Abstract:
An image sensor may have a pixel array and an imaging lens for forming an image on the pixel array. The sensor may also include a pixel readout unit for enabling individual pixel values to be readout. The sensor may further include a pixel selection unit wherein at least one pixel sub-array is selected according to the pixel values readout and the at least one sub-array is used for reading the image.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to image sensors with a pixel array, and, in particular, to such image sensors having selectable sub-arrays.  
       BACKGROUND OF THE INVENTION  
       [0002]     Devices containing image sensors, as a primary or secondary function, are now extremely common. For example, an optical mouse has an image sensor that allows the mouse to detect movement over a surface and many mobile cellular telephones have an integral camera. Image sensors that are used for these types of devices should generally be low cost and use components that minimize cost.  
         [0003]     U.S. Pat. No. 6,763,157 discloses a system for utilizing a pixel array to receive data transmission from one or more optical fibers.  
       SUMMARY OF THE INVENTION  
       [0004]     According to a first aspect, there is provided an image sensor that may have a pixel array and an imaging lens for forming an image on the pixel array. The sensor may further comprise a pixel readout unit or readout means for enabling individual pixel values to be readout and a pixel selection unit or selection means, wherein at least one pixel sub-array may be selected according to the pixel values readout with the at least one sub-array being used for reading the image. Preferably, pixels not selected in the at least one sub-array may not be read out, the readout unit or readout means thereby only reading out pixel values of the at least one sub-array.  
         [0005]     Alternatively, pixels not selected in the at least one sub-array may be disconnected, the readout unit or readout means thereby only reading out pixel values of the at least one sub-array. Alternatively, pixels not selected in the at least one sub-array may have their pixel values ignored. Preferably, the pixel selection unit or selection means may maintain a map of selected pixels.  
         [0006]     According to a second aspect, there is provided a method of selecting a pixel sub-array from an image sensor that may comprise a pixel array and an imaging lens. The method may comprise illuminating the pixel array with a set or predetermined illumination, reading out pixel values from the pixel array, and selecting a sub-array according to the pixel values from the set or predetermined illumination.  
         [0007]     Preferably, the set or predetermined illumination may comprise a continuous, regular or predetermined illumination test pattern. Preferably, the predetermined illumination test pattern may comprise an image test chart with centration feature or features. Preferably, the image test chart may be a Local Contrast Normalization chart.  
         [0008]     Preferably, the output from the pixel readout unit or readout means may also be utilized to focus the image sensor. Preferably, the sub-array may be selected according to expected pixel values for the set or predetermined illumination which may be the highest pixel values, the lowest pixel value, the predefined pixel values, or within a range of predefined pixel values.  
         [0009]     According to a third aspect, there is provided a method of identifying “bad” pixels from an image sensor that may comprise a pixel array and an imaging lens. The method may comprise illuminating the pixel array with a set or predetermined illumination, reading out pixel values from the pixel array, and identifying pixels with excessive or decreased photo response. Preferably, the image sensor may store a map of the “bad” pixels to enable compensation of the output for the “bad” pixels.  
         [0010]     According to a fourth aspect, there is provided an optical pointing device that may comprise an image sensor according to the first aspect. Preferably, the optical pointing device may comprise an optical mouse.  
         [0011]     According to a fifth aspect, there is provided a mobile device that may comprise an image sensor according to the first aspect of the invention. Preferably, the mobile device may comprise at least one of a mobile cellular telephone, a camera, a portable computer, a personal digital assistant device, and a Web Cam.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which;  
         [0013]      FIG. 1  illustrates an image sensor semiconductor package according to the invention;  
         [0014]      FIG. 2  illustrates an image sensor according to the invention including semiconductor package, imaging lens and lens holder; and  
         [0015]      FIG. 3  illustrates a flow diagram of an image sensor sub-array pixel selection unit or selection means. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     A typical image sensor may have a rectangular pixel array, usually with a 4:3 aspect ratio. Imaging lenses are typically circular and consequently form a circular image. To ensure that the output from an image sensor does not have redundant areas, the image sensor is designed so that the imaging lens forms an image that is larger than the pixel array.  
         [0017]     Semiconductor chips are often pad-limited in size, that is, they should be a particular size for enabling connections to be made in the semiconductor package. This means that there can be additional space on the semiconductor chip. As pixel size has steadily decreased with advances in semiconductor technology, it is possible to have a larger pixel array than is required for an application. Furthermore, applications which do not pad-limit the size of the semiconductor chip can benefit from a larger than required pixel array due to cost savings in lens components, such as an imaging lens and lens holder, due to the improved tolerances of an image sensor in accordance with the invention.  
         [0018]     Referring now to  FIGS. 1 and 2 , an image sensor  10  comprises a semiconductor image sensor package  11 , a lens holder  13 , and an imaging lens  15 . The semiconductor image sensor package  11  comprises a pixel array  12 , a pixel readout unit or readout means  14 , a pixel selection unit or selection means  16 , and control circuitry  18 .  
         [0019]     An image outline  20 , formed by the imaging lens  15 , is projected from the imaging lens  15  on to the pixel array  12 . The imaging lens  15  has a focal length and a field of view such that the image outline  20  is smaller than the pixel array  12 . The pixel array  12  has an array center  17  and the imaging lens  15  has a lens center  19 .  
         [0020]     In prior art systems, the pixel array  12  and the imaging lens  15  had to be aligned as closely as possible. The tolerance in prior art systems had to take into account imaging lens tilt, imaging lens to lens holder tolerances, and positioning of the lens holder in relation to semiconductor image sensor package  11 .  
         [0021]     In the sensor package  11 , however, alignment of the array center  17  and lens center  19  is not crucial. The lens center  19  is found on the pixel array  12  during a system test. The system test may be during unit manufacture, system initialization, or periodically during normal operation. For example, the system test may be performed periodically during normal operation by raising the exposure level of a scene with a set or predetermined amount of light allowing an approximation of continuous illumination. A system test performed during manufacture is described below.  
         [0022]     Referring to  FIG. 3 , the image sensor  10  undergoes a system test  30  during manufacture. Firstly, the image sensor enters a test mode  32  and then performs a dark calibration or offset adjustment  34 .  
         [0023]     At this stage the image sensor  10  is illuminated by a set or predetermined illumination. This may be a continuous illumination for a specified period or a regular pattern or predetermined image test chart with centration feature or features. For example, an image test chart could be a single dot on a uniform background,  2  features on an image axis (horizontal or vertical), a resolution chart or Local Contrast Normalization (LCN) chart, as defined by the Standard Mobile Imaging Architecture (SMIA). Using a LCN chart is particularly advantageous as an image sensor is typically illuminated with a LCN chart during manufacture to allow the imaging lens to be correctly focused on the pixel array. The output from the LCN chart could also be directed towards the system test for identifying a sub-array.  
         [0024]     The system test  30 , in this case, uses a continuous white illumination. The entire pixel array  12 , using the pixel readout unit or readout means  14 , is read out in a read step  36 . In a calculation step  38 , the pixel selection unit or selection means  16  analyzes the pixel values of each pixel or a predetermined size group of pixels of the pixel array  12  to identify the pixel intensity mean. Then, during a comparison step  40 , the pixel intensity means are compared and an identification step  42  identifies pixels with the highest mean intensity that comprise a predetermined sub-array size. For example, the predetermined sub-array could be a VGA (640×480) sub-array within a pixel array  12 , which is a SVGA (800×600) array.  
         [0025]     In alternative embodiments, the steps  38  to  42  may use other methods to identify the sub-array of interest, which may be highest value, lowest value, predetermined value or within a predetermined range of values. The system test  30  also performs a “bad” pixel identification step  44 . The step  44  identifies pixels with excessive or decreased photo response, as these pixels comprise the defective or “bad” pixels.  
         [0026]     If required, a repeat step  46  can repeat steps  36  to  42 / 44  two to N times. The results from the repetition can then be averaged to reduce noise and improve accuracy.  
         [0027]     A final store step  48  stores the location of the identified sub-array  22 . The store can be performed using external memory, internal memory or on-die fuses. The location of selected pixels and defective pixels can be stored using a map that may be run-length encoded map.  
         [0028]     Once the sub-array  22  has been selected for operation, unused pixels are disconnected, their values are ignored or they are simply not addressed during readout. In this example, the pixel selection unit or selection means is preconfigured to select a 3 by 3 square of pixels and consequently, as shown in  FIG. 1 , selects a sub-array  22 . In this case, the sub-array  22  represents the area of pixels that have responded as expected to the set or predetermined illumination.  
         [0029]     As the pixel array  12  is larger than the image outline  20 , the tolerances required to position the imaging lens are not required to be as small as if the pixel array  12  was the same size or smaller than the image outline  20 . The typical alignment tolerance of a prior-art image sensor is in the region of ±200 μm between the image sensor semiconductor package, the imaging lens and lens holder. With an image sensor as described herein, an alignment tolerance of ±10 μm can be achieved between the image sensor semiconductor package, the imaging lens and lens holder using the same imaging lens and lens holder. This is due to the alignment of the center of the sub-array  22  being calibrated after the image sensor has been manufactured and is facilitated by the pixel array  12  being larger than the size of the image  20 .  
         [0030]     As such, the imaging lens can be integrated into a low-cost package with resultant cost savings. Furthermore, the selectable sub array will enable the maximum yield to be obtained; avoid accurate lens to array positioning; and improve image quality by enabling the region with the optimum relative illumination to be utilized.  
         [0031]     In addition, the pixel selection unit or selection means  12  may also identify pixel values that are inconsistent with surrounding pixels. These inconsistent pixel values may be identified as “bad” pixels. The pixel selection unit or selection means can maintain a map of bad pixels, which allows them to be better compensated for, thus improving the manufacturing yield.  
         [0032]     Improvements and modifications may be incorporated without departing from the scope of the invention.