Abstract:
An optical navigation device includes a navigation radiation source capable of producing a beam of radiation, and an optical element. The optical navigation device is for identifying movement of an object to thereby enable a control action to be carried out. A sensor is for receiving an image based upon reflected radiation from the navigation radiation source, and at least one illumination radiation source is for illuminating the optical navigation device. The device is operable, in recurrent cycles of at least reset, expose, and readout phases, to activate the at least one illumination radiation source during the reset phase. The reset phase may have the longest duration of the phases.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to optical navigation devices, and, particularly, but not exclusively, to miniature devices for use in a variety of different applications. 
       BACKGROUND OF THE INVENTION 
       [0002]    Current mobile or computer devices use a variety of different navigation mechanisms. Some of these present a mousing surface for direct manipulation by a finger (or other part of the body), and work by producing images of the finger as it moves on the mousing surface, translating this finger movement to movement of a cursor (for example) on a screen. In this respect a navigation device of this type works in a similar fashion to an optical mouse, and is referred to as an optical navigation device. 
         [0003]    It is often desirable to have an optical navigation device backlit. However, this can lead to performance degradation should the backlight remain on constantly. This means that the backlight illuminates the mousing surface and degrades the contrast of a fingerprint, thereby reducing the signal-noise ratio of the acquired image. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object to address the above drawbacks. In a first aspect, there is provided an optical navigation device comprising a navigation radiation source capable of producing a beam of radiation, and an optical element for identifying movement of an object to thereby enable a control action to be carried out. The optical navigation device also includes a sensor for receiving an image based upon reflected radiation from the navigation radiation source, and at least one illumination radiation source for illuminating the optical navigation device. The device is operable in recurrent cycles of at least reset, expose and readout phases and to activate the at least one illumination radiation source only during the reset phase, the reset phase having the longest duration of the phases. 
         [0005]    The device preferably also operates in a calibration phase. The device may operate in cycles of the reset, calibration, expose and readout phases, the reset phase being operable to reset the sensor to a predetermined level, the calibration phase being operable to determine the appropriate sensor output level in the absence of light, the expose phase for obtaining the image and the readout phase for outputting the image. 
         [0006]    The reset phase may have a duration comprising 50% or more of the total duration of each cycle of phases. The device may be operable to switch off the illumination radiation source shortly before the end of the reset period. The device may be operable not to activate the illumination radiation source during cycles in which the sensor receives images indicative of movement of the object and/or images with detectable features. 
         [0007]    The device may be operable such that the duration that the illumination radiation source is activated during each reset cycle is variable so as to control illumination intensity. Preferably, the device is further operable such that the duration that the illumination radiation source is activated does not alter the duration of the reset phase. 
         [0008]    The device may be operable such that the illumination radiation source is controllable by the navigation device and/or a host device in which the navigation device is comprised. The illumination radiation source may be activated dependent upon a status of the navigation device and/or the host device. 
         [0009]    In addition, the device may be operable such that the duration of each of the cycles of phases may be variable, such that the device may operate in a number of modes. The device may comprise a plurality of illumination radiation sources. 
         [0010]    Each one of the plurality of illumination radiation sources may be arranged to illuminate different portions of the navigation device. The plurality of illumination radiation sources may output visible light, each with a different colour. In a main embodiment, there may be three such illumination radiation sources, each outputting an additive primary colour. 
         [0011]    The device may be operable to turn one of the plurality of illumination radiation sources on at one time. The device may be further operable to allow two or more of the plurality of illumination radiation sources to be turned on during a single reset period, such that the ratio of durations that each illumination radiation source is turned on determines the perceived illumination colour. The ratio may be controllable by the navigation device and/or a host device in which the navigation device is comprised. The ratio may be dependent upon a status of the navigation device and/or the host device. The device may be operable to set the ratio proportionally, with a further parameter to set illumination intensity. 
         [0012]    The device may be operable such that each one of the illumination radiation sources is able to be turned on for substantially the full reset phase duration. Alternatively, the device may be operable such that each one of the illumination radiation sources is able to be turned on for only a proportion of the reset phase duration, the proportion being substantially the reciprocal of the number of illumination radiation sources. 
         [0013]    The optical element may be formed from a single piece construction and may include at least one frustrated total internal reflection (F-TIR) surface capable of causing frustrated total internal reflection of the beam of radiation when the object contacts the first surface to thereby generate the image which is capable of being translated into the control action. The optical element may comprise at least one further surface for directing radiation from the radiation source to at least one F-TIR surface. The optical element may comprise at least one additional surface for directing radiation from the F-TIR surface to the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a cross-sectional diagram of an optical navigation device, suitable for application of the present invention, given by way of example, 
           [0016]      FIG. 2  is an optical diagram of the  FIG. 1  device, given by way of example, 
           [0017]      FIG. 3  is a timing diagram for a prior art optical navigation device with illumination; 
           [0018]      FIG. 4  is a timing diagram for an optical navigation device with illumination according to a first embodiment of the invention; 
           [0019]      FIG. 5  is a schematic drawing of an optical navigation device according to the first embodiment of the invention; 
           [0020]      FIG. 6  is a timing diagram for an optical navigation device with illumination according to a second embodiment of the invention; 
           [0021]      FIG. 7  is a timing diagram for an optical navigation device with illumination according to a third embodiment of the invention; 
           [0022]      FIG. 8  is a schematic drawing of an optical navigation device according to the second and third embodiments of the invention; and 
           [0023]      FIG. 9  is a schematic drawing of an optical navigation device according to a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The present invention relates to an optical navigation device module for use in a device. The device may be of any appropriate type and may include mobile phones, PDAs, Laptop computers, TV remotes, games controls and other such devices herein referred to as a computer application. 
         [0025]      FIGS. 1 and 2  show a particular type of optical navigation device module suitable for the application of the present invention. This optical navigation device module is described in detail in co-pending application number GB0908899.8. It should be noted that this optical navigation device is described by way of example only, and any suitable optical navigation device using reflection off a mousing surface can be used. 
         [0026]    The optical navigation device module  100  includes a base plate  102 , an optical element  106  which incorporates a clip  104  (the optical element will be described in greater detail below), an illumination source  108 , such as an LED, and a sensor  110 . The overall construction of the optical navigation device module  100  is of a low profile construction making it suitable for mobile devices. The actual size of the overall package containing the optical navigation device sits under a cap of about 7 mm in diameter and the module itself has a maximum dimension of about 5.8 mm. 
         [0027]    The optical element  106  may be molded from a single piece of plastic and provides a so called mousing surface  112 . An example of the type of plastic material is a monolithic optical block made of a plastics material such as poly (methyl methacrylate) also known as PMMA, although it will be appreciated other materials (such as Polycarbonate, Cycicyclophane copolymer) may alternatively be used. 
         [0028]    As can be seen in  FIG. 2 , this particular type of optical navigation device uses the optical layout for a frustrated total internal reflection (F-TIR) device  200 , although direct imaging systems and other systems are equally applicable to be used in place of the F-TIR device  200 . 
         [0029]    The F-TIR device  200  includes an LED  202  which emits a source of radiation directed by optical element  204  to the internal surface  210  of the mousing surface  112 . A further optical element  208  then directs any reflected radiation from surface  210  on to sensor  206 . 
         [0030]    The LED  202  may be of any appropriate type and may be mounted on a substrate. In a particular example the LED emits in the near infrared range for example between about 700 to 900 nm. 
         [0031]    The optical element  204  directs the LED illumination into the monolithic optical block which forms the optical element  106 . The optical element  204  may be in any appropriate to form, for example, a single convex surface, a series of lenslets configured as a “fly eye” structure, or any other suitable structure capable of providing near collimated illumination at the internal surface  210 . The optical element  204  may be capable of being tilted in order to control the illumination pattern and direction of the beam at the mousing surface. 
         [0032]    The mousing surface  112  includes an internal surface  210  and an external surface  212 . At the mousing surface, any object with a refractive index which is placed in contact with the external surface  212  may frustrate the total internal reflection of the beams  214  at the internal surface  210 . A suitable object may include a finger, pointer, hand or other object or feature. A so-called frustrated reflection will thus be generated and the resulting pattern is focused by optical element  208  onto the sensor  206 . 
         [0033]    The internal surface  210  is relatively smooth when compared to the features which give rise to the F-TIR. Illumination reflected from the internal surface  210  when there is no object close to the mousing surface is virtually 100% reflected. However, when the reflection is frustrated, about 10% or less of the illumination is reflected, thus resulting in contrast ratio of about 1:10 in the present example. Note that at 850 nm most of returned signal is from scattering at the object in contact with the optical element  106  (e.g. the skin of the user&#39;s finger). 
         [0034]    The frustrated reflection directed on to the sensor is detected in order to identify the point or points of contact of the object in contact with the external surface  212 . Subsequent measurements of the point or points of contact provide information corresponding to the movement of object on the external surface  212 . The action or movement of the object can then be translated into actions to operate a mobile personal computer. 
         [0035]    The optical navigation device may have a backlight provided by an LED separate to the LED  202 , which provides the radiation source for the sensor  206 . However, should the backlight remain on constantly, performance degradation can result. This is because the backlight illuminates the mousing surface and degrades the contrast of a fingerprint, thereby reducing the signal-noise ratio of the acquired image. 
         [0036]    For good contrast of the fingerprint, the illumination optics ideally produces a collimated beam of light at an oblique angle (i.e. as close as possible to the plane of the surface) as possible. In comparison, the backlighting optics produces a smooth distribution of light over the surface and is as homogenous as possible. Hence the photons from the backlighting system do not provide good contrast of surface texture. 
         [0037]    The backlight naturally uses wavelengths in the visible band while it is common for the navigation LED to be infra-red (IR). The longer wavelength IR photons penetrate the silicon deeper before generating electron-hole pairs. A significant portion of the photo generated charge from the IR/navigation source will be generated in the bulk of the substrate which is usually highly doped. As a result the electron-hole pairs will recombine quickly and not be collected, and the device will have a reduced quantum efficiency. Hence, if the photon flux for the visible (backlighting) LED is the same as the IR (navigation) LED, the detector will receive a greater amount of (unwanted) signal from the visible (backlit) LED compared to the signal from the IR (navigation) LED and the image contrast will be further degraded. 
         [0038]      FIG. 3  shows a typical timing diagram of prior art sensor operation. It shows a sensor operating in repeated cycles of four phases, during which the levels of the sensor photodiode voltage VPD, the reset line RST, the navigation LED NAVLED and the Backlight LED, BACKLED are shown. The four phases are: a reset phase where the sensor&#39;s pixels are all reset to a predetermined voltage level, a black calibration phase whereby the sensor is calibrated so as to determine the appropriate output level in the absence of light, an expose phase where the sensor obtains an image and a readout phase, where the obtained image is read out for comparison to previous images to determine motion. This basic cycle of phases is well known for any imaging sensor. 
         [0039]    The timing arrangement of  FIG. 3  attempts to address the above signal degradation issue by turning the backlighting LED off during part of the frame so it does not interfere with the navigation operation. While it is possible to turn the backlighting LED on during dark calibration, this is disadvantageous as it would produce an offset into the pixel and reduce the dynamic range. Turning the backlight LED on during either exposure (Texpose) or readout (Treadout) will cause the degradation problems described above. Hence if the backlight is turned on during the reset phase (Treset 1 ) the may backlighting not degrade the imaging/navigation system. 
         [0040]    Normally this reset period (Treset 1 ) is a short period (10 μs) compared to the total frame period (100 μs). Consequently, if this period is used to provide backlighting, the resulting duty cycle of the backlight is low (10% typical). This would produce a dim back light unless the peak current through the backlighting LED was increased accordingly. It is common for LEDs to be operated in this mode and their maximum current is typically dependent on the duty cycle of operation (thermal dissipation limits, so 1 A at duty cycle of 10% equals 100 mA constant). 
         [0041]    However, this operation is undesirable in a mobile device as the large maximum current presents additional demands on the device&#39;s power management circuitry and the effect of voltage drops becomes more significant. 
         [0042]    A mouse/navigation device sensor operation is different to that of a still camera/video camera/webcam in that the mouse/navigation device sensor can control the lighting. A significant further difference is that of frame-rate. For good performance (especially to avoid temporal aliasing—c.f. spokes on a wheel appearing to rotate backwards on TV/film) the mouse sensor operates at 1 kHz-10 kHz frame rate compared to 10-50 Hz of a DSC/webcam. To achieve high frame rates, not only should the pixel be very sensitive to light (U.S. Pat. No. 6,969,879, incorporated herein by reference describes such a sensitive pixel) but the mode of operation should be such that for most of the frame time it is sensitive to light, and the time of the non-imaging periods should be reduced. 
         [0043]    For example,  FIG. 3  shows equal time for the black calibration phase Tblackcal and readout phase Treadout, however black calibration phase Tblackcal is typically shorter than readout phase Treadout, using the technique described in U.S. Pat. No. 7,502,061 (hereby incorporated by reference). 
         [0044]      FIG. 4  is a timing diagram illustrating the operation of a navigation device with backlighting system according to an embodiment of the invention. The lines shown and phases (other than duration) are the same as that illustrated in  FIG. 3 . However, the reset phase (Treset 2 ) has now been lengthened and is now the longest phase in the system. It is preferable that the period for which the backlight LED is on, TbLED 2  is shorter than the reset phase Treset 2 , so that (a) the navigation device photo sensor does not detect any photons from the backlighting system, and (b) the backlighting LED is turned off in advance of the reset pulse so that the disturbance on the supply rails caused by turning off the current through the LED has disappeared before the pixel comes out of reset. 
         [0045]    It will be appreciated that the above operational embodiment will result in a significant reduction of the frame rate. However, the inventor has determined that the higher frame rates used for a conventional optical desktop mouse are not necessarily used for smaller optical navigation devices such as illustrated in  FIGS. 1 and 2  (although a frame rate much higher than cameras etc. is still required). This is because finger motor skills used when operating a finger operated device tend to be slower then arm motor skills used when operating a desktop mouse. 
         [0046]    The width of the backlight LED (TbLED 2 ) pulse can be varied to change the average intensity of the backlighting. Preferably, the reset (Treset 2 ) pulse stays the same length and does not vary as the backlight LED (TbLED 2 ) pulse varies. This keeps the frame length (Tframe 2 ) constant which simplifies the calculation of speed of the mouse. If desired (e.g. in power saving modes), there may be provided a set of frame (Tframe 2 ) periods so that the frame rate of the mouse can be adapted to the mode of operation (e.g. high speed motion, medium speed motion, low speed motion, no motion &amp; short sleep, no motion and medium sleep and no motion and long sleep period). 
         [0047]    Instead of having just one output from the navigation device sensor, it is possible to have several outputs to control several LEDs. These may be of the same colour and designed to co-operate with the illumination optics to each illuminate only part of the surface (e.g. brighter centre and darker periphery or vice-versa), or else they may be of different colours for aesthetic or technical reasons. 
         [0048]      FIG. 5  shows an optical navigation device according to said first embodiment of the invention. It comprises optical sensor  10 , a navigation Light Emitting Diode (LED)  20  and a white or tricolour backlighting LED  30 . 
         [0049]      FIG. 6  illustrates such an arrangement where there are 3 separate illumination LED outputs, BACKLEDR, BACKLEDG, and BACKLEDB, each with different pulse widths. Although this represents a possible solution, it is not optimal as it results in either 0, 1, 2 or 3 backlighting LEDs illuminating at any one time, hence there is a large variation in the total supply current. This may pose issues for the system&#39;s power management block. 
         [0050]      FIG. 7  illustrates an alternative arrangement to that illustrated in  FIG. 6 . In this embodiment the pulses for the backlighting LED channels BACKLEDR, BACKLEDG, BACKLEDB are non-overlapping. They are set such that as one turns off another is turned on. Not only does this reduce the maximum current required for backlighting to ⅓ of the system shown in  FIG. 6  (assuming same current through each LED colour), but it keeps the current drain roughly constant during the reset period. In both cases this helps to minimise the inrush current into the mouse module. 
         [0051]      FIG. 8  shows an optical navigation device according to said first embodiment of the invention. It comprises optical sensor  10 , a navigation Light Emitting Diode (LED)  20  and three backlighting LEDs, red  30   r , green  30   g  and blue  30   b.    
         [0052]    In the illustrated example, the LEDs are red, green and blue, these being additive primary colours that allow an illumination output in the other visible colours of the spectrum. In the specific example shown, the blue LED signal (BACKLEDB) is on for about the same period as the green LED signal (BACKLEDG) but only half the time of the red LED signal (BACKLEDR), i.e. duration of signal TbbLED 4  equals duration of signal TbgLED 4  equals 2× duration of signal TbrLED 4 . The ratio of the periods sets the colour of the backlight. For optimal navigational performance the total duration of signals TbbLED 4 , TbgLED 4  and TbrLED 4  is less than the duration of Treset 4 . The actual width/duty cycle of the backlighting pulse can be set in variety of manners. 
         [0053]      FIG. 9  shows a variation on the optical navigation device according to  FIG. 8 . In this, two illumination LEDs are provided, a red and green bi-colour LED  30   rg  and a blue LED  30   b.    
         [0054]    The navigation device has, in some embodiments, a memory (e.g. I2C or SPI register) which the host controller of the host device (for example, phone, MP3 player or similar portable media player (PMP), Digital Still camera, satellite navigation system, remote control) can access to program the brightness/colour/width of the backlighting LED pulse. This can be useful for a variety of applications/enhancements of the device, for example: 
       For a Cellular Phone: 
       [0000]    
       
         
           
             Backlight illuminates on incoming phone call or SMS message. 
             Backlight changes colour depending on if there is phone call or SMS message (e.g. white for incoming call and red for incoming text message). 
             Backlight illuminates with colour depending on caller ID of incoming phone call/SMS message (e.g. white for most calls, red for certain persons or group of persons, green for others etc.). 
             Backlight colour indicates battery power level—e.g. if battery levels are below a threshold then the navigation device backlight is commanded to illuminate red. 
             Backlight illuminates when the phone is being charged. It could be commanded to change colour to indicate either the level of charge or time remaining to fully charge the unit. 
             Backlight colour indicates RF signal strength—e.g. if received signal strength is above a certain threshold then the backlight could be commanded to show green light, similar to the number of “bars” on a conventional display. 
             Backlight could be commanded to illuminate or change colour when keys are depressed—The colour of the backlight could be commanded to change if the keys are depressed while the keylock is engaged. 
           
         
       
     
       For a MP3 Player or Personal Music Player: 
       [0000]    
       
         
           
             Backlight colour indicates battery power level—e.g. if battery levels are below a threshold then the navigation device backlight is commanded to illuminate red. 
             Backlight illuminates or changes colour when the player is charging. 
             Backlight illuminates or changes colour when data is uploaded to the media player (e.g. red). 
             Backlight illuminates or changes colour when data is downloaded from the media player (e.g. green). 
             Backlight illuminates to indicate an “illegal operation” e.g. trying to reduce the volume control to below zero or above the maximum level. 
             Backlight illuminates or changes colour to warn the user—e.g. the volume is commanded to exceed a preset level. 
             The intensity or colour of the backlight is used to indicate the volume level. 
             The intensity and/or colour of the backlight can change in harmony with the sound being played, e.g. high average sound level produces a brighter backlight and the spectral content of the audio is mapped onto the colour of the illumination (e.g. predominantly low-frequency audio tones control the red LED, mid-tones to green LED and high frequency tones the blue LED). 
             Backlight colour/intensity indicates that the player has successfully connected to a host computer. 
           
         
       
     
       For a Camera: 
       [0000]    
       
         
           
             Backlight illuminates or changes colour when the camera is acquiring an image. 
             Backlight illuminates or changes colour when the camera is processing an image. 
             Backlight illuminates, flashes or changes colour when the camera is in “self timer” mode, waiting to acquire an image (e.g. 8× green backlighting pulses at 1 second intervals followed by 2× red backlighting flashes just before the image is captured). 
             Backlight illuminates, flashes or changes colour to indicate an error condition—e.g. under exposure, over exposure, long exposure (longer than can be comfortably handheld), camera&#39;s memory store (SD/CF/xD/MS/SM) is full. 
             Backlight illuminates, flashes or changes colour to indicate that the camera is in a nonstandard or unusual mode of operation (e.g. manual focus, manual white balance, manual exposure). 
             Backlight colour indicates battery power level—e.g. if battery levels are below a threshold then the navigation device backlight is commanded to illuminate red. 
             Backlight colour/intensity indicates the camera has successfully connected to a host computer. 
           
         
       
     
       For a Satellite Navigation System: 
       [0000]    
       
         
           
             Backlight illuminates, flashes or changes colour to indicate that the user is approaching their destination 
             Backlight illuminates, flashes or changes colour to warn the driver of an approaching manoeuvre 
             Backlight illuminates, flashes or changes colour to warn the driver of an approaching road traffic restriction (e.g. change in speed limit). 
             Backlight colour indicates battery power level—e.g. if battery levels are below a threshold then the navigation device/fingermouse backlight is commanded to illuminate red 
             Backlight colour/intensity indicates the SatNav device has successfully connected to a host computer 
           
         
       
     
       For a Remote Control 
       [0000]    
       
         
           
             Backlight colour indicates battery power level—e.g. if battery levels are below a threshold then the navigation device backlight is commanded to illuminate red. 
             Backlight colour/intensity indicates which controlling mode the remote control is operating in (e.g. red for DVD, blue for BlueRay, white for TV, green for radio etc.) 
           
         
       
     
         [0085]    The backlighting can also be controlled by the navigation device itself, as illustrated by the following examples. When the device detects that there is motion over the surface of the sensor, the backlighting can be inhibited to reduce energy consumption (as the user&#39;s finger is probably obscuring the surface, it is not sensible to use energy to illuminate the user&#39;s finger as the lighting cannot be observed). It is common for navigation device engine to detect the presence or absence of features on the surface. This signal can also be used to control (e.g. enable/disable) the backlighting. 
         [0086]    As well as independent control by either the host or the navigation sensor, it is desirable to have co-operative control. In this mode the host would command the sensor to illuminate one/several/all LEDs, but this be gated with information from the sensor. For example,
       BACKLEDR=I2C_register_RED AND NOT (MOUSE_MOTION)       
 
         [0088]    In the example shown above, the host would request that the RED LED was turned on (for example to indicate a. low battery warning), but at the appropriate time, that is in the absence of motion over the surface of the sensor (i.e. finger not moving on the surface). This gating can be easily controlled by setting/clearing appropriate control bits via I2C or SPI register command. 
         [0089]    The pulse widths of the LED may be set via a dedicated I2C memory location (e.g. 0x01=2 μs, 0x02=4 μs etc.) or else there may be proportional or percentage values stored, along with a global fader value, for example 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Register Address 
                 Register Name 
                 Register Function 
               
               
                   
               
             
             
               
                 N 
                 REDPERCENT 
                 0 = LED off, 255 (FF) = LED 
               
               
                   
                   
                 maximum 
               
               
                 N + 1 
                 GREENPERCENT 
                 0 = LED off, 255 (FF) = LED 
               
               
                   
                   
                 maximum 
               
               
                 N + 2 
                 BLUEPERCENT 
                 0 = LED off, 255 (FF) = LED 
               
               
                   
                   
                 maximum 
               
               
                 N + 3 
                 FADER 
                 0 = All LEDs 
               
               
                   
                   
                 off, 255(FF) = LED setting 
               
               
                   
               
             
          
         
       
     
         [0090]    The host could then program in a particular colour, e.g. pink
       N←0xFF # Red value   N+1←0xC0 # Green value   N+2←0xCB # Blue value       
 
         [0094]    This could be turned on with
       N+3←0xFF
 
and the navigation device LED control would calculate the periods for TbrLED 4  TbgLED 4  &amp; TbbLED 4  such that the total of these periods is less than the period of Treset 4 .
       
 
         [0096]    If the period Treset 4  changes (e.g. due to change from navigation engine), the device would automatically adjust the periods of TbrLED 4  TbgLED 4  &amp; TbbLED 4  to maintain the correct ratio. Further, if the host wished to dim the backlight and keep the same colour, a single register write is used. 
         [0097]    Should the navigation device sensor integrate an ambient light detector (for an example, see applicant&#39;s co-pending application—attorney ref P113944.GB.01) not only should the navigation LED be disabled when device is measuring ambient light, but also the background illumination should be disabled when the device is measuring ambient light. The sensor for detecting ambient light may, in an embodiment, be the same sensor as the navigation device sensor. 
         [0098]    The above embodiments are for illustration only and other embodiments and variations are possible and envisaged without departing from the spirit and scope of the invention. For example the actual type of optical navigation device is not relevant so long as it is of a type that uses a radiation device and sensor to determine movement and at least a further radiation device for illumination. Also, the above non-exhaustive list of possible examples are purely illustrative of the utility of the invention disclosed herein.