Patent Publication Number: US-8530815-B2

Title: Imaging device with varying optical signal at a location of a multi-dimensional array of light sensitive elements

Description:
BACKGROUND 
     A challenge exists to deliver quality and value to consumers, for example, by providing various capabilities in imaging and printing devices while maintaining cost effectiveness and output speed. Further, imaging and printing businesses may desire to enhance the functionality of their devices. For example, such businesses may desire to provide enhanced image reproduction capability without requiring additional effort on the part of such consumers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description references the drawings, wherein: 
         FIGS. 1   a - 1   d  illustrate a rolling reset and rolling readout of a two-dimensional sensor array, as used in one example. 
         FIG. 2  is a graph illustrating a decrease of a percent of illumination intensity as a function of the cosine of the angle off-center from an optical axis, raised to the fourth power, as used in one example. 
         FIG. 3  shows the uncorrected illumination profile for a rectangular document captured by an 82 degree total field-of-view lens, prior to correction in one example. 
         FIG. 4  is a block diagram and example of an imaging device. 
         FIG. 5  is an example of an illumination level profile. 
         FIG. 6  shows an example of a corrected illumination profile at a multi-dimensional array of light sensitive elements for a rectangular document captured by an 82 degree total field-of-view lens. 
         FIG. 7  is another example of an illumination level profile. 
         FIG. 8  shows another example of a corrected illumination profile at a multi-dimensional array for a rectangular document captured by an 82 degree total field-of-view lens. 
         FIG. 9  illustrates examples of some of the devices in which the imaging device of  FIG. 4  may be used. 
         FIG. 10  is a block diagram and example of a method for use in an imaging device. 
         FIG. 11  is a block diagram and example of additional elements of the method shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Imaging devices ideally capture images and reproduce them as accurately as possible. These captured images can include things such as photographs and scanned documents. However, realistic reproduction can be difficult because of challenges and limitations associated with a particular design. 
     For example, in some optical imaging systems, light from an image is focused by optics onto a two-dimensional sensor array. The two-dimensional sensor array is divided into rows of light sensitive pixels. A rolling reset first proceeds through the two-dimensional sensor array and successively resets each row. After an appropriate exposure time has elapsed, a rolling readout operation proceeds through the sensor array to capture the exposure value for the pixels in that array. An image is then constructed from these exposure values. 
     For example, as shown in  FIG. 1   a , a rolling reset starts at row one  10  of two-dimensional sensor  20 , at time zero (t=0), and then proceeds down one row per time period in the direction of arrow  30 , as shown in  FIG. 1   b . The time period is set to be the amount of time the system requires to read one row. In this example, the exposure time is set to five such that row one  10  is read out by the system after five time periods, as shown in  FIG. 1   c . Subsequently, row two  40  is read out at time six (t=6), as shown in  FIG. 1   d . In a typical system, all rows in the sensor array have an identical exposure duration using a combination of the rolling shutter and the subsequent rolling readout. 
     In such optical imaging systems, the collected light is lower for portions of an image off-center from the optical axis. The illumination intensity decreases as a function of the cosine of the angle off-center from the optical axis, raised to the fourth power.  FIG. 2  illustrates this effect graphically where the percent illumination  50  is graphed as a function of the angle theta in degrees off-center from the optical axis  60 . 
     This “cosine-four impact” causes the collected light and corresponding signal-to-noise ratio to be significantly lower for the outer portions of the image because illumination is controlled to prevent the central portion of an image from being saturated. This results in low image quality at the outer edges of the image. For systems using a two-dimensional imaging sensor, the signal-to-noise at the corners of the image can be more than three-times lower than the signal-to-noise in the center of the image. 
     In addition, non-uniformity of the illumination source can further reduce the signal-to-noise at the corners of the image. This occurs because in many illumination sources the output signal or intensity is typically brighter towards the center of an illuminated area and is reduced in intensity towards the edges of the illuminated area. 
       FIG. 3  shows the uncorrected illumination profile  70  for a rectangular document captured by an 82 degree total field-of-view lens. For a uniform target, the illumination intensity or level  75  at the corners  80  of the image is approximately 32% of the illumination intensity at the center  90  of the optical field.  FIG. 3  illustrates the results for an image sensor using a 44×33 grid (44 columns  100  and 33 rows  110 ) which corresponds to a 4:3 aspect ratio used in some image sensors. 
     A solution to this “cosine-four impact” is to apply analog or digital gain to the outer portions of the image to increase the signal and normalize the image. However, such a gain technique increases both the signal and the noise such that the signal-to-noise ratio of the outer portions is typically very poor. Alternatively, a system can capture multiple images with different illumination levels and construct a composite image. However, this method is computationally intensive and it is difficult to achieve proper uniformity and linearity in the image. It can also add cost due to the additional needed computing capability and can lower the speed at which images can be produced. 
     To optimize the signal-to-noise in the image, the illumination of the captured image should be as uniform as possible over the image field of the sensor. An example of an imaging device  120  designed with this goal in mind is shown in the block diagram of  FIG. 4 . Imaging device  120  includes a multi-dimensional array of light sensitive elements  130 , an illumination source  140  that outputs an optical signal  150 , an optical element  160 , and an illumination controller  170 . As shown in  FIG. 4 , illumination source  140  may include one or more light-emitting diodes (LEDs)  145 . Multi-dimensional array  130  includes a plurality of rows  180  and columns  190  of light sensitive elements that are arranged so as to be positioned relative to a central location  200 . In this particular example, array  130  includes thirty-three (33) rows  180  and forty-four (44) columns  190 . 
     Illumination source  140  generates an optical signal  150  that illuminates object  155 . Optical element  160  is designed to image object  155  which is illuminated by optical signal  150  on multi-dimensional array of light sensitive elements  130 . Optical element  160  can be a variety of designs and include one or more lenses, mirrors, or a combination of the two. Illumination controller  170  is designed to vary an output of optical signal  150  of illumination source  140  to control exposure at one or more locations of array  130 , as generally indicated by double arrow  210 . For example, illumination controller  170  may vary the output of optical signal  150  based on distance from central location  200 . As another example, illumination controller  170  may vary the output of optical signal  150  based upon a location of a row  180  within array  130  or based upon a location of a column  190  within array  130 . 
     Imaging device  120  utilizes a rolling shutter that consists of two synchronized events, a rolling reset and a rolling readout, as described earlier in  FIGS. 1   a - 1   d , both of which occur on multi-dimensional array  130  of light sensitive elements. First the rolling reset proceeds through array  130  and successively resets each of the rows  180  to an initial value. After the appropriate exposure time has elapsed, the rolling readout operation proceeds through each of the rows  180  of array  130 . Illumination controller  170  synchronizes optical signal  150  of illumination source  140  with the rolling shutter such that the output of optical signal  150  is increased while exposing rows  180  or columns  190  further away from central location  200  and decreased during exposure of central location  200 . This technique allows the illumination level to be increased for the outer portions of the image and reduces the “cosine-four impact”, discussed above. 
     Illumination controller  170  can be of a variety of designs and include a processor that executes instructions stored on a non-transitory computer-readable medium (CRM)  220 , as generally indicated by double arrow  230 . Computer-readable medium  220  can include any type of memory device or combination thereof, such as a hard drive, read only memory (ROM), random access memory (RAM), flash drive, etc. 
     For example, because multi-dimensional array of light sensitive elements is divided into thirty-three (33) rows  180 , the illumination level of optical signal  150  of illumination source  140  for the first row can be set to 1.65 times the illumination level for the row at central location  200 . The illumination level of optical signal  150  of illumination source  140  for the second row is then set to 1.56 times the illumination level for the row at central location  200 . The illumination level of optical signal  150  of illumination source  140  is thus selected, as illustrated in  FIG. 5 , This helps optimize the illumination for each row to get a nominal exposure. The nominal exposure is the amount of light that increases the center pixel of a row to be substantially similar to the pixels in the optical center of the field for a uniform target. 
     If illumination controller  170  causes illumination source  140  to produce an optical signal  150  across rows  180  of array  130  with the illumination level profile shown in  FIG. 5 , the illumination at multi-dimensional array of light sensitive elements  130  is improved to the level shown in graph  240  of  FIG. 6 . With this implementation, the illumination level  250  at edges  260  is now the worst case as opposed to the corners  80  in the non-optimized system shown in  FIG. 3 . For corners  270  of the image represented by graph  240  shown in  FIG. 6 , the illumination ratio increases from approximately 32% to approximately 53%. This increases the signal-to-noise ratio by approximately 1.6× for corners  270 . The worst case illumination is now located at edges  260  in row  17  and is approximately 46% of the central location  280  illumination level  250 . The illumination level  250  remains substantially the same for central location  280  because the level of optical signal  150  is selected to be substantially equal to the non-optimized system illustrated in  FIG. 3 . 
     As an alternative example, illumination controller  170  can cause illumination source  140  to proceed, in an orthogonal direction to that illustrated in  FIGS. 5 and 6 , to produce an optical signal  150  across the forty-four (44) columns  190  of array  130 . In this implementation, the rolling shutter is designed to move through the columns  190  of array  130 . This approach improves the illumination level to an even greater extent than that illustrated in  FIGS. 5 and 6 .  FIG. 7  shows an illumination level profile  290  utilizing this approach for each of the forty-four (44) columns  190  of array  130 .  FIG. 8  shows a graph  300  of the illumination level at multi-dimensional array of light sensitive elements  130  that results from the utilization of the illumination profile illustrated in  FIG. 7  for the forty-four (44) columns  190  of array  130 . 
     As can be seen in  FIG. 8 , the minimum illumination level  310  for the corners  320  of the image is approximately 70% versus approximately 32% for the non-optimized system shown in  FIG. 3 . This corresponds to an approximate 2.16× increase in signal-to-noise ratio of the corners  320 . The illumination level remains substantially the same for the central location  330  because the level of optical signal  150  is selected to be substantially equal to the non-optimized system shown in  FIG. 3 . However, the minimum illumination level  310  is approximately 61% for edges  340  of graph  300  for this optimized system versus approximately 32% for corners  80  of the non-optimized system illustrated in  FIG. 3 . This means the worst case signal-to-noise ratio is improved by approximately 90% for this implementation. 
     The illumination level profile can be different from those used in the examples provided above and discussed with respect to  FIGS. 5-8 . The particular illumination level profile is selected to optimize the illumination level at multi-dimensional array of light sensitive elements  130  based on the particular characteristics of an imaging device (e.g., the field-of-view, geometry of the multi-dimensional array of light sensitive elements  130 , etc.). These illumination profiles can be determined in a variety ways and derived computationally as needed or stored in a look-up table on computer-readable medium  220 . 
     As shown in  FIG. 9 , there are several applications for the imaging device  120  illustrated in  FIG. 4 . For example, it can be used in a camera  345 , printing device  350  and a scanner  360 . Although not illustrated, it is to be understood that other applications are also possible. 
     An example of a method  370  for use in an imaging device  120  is illustrated in  FIG. 10 . Method  370  begins by illuminating an object with an optical signal from an illumination source, as shown by block  380 . Next, method  370  proceeds by starting a rolling shutter moving across a multi-dimensional array of light sensitive elements and enabling a first portion of the array to collect light provided by the optical signal from the illumination source, as shown by block  390  in  FIG. 10 . Next, method  370  records the signal from this first portion of the array, as shown in block  400  in  FIG. 10 . Method  370  adjusts a level of the optical signal from the illumination source so that the level is appropriate for a next portion of the array, as shown by block  410  of  FIG. 10 . The level of the optical signal at the first portion of the multi-dimensional array of light sensitive elements may be increased relative to the level of the optical signal at the second portion of the multi-dimensional array of light sensitive elements. 
     Next, method  370  moves the rolling shutter to a next portion of the multi-dimensional array of light sensitive elements, as generally indicated by block  420  in  FIG. 10 . Next, method  370  records the signal for the next portion of the array, as shown by block  430  in  FIG. 10 . Method  370  then determines if all portions of the array have been read, as generally indicated by block  435  in  FIG. 10 . If all portions have been read, method  370  ends. If not, then method  370  returns back to block  410  and continues, as indicated in  FIG. 10 . 
     Method  370  may also construct an image from the signals recorded from all the portions of the array, as shown by block  440  in  FIG. 11 . Finally, method  370  may conclude by printing the constructed image, as indicated by block  450  of  FIG. 11 . 
     Although several examples have been described and illustrated in detail, it is to be clearly understood that the same are intended by way of illustration and example only. These examples are not intended to be exhaustive or to limit the invention to the precise form or to the exemplary embodiments disclosed. Modifications and variations may well be apparent to those of ordinary skill in the art. For example, the rolling reset and/or the rolling readout operation can be performed at a varying speed, rather than a uniform one. As an additional example, the illumination source  140  may include other optical signal sources such as one or more bulbs, rather one or more LEDs  145 . The spirit and scope of the present invention are to be limited only by the terms of the following claims. 
     Additionally, reference to an element in the singular is not intended to mean one and only one, unless explicitly so stated, but rather means one or more. Moreover, no element or component is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.