Abstract:
An orientation sensor for use with an image sensor is provided, which includes at least two polarizers with different orientations and associated photodetectors and a signal processing unit. The orientation sensor can be incorporated in a digital camera. When the camera is exposed to daylight, which is polarized, the relative outputs from the differently oriented polarizers can be compared to record the orientation of the camera. This orientation can be stored with the image data so that a user does not have to manually change the orientation of an image on an image display device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an orientation sensor, and in particular, to an orientation sensor for use with an image sensor. 
     BACKGROUND OF THE INVENTION 
     Digital still cameras are used to take photographs which are usually rectangular, with the dimensions of the rectangle being defined by a long axis and a short axis. The user of a digital still camera (DSC) can operate the camera at any orientation. However, most photographs are taken either in a landscape orientation, where the long axis of a photograph is horizontal, or in a portrait orientation, where the long axis of a photograph is vertical. 
     Photographs that are taken are subsequently displayed on a display device such as a computer, which will usually present the photographs in a landscape orientation. Thus, any photographs that have been taken in the portrait orientation have to be rotated to be viewed in the correct orientation. This is time-consuming and annoying for a user of the camera and/or the display device. 
     Some cameras incorporate a mechanical device for measuring the orientation of the camera, such as a mercury or ball based tilt switch. However, these are physically large, which is a major disadvantage for incorporation in a modern DSC, and there are also environmental concerns with the use of mercury. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided an orientation sensor for use with an image sensor including at least two polarizers with different orientations, a photodetector associated with each polarizer, and a signal processing unit or means capable of determining the orientation of the image sensor from output signals of the photodetectors when the image sensor is exposed to polarized light. 
     In further aspects, the invention provides for an image sensor having an image sensing array and at least one orientation sensor. 
     A method of determining the orientation of an image sensor includes providing at least two polarizers with different orientations, providing a photodetector associated with each polarizer, exposing the image sensor to polarized light, and obtaining and processing output signals from the photodetectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic plan view illustrating an orientation sensor according to a first embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating the system implementation used with the orientation sensor of  FIG. 1 ; 
         FIG. 3  is a schematic plan view illustrating an orientation sensor according to a second embodiment of the present invention; 
         FIG. 4  is a schematic plan view illustrating an image sensor according to a third embodiment of the present invention; 
         FIG. 5  is a schematic plan view illustrating an image sensor according to a fourth embodiment of the present invention; and 
         FIG. 6  is a schematic diagram illustrating the system implementation used with the image sensor of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention relies on the principle that daylight is polarized, due to the scattering of light by dust particles in the atmosphere. This polarization is horizontal in normal daylight situations. 
       FIGS. 1 and 2  illustrate a first embodiment of the present invention. An orientation sensor  10  comprises a first polarizer  12  with an associated light sensor (not shown) and readout  16  and a second polarizer  14  with an associated light sensor (not shown) and readout  18 . Each polarizer with its associated light sensor can be termed to be a polarization “detector”. 
     By “polarizer” it is meant any suitable structure which provides a polarizing effect to light in the visible spectrum that passes through it. The polarizer could take any suitable form, for example a sheet of polarizing film, or a direct metal etching. A polarizer is defined by parallel spaced lines which are alternately transparent and opaque to incident light, and the orientation of a polarizer is defined as the longitudinal axis of these lines, i.e. as the direction of oscillation of the electromagnetic field of incident radiation that is allowed to pass through the polarizer. 
     In the first embodiment, the first and second detectors  12 ,  14  are used to measure the polarization of the light when the image is taken. The relative outputs of the detectors  12 ,  14  are then used to determine the orientation of the image sensor. When the invention is implemented in a DSC, this information can be stored with the picture (for example, with the standard EXIF format) and used later, for example by computer which is operated by image display software, to correct for the orientation of the polarizers  12 ,  14 . 
     The outputs from the readouts  16 ,  18  can be manipulated. in any suitable way to determine the orientation of the image sensor. One such way is illustrated in  FIG. 2 . The outputs from the two detectors  12 ,  14  are compared using a window comparator  20 , and an orientation decision is made based on whether the difference between the outputs is greater than a predetermined threshold, as follows: If (output( 12 )−output( 14 ))&gt;threshold then Portrait mode; If (output( 14 )−output( 12 ))&gt;threshold then Landscape mode; Else Unknown. 
     This assumes that light incident on the camera is horizontally polarized, and that detector  14  is horizontally polarized. With different polarizations, the portrait/landscape decision could be reversed. The definitions of landscape and portrait depend on a rectangular image being the result of image detection, as is the case in a DSC. However, it will be appreciated that the decision could be between two arbitrary first and second orientations such that the orientation of an image sensing array of any shape or size can be detected. The concept is thus extendible to other products other than a DSC, for example, an optical mouse, or to a mobile phone incorporating a digital camera, or other non-standard image sensors for specialized applications. 
     Typically, the threshold will be dependent on the signal levels used to make the system work over a wide range of illumination levels. The signal levels from the detectors themselves can be used for this function. 
     As a further option, a summer  22  can be provided to infer the brightness of the imaged scenes in which case the decision logic can be represented by the following: If (output ( 12 )−output( 14 ))&gt;(threshold*(output( 12 )+output( 14 ))) then Portrait mode; If (output ( 14 )−output( 12 ))&gt;(threshold * (output( 12 )+output( 14 ))) then Landscape mode; Else Unknown. 
     If the summer  22  is not provided or implemented, the Maximum signal can be used as a crude approximation to scene brightness, the decision logic being represented by: If (output ( 12 )−output( 14 ))&gt;(threshold*Max(output( 12 ), output( 14 ))) then Portrait mode; If (output ( 14 )−output( 12 ))&gt;(threshold*Max(output( 12 ), output( 14 ))) then Landscape mode; Else Unknown. 
     The schemes described above are simple, “hard-wired” systems. This decision logic could be effectively implemented using a fuzzy logic or neural network type of logic. 
     As the present invention relies on the detected signal brightness, the configuration shown in  FIG. 1  is sensitive to shading across the scene which could be mistaken for variation in polarization. Errors caused in this way are reduced by a second embodiment of the invention, which is illustrated in  FIG. 3 . As seen in  FIG. 3 , an orientation sensor  30  comprises first and second vertical detectors  32 ,  34  and first and second horizontal detectors  36 ,  38 , with associated readouts  40 ,  42 ,  44  and  46 . 
     The vertically polarized detectors  32 ,  34  and the horizontally polarized detectors  36 ,  38  share a common center. If there is a shading (for example if left is brighter than right), then the left hand vertically polarized detector  32  will have a higher output than the right hand vertically polarized detector  34 , and the left hand horizontally polarized detector  38  will have a higher output than the right hand horizontally polarized detector  36 . 
     Although not shown, these differences can be compensated using summers to sum the output from the readout channels with the same polarization before using a comparator to compare with the opposite polarization. An optional summer may be provided, which can be used in the same manner as the summer  22  described in the first embodiment of the present invention. 
     It will be appreciated that the above first embodiment, comprising two polarizers having opposite orientations, and the above second embodiment, comprising two pairs of polarizers, each pair having opposite orientations, are only specific examples which serve to illustrate the scope of the invention. In variations of these embodiments, any or all of the number of polarizers, the size of a grid of polarizers, and the orientations of polarization of the detectors can be varied. 
     A third embodiment of the invention is illustrated in  FIG. 4 , which shows an integrated DSC image sensor and orientation sensor. An orientation sensor  60  is placed adjacent to an image sensor array  62 . The orientation sensor  60  can be an orientation sensor  10 ,  30  according to either of the above described first and second embodiments, or a variation thereof. 
     The orientation sensor  60  is preferably formed on the same substrate as the imaging device. The image plane is usually rectangular and fits inside the image circle  64  of the lens (not shown). If the orientation sensor  60  is close to the imaging array  62 , it will be inside the image circle  64  of the camera&#39;s optics system. This has the cost advantage of not having to modify the existing optics system. 
     Although outside the normal image plane, the polarization sensor  10 ,  30 ,  60  is larger than the imaging pixels and so does not require an image which is as sharp or bright. Although the orientation sensor can be on any side of the image array  62 , it is preferable to have it at the lower side (as mounted in the camera). With this arrangement, it will image the upper part of the camera&#39;s field of view. Usually, this will be the sky—which shows the highest amount of polarization. Although the configuration shown in  FIG. 4  is a workable system, it has the disadvantage of not being able to distinguish the sense of rotation of a camera, i.e. to distinguish between 90° C. rotation clockwise or anticlockwise. 
       FIGS. 5 and 6  illustrate a fourth embodiment of the invention, which helps overcome this problem in certain situations. When a photograph is taken in daylight, the lightest part of the screen, usually the sky, is normally at the top of an image. Thus, the signals from three orientation sensors  70 ,  72 ,  74  can be processed as illustrated in  FIG. 6 . The portrait and landscape outputs from the three orientation sensors  70 ,  72 ,  74  are summed and the majority decision from these outputs is used to determine between landscape and portrait modes. 
     If the system has determined that the camera is in portrait mode, then the brightness outputs from the two orientation. sensors  70 ,  74  on the short axes of the sensor  66  are compared. Assuming the configuration shown in  FIG. 6  is a view facing the image, then if orientation sensor  70  has a higher scene brightness output than orientation sensor  74 , then it is determined that the image sensing array  66  (and hence the image sensor) has been rotated 90° C. clockwise (as viewed by the cameraman, to the rear of the camera), which puts orientation sensor  70  at the physically lower part of the camera—the lens inversion imaging the upper part of the scene on orientation sensor  70 . 
     For all the above embodiments, the polarizers  12 ,  14 ,  32 ,  34 ,  36 ,  38  can be manufactured as an integrated photodetector and polarizing assembly. This leads to a smaller and cheaper DSC, and as the polarizer assemblies are stand-alone and use no mechanical parts, the reliability of their operation is therefore enhanced when compared with known polarizers, which are manufactured as separate components to be added to the optical stack. It would of course be possible to fabricate the polarizers separately from the sensors if this is more practical. 
     The invention has been described above with particular reference to a DSC implementation. However, it will be appreciated that the principles of the invention have a wider application and as such can be considered to extend to other products such as an optical mouse, or a mobile telephone incorporating a digital camera. 
     Improvements and modifications can be made to the above without departing from the scope of the present invention.