Abstract:
Look ahead shutter pointer used for exposure control determination. One shutter is used to remove the pixels from reset. This begins the integration process. The pixels are integrated by turning them on one by one and sampling their outputs sometime later. The amount of time between turning them on and sampling them can be varied. This can be done while the system is being used. In the preferred embodiment the time of exposure is changed at the interface between two frames to avoid problems in the image.

Description:
BACKGROUND OF THE INVENTION 
     Image sensors are of ten embodied by a plurality of photo sensitive pixels. Each pixel receives information indicative of a portion of the image. The information is in the form of incoming photocarriers indicative of the incoming light photons. Each pixel accumulates photocarriers indicative of the amount of light impinging on that pixel. That amount of time is often called the exposure. 
     SUMMARY OF THE INVENTION 
     The present invention describes varying the pixel exposure by varying the time between enabling the pixel to integrate and reading the pixel. The pixels are formed into an array of pixels. Two counters are used to address the array. One counter, called the shutter pointer, keeps track of the address of the pixels that will next be enabled. Once enabled, those pixels will begin to integrate. The other counter called the read counter keeps track of the row address to be read. The time between enabling and reading determines the exposure. 
     For any given exposure, the difference between these two counters is always the same. Each pixel in the array hence gets the same amount of exposure, thereby avoiding one portion of the image having a different look than another. 
     The exposure can be changed by an automatic exposure technique or by manual input from the user. The change of exposure changes the difference between the enable and read addresses. 
     One aspect is to change this exposure using a seamless technique in which no one image/field has internally-varying characteristics. This is done without changing the frame rate or diffusing the exposure difference for pixels within the same frame. 
     Another aspect uses a third counter to look ahead to the next frame while the read pointer and shutter are processing the current frame. This third counter evaluates the program exposure at the start of a new frame. When the exposure changes, the third counter is advanced to the next frame. The current frame with the old exposure is simultaneously processed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These techniques will be described with reference to the accompanying drawings wherein: 
     FIG. 1 shows the system with the shutter closed and the array and the reset state; 
     FIG. 2 shows the start of exposure; 
     FIG. 3 shows the advance of the shutter; 
     FIG. 4 shows how each row is read into an analog to digital converter and the shutter is closed behind it; 
     FIG. 5 shows the positions of the read pointer and shutter pointer as the process continues; 
     FIG. 6 shows the first step in a process of changing the exposure; 
     FIG. 7 shows the shutter conditions at the bottom of that window; 
     FIG. 8 shows the bottom of the window and the beginning of the next window; 
     FIG. 9 shows the complete new frame with the set up shutter width; 
     FIG. 10 shows continuation of the reading; 
     FIG. 11 shows the new pointer initiating a new shutter width; 
     FIG. 12 shows the continuation of the reading with both shutters enabled; 
     FIG. 13 shows the first shutter pointer wrapping around loaded with the second shutter pointer value; 
     FIG. 14 shows the new frame starting with the updated shutter width; 
     FIG. 15 shows the beginning point of a user entering a new shutter width which is smaller than the old shutter width; 
     FIG. 16 shows the shutters enabled and disabled as the bottom of the current array is reached; 
     FIG. 17 shows the shutter pointer wrapping around and waiting for the read pointer to collapse the shutter width; 
     FIG. 18 shows the continuation of the shutter pointer as it continues to wait; 
     FIG. 19 shows the shutter pointer starting the next frame with the new collapsed shutter width; and 
     FIG. 20 shows the new frame with the updated shutter path. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is specifically intended for use in an image sensor of the active type. The specific embodiment uses an active pixel sensor of the type described in U.S. Pat. No. 5,471,515, the disclosure of which is incorporated by reference to the extent necessary for understanding of the invention. However, any CMOS image sensor or, more generally, any image sensor of any type could be used according to these techniques. 
     FIG. 1 shows a block diagram of the active pixel sensor array, along with a sample schematic of the way the array can operate. While this system shows a photodiode pixel, it should be understood that this method applies to any kind of pixel including, for example, a photogate pixel. 
     When the array is in the reset state, the APS array  100  shows all pixels as being shaded, meaning that all are in the “don&#39;t care” state. 
     The pixels are collectively sampled into an A/D converter using a system with a floating diffusion  102  that is sampled into a source follower  104 . The output of the source follower  104  is also sampled through a row selector transistor  106  onto column bus  108 . The column bus  108  is coupled into the A/D converter array described herein. 
     The control circuit is shown in block diagram form in FIG.  1 A. Two shutter pointers: shutter A  150  and shutter B  152  are provided. These pointers respectively begin the row integration process by releasing the row from reset. The row is reset by providing a VDD potential on the gate  110  that connects the floating diffusion  102  to a sink well  112 . When the reset value is released, the floating diffusion  102  can accumulate charge from incoming photocarriers. 
     Counter  154  represents the read counter which selects the transistor  106  and thereby provides the output signal to the A/D converters. 
     In FIG. 1, the shutter is closed meaning that the gate  110  is in the reset state. The potentials of the various elements are shown in state diagram  115 : all incoming photocarriers are sinked to the well  117 . 
     In FIG. 2, the exposure is started. This is done by advancing the shutter counter (shutter A  150 ) through the array selectively releasing the array from reset. The rate at which the opening is advanced through the array is equal to the read row rate. The operation occurs by releasing the reset to each row in a timed manner. The timing is controlled by controller  160  which can be dedicated logic or a microprocessor. 
     FIG. 2 shows how the states change when reset is released. Each floating diffusion begins to integrate charge once released. The read pointer is disabled, to avoid any false reads. 
     In this example of FIG. 2, column  4 , shown by pointer  200 , is being advanced cyclically. Therefore, column  1  has been opened for the time of 4 clock rate exposures. 
     FIG. 3 shows the operation continuing. Pointer  200  has now reached the 15th row and in this example, we assume that 15 exposure cycles are desired for a total exposure time of 15-N where N is a time between subsequent advances of the shutter. When N=15, the read pointer  154  is enabled; shown as element  300 . The enabling of read pointer  300  enables the row select  106  and thereby provides the charge from that row onto the column bus  108  into associated A/D converter  302 . In this example, therefore, the first row of the array is read into the column processing A/D circuits. The shutter advancing speed matches the row processing speed. Here, the shutter width equals 14 rows and the pixel integration time equals 14 times the process time per row. 
     After reading out the row, the reset for that particular row is again brought high to maintain the pixel in reset. 
     FIG. 4 shows the continuation of the process. The read pointer  300  is always 15 rows behind the shutter pointer  200 . Each row is read into the A/D converter, and the shutter is closed behind it by bringing the appropriate reset transistor  110  into reset. 
     FIG. 5 shows the way in which the pointers wrap around. When the shutter pointer reaches the bottom  500  of the array, it wraps back around to the top  502  of the array. The read pointer is still reading behind the time of the shutter pointer enablement. 
     The operation of selecting a new frame is shown in the flow chart of FIG.  5 A and with reference to FIGS. 6-14. FIG. 6 shows the beginning of a new frame. As shown in step  550  of the flow chart of FIG. 5A, the shutter pointer  1  is set to the desired width, pointer  2  is disabled, and the read pointer is enabled. The read pointer tells the pixels to be read a specified amount of time after the shutter pointer has enabled them. FIG. 6 shows the system operating with the current shutter width of three rows. 
     Some time after the operation of FIG. 6, the user enters a new shutter width as shown in step  552 . The new shutter width in this example is 14 rows although it could be any value. FIG. 7 shows the shutter reaching the bottom of the window. FIG. 8 shows the read pointer reaching the bottom of the window and FIG. 9 shows the read pointer reaching the top of the window. When the read pointer reaches the top, as detected at step  554 , the shutter pointer number  2  is set to the new width. However, pointer number  2  remains disabled at this time and through the operation of FIG. 10 where it moves in step with pointer  1  and the read pointer. This is represented as step  556  in FIG.  5 A. When pointer number  2  reaches the end of the row, as detected at step  558  and shown in FIG. 11, it is enabled to initiate the new shutter width. The read pointer  200  is still reading three rows behind the shutter pointer  1   300 . However, subsequent top rows such as  1000  are being turned on by shutter pointer  2   1002 . This is shown generally as step  560  in FIG.  5 A. The reads continue as shown in FIG.  12 . When pointer  1  reaches the bottom of the row, in step  562  in FIG. 13, pointer  1  receives the contents of pointer  2  (step  564 ) and wraps around loaded with the value of pointer  2 . Therefore, when the read pointer  200  reaches the bottom of the row, the bottom-most row receives the last three row exposure. The top of the next row, shown in FIG. 14, has a 14-row shutter width. 
     The opposite operation is carried out for shortening the row as shown in FIG.  15 . FIG. 15 represents the time when the user enters a new shutter width that is smaller. FIG. 15 shows a 14-width row, and the new shutter width is 8 rows. An analogous operation occurs, with FIG. 16 showing the read pointer  200  and 2 shutter pointers. 
     When the shutter pointer wraps around, the read pointer is still 14 columns behind the shutter pointer. The shutter pointer, however, does not change value until the read pointer collapses the shutter width to 8 columns as shown in FIG.  18 . At that time, the shutter pointer starts moving again as shown in FIG.  19 . The new frame with updated shutter width is shown in FIG.  20 . 
     The system described herein could be carried out using a processor or hard-wired logic. The preferable way to do this is digital control of the logic using gates defined in hardware description language or HDL. 
     Embodiments are contemplated and are supported by the claims.