Patent Description:
In one prior art system, the computer that controls the lens searches for the focal position that maximizes the high spatial frequency content of the image. Since an out-of-focus image is blurred, the spatial frequency spectrum associated with images of scenes that contain sharp edges and other high spatial frequency generating elements has less power in the high frequency portion of the spectrum than an image of the scene when in focus. Accordingly, these schemes iteratively search the focal distance for the focus that generates the image having the highest ratio of high spatial frequency energy to average spatial frequency energy. The time to perform the search presents challenges when this algorithm is applied to a rapidly changing scene that is being captured by a motion picture camera.

A second class of prior art autofocus systems that avoids this search time utilizes a measurement of the phase difference between pixels that view the image through different portions of the camera lens. These schemes utilize a dedicated imaging array that is separate from the imaging array that generates the photograph or special pixel sensors in the array to sense this phase difference. These special autofocus pixels replace the conventional pixels that record the image; hence, the image recorded by the array includes "holes" at the locations corresponding to the autofocus pixels. These holes are filled by interpolating the results from the surrounding pixels.

<CIT> discloses an image sensor that includes an array of pixels arranged in rows and columns. Each pixel may include a number of adjacent sub-pixels covered by a single microlens. The adjacent sub-pixels of each pixel may include color filter elements of the same color. Image signals from the sub-pixels may be used to calculate phase information in each pixel in the array.

The present invention is defined by claim <NUM>. Further aspects are defined by the dependent claims.

The present invention is based on two observations. First, each pixel sensor in the imaging array includes a floating diffusion node that can be used for the autofocus measurements without losing any pixels from the imaging array. Second, by varying the position of the floating diffusion node, the autofocus measurement can be made without blocking light from the autofocus pixels to provide the asymmetry needed for a phase autofocus measurement.

To simplify the following discussion, a pixel sensor is defined to be a circuit that converts light incident thereon to an electrical signal having a magnitude that is determined by the amount of light that was incident on that circuit in a period of time, referred to as the exposure. The pixel sensor has a gate that couples that electrical signal to a readout line in response to a signal on a row select line.

A rectangular imaging array is defined to be a plurality of pixel sensors organized as a plurality of rows and columns of pixel sensors. The rectangular array includes a plurality of readout lines and a plurality of row select lines, each pixel sensor being connected to one row select line and one readout line, the electrical signal generated by that pixel being connected to the readout line associated with that pixel in response to a signal on the row select line associated with that pixel sensor.

The manner in which the present invention provides its advantages can be more easily understood with reference to <FIG>, which illustrates a two-dimensional imaging array according to one embodiment of the present invention. Rectangular imaging array <NUM> includes a pixel sensor <NUM>. Each pixel sensor has a main photodiode <NUM> and a parasitic photodiode <NUM>. The manner in which the pixel sensor operates will be discussed in more detail below. The reset circuitry and amplification circuitry in each pixel is shown at <NUM>. The pixel sensors are arranged as a plurality of rows and columns. Exemplary rows are shown at <NUM> and <NUM>. Each pixel sensor in a column is connected to a readout line <NUM> that is shared by all of the pixel sensors in that column. A calibration source <NUM> is optionally included on each readout line. Each pixel sensor in a row is connected to a row select line <NUM> which determines if the pixel sensor in that row is connected to the corresponding readout line.

The operation of rectangular imaging array <NUM> is controlled by a controller <NUM> that receives a pixel address to be read out. Controller <NUM> generates a row select address that is used by row decoder <NUM> to enable the readout of the pixel sensors on a corresponding row in rectangular imaging array <NUM>. The column amplifiers are included in an array of column amplifiers <NUM> which execute the readout algorithm, which will be discussed in more detail below. All of the pixel sensors in a given row are read out in parallel; hence there is one column amplification and analog-to-digital converter (ADC) circuit per readout line <NUM>. The column processing circuitry will be discussed in more detail below.

When rectangular imaging array <NUM> is reset and then exposed to light during an imaging exposure, each photodiode accumulates a charge that depends on the light exposure and the light conversion efficiency of that photodiode. That charge is converted to a voltage by reset and amplification circuitry <NUM> in that pixel sensor when the row in which the pixel sensor associated with that photodiode is read out. That voltage is coupled to the corresponding readout line <NUM> and processed by the amplification and ADC circuitry associated with the readout line in question to generate a digital value that represents the amount of light that was incident on the pixel sensor during the imaging exposure.

<FIG> is a schematic drawing of a typical prior art pixel sensor in one column of pixel sensors in an imaging array. Pixel sensor <NUM> includes a photodiode <NUM> that measures the light intensity at a corresponding pixel in the image. Initially, photodiode <NUM> is reset by placing gate <NUM> in a conducting state and connecting floating diffusion node <NUM> to a reset voltage, Vr. Gate <NUM> is then closed and photodiode <NUM> is allowed to accumulate photoelectrons. For the purposes of the present discussion, a floating diffusion node is defined to be an electrical node that is not tied to a power rail, or driven by another circuit. A potential on gate <NUM> sets the maximum amount of charge that can be accumulated on photodiode <NUM>. If more charge is accumulated than allowed by the potential on gate <NUM>, the excess charge is shunted to ground through gate <NUM>.

After photodiode <NUM> has been exposed, the charge accumulated in photodiode <NUM> is typically measured by noting the change in voltage on floating diffusion node <NUM> when the accumulated charge from photodiode <NUM> is transferred to floating diffusion node <NUM>. Floating diffusion node <NUM> is characterized by a capacitance represented by capacitor <NUM>'. In practice, capacitor <NUM>' is charged to a voltage Vr and isolated by pulsing the reset line of gate <NUM> prior to floating diffusion node <NUM> being connected to photodiode <NUM>. The charge accumulated on photodiode <NUM> is transferred to floating diffusion node <NUM> when gate <NUM> is opened. The voltage on floating diffusion node <NUM> is sufficient to remove all of this charge, leaving the voltage on floating diffusion node <NUM> reduced by an amount that depends on the amount of charge transferred and the capacitance of capacitor <NUM>'. Hence, by measuring the change in voltage on floating diffusion node <NUM>, the amount of charge accumulated during the exposure can be determined. The voltage on floating diffusion node <NUM> is measured by a column amplifier <NUM> when the pixel sensor in question is connected to the readout line <NUM> in response to a signal on bus <NUM>.

The present invention is based on the observation that a pixel of the type discussed above can be modified to include a second parasitic photodiode that is part of the floating diffusion node and has a significant photodiode detection efficiency. Normally, the light conversion efficiency of the parasitic photodiode is minimized by shielding the floating diffusion node from light. However, as pointed out in co-pending <CIT>, filed on <NUM>/<NUM>/<NUM>, the light conversion efficiency of the parasitic photodiode can be increased by adjusting the spacings of the components in the vicinity of the floating diffusion node.

To distinguish the parasitic photodiode from photodiode <NUM>, photodiode <NUM> and photodiodes serving analogous functions will be referred to as the "conventional photodiodes". Refer now to <FIG>, which illustrates a pixel sensor in which the parasitic photodiode is utilized in an image measurement. To simplify the following discussion, those elements of pixel sensor <NUM> that serve functions analogous to those discussed above with respect to <FIG> have been given the same numeric designations and will not be discussed further unless such discussion is necessary to illustrate a new manner in which those elements are utilized. In general, parasitic photodiode <NUM> has a detection efficiency that is significantly less than that of photodiode <NUM>. The manner in which the ratio of the photodiode detection efficiencies of the two photodiodes is adjusted is discussed in more detail in co-pending <CIT>, filed on <NUM>/<NUM>/<NUM>. In one exemplary embodiment, the ratio of the conversion efficiency of the main photodiode to the parasitic photodiode is <NUM>:<NUM>. Other embodiments in which this ratio is <NUM>:<NUM> or <NUM>:<NUM> are useful.

The photocharge that accumulates on the parasitic photodiode during an exposure can be determined separately from the photocharge that accumulated on the main photodiode during the exposure. The process may be more easily understood starting from the resetting of the pixel sensor after the last image readout operation has been completed. Initially, main photodiode <NUM> is reset to Vr and gate <NUM> is closed. This also leaves floating diffusion node <NUM> reset to Vr. If a correlated double sampling measurement is to be made, this voltage is measured at the start of the exposure by connecting floating diffusion node <NUM> to column amplifier <NUM>. Otherwise, a previous voltage measurement for the reset voltage is used. During the image exposure, parasitic photodiode <NUM> generates photoelectrons that are stored on floating diffusion node <NUM>. These photoelectrons lower the potential on floating diffusion node <NUM>. At the end of the exposure, the voltage on floating diffusion node <NUM> is measured by connecting the output of source follower <NUM> to column amplifier <NUM>, and the amount of charge generated by parasitic photodiode <NUM> is determined to provide a first pixel intensity value. Next, floating diffusion node <NUM> is again reset to Vr and the potential on floating diffusion node <NUM> is measured by connecting the output of source follower <NUM> to column amplifier <NUM>. Gate <NUM> is then placed in the conducting state and the photoelectrons accumulated by main photodiode <NUM> are transferred to floating diffusion node <NUM>. The voltage on floating diffusion node <NUM> is then measured again and used by column amplifier <NUM> to compute a second pixel intensity value.

The basic principle of a phase detection autofocus system can be more easily understood with reference to <FIG>, which illustrate the manner in which the distance from the camera lens to the imaging array can be detected. Referring to <FIG>, consider a point <NUM> in a scene that is to be captured by the imaging array of a camera through a lens <NUM>. For the purpose of this example, it will be assumed that lens <NUM> is masked by a mask <NUM> that blocks all of the light except for light passing through the two edge windows shown at <NUM> and <NUM>. The light from windows <NUM> and <NUM> is imaged onto two linear arrays of pixel sensors shown at <NUM> and <NUM>. For the purposes of the present discussion, it will be assumed that the pixel sensors in array <NUM> can only "see" light from window <NUM>, and the pixel sensors in array <NUM> can only "see" light from window <NUM>. In <FIG>, the light from window <NUM> is detected at pixel sensor <NUM> in array <NUM>, and the light from window <NUM> is detected at pixel sensor <NUM>.

The distance from lens <NUM> to the plane of arrays <NUM> and <NUM> is denoted by D. The pixel sensors at which the light is imaged onto the two arrays depends on the distance, D. In the example shown in <FIG>, lens <NUM> images the plane in the scene containing point <NUM> to a point below the plane of the arrays. Hence, the image of the plane in the scene is out of focus. If the lens is moved toward arrays <NUM> and <NUM>, the pixel sensors that now detect the light are located toward the middle of arrays <NUM> and <NUM>. In the case in which lens <NUM> focuses the light onto the plane of arrays <NUM> and <NUM>, the location of the pixel sensors receiving the light is in the middle of the array nearest to the optical axis <NUM> of lens <NUM>. <FIG> illustrates the lens is at the proper distance and the pixel sensors receiving the light are shown at <NUM> and <NUM>. Refer now to <FIG>. In this case, lens <NUM> is too close to the plane of arrays <NUM> and <NUM>, and the pixel sensors receiving the light are again separated along the length of the arrays as shown at <NUM> and <NUM>.

Conversely, if one could determine the identity of the pixel sensors receiving light from the two windows in the lens, the distance needed to properly focus point <NUM> onto the imaging arrays could be determined. If the pixel sensors receiving the light are known, the distance that the lens must be moved to arrive at the correct focus can be determined from a lookup table, and hence, no iteration of the lens distance is needed. Hence, this type of autofocus scheme can perform the autofocus adjustments in a much shorter time than that available with schemes that optimize the high frequency spatial composition of the image.

Adapting this autofocus scheme to imaging arrays in which the arrays of pixel sensors are within the imaging array used to form the image of the scene being photographed presents two challenges. First, the imaging lens is not masked. This problem can be overcome by using pixel sensors that only measure the light transmitted by one half of the camera lens. If the autofocus pixel sensors are separate from the pixel sensors that actually detect the image, pixel sensors that satisfy this constraint can be obtained by masking a microlens that is located over the pixel sensor. However, such schemes effectively remove pixel sensors from the imaging array. The manner in which this is accomplished in the present invention without sacrificing pixel sensors within the imaging array will be discussed in more detail below.

Second, the light projected onto the autofocus linear arrays is not a single point of light, but rather lines from the scene. Accordingly, merely detecting the identity of the pixel sensor receiving the most light in each array does not provide the needed information for determining the proper D. This problem can be overcome by computing an image correlation value that can be mapped to the distance between the lens and the imaging array.

The manner in which the first challenge is overcome by the present invention can be more easily understood with reference to <FIG>, which is a top view of a portion of an embodiment of an imaging array <NUM> that utilizes the pixels shown in <FIG> as taught in the above-mentioned US patent application. To simplify the drawing, the various gates and control lines have been omitted from the drawing. The pixel sensors are arranged in a rectangular array. The elements of a typical pixel sensor are labeled at <NUM>. In particular, pixel sensor <NUM> has a main photodiode <NUM> and a parasitic photodiode <NUM>. Both of these photodiodes receive light from a microlens <NUM> that overlies the silicon surface in which the photodiodes are constructed. The pixel sensors are typically arranged in groups of four pixel sensors such as group <NUM>. In an array for utilization in a color camera, each pixel sensor is covered by a color filter. Typically, one pixel sensor is covered by a red filter as denoted by the "R"; one pixel sensor is covered by a blue filter as denoted by the "B", and two pixel sensors are covered by green filters as denoted by the "G". The color processing is not relevant to the present discussion, and hence, will not be discussed here.

The present invention is based on the observation that the parasitic photodiodes associated with floating diffusion nodes can be used to form the linear imaging arrays needed for the autofocus system without altering the main photodiodes, and hence, the pixel losses associated with prior art schemes can be avoided.

Refer now to <FIG>, which is a cross-sectional view of pixel sensors <NUM> and <NUM> through line <NUM>-<NUM> shown in <FIG>. Again, the various gates and wiring structures for connecting the gates and the photodiodes to the bit lines have been omitted to simplify the drawing. The main photodiodes are shown at <NUM> and <NUM>, respectively. The corresponding floating diffusion nodes with their parasitic photodiodes are shown at <NUM> and <NUM>. The wiring layers over the substrate in which the photodiodes are constructed include a number of patterned metal layers <NUM> and <NUM> that form an aperture for limiting the light from the microlenses <NUM> and <NUM> that can reach the photodiodes. Color filters <NUM> and <NUM> are deposited over the wiring layer and under the microlenses. It should be noted that in this configuration, both of the parasitic photodiodes receive light preferentially from the same half of the microlens, i.e., halves 64A and 72A. Hence, the parasitic photodiodes in this arrangement are not suitable for the autofocus pixel sensors.

Refer now to <FIG>, which is a top view of a portion of an imaging array according to one embodiment of the present invention. Imaging array <NUM> differs from imaging array <NUM> shown in <FIG> in that every third row of pixel sensors is the mirror image of the corresponding row in imaging array <NUM>. This creates two arrays of floating diffusion nodes as shown at <NUM> and <NUM>. As a result, the floating diffusion nodes in one of these rows, e.g. row <NUM>, receive light preferentially from one side of the microlens in the pixel sensor in which the floating diffusion node is located, and the floating diffusion nodes in the other of these rows, e.g. <NUM>, receive light preferentially from the other side of the microlens.

Refer now to <FIG>, which is a cross-sectional view through line <NUM>-<NUM> shown in <FIG>. The floating diffusion node <NUM> in pixel sensor <NUM> which is part of row <NUM> receives light from the half of microlens <NUM> shown at 141A and receives substantially less light from the other half of microlens <NUM>. In contrast, floating diffusion node <NUM> in pixel sensor <NUM> receives light preferentially from the half of microlens <NUM> shown at 142A. Hence, the floating diffusion nodes in these two rows of pixel sensors can be used as an autofocus sensing array.

To simplify the following discussion, the pixel sensors whose floating diffusion nodes are used for autofocus purposes will be referred to as autofocus pixel sensors. Those autofocus pixel sensors that are in the rows analogous to row <NUM> will be referred to as the top autofocus pixel sensors. Those autofocus pixel sensors that are in rows in the positions that are analogous to row <NUM> will be referred to as the bottom autofocus pixel sensors. The labels "top" and "bottom" are merely labels and not intended to denote a position relative to the Earth. In general, the region of the imaging array that generates the image in a particular region of the field of view that is to be maintained in focus will have a two-dimensional array of autofocus pixel sensors that can be used to make the autofocus measurement. This region will be referred to as an autofocus region in the following discussion. Any particular autofocus pixel sensor can be identified by a pair on indices, (I,J), denoting the position of that autofocus pixel sensor in the two-dimensional imaging array. The signals from the floating diffusion nodes in the bottom autofocus pixel sensors will be denoted by B(I,J), and those from the floating diffusion nodes in the top autofocus pixel sensors will be denoted by T(I,J). Since each top autofocus pixel sensor has a corresponding bottom autofocus pixel sensor, the indices are chosen such that B(I,J) is the autofocus pixel sensor corresponding to T(I,J). The autofocus region signals will correspond to some set of the possible A(I,J) and B(I,J) signals.

It should be noted that using the floating diffusion nodes that are part of the imaging array that generates the image of the scene requires that the floating diffusion nodes operate under the color filters. Any distortions introduced by the color filters can be removed by using multiple pairs of lines of the autofocus pixel sensors. Referring again to <FIG>, the top autofocus pixel sensors in array <NUM> are covered by red or green filters, but not blue filters. Similarly the bottom autofocus pixel sensors are covered by blue and green filters, but not red filters. However, the autofocus measurement is made with both arrays <NUM> and <NUM>, then all possible combinations are obtained. In one aspect of the present invention, the collection of top autofocus pixel sensors used for the autofocus measurement include substantially equal numbers of pixel sensors with red, blue, and green filters. Similarly, the collection of bottom autofocus pixel sensors used for the autofocus measurement includes substantially equal numbers of pixel sensors with red, blue, and green filters. For the purposes of the present discussion, the number of filters of each color that are included will be defined to be substantially equal if the auto focus adjustment obtained from the autocorrelation measurement discussed below is not altered by any lack of equality in the numbers.

As noted above, the camera lens is not masked, and hence, the autofocus pixel sensors receive light from a number of different points in the scene. Accordingly, some form of cross-correlation function must be used to determine the top and bottom pixel locations from which the lens position correction is to be determined. <MAT> Here, TA(x,y), and BA(x,y) are the average values of T(x,y) and B(x,y), respectively, over the autofocus pixel sensors. The summations are performed over the set of autofocus pixel sensors that are being used to focus the chosen region of the image. The (u,v) value for which p(u,v) is maximum provides a value that can be used to access the camera lens movement needed to bring the region of the scene being imaged onto the autofocus pixel sensors into focus. In the case of a simple lens, the distance the lens is to move is determined. Alternatively, the focal length of a more complex imaging lens could be altered to bring the image into focus. In this case, the change in focal length would be determined. In one aspect of the invention, the controller stores a focus table that maps this determined (u,v) value to a camera lens movement or focal length change needed to bring the scene into focus.

Typically, the lens is moved such that a particular region of the image is in focus. This is usually a region near the center of the image. In the present invention, autofocus pixel sensors are available over essentially the entire imaging array. Hence, a plurality of regions that can provide autofocus data are present. A region having sufficient autofocus pixel sensors to perform the focal adjustment will be referred to as an autofocus zone in the present discussion. Refer now to <FIG>, which illustrates an imaging array having multiple autofocus zones. Imaging array <NUM> is organized as a rectangular array having autofocus pixel sensor arrays on the rows. Essentially, two out of every three rows contain autofocus pixel sensors, as shown at <NUM>-<NUM>. An autofocus zone can be as small as a portion of two of the autofocus pixels sensor rows as shown at <NUM>-<NUM>, or an autofocus zone can include portions of four or more of the autofocus pixel sensor rows as shown at <NUM>.

In practice, autofocus controller <NUM> is programmed to use one of the autofocus zones to set the focal properties of lens <NUM>. Autofocus controller <NUM> can be implemented in the overall camera controller or as a separate controller that communicates with a master camera controller such as <NUM> shown in <FIG>. Controller <NUM> then sends signals to actuator <NUM> to move lens <NUM> such that the autofocus zone is in focus. As noted above, the autofocus zone normally used is one near the center of the imaging array. However, the correlation function used to set the lens focus can be computed at a large number of autofocus zones in the imaging array and transmitted with the image that is measured after the autofocus control has brought the autofocus zone of interest into focus. This additional information can be used to provide a measurement of the distance of the corresponding regions of the scene from the region on which the camera is focusing.

In one aspect of the invention, a motion picture sequence of images is acquired by making an autofocus measurement before each frame of the motion picture sequence. Hence, the time that can be devoted to making the autofocus adjustment is limited. The time needed to perform the autofocus adjustment will be referred to as the autofocus time period. This time period includes the time needed to expose the autofocus pixel sensors, the time needed to read out those pixel sensors and perform the correlation computation, and the time needed to move the lens. Typically, there is some region of the imaging array, such as the central region, that is to be kept in focus by the autofocus system. It is advantageous to reduce the autofocus exposure time. The autofocus exposure time depends on the number of autofocus pixel sensors in the region of interest that are used in the autofocus focus computation and the light levels in the scene. If the light levels are low or the autofocus exposure time is too short, the resulting autofocus pixel sensor outputs will have significant amounts of noise. The autofocus exposure computation depends on a correlation measurement such as the p(u,v) computation discussed above. As more pixels are added to that computation, the effects of noise are reduced. Since more than half of the pixel sensors in the array are autofocus pixel sensors, the present invention can reduce the autofocus exposure period and use the outputs from more autofocus pixel sensors to compensate for the increased noise. It should be noted that this is a significant advantage of the present invention over systems that have a small number of dedicated autofocus pixel sensors embedded in the imaging array in place of pixel sensors that record the image. In one exemplary embodiment, the number of autofocus pixel sensors used to determine the correct focal adjustment is greater than <NUM>. In another exemplary embodiment, the number of autofocus pixel sensors used to determine the correct focal adjustment is less than or equal to <NUM>.

In one aspect of the invention, the area in the center of the imaging array is used for setting the camera lens distance from the imaging array. However, it should be noted that a "focal map" of the entire scene that is projected onto the imaging array can be computed by repeating the distance computation over small segments of the imaging array at locations throughout the imaging array. Such a map would be useful in constructing a three-dimensional image of the scene. Hence, in one aspect of the invention, the signals from the autofocus pixel sensors used to set the lens distance prior to taking an image are output as a separate image for use in later post-processing of the image.

The above-described US patent application describes a method for extending the range of a pixel sensor by using the floating diffusion node to provide a second light measurement of the light received by the pixel sensor during the imaging exposure. The floating diffusion nodes in that method have light conversion efficiencies that are typically <NUM>/30th of the light conversion efficiency of the main photodiode, and hence, provide a measurement of the received light when the pixel is subjected to light intensities that cause the main photodiode to saturate. The floating diffusion nodes in the present invention can likewise be used to extend the dynamic range of the pixel sensor.

It should be noted that the main photodiodes and the microlenses in the above-described embodiments form a regular array with equal spacing in both the column and row directions. The floating diffusion nodes are not uniformly distributed over the imaging array, some post imaging processing may be required. For example, the image as seen by the floating diffusion nodes could be re-sampled to provide an image on a uniform grid. The values of this re-sampled floating diffusion node image would then be combined with the corresponding values in the image generated by the main photodiodes to provide the extended light intensity measurements. To perform the post-processing, the image as seen by the floating diffusion nodes must be outputted and saved with the image as seen by the main photodiodes.

In the above-described embodiments, the floating diffusion nodes in the autofocus pixel sensors are positioned such that the floating diffusion nodes receive light from only one side of the microlens. However, embodiments in which the floating diffusion nodes receive light preferentially from one side of the microlens can also be constructed. For example, the floating diffusion nodes are positioned such that <NUM> percent of the light comes from one side of the microlens and <NUM> percent of the light received by the floating diffusion node comes from the other side of the microlens. In another exemplary embodiment, the floating diffusion nodes are positioned such that <NUM> percent of the light comes from one side of the microlens and <NUM> percent of the light received by the floating diffusion node comes from the other side of the microlens. Using additional autofocus pixel sensors in the autofocus cross-correlation method can compensate for this lack of light separation.

While the autofocus system of the present invention tolerates noise in the autofocus pixel sensors, the floating diffusion nodes in the autofocus pixel sensors must have sufficient light conversion efficiency to measure the light levels in the autofocus region of the imaging sensor. Hence, the light conversion efficiency of the floating diffusion nodes is preferably adjusted to be somewhat higher than <NUM>/30th of the main photodiode light conversion efficiency discussed above. Mechanisms for adjusting the light conversion efficiency of the floating diffusion nodes are discussed in the above-referenced US patent application, which is hereby incorporated in its entirety by reference. Increasing the light conversion efficiency of the floating diffusion nodes, however, reduces the improvement in the dynamic range that is achievable by utilizing the floating diffusion nodes as a second photodiode during the exposure of the image. In one embodiment, the floating diffusion node light conversion efficiency is set to be greater than <NUM>/10th of the main photodiode light conversion efficiency. In another embodiment, the floating diffusion node light conversion efficiency is set to be greater than <NUM>/30th of the main photodiode light conversion efficiency.

The above described embodiments refer to rows and columns of pixel sensors; however, it is to be understood that the rows and columns could be interchanged in other embodiments. In addition, the autofocus pixel sensors could be organized such that columns of floating diffusion nodes form the two linear arrays used for autofocus purposes.

To simplify the following discussion, the photodiodes used in the autofocus adjustment will be referred to as the autofocus photodiodes. In the above-described embodiments the parasitic photodiodes associated with the floating diffusion nodes are the autofocus photodiodes. These embodiments do not increase the area of the pixel sensors, and hence, provide significant advantages. However, the parasitic photodiodes are not pinned photodiodes, and hence, have increased noise relative to the main photodiodes. These noise issues can be reduced by using a separate small pinned photodiode in place of the parasitic photodiode of the floating diffusion node. In such embodiments, the light conversion efficiency of the floating diffusion node would be intentionally reduced as is the case with conventional imaging arrays.

Refer now to <FIG>, which is a schematic drawing of a pixel sensor having two photodiodes that could be used in such two photodiode autofocus embodiments. Pixel sensor <NUM> includes a main photodiode <NUM> and an auxiliary photodiode <NUM>. The area of auxiliary photodiode <NUM> is chosen to be much smaller than that of photodiode <NUM>. For example, auxiliary photodiode <NUM> has an area less than <NUM> times that of main photodiode <NUM> in one embodiment. Both photodiodes can be separately connected to floating diffusion node <NUM> by controlling gates <NUM> and <NUM>. Since auxiliary photodiode <NUM> has a much smaller area than main photodiode <NUM>, an anti-blooming gate is not needed. The two photodiodes can be read out in a manner analogous to that discussed above with respect to the parasitic photodiode embodiments. During non-autofocus operations, the photocharge accumulated on auxiliary photodiode <NUM> can be used to extend the dynamic range of pixel sensor <NUM> in a manner analogous to that described above. For the purposes of the present discussion, the important aspect of pixel sensor <NUM> is the relative placement of main photodiode <NUM> and auxiliary photodiode <NUM> within pixel sensor <NUM>.

Refer now to <FIG>, which is a top view of a portion of an imaging array according to one embodiment of the present invention that utilizes the pixel sensor design shown in <FIG>. Imaging array <NUM> differs from imaging array <NUM> shown in <FIG> in that every third row of pixel sensors is the mirror image of the corresponding row in imaging array <NUM>. This creates two arrays of auxiliary photodiodes as shown at <NUM> and <NUM>. As a result, the auxiliary photodiodes in one of these rows, e.g. row <NUM>, receive light preferentially from one side of the microlens in the pixel sensor in which the auxiliary photodiode is located, and the auxiliary photodiodes in the other of these rows, e.g. <NUM>, receive light preferentially from the other side of the microlens.

Refer now to <FIG>, which is a cross-sectional view through line <NUM>-<NUM> shown in <FIG>. The auxiliary photodiode <NUM> in pixel sensor <NUM> which is part of row <NUM> receives light from the half of microlens <NUM> shown at 441A and receives substantially less light from the other half of microlens <NUM>. In contrast, auxiliary photodiode <NUM> in pixel sensor <NUM> receives light preferentially from the half of microlens <NUM> shown at 442A. Hence, the auxiliary photodiodes in these two rows of pixel sensors can be used as an autofocus sensing array. While the auxiliary photodiodes are asymmetrically placed, the main photodiodes <NUM> and <NUM> form a regular rectangular array.

The manner in which the auxiliary photodiodes are used in the autofocus procedure is analogous to that described above with respect to the parasitic photodiodes. To simplify the following discussion, the pixel sensors whose auxiliary photodiodes are used for autofocus purposes will again be referred to as autofocus pixel sensors. Those autofocus pixel sensors that are in the rows analogous to row <NUM> will be referred to as the top autofocus pixel sensors. Those autofocus pixel sensors that are in rows in the positions that are analogous to row <NUM> will be referred to as the bottom autofocus pixel sensors. The labels "top" and "bottom" are merely labels and not intended to denote a position relative to the Earth. In general, the region of the imaging array that generates the image in a particular region of the field of view that is to be maintained in focus will have a two-dimensional array of autofocus pixel sensors that can be used to make the autofocus measurement. This region will be referred to as an autofocus region in the following discussion. Any particular autofocus pixel sensor can be identified by a pair on indices, (I,J), denoting the position of that autofocus pixel sensor in the two-dimensional imaging array. The signals from the auxiliary photodiodes in the bottom autofocus pixel sensors will be denoted by B(I,J), and those from the auxiliary photodiodes in the top autofocus pixel sensors will be denoted by T(I,J). Since each top autofocus pixel sensor has a corresponding bottom autofocus pixel sensor, the indices are chosen such that B(I,J) is the autofocus pixel sensor corresponding to T(I,J). The autofocus region signals will correspond to some set of the possible A(I,J) and B(I,J) signals. The autofocus adjustment is then carried out as described above with reference to the parasitic photodiodes.

Other layouts of the autofocus photodiodes, either the parasitic photodiode of the floating diffusion node, or a separate photodiode, than those discussed above are also possible.

Claim 1:
An apparatus comprising a two dimensional array (<NUM>) of pixel sensors (<NUM>, <NUM>) and a controller (<NUM>), each pixel sensor comprising:
a single main photodiode (<NUM>);
an autofocus photodiode (<NUM>); and
a microlens (<NUM>) that concentrates light onto said main photodiode and said autofocus photodiode,
said two-dimensional array of pixel sensors comprising first and second autofocus arrays of pixel sensors , said pixel sensors in said first autofocus array of pixel sensors having said autofocus photodiode positioned such that each autofocus photodiode receives light preferentially from one half of said microlens in that pixel sensor and said pixel sensors in said second autofocus array of pixel sensors having each autofocus photodiode positioned such that each autofocus photodiode receives light preferentially from said other half of said microlens in that pixel sensor; wherein the controller is configured to generate an autofocus signal, wherein generating said autofocus signal comprises computing a cross-correlation function of signals from said autofocus photodiodes in said first autofocus array of pixel sensors with a signal from said autofocus photodiodes in said second autofocus arrays of pixel sensors;
characterized in that
said autofocus photodiode comprises a parasitic photodiode associated with a floating diffusion node (<NUM>) in each of said pixel sensors.