 30 and, following expiry of the delay timer 30, in step 24 triggers NIR light emission by the NIR emitter 8 for a predefined time period. The emitted light will then pass through the OLED display 2 to interact with any object which may be directly in front of the display 2. Any light reflected back through the OLED display 2 then is detected by the detector 10, thereby producing a proximity signal.

In FIG. 3, the respective components are shown schematically. The OLED display driver 4 is functionally connected with the optical proximity sensor module 6 which comprises the integrated delay circuitry 12 and, embedded therein, the delay timer 30. The VSYNC signal is shown being transmitted from the display driver 4 to the delay circuitry 12 of the module 6.

FIG. 4 illustrates an exemplary timing scheme. The upper track 40 therein illustrates the VSYNC signal in form of respective peaks 42 as generated by the OLED display driver 4. Each VSYNC pulse, as represented by the peaks 42, initiates a synchronization period 44, shown in track 46, during which the content of the display 2 is updated. In particular, at a given time ti within the synchronization period 44 and after the leading edge of a VSYNC pulse, as represented by the peaks 42, the display brightness above the proximity sensor module 6 is temporarily decreased as the corresponding pixel lines are refreshed or 'blanked' . To take advantage of this state, which allows passing of the proximity signals emitted by the NIR emitter 8 through the display 2 at low distortion, the emission of the proximity signals is triggered to happen within or in proximity to this "blank time". Consequently, the delay timer 30 is set upon receipt of the VSYNC pulse in the delay circuitry 12, and after expiry of the delay timer 30, i. e. after an appropriately selected time delay from the leading edge of the VSYNC pulse, the delay circuitry 12 generates an emitter drive pulse 48 to drive the emitter 8 of the proximity sensor module 6. In the diagram of FIG. 4, the emitter pulses 48 are shown in the lower most track 50. Each drive pulse 48 has a duration time associated therewith. The time periods for the time delay and the duration time of the drive pulse in one aspect of the invention are optimized to minimize the visual distortion created by the NIR light pulse in the display 2. For example, timing may result in the NIR light pulse being generated very shortly before the blanking of the overlying display pixels.

In regular systems, the respective drive pulse 48 for example may be triggered by the delay timer 30 at a time after the respective VSYNC pulse corresponding to a selected percentage of the associated synchronization period 44, e. g. at a delay time of 80% of the synchronization period 44 after the respective VSYNC pulse. This, however, in the case of changes of operational modes as described above, may lead to dissatisfying results.

For further explanation, time line 52 in the diagram for FIG. 4 represents a point in time at which a change of the functional mode of the smartphone 1 occurs. For times before this event (area left of time line 52 in FIG. 4) , the smartphone 1 is operated in a normal performance mode (e. g. when running standard, slow applications) with a vertical sync rate of 60Hz. In this mode, the synchronization periods 44 have a length of about 16.66 ms. Correspondingly, the emitter drive pulses 48 are triggered at a delay of e.g. 13 ms after each peak 42 representing the respective incoming VSYNC signal. As can be seen from the diagram in FIG. 4, however, directly after the change of the functional mode to a "slow mode" with a vertical sync rate of only 30 Hz, and the corresponding synchronization periods 44 becoming a length of about 33.33 ms (areas to the right of time line 52 in FIG. 4, even though the delay for the emitter drive pulses 48 may be adjusted to about 25 ms in this mode, the emitter drive pulse 48a directly after time line 52 is triggered in the "former mode" and thus too early.

In order to compensate for effects of such type, in one aspect the invention suggests to further provide an Ignore- timer-functionality, as represented in the schematic diagram shown in FIG. 5. Again, in analogy to previous FIG. 4, FIG. 5 also shows the upper track 40 depicting the VSYNC signals in form of respective peaks 42 as generated by the OLED display driver 4. Corresponding to each VSYNC pulse, as represented by the peaks 42, a delay time period 54 (in the embodiment shown 40% of the cycle time, i.e. about 5ms) shown in track 56 is initiated, at the end of which an emitter drive pulse 48 for the proximity measurement is initiated (see track 50 in FIG. 5) . In addition to the set up shown in FIG. 4, however, in the embodiment of the invention represented by FIG. 5 an ignore timer 60 is provided. In this set up, each VSYNC pulse, as represented by the peaks 42, by triggering the ignore timer 60 further initiates an ignore time period 62, shown in track 64, during which the initiation of an emitter drive pulse 48 is suppressed, or the triggering of a proximity measurement is blocked in any other way. Rather, the next proximity measurement may only initiated after the respective ignore timer 60 has expired. In other words, in this set up, the ignore timer 60 starts with a detected incoming VSYNC- pulse, which triggers a proximity measurement, wherein the ignore timer 60 masks further incoming VSYNC trigger pulses until it expires.

In particular, in one aspect of the invention, and for implementations into regular smartphones 1 or wearable devices, the ignore time period 62 to which the ignore timer 60 is set may be chosen to be about 15 ms. By using an ignore timer 60 set to 15 ms, effectively leading to ignoring any VSYNC-pulse as trigger for the NIR emitter 8 within a time of 15 ms after a previously detected VSYNC-pulse, the proximity measurement rate may be kept constant for display VSYNC rates of 30Hz, 60Hz, 90Hz, 120Hz or 240Hz. The SYNC-Delay in this embodiment may still be a fraction of the SYNC-period defined by the VSYNC-pulses without ignore-time.

For further explanation, in the diagram of FIG. 5 a time line 66 representing point in time at which a change of the functional mode of the smartphone 1 occurs is also shown. In FIG. 5, time line 66 represents the change of a functional mode with a vertical sync rate of 60 Hz (area left of time line 66 in FIG. 5) , to a high performance functional mode (e. g. when running gaming applications) with a vertical sync rate of 120Hz. As can be seen from FIG. 5, in this mode the synchronization periods 44 as given by the time difference between two subsequent VSYNC peaks 42H have a length of about 8.44 ms. Even so, the lengths of the delay time periods 54 may be left unchanged at 5 ms, and the lengths of the ignore time periods 62 may be left unchanged at about 15ms. As a consequence, in the regime of enhanced vertical sync rates of 120Hz, after a first VSYNC pulse as represented by peak 42 has triggered both the delay timer 30 and the ignore timer 60, the subsequent VSYNC pulse as represented by peak 42S occurs while the ignore time period 62 is still active, and therefore is suppressed from detection.

In one aspect of the invention, the use of the ignore timer 60 may be adjustable. For example, in case of low-pass filtering used for the proximity result data, such as for averaging of the latest result numbers, the ignore-time period 62 might be lowered or set to zero when the latest unfiltered result exceeds the threshold in either direction for a number of measurements equal to the number of data used in the averaging filter. This helps to speed up the response while allowing worse display distortion just for a short time when a target approaches or leaves the threshold region.

In yet another aspect of the invention, considered independently inventive or an inventive combination with any of the aspect explained above, the method may be modified to appropriately address a situation in which the display 2 is used with particularly low update rates. In such situations, the synchronization period 44 may extend over a number of delay time periods 54, as indicated in the schematic representation of FIG. 6. Again, in analogy to previous FIGs. 4 and 5, FIG. 6 also shows the upper track 40 depicting the VSYNC signals in form of respective peaks 42 as generated by the OLED display driver 4. The delay time periods 54 shown in track 56 of FIG. 6 in this embodiment may be set at regular periods, whereby in the example shown in FIG. 6 the synchronization period 44 due to the slow update rate extends over more than three delay time periods 54. The gap resulting therefrom may result in an undesired gap in the sequence of emitter drive pulses 48.

In order to compensate for this, in accordance with this aspect of the invention, a repetition timer 70 may be provided. This repetition timer 70 may be used to add additional trigger pulses 72 initiating the execution of an additional proximity measurement and, accordingly, adding a further delay time period 74 to the sequence as shown in track 56. The SYNC-Delay in this embodiment should be derived from the SYNC-Period of the display 2, and a repetition delay counter 76 should be restarted at this time. In particular, the SYNC- Period divided by the TREP DELAY may be an Integer and the PWM-Rate of the display is equal or a multiple of 1/TREP_DELAY.

In FIG. 6, for this embodiment an example with long (FIG. 6a) and short (FIG. 6b) Sync-Delay is shown. In both cases, after the initial trigger pulse 72 has been initiated and the further delay time period 74 is still ongoing, the respective synchronization period 44 ends, thereby initiating the subsequent emitter drive pulse 48 of the "regular" sequence of pulses, thereby effecting a "regular restart" of updates for the display 2 at restart time 78.

Similar to the options for the ignore timer 60 mentioned above, in one aspect of the invention, the use of the repetition timer 70 also may be adjustable. For example, in case of any low-pass filtering used for the proximity result data like averaging of the latest result numbers, the repetitiontime might be lowered with division by 2 or 4 when the latest unfiltered result exceeds the threshold in either direction for a number of measurements equal to the number of data used in the averaging filter. This helps to speed up the response while allowing worse display distortion just for a short time when a target approaches or leaves the threshold region.

The embodiments of the method discussed herein have been disclosed for the purpose of familiarizing the reader with novel aspects of the idea. Although preferred embodiments have been shown and described, many changes, modifications, equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims.

In particular, the disclosure is not limited to the disclosed embodiments, and gives examples of as many alternatives as possible for the features included in the embodiments discussed. However, it is intended that any modifications, equivalents and substitutions of the disclosed concepts be included within the scope of the claims which are appended hereto .

Features recited in separate dependent claims may be advantageously combined. Moreover, reference signs used in the claims are not limited to be construed as limiting the scope of the claims.

Furthermore, as used herein, the term "comprising" does not exclude other elements. In addition, as used herein, the article "a" is intended to include one or more than one component or element, and is not limited to be construed as meaning only one.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred.

LIST OF REFERENCE NUMERALS

1 smartphone

2 display

4 display driver

6 proximity sensor module

8 NIR emitter

10 detector

12 delay circuitry

21 , 22 , 23 , 24 step

30 delay timer

40 track

42 peak

44 synchroni zation period

46 track

48 emitter drive pul se

50 track

52 time line

54 delay time period

56 track

60 ignore timer

62 ignore time period

64 track

66 time line

70 repetition timer

72 trigger pul se

74 delay time period

76 repetition delay counter

78 restart time

Claims

1. A method of operating an optical proximity sensor (6) of a computer device having a display (2) , where the optical proximity sensor (6) is located beneath or otherwise adjacent to the display (2) , the method comprising: obtaining a vertical synchronization signal from a display driver; synchronizing periodic illumination of a light emitter (8) of the optical proximity sensor (6) with the vertical synchronization signal, wherein said synchronization comprises introducing a delay time period (54) to generate emitter drive pulses (48) to drive the light emitter (8) ; and wherein said synchronization further comprises introducing an ignore time period (62) to suppress generation of emitter drive pulses (48) .

2. The method of claim 1, wherein said ignore time period (62) is adjustable.

3. The method of claim 1 or 2, wherein said synchronization further comprises introducing further trigger pulses (72) initiated by a repetition timer (70) to initiate the generation of additional emitter drive pulses (48) .

4. The method of claim 3, wherein the generation of said further trigger pulses (72) is adjustable.

5. The method of any one of the previous claims, wherein said display (2) is an Organic Light Emitting Diode display.

6. The method of any one of the previous claims, wherein said optical proximity sensor (6) is an Infra-Red optical proximity sensor (6) , for example a Near Infra-red optical proximity sensor.

7. A computer device (1) comprising a display (2 ) , an optical proximity sensor (6) located beneath or otherwise adjacent to the display (2) and comprising a light emitter (8) , a display driver (4) for generating a vertical synchronization signal, said driver (4) being configured to synchronize periodic illumination of the light emitter with the vertical synchronization signal, wherein said synchronization comprises introducing a delay time period (54) to generate emitter drive pulses (48) to drive the light emitter (8) ; and wherein said synchronization further comprises introducing an ignore time period (62) to suppress generation of emitter drive pulses (48) .

8. The device (1) of claim 7, wherein said display (2) is an Organic Light Emitting Diode display.

9. The device (1) of claim 7 or 8, the device (1) being one of a smartphone (1) , tabletop or laptop computer.