Abstract:
A vignetting-type chromatic error, particularly found in image sensors having pixel dimensions of a few microns, may be corrected by applying to each color channel a gain factor which increases with the radial position of the pixel. The gain factor may be also controlled based upon ambient color temperature

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the correction or reduction of vignette-type errors produced in solid state image sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vignetting is well known in imaging systems, being the effect which causes image brightness to reduce with increasing distance from the optical axis. In traditional film cameras, vignetting results from the lens design producing non-uniform brightness, and from the presence of external obstructions such as lens hoods. Such vignetting can only be reduced by improved lens design and by avoiding external obstructions at the edge of the field of view. 
         [0003]    Solid state image sensors can suffer vignetting for the same reasons. However, in this case the vignetting can be counteracted electronically by signal processing. It is known to do so by controlling the gain in the output signal channels as a function of the radial distance of the relevant pixel from the optical axis. In known color sensors, this gain control is applied equally to each of the color channels, and is basically a grey scale variation. 
         [0004]    The known anti-vignetting arrangements may not produce acceptable results under all conditions. This is particularly the case with physically small sensors where the individual pixel sizes may be of the order of a few microns, and where simple one- or two-element plastic lenses are used. This situation is common in cameras incorporated in, for example, mobile phones. 
         [0005]    It has been found that in these situations the final image may contain vignetting-type errors wherein color errors are present which increase in significance with increasing radial distance from the optical axis. These errors are of a nature which is noticeable and obtrusive to the viewer. It is believed that this effect is caused by diffraction effects at the surface of the image sensor produced by its small feature size. There may also be a contribution from the use of simple and inexpensive lenses which have non-uniform color dispersion. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to eliminate, or at least reduce, the vignette-type errors discussed above. 
         [0007]    The invention provides a method of correcting chromatic vignetting in the output of a solid state image sensor having a pixel array, the method including:(a) detecting the ambient light temperature; and (b) adjusting the gain of the output signal by varying gain as a function of radial distance from the optical axis; and wherein (c) the gain adjustment is selected based upon the ambient light temperature. 
         [0008]    Typically, the output signal comprises RGB color channels, and the gain of all channels is adjusted for white balance before carrying out step (by. Preferably the gain adjustment of step (b) is carried out on at least the red and blue of the RGB channels. The ambient light temperature may suitably be derived from data relating to white balance. The white balance data may be used to classify the ambient lighting conditions as one of a number of standard conditions, such as daylight, dusk, tungsten, and fluorescent. The channel gains to be applied are preferably read from a look-up table. 
         [0009]    From another aspect, the invention provides an image sensor including: a pixel array; a readout block or means for reading out pixel values to form an image signal; a unit or means for detecting the ambient light temperature; and a gain adjustment block or means for adjusting the gain of the image signal as a function of the radial distance from the optical axis by a factor which is dependent on the ambient light temperature. 
         [0010]    Preferably, the pixel array comprises RGB pixels, the image signal comprises RGB channels, and the gain adjustment means operates on at least two of the RGB channels independently; and the image sensor further includes a white balance unit or means operable to adjust the gain of all channels to provide white balance. The white balance means preferably also acts as the ambient light temperature detecting means. The gain adjustment means may be controlled by a look-up table, which may define gain factors for a number of standard ambient lighting conditions, such as daylight, dusk, tungsten, and fluorescent. 
         [0011]    The invention further provides a camera incorporating an image sensor as defined above; and a mobile telephone incorporating such a camera. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: 
           [0013]      FIG. 1  is a schematic block diagram illustrating one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring to  FIG. 1 , an image sensor has a pixel array  10  controlled by a control circuit  12 , and a readout circuit  14  which reads out pixel values to provide RGB signals at  16 . The precise form of these elements is not significant to the present invention; typically they will be incorporated in a single-chip CMOS image sensor, and the readout circuit will perform image processing functions such as reduction of kT/C noise by correlated double sampling or other known techniques. 
         [0015]    A white balance circuit  18  is provided, as is well known per se. This provides auto white balance by statistically analyzing the content or bias of red, green and blue in any given scene. The model assumes that the average content in any given scene will be equal (i.e. overall gray), and the deviation from this gray model is used to compensate the image signals in such a way as to cancel the effect of any dominant lighting. In  FIG. 1  this is indicated by gain adjustment  20 , in which the gain of each of the RGB channels is altered by a factor selected to produce a ‘gray’ average. It will be appreciated that such gain adjustment applies a gain selected for the given channel which is the same across all the pixels of that channel in the pixel array  10 . 
         [0016]    The white-balanced ROB signals are then further adjusted at  22  in response to an anti-vignetting control  24 . In this stage, the gain in a given channel is adjusted as a function of the radial distance of the relevant pixel from the optical axis, in other words pixels further from the axis will be subject to a greater gain than those closer to the axis. The variation in gain will typically be linear, but powers other than unity may be required; this can be established empirically. 
         [0017]    We have established that the chromatic vignette-type aberrations which this invention addresses vary with the nature of the ambient light, and may require a different balance of RGB corrections also varying with ambient light. Specifically, the required corrections can be defined by reference to the ambient color temperature, and accordingly the gain adjustment  22  is controlled in dependence on ambient color temperature. 
         [0018]    A convenient means of supplying ambient color temperature information to the control  24  is to derive this from the existing auto white balance provision. Since the white balance circuit examines the balance of RGB in the overall scene, it contains information which approximates to the color temperature of the scene. This information can be conveniently defined as a position on a color temperature locus, or as a vector in a two-dimensional color space, and the chosen parameter may be used as a control input for the anti-vignetting gain adjustment. 
         [0019]    In the present embodiment, the auto white balance information is applied to a look-up table  26  to obtain predefined gain factors for the three channels. These may be based, for reasons of simplicity, and assigning the white balance information to one of a limited number of defined categories. In practical terms, a reasonable degree of correction can be obtained by defining four categories of lighting conditions, namely daylight, dusk, tungsten and fluorescent, which will cover most user situations. For some applications fewer or more categories could be used. 
         [0020]    Although certain parts in the foregoing description have been described as “circuits”, it will be appreciated that the relevant functions may be performed by a mixture of firmware and software rather than dedicated circuits, as is well known in the art, and that the whole system will normally be incorporated in a single chip. 
         [0021]    The embodiment described uses a RGB color separation scheme, but the invention applies equally to other forms of color separation. The invention may also find application in monochrome image sensors if very small pixel sizes are used, in view of the diffraction effects discussed above. 
         [0022]    In the above embodiment, the ambient color temperature is derived from the white balance information. However, it is equally possible to derive a measure of the ambient color temperature in some other way, for example by using a separate RGB sensor (i.e. separate from the pixel array) measuring the overall RGB values of the scene; such sensors are known and commercially available. It would also be feasible to measure only red and blue in the scene, since in many real life situations green level is approximately constant under varying lighting conditions. 
         [0023]    The embodiments described herein thus provide a small and low-cost image sensor that avoids the above-discussed image degradation. The invention is of particular utility in low cost portable devices such as mobile telephones and webcams.