Cross Reference to Related Application

[0001] This Patent Cooperation Treaty patent application claims priority to U.S. non- provisional application No. 13/782,532, filed March 1 , 2013, and titled "Exposure Control for Image Sensors", the contents of which are incorporated herein by reference in their entirety.

Technical Field

[0002] The present invention relates generally to image sensors, and, more specifically, to exposure control in image sensors.

Background

[0003] Electronic image sensors such as charge coupled devices (CCDs) and

complementary metal oxide semiconductor (CMOS) sensors typically include one or more photodiodes for each pixel that, when exposed to light, accumulate electrical charge in proportion to the brightness of the light incident on the photodiode of the pixel. Some electronic image sensors are used with mechanical shutters to help control the duration of time that the photodiodes are exposed to a light source, whereas other electronic image sensors (including certain of those on, for example, cell phones, tablet computers, and other electronic devices) are not used with a mechanical shutter and hence rely on some form of electrical shuttering to control the charge generated in the photodiodes when exposed to light. [0004] One way in which electrical shuttering can be implemented is by controlling gates (e.g., transistors) that couple the photodiode to other parts of the sensor. For example, gates coupled to the photodiode may continuously drain away any charge generated in the photodiode when no image is to be captured by the sensor. Controlling the flow of charge generated in the photodiode may generally be referred to as exposure control because it controls the effective exposure of the photodiode, even though without a mechanical shutter, the sensor may constantly be exposed to light. In other words, because of the difficulty of mechanically controlling exposure of photodiodes, the effective exposure of some electronic image sensors may be controlled by selectively accumulating and selectively discarding charge generated by incident photons in those areas. [0005] It can be useful to control exposure of pixels in an image sensor based on different lighting and color conditions of a particular scene to be imaged. For example, in a bright light situation, the exposure time of pixels on the sensor may be reduced to avoid saturating the wells of the photodiodes of the pixels (which, if not corrected, may lead to blooming). Conversely, in a low light situation, the exposure time may be increased in order to allow sufficient charge to collect in the pixel to maintain a sufficient signal-to-noise ratio. However, changing exposure times may be challenging for several reasons, particularly for image sensors that capture video at a set frame rate and thus have limited amount of time within which to expand exposure time. Continuing with the video example, on one hand, if the exposure time is shortened too severely within each video frame, severe motion artifacts may occur between frames of the video causing aliasing or a stroboscopic effect between frames. If, on the other hand, exposure time is lengthened too much, pixels may saturate, and may even bloom into adjacent pixels.

[0006] Color image sensors typically include some type of color filter array (CFA), such as a Bayer pattern having a red-green-blue-green (RGBG) arrangement of pixels. For a Bayer pattern, the pixels of the image sensor may be arranged into groups of 2x2 sub-arrays, with each 2x2 sub-array including one red pixel and one green pixel in one row, and one blue pixel and one green pixel in the other row. In such an arrangement, the two green pixels are generally located diagonally from one another in the 2x2 sub-array. Usually, all four pixels in each 2x2 sub-array are identical other than a color filter placed over the top of the pixel; in other words, the pixels usually have photodiodes with similar well capacity. However, the color filters cause different wavelengths of light to be detected by the different color pixels. The sensitivity of the photodiodes of the pixels to incident photons may, however, vary based on the wavelength of light incident thereon, and so different colored pixels may have different sensitivities to identical brightness of different colors of light. For example, in the Bayer pattern, the green pixels are usually the most sensitive. Because the green pixels are the most sensitive, the green photodiodes will typically fill up with charge faster than the red and blue photodiodes for most neutral color scenes to which the image sensor is exposed.

[0007] Recently there has been interest in adding a clear pixel, which may not have a colored filter placed over the top of it. The clear pixel is also known as a "white pixel."

Adding clear pixels to an image sensor may provide a higher luminance sensitivity and wider overall spectrum for the image sensor. However, the clear pixel is usually more sensitive than any of the red, green, or blue pixels to incident light, and thus the clear pixel typically saturates faster than any of the other color pixels. [0008] One technique to help prevent image sensor pixels from saturating and potentially spilling accumulated charge over into adjacent pixels (a phenomenon known as blooming) is to drain excess charge away from the photodiodes using a form of anti-blooming control.

[0009] In most image sensors today, exposure control and anti-blooming are applied in a generally uniform manner. For example, most image sensors set a single, common exposure time for all of the different colors of pixels, and the exposure time is a continuous block of time. The exposure time is generally chosen responsive to the most sensitive color channel, with the exposure time being shorter than the time during which the most sensitive color pixels are expected to saturate with accumulated charge. However, cutting short the effective exposures of the other, less-sensitive color pixels may cause less than optimal results (possibly with low signal to noise ratios for those colors, as signal to noise ratio is generally related to the amount of charge accumulated in the photodiode). Furthermore, the uniform application of anti-blooming control among all of the different colors of pixels also prevents more fine-tuned blooming mitigation techniques from being applied.

Summary [0010] One example of the present disclosure may take the form of a method of operating an image sensor. The method may include selectively storing, responsive to a first control signal, charge accumulated in a photodiode during a first sub-exposure in a storage node, and selectively discarding, responsive a second control signal, charge accumulated in the photodiode during a first reset. The method may also include selectively storing, responsive to the first control signal, charge accumulated in the photodiode during a second sub- exposure in the storage node, and transferring, responsive to a third signal, charge stored in the storage node from the first and second sub-exposures to a floating diffusion node.

[0011] Another example of the disclosure may take the form of an image sensor pixel that includes a photodiode, a storage node, and a storage gate coupled between the photodiode and storage node with the storage gate selectively receiving an exposure control signal and being responsive thereto. The image sensor pixel may also include a floating diffusion node and a transfer gate coupled between the storage node and floating diffusion node with the transfer gate selectively receiving a transfer control signal and being responsive thereto. The image sensor pixel may also include control circuitry configured to provide the exposure control signal to the storage gate such that the charge accumulated in the photodiode during a plurality of sub-exposures is transferred to the storage node after or during each sub- exposure, the control circuitry further configured to provide the transfer signal to the transfer gate following the plurality of sub-exposures such that charge stored in the storage node is transferred to the floating diffusion node. [0012] Another example of the present disclosure may take the form of a method of operating an image sensor. The method may include selectively accumulating charge in a first photodiode associated with a first color channel during a first period of time responsive to a first exposure control signal. The method may also include selectively accumulating charge in a second photodiode associated with a second color channel during a second period of time responsive to a second exposure control signal. The first color channel may be different than the second color channel and the first period of time may be shorter than the second period of time, with the first and second periods of time being at least partially contemporaneous with each other.

[0013] Another example of the present disclosure may take the form of an image sensor. The image sensor may include a first pixel associated with a first color channel and configured to accumulate charge for a first period of time responsive to a first control signal. The image sensor may also include a second pixel associated with a second color channel and configured to accumulate charge for a second period of time responsive to a second control signal. The first color channel may be different than the second color channel and the first period of time may be shorter than the second period of time.

[0014] Another example of the present disclosure may take the form of a method of operating an image sensor. The method may include selectively accumulating charge in a first photodiode associated with a first color channel during a first plurality of sub-exposures. The method may also include selectively transferring, responsive to a first control signal, charge accumulated in the first photodiode during each of the first plurality of sub-exposures to a first node. The method may also include selectively accumulating charge in a second photodiode associated with a second color channel during a second plurality of sub- exposures. The method may also include selectively transferring, responsive to a second control signal, charge accumulated in the first photodiode during each of the second plurality of sub-exposures to a second node. The first color channel may be different than the second color channel and at least one of the first plurality of sub-exposures may be shorter than at least one of the second plurality of sub-exposures. The first and second plurality of exposures may be at least partially contemporaneous.

[0015] Another example of the present disclosure may take the form of a method of operating an image sensor. The method may include partitioning a first exposure of a first pixel associated with a first color channel into a first plurality of sub-exposures, each of the first plurality of sub-exposures having a respective duration. The method may also include partitioning a second exposure of a second pixel associated with a second color channel into a second plurality of sub-exposures, each of the second plurality of sub- exposures having a respective duration. The duration of at least one of the first plurality of sub-exposures may be shorter than at least one of the second plurality of sub-exposures.

Brief Description of the Drawings

[0016] Fig. 1 A is a front perspective view of an electronic device including one or more cameras. [0017] Fig. 1 B is a rear perspective view of the electronic device of Fig. 1 A.

[0018] Fig. 2 is a simplified block diagram of the electronic device of Fig. 1 A.

[0019] Fig. 3 is a simplified schematic cross-section view of the electronic device of Fig. 1 A taken along line 3-3 in Fig. 1 A. [0020] Fig. 4A is a simplified diagram of an image sensor architecture for a camera of the electronic device.

[0021 ] Fig. 4B is an enlarged view of a pixel architecture of Fig. 4A illustrating a single pixel.

[0022] Fig. 5 is a simplified schematic view of a pixel cell having a rolling shutter operation.

[0023] Fig. 6 is a simplified schematic view of a pixel cell having a global shutter configuration.

[0024] Fig. 7 is a simplified schematic view of four pixel cells having a rolling shutter configuration. [0025] Fig. 8 is a simplified schematic view of four pixel cells having a global shutter configuration.

[0026] Fig. 9 is a timing diagram for one embodiment of exposure control. [0027] Fig. 10 is a timing diagram for another embodiment of exposure control. [0028] Fig. 1 1 is a timing diagram for still another embodiment of exposure control. [0029] Fig. 12 is a timing diagram for still another embodiment of exposure control. Specification Overview

[0030] In some embodiments herein, apparatuses and methods for controlling effective exposure in electronic image sensors are disclosed. In one embodiment, the overall effective exposure for one or more frames may be divided or partitioned into a plurality of sub-exposures. In another embodiment, the effective exposure of photodiodes in a color filter array may be varied among the different color channels of the array. In still another embodiment, the overall effective exposure for one or more frames may be divided or partitioned into a plurality of sub-exposures for pixels in one or more color channels, with the sub-exposures between the different color channels having different durations. [0031] Turning now to the figures, the image sensor and an illustrative electronic device for incorporating the image sensor will be discussed in more detail. Fig. 1 A is a front elevation view of an electronic device 100 including the image sensor. Fig. 1 B is a rear elevation view of the electronic device 100. The electronic device 100 may include any or all of a first camera 102, a second camera 104, an enclosure 106, a display 1 10, and an input/output button 108. The electronic device 100 may be substantially any type of electronic or computing device, such as, but not limited to, a computer, a laptop, a tablet, a smart phone, a digital camera, a printer, a scanner, a copier, or the like. The electronic device 100 may also include one or more internal components (not shown) typical of a computing or electronic device, such as, but not limited to, one or more processors, memory components, network interfaces, and so on. Examples of such internal components will be discussed with respect to Fig. 2.

[0032] As shown in Fig. 1 , the enclosure 106 may form an outer surface and protective case for the internal components of the electronic device 100 and may at least partially surround the display 1 10. The enclosure 106 may be formed of one or more components operably connected together, such as a front piece and a back piece, or may be formed of a single piece operably connected to the display 1 10.

[0033] The input member 108 (which may be a switch, button, capacitive sensor, or other input mechanism) allows a user to interact with the electronic device 100. For example, the input member 108 may be a button or switch to alter the volume, return to a home screen, and the like. The electronic device 100 may include one or more input members 108 and/or output members, and each member may have a single input or output function or multiple input/output functions.

[0034] The display 1 10 may be operably connected to the electronic device 100 or may be communicatively coupled thereto. The display 1 10 may provide a visual output for the electronic device 100 and/or may function to receive user inputs to the electronic device 100. For example, the display 110 may be a multi-touch capacitive sensing screen that may detect one or more user inputs.

[0035] The electronic device 100 may also include a number of internal components. Fig. 2 is a simplified block diagram of the electronic device 100. The electronic device 100 may also include one or more processors 1 14, a storage or memory component 1 16, an input/output interface 1 18, a power source 120, and one or more sensors 122, each will be discussed in turn below.

[0036] The processor 1 14 may control operation of the electronic device 100. The processor 1 14 may be in communication, either directly or indirectly, with substantially all of the components of the electronic device 100. For example, one or more system buses 124 or other communication mechanisms may provide communication between the processor 1 14, the cameras 102, 104, the display 1 10, the input member 108, the sensors 122, and so on. The processor 1 14 may be any electronic device cable of processing, receiving, and/or transmitting instructions. For example, the processor 1 14 may be a microprocessor or a microcomputer. As described herein, the term "processor" is meant to encompass a single processor or processing unit, multiple processors, or multiple processing units, or other suitably configured computing element.

[0037] The memory 1 16 may store electronic data that may be utilized by the electronic device 100. For example, the memory 1 16 may store electrical data or content e.g., audio files, video files, document files, and so on, corresponding to various applications. The memory 1 16 may be, for example, non-volatile storage, a magnetic storage medium, optical storage medium, magneto-optical storage medium, read only memory, random access memory, erasable programmable memory, or flash memory. [0038] The input/output interface 1 18 may receive data from a user or one or more other electronic devices. Additionally, the input/output interface 1 18 may facilitate transmission of data to a user or to other electronic devices. For example, in embodiments where the electronic device 100 is a phone, the input/output interface 1 18 may be used to receive data from a network, or may be used to send and transmit electronic signals via a wireless or wired connection (Internet, WiFi, Bluetooth, and Ethernet being a few examples). In some embodiments, the input/output interface 1 18 may support multiple network or communication mechanisms. For example, the network/communication interface 1 18 may pair with another device over a Bluetooth network to transfer signals to the other device, while simultaneously receiving data from a WiFi or other network. [0039] The power source 120 may be substantially any device capable of providing energy to the electronic device 100. For example, the power source 120 may be a battery, a connection cable that may be configured to connect the electronic device 100 to another power source such as a wall outlet, or the like.

[0040] The sensors 122 may include substantially any type of sensor. For example, the electronic device 100 may include one or more audio sensors (e.g., microphones), light sensors (e.g., ambient light sensors), gyroscopes, accelerometers, or the like. The sensors 122 may be used to provide data to the processor 1 14, which may be used to enhance or vary functions of the electronic device 100.

[0041 ] With reference again to Figs. 1 A and 1 B, the electronic device 100 may also include one or more cameras 102, 104 and optionally a flash 1 12 or light source for the cameras 102, 104. Fig. 3 is a simplified cross-section view of the first camera 102, taken along line 3-3 in Fig. 1 A. Although Fig. 3 illustrates the first camera 102, it should be noted that the second camera 104 may be substantially similar to the first camera 102. In some embodiments, one camera may include a global shutter configured image sensor and one camera may include a rolling shutter configured image sensor. In other examples, one camera may have an image sensor with a higher resolution than the other. Likewise, it should be appreciated that the structure shown in Fig. 3 is but one possible structure for either of the first and second cameras.

[0042] With reference to Fig. 3, the cameras 102, 104 may include a lens 126 in optical communication with an image sensor 130. The lens 126 may be operably connected to the enclosure 106 and positioned above the image sensor 130. The lens 126 may direct or transmit light 128 within its field of view on to a photodiode layer (discussed in more detail below) of the image sensor 130.

[0043] The image sensor 130 may be supported beneath the lens 126 by a substrate 132 or other support structure. The image sensor 130 may convert light 128 into electrical signals that may represent the light from the captured scene. In other words, the image sensor 130 captures the light 128 optically transmitted via the lens 126 into electrical signals.

Image Sensor Architecture

[0044] An illustrative architecture for the image sensor 130 will now be discussed in more detail. Fig. 4A is a simplified schematic of one possible architecture for the image sensor 130. Fig. 4B is an enlarged view of a pixel of the pixel architecture of Fig. 4A. Fig. 5 is a simplified schematic view of the pixel of Fig. 4A. With reference to Figs. 4A-5, the electronic device 100 may include an image processing component having a pixel architecture defining one or more pixels 136 and/or groups of pixel cells 138 (e.g., groups of pixels 136 grouped together to form a Bayer pixel or other set of pixels). The pixel architecture 134 may be in communication with a column select 140 through one or more column output lines 146 and a row select 144 through one or more row select lines 148.

[0045] The row select 144 and/or the column select 140 may be in communication with an image processor 142. The image processor 142 may process data from the pixels 136 and provide that data to the processor 1 14 and/or other components of the electronic device 100. It should be noted that in some embodiments, the image processor 142 may be incorporated into the processor 1 14 or separate therefrom. The row select 144 may selectively activate a particular pixel 136 or group of pixels, such as all of the pixels 136 on a certain row. The column select 140 may selectively receive the data output from select pixels 136 or groups of pixels 136 (e.g., all of the pixels with a particular column). [0046] With reference to Fig. 5, each pixel 136 may include a transistor array 152 or control circuitry and a photodiode 154. The photodiode 154 may be in optical

communication with the lens 126 to receive light transmitted therethrough. The photodiode 154 may absorb light and convert the absorbed light into an electrical signal. The photodiode 154 may be an electron-based photodiode or a hole-based photodiode.

Additionally, it should be noted that the term "photodiode," as used herein, is meant to encompass substantially any type of photon or light detecting component, such as a photogate or other photo-sensitive region. The photodiode 154 is coupled to a transfer gate 158, which selectively connects the photodiode 154 to the remaining control circuitry 152 of the pixel 136.

[0047] The transfer gate 158 is coupled to a reset gate 156 and a source follower (SF) gate 160. A reset gate 162 and the SF gate 160 are coupled to a reference voltage node 164 which connects the two gates to a reference voltage source (Vdd) 166. The row select gate 162 is coupled to a row select line 148 for the pixel 136. A floating diffusion (FD) node 163, including a charge storage component 168, may be coupled between the transfer gate 158 and the reset gate 156 and SF gate 160. The control circuitry 152 (one example of which is the aforementioned transistor array) may include additional gates other than those shown in Fig. 5. For example, an anti-blooming gate may be in communication with the photodiode 154 to drain excess charge over saturation from the photodiode. [0048] Generally, in operation, when one of the cameras 102, 104 is actuated to capture an image, the reference voltage 166 is applied to the reset gate 156 and the transfer gate 158. When the transfer gate 158 is open, the charge within the photodiode 154 is drained to deplete the photodiode. In some embodiments, the cameras 102, 104 may not include a shutter over the lens 126, and so the image sensor 130 may be constantly exposed to light. In these embodiments, the photodiode 154 may have to be reset or depleted before a desired image is to be captured. Once the charge from the photodiode 154 has been depleted, the transfer gate 158 and the reset gate 156 may be turned off, isolating the photodiode 154. The photodiode 154 may then begin integrating and collecting light 128 transmitted to the image sensor 130 from the lens 126. As the photodiode 154 receives light, it starts to collect charge (e.g., a depletion region reduces as electrons from the light are received). However, the charge within the photodiode 154 may remain within a well of the photodiode 154 because the transfer gate 158 (connecting the photodiode 154) to the control circuitry 150 and other gates is off.

[0049] Once integration is complete and the photodiode 154 has collected light 128 from the lens 126, the reset gate 126 may be turned on to reset the floating diffusion node 163. Once the floating diffusion 162 has been reset, the reset gate 156 may be turned off and the transfer gate 158 may be turned on. The charge from the photodiode 154 can then be transferred to the floating diffusion node 163 and stored in the storage component 168. To read out the charge from the photodiode 154 (via the floating diffusion 163), the row select gate 152 and the SF gate 160 may be activated, and the SF gate 160 amplifies the charge within the floating diffusion 163 and through the row select gate 162, the signal or charge is provided to the column output line 146.

[0050] In a rolling shutter operation, the photodiodes 154 in different rows may be exposed at different times. Accordingly, if one or more objects within a scene are moving, a first row may capture a different position of the image than a second row, as they are exposed sequentially, which may cause motion artifacts in the sensed image. In a global shutter operation, additional storage gates may be added to store charge from the photodiode 154. In the global shutter operation, each row within the pixel architecture 134 may be reset and exposed at substantially the same time. Each storage node may also simultaneously transfer the charge from the photodiode 154 to a storage node, and then each pixel 136 may be read out row by row. Some embodiments may implement a rolling shutter architecture on a column basis, with appropriate changes to the associated circuitries.

[0051] With reference now to Fig. 6, in some embodiments, the image sensor 130 may include one or more global shutter pixels 136. Fig. 6 is a simplified diagram of the photodiode 154 and the control circuitry 152, which includes a storage gate 190. With reference to Fig. 6, each pixel 136 may include a storage gate 190 coupled between a photodiode 154 and a storage node 192. The storage node 192 may store charge from the photodiode 154 to allow a global shutter operation, and may in some examples be electrically and/or optically shielded so as to prevent stray charge and/or light from corrupting the contents of the storage node 192. For example, in a global shutter operation, each of the pixels 136 of image sensor 130 may begin charge integration at the same time. During the integration period, each of the photodiodes 154 may accumulate charge corresponding to light transmitted through the lens 126 that encounters each photodiode 154. After integration, the storage gate 190 may be activated and the charge from the photodiode 154 may be transmitted to the storage node 192. The data from the photodiode 154 may be held at the storage node 192 until the particular pixel 136 is ready to be read out.

[0052] When the pixel 136 is to be read out, the transfer gate 158 may be activated to transfer the charge from the storage node 192 to the floating diffusion node 163. Once the data is stored in the floating diffusion 163, the pixel 136 may be operated in substantially the same manner as described above with respect to Fig. 6, i.e., every row may be read out sequentially. During a global shutter operation, all of the pixels 136 may capture light at substantially the same time, which may decrease artifacts in an image due to an object moving. For example, during a rolling shutter operation where each row is sequentially exposed and read out, if an object moves between a first pixel row integration and a last row integration, the image may be blurred. In the image sensor 130 of Fig. 6, the pixels 136 for the entire sensor 130 may be exposed to light and integrate charge at substantially the same time.

[0053] In general, many different types of image sensor architectures are contemplated, some of which may include stacked chips, sharing on various levels (e.g., shared floating diffusion nodes, shared storage nodes, shared transfer gates, etc.), global shutter architectures, rolling shutter architectures, still and video image sensors, and so forth as described for example in co-pending application 13/756,459 entitled "Vertically Stacked Image Sensor" filed on January 31 , 2013, the entirety of which is hereby incorporated by reference for all purposes. [0054] With reference now to Figs. 7 and 8, two examples of pixel sub-arrays are illustrated. In Fig. 7, four rolling shutter pixels 136-1 , 136-2, 136-3, 136-4 are shown, similar to the pixel 136 illustrated in Fig. 5. In some examples, the four pixels 136-1 , 136-2, 136-3, 136-4 in Fig. 7 may be arranged in a 2x2 sub-array and may be one cell of a Bayer CFA; for example, the first pixel 136-1 may be red, the second pixel 136-2 may be green, and the first and second pixels 136-1 , 136-2 may share a row select signal. The third pixel 136-3 may be green, the fourth pixel 136-4 may be blue, and the third and fourth pixels 136-3, 136-4 may share a row select signal. Alternatively, one of the second or third pixels 136-2, 136-3 may be a clear pixel instead of a green pixel. In general, the four pixels 136-1 , 136-2, 136-3, 136- 4 may be associated with one or more different color channels, and any different color filter array may be used.

[0055] With reference to Fig. 8, four global shutter pixels 136-1 , 136-2, 136-3, 136-4 are shown, each similar to the global shutter pixel 136 illustrated in Fig. 6, and with the optional addition of an anti-blooming gate coupled to the photodiode of each pixel. In some examples, the four pixels 136-1 , 136-2, 136-3, 136-4 in Fig. 8 may be arranged in a 2x2 sub- array and may form one cell of a Bayer CFA; for example, the first pixel 136-1 may be red, the second pixel 136-2 may be green, and the first and second pixels 136-1 , 136-2 may share a row select signal. The third pixel 136-3 may be green, the fourth pixel 136-4 may be blue, and the third and fourth pixels 136-3, 136-4 may share a row select signal.

Alternatively, one of the second or third pixels 136-2, 136-3 may be a clear pixel instead of a green pixel. In general, the four pixels 136-1 , 136-2, 136-3, 136-4 may be associated with one or more different color channels, and any different color filter array may be used. [0056] Although various embodiments of image sensors may be described herein with reference to the four pixels shown in Fig. 7 or with reference to the four pixels shown in Fig. 8, it is to be understood that the methods of operating image sensors disclosed herein generally may be employed in either global or rolling shutter architectures and the specific examples given herein are merely for illustration.

Pulsed Exposure

[0057] In one embodiment of exposure control, which may be called "pulsed exposure," the overall effective exposure for one or more frames may be divided or partitioned into a plurality of sub-exposures. Subdividing an exposure over a plurality of sub-exposures may extend the absolute time over which an exposure effectively occurs. Such a method may be particularly useful for taking video in bright light situations, where, in order to avoid saturation, the exposure time for each frame needs to be reduced to be only a fraction (1/2, 1/3, 1/4, 1/10, 1 /20, 1/50, 1 /100, etc.) of the total length of the frame in order to reduce the probability of saturating one or more pixels. Dividing the exposure into multiple sub- exposures may help make the light that is exposed during the video frames more uniform in order to reduce any stroboscopic effects that may otherwise occur if, for example, the pixels of the image sensor are only exposed for the first 5% of each frame. In general, though, pulsed exposure is not limited to use in bright light video situations, and generally may be applied in video or still imaging and in high, medium, or low lighting conditions. [0058] With reference to the pixels 136-1 , 136-2, 136-3, 136-4 shown in Fig. 8 and also with reference to the timing diagram in Fig. 9, one or more control signals may be pulsed (e.g., provided alternatingly as logic high 179 and logic low 178, or alternatingly between any two or more different voltages) to control exposure of the photodiodes 154. The pulsed control signal may be an anti-blooming signal 171 provided to anti-blooming gates, a storage gate transfer signal 172 provided to storage gates 190, a transfer signal 173, generally any control signal, or some combination of these. In general, the same control signals may be applied across the different pixels 136-1 , 136-2, 136-3, 136-4 in some embodiments; for example, a single storage gate transfer signal may be applied to the storage gates 190 of each of the pixels 136-1 , 136-2, 136-3, 136-4. In other words, all of the color channels of the image sensor may, in some embodiments, receive the same exposure control signals. In other examples, different control signals are provided to different ones of the pixels 136-1 , 136-2, 136-3, 136-4.

[0059] The exposure control signals may selectively cause charge to accumulate during one or more sub-exposures 176 and/or cause charge to be discarded during one or more reset periods 177 (e.g., by coupling the photodiode to a power supply). In one example, and with reference to Fig. 9, one control signal SG1 172-1 selectively causes charge that has accumulated in or is accumulating in the photodiode 154 to be transferred to a storage node 192 to be stored. When the control signal SG1 172-1 is asserted as logic high 179, any charge that is generated in the photodiode 154 may be transferred through the storage gate 190 to the storage node 192. Conversely, when the control signal SG1 172-1 is provided as logic low, the storage gate 190 may be closed and no charge may pass from the photodiode 154 to the storage node 192. Still referring to the timing diagram of Fig. 9, another control signal AB1 171 -1 causes a photodiode to be selectively electrically coupled to an anti- blooming node (which may be a power supply), so that when the control signal AB1 171 -1 is asserted as logic high 179, any charge that is generated in the photodiode does not accumulate but is discarded (which may be referred to as a reset period 177). When the control signal AB1 171 -1 is provided as logic low 178, the photodiode is decoupled from the anti-blooming node and charge is allowed to accumulate in the photodiode and/or is otherwise captured (which may be referred to as a sub-exposure 176). In other words, the control signal AB1 171 -1 may control the effective exposure or integration time of the pixel 136-1 .

[0060] In general, a frame may include any number of sub-exposures 176 and any number of reset periods 177. For example, one frame (which may be representative of, for example, one still image standing alone or one still image in a series of images for a video) may be divided into 2, 3, 4, 10, 50, 100, 1000, 10,000 sub-exposures 176 or any other suitable number, and the charge accumulated during the two or more sub-exposures 176 may all be collected in the storage node 192 prior to being transferred to the floating diffusion node 163. In some examples, all of the two or more sub-exposures 176 may have substantially the same duration 176t (as one non-limiting example, each has a duration of 1 ms), or may have different durations (e.g., some sub-exposures are longer than others). Also, in some examples, one or more of the sub-exposures 176 may have substantially the same duration as one or more reset periods 177, or the duration 176t of one or more of the sub-exposures 176 may be substantially more or less than the duration 177t one or more of the reset periods 177. For one example, one or all of the reset periods 177 may be .1 , .5, 1 .5, 2, 5, 10, 100, 1000, 10,000 times that of one or more of the sub-exposures 176.

[0061] After charge has accumulated and been transferred to the storage node 192 after two or more sub-exposures 176, a transfer signal TX1 173-1 may be provided to the transfer gate 158 coupled between the storage node 192 and the floating diffusion node 163. The transfer signal TX1 173-1 may be asserted as logic high 179 to effectuate the transfer in some embodiments and with reference to Fig. 8. Because all of the charge accumulated in each of the plurality of sub-exposures 176 is collected in the storage node 192, the pixel sums charge in the analog domain, rather than digitally sampling the charge collected during each sub-exposure 176 and summing the digital data. Such analog domain charge summing may require less power than would be needed to digitally sample sub-exposure charge and then sum the digital samples, and analog domain charge summing may be more accurate because it may reduce the amount of noise introduced into the summing as compared with the digital sampling and summing.

[0062] With reference to Figs. 8 and 9, and as previously mentioned, the control signals AB1 , SG1 , TX1 provided to one pixel 136-1 may be sub substantially identical to the control signals AB2, SG2, TX2 provided to the second pixel 136-2, the control signals AB3, SG3, TX3 provided to the third pixel 136-3, and the control signals AB4, SG4, TX4 provided to the fourth pixel 136-4. The control signals may be provided to the various pixels 136-1 , 136-2, 136-3, 136-4 in generally any manner. Although Fig. 9 shows the transfer signals TX1 , TX2, TX3, TX4 as being provided to each of the pixels 136-1 , 136-2, 136-3, 136-4 at substantially the same time, in another embodiment, the transfer signals may be staggered in order to allow for sharing of a floating diffusion node and/or to allow for a row-by-row rolling readout.

Per Color Channel Exposure

[0063] In another embodiment of exposure control, which may be called "per color channel exposure," the effective exposure of photodiodes for one or more frames may vary among different color pixels. As described above, different color pixels may have different sensitivities to similarly bright light, and thus it may be advantageous to provide different effective exposure times (also known as integration times) among the different colored pixels to help overcome the fundamental differences in sensitivity between the different color channel pixels. Such method may be particularly useful for taking still image shots of static scenes because it does not necessarily require pulsed exposure as explained above and because the objects to be imaged may not be in motion. In general, though, per color channel exposure is not limited to use for still images of static scenes, and generally may be applied in video or still imaging, in dynamic or static scenes, in high, medium, or low lighting conditions and so forth.

[0064] With reference to the pixels 136-1 , 136-2, 136-3, 136-4 shown in Fig. 8 and also with reference to the timing diagram in Fig. 10, one or more exposure control signals may be provided to each of the pixels 136-1 , 136-2, 136-3, 136-4 to control the effective exposure of the corresponding photodiodes 154. The exposure control signal may be an anti-blooming signal 171 provided to an anti-blooming gate, a storage gate transfer signal 172 provided to a storage gate 190, a transfer signal 173, generally any control signal, or some combination of these. In general, the same control signals may not be provided across the different pixels 136-1 , 136-2, 136-3, 136-4 in some embodiments. For example, a first storage gate transfer signal SG1 172-1 and/or a first anti-blooming signal AB1 171 -1 may be provided to the storage gate 190 and anti-blooming gate of the first pixel 136-1 , while a second storage gate transfer signal SG2 172-2 and/or a second anti-blooming signal AB2 171 -2 may be provided to the storage gate 190 and anti-blooming gate of the second pixel 136-2, and so forth.

[0065] The exposure control signals may selectively cause charge to accumulate and/or cause charge to be discarded (e.g., by electrically coupling the photodiode to a power supply). In one example, and with reference to Fig. 10, the first pixel 136-1 may be associated with a first color channel, for example clear or green. One control signal AB1 171 -1 may cause the photodiode to selectively be coupled to an anti-blooming node (which may be a power supply), so that when the control signal AB1 171 -1 is asserted as logic high 179, any charge that is generated in the photodiode does not accumulate but is discarded (which may be referred to as a reset period 177). When the control signal AB1 171 -1 is provided as logic low 178, the photodiode is decoupled from the anti-blooming node and charge is allowed to accumulate in the photodiode and/or is otherwise captured (which may be referred to as the effective exposure 176 of the photodiode). Still referring to the timing diagram of Fig. 10, another control signal SG1 172-1 selectively causes charge that has accumulated in or is accumulating in the photodiode 154 to be transferred to a storage node 192 to be stored. When the control signal SG1 172-1 is asserted as logic high 179, any charge that is generated in the photodiode 154 may be transferred through the storage gate 190 to the storage node 192, whereas when the control signal SG1 172-1 is provided as logic low 178, the storage gate 190 may be closed and no charge may pass from the photodiode 154 to the storage node 192. [0066] With reference still to Figs. 8 and 10, similar exposure control signals AB2 171 -2 and SG2 172-2 are provided to the second pixel 136-2, which may be associated with a color channel different from the color channel with which the first pixel 136-1 is associated. The second pixel 136-2 may be associated with, for example, red or blue, or possibly green if the first pixel 136-1 is associated with a clear color channel. The exposure control signal AB2 171 -2 provided to the second pixel 136-2 is, however, different from the exposure control signal AB1 171 -1 provided to the first pixel 136-1 in that the exposure control signal AB2 171 -2provided to the second pixel 136-2 allows for a longer effective exposure 176 of the second pixel 136-2 than the first pixel 136-1 . In other words, the exposure control signal AB1 171 -1 causes charge to accumulate in the photodiode 154 of the first pixel 136-1 for a shorter period of time 176t than the period of time 176t during which exposure control signal AB2 171 -2 allows for charge to accumulate in the photodiode 154 of the second pixel 136-2. The periods of time during which the first and second pixels 136-1 , 136-2 accumulate charge may be at least partially contemporaneous with one another in some examples, but may not be in other examples.

[0067] Third and fourth groups of similar exposure control signals are provided to the third and fourth pixels 136-3, 136-4 in Fig. 8, with the effective exposure 176 of the third pixel 136-3 being longer than the effective exposure 176 of the second pixel 136-2 and the effective exposure 176 of the fourth pixel 136-4 being longer than the effective exposure 176 of the third pixel 136-3. The effective exposure 176 of the fourth pixel 136-4 may be as long as practically possible based on a required frame rate or user selected exposure time in some embodiments. As just one example, the effective exposure 176 of the fourth pixel

136-4 may be 15 milliseconds and the fourth pixel 136-4 may be associated with a red color channel. The effective exposure 176 of the third pixel 136-3 may be 1 1 milliseconds, and the third pixel may be associated with a blue color channel. The effective exposure 176 of the second pixel 136-2 may be 8 milliseconds, and the second pixel may be associated with a green color channel. The effective exposure 176 of the first pixel may be 3 milliseconds, and the first pixel may be associated with a clear color channel. In general, the durations of effective exposure 176 for the different pixels 136-1 , 136-2, 136-3, 136-4 may be determined based on the relative sensitivities of the different color channels associated with the different pixels 136-1 , 136-2, 136-3, 136-4. Determining the durations of the effective exposures 176 may include in some examples, setting the least sensitive color channel pixels to have as long of effective exposure time as allowed, and then reducing the other color channel pixels to have shorter effective exposure times based on the sensitivities of those other color channels relative to the least sensitive color channel. Also, if two of the pixels are associated with the same color channel, they may have similar effective exposure times. [0068] Some of the exposure control signals AB1 171 -1 , AB2 171 -2, AB3 171 -3, AB4 171 - 4 may cause charge to be drained or discarded from the respective photodiodes 154 of the pixels 136-1 , 136-2, 136-3, 136-4 after each respective effective exposure time has ended. For example, with reference to the first pixel 136-1 in Fig. 10, following the effective exposure 176 of the photodiode 154, the exposure control signal SG1 172-1 may transition to logic low 178, and the control signal AB1 may transition to logic high 179. Providing the control signal AB1 171 -1 as logic high may end the effective exposure 176 and induce drainage of any further charge generated in the photodiode 154 of the first pixel 136-1 , with the drainage beginning after the end of the effective exposure period of the first pixel 136-1 but before the end of effective exposure of the second pixel 136-2. In other words, the photodiode 154 of the second pixel 136-2 may continue to be effectively exposed and accumulate charge while generated charge is being drained away from the photodiode 154 of the first pixel 136-1 .

[0069] After charge has accumulated and been transferred to the storage node 192 in each of the pixels 136-1 , 136-2, 136-3, 136-4, a transfer signal 173 may be provided to the transfer gate 158 coupled between the storage node 192 and the floating diffusion node 163 for each pixel, as illustrated in Fig. 10. The transfer signal 173 may be asserted as logic high 179 to effectuate the transfer. In the global shutter implementation illustrated in Figs. 8 and 10, even though the effective exposures 176 of the different pixels 136-1 , 136-2, 136-3, 136-4 may span different times and have different effective exposure durations (though each pixel 136-1 , 136-2, 136-3, 136-4 may be effectively exposed substantially

contemporaneously during at least the effective exposure 176 of the first pixel 136-1 as illustrated in Fig. 10), the global transfer of accumulated charge from each respective storage node to each respective floating diffusion node may happen substantially contemporaneously (e.g., may be a global transfer). In another embodiment, however, the transfer signals TX1 173-1 , TX2 173-2, TX3 173-3, TX4 173-4 provided to the pixels 136-1 , 136-2, 136-3, 136-4 may be staggered in order to allow for sharing of a floating diffusion node and/or to allow for a row-by-row rolling readout.

[0070] As mentioned above, any of the exposure control methods described herein may be applicable to any type of image sensor architecture, including both rolling and global shutters. With reference to Figs. 5, 7, and 1 1 , for example, if per color channel exposure is used in an image sensor with rolling shutter, a signal 173 provided to the transfer gate 158 may function as a combination of one or more of an anti-blooming signal, a storage signal, a transfer signal, and so forth, with the signal being provided at a plurality of different voltage levels. For example, the signal 173 may function as an integration signal when it is strongly negative 178 (e.g., -1 .2 volts) so that the photodiode 154 is isolated from the floating diffusion node 163, may function as a transfer signal when it is strongly positive 179 (e.g., +2.8 volts) so that charge accumulated in the photodiode 154 is transferred to the floating diffusion node 163, and may function as an anti-blooming signal when it is slightly positive 180 (e.g., +.2 volts) because it is slightly leaky. [0071] Referring to the timing diagram illustrated in Fig. 1 1 , in some embodiments, a reset signal RST1 174-1 may be provided to the reset gate 156 of the first pixel 136-1 (which may be a clear or other sensitive pixel color) as logic high 179, while the transfer signal TX1 173- 1 is provided to the transfer gate 158 at a slightly positive voltage 180 so as to leak accumulated charge away (i.e., to prevent blooming). The transfer signal TX1 173-1 may be pulsed to a logic high 179, and then be provided as logic low 178 at a strongly negative voltage. At approximately the same time as the transfer signal TX1 173-1 transitions to logic low 178, the reset signal RST1 may also transition to logic low 178. This may then begin the exposure or integration of the first pixel 136-1 . Shortly before the end of the exposure of the first pixel 136-1 , the reset signal RST1 may briefly be pulsed to logic high 179 and then back to logic low 178 in order to reset the floating diffusion node 163, and optionally to read out a voltage in the reset floating diffusion node. Then, the transfer signal TX1 173-1 is pulsed to logic high 179 then back to logic low 178 in order to transfer the charge to the floating diffusion node 163 and read out the same. Then, the transfer signal TX1 173-1 is set to the slightly positive voltage 180 in order to again enter an anti-blooming mode. Similar control signals may be provided to the other pixels 136-2, 136-3, 134, except that those pixels may remain in the anti-blooming mode, with the slightly positive voltage 180 being provided to the transfer gate 158, for different amounts of times. For example, the second pixel 136-2 may remain in anti-blooming mode for slightly less time than the first pixel 136-1 , with the third and fourth pixels 136-3, 136-4 remaining in anti-blooming mode for even less time. If all of the pixels 136-1 , 136-2, 136-3, 136-4 are read out at substantially the same time, as illustrated in Fig. 1 1 , the effective exposures 176t of the pixels 136-1 , 136-2, 136-3, 136-4 will be different because of the different amounts of time during which the slightly leaky, anti- blooming voltage 180 was provided to the transfer gate 158. In this manner, less sensitive pixels (e.g., 136-4) may be exposed for a longer period of time 176t than more sensitive pixels (e.g., 136-1 ) so that the more sensitive pixels do not over fill with charge.

Furthermore, because the anti-blooming voltage 180 is provided to the pixels when they are not being exposed, anti-blooming may be reduced or eliminated.

[0072] With reference now to Figs. 10 and 1 1 , it will be understood that references to signals being 'before' or 'after' another signal are relative, and some embodiments may include signals provided in a switched context. For example, although Fig. 10 illustrates beginning the exposure periods 176 for each of the pixels 136-1 , 136-2, 136-3, 136-4 at substantially the same time and ending the exposure periods of the pixels 136-1 , 136-2, 136- 3, 136-4 at staggered times (e.g., the first pixel 136-1 ends its exposure before the second pixel 136-2 ends its exposure, and so forth), in another example, the pixels 136-1 , 136-2, 136-3, 136-4 start exposure periods 176 at staggered times, and end exposure periods 176 at substantially the same time, as illustrated in the rolling shutter example in Fig. 1 1 . In general, the exposure periods 176 for the pixels 136-1 , 136-2, 136-3, 136-4 may begin or end 'before' or 'after' one another in a large variety of combinations to achieve the operation and results described.

Pulsed Per Color Channel Exposure [0073] One other embodiment of exposure control, called "pulsed per color channel exposure," may, to a certain extent combine elements of both "pulsed exposure" control and "per color channel exposure" control. More specifically, in pulsed per color channel exposure control, the overall effective exposure for one or more frames may be divided or partitioned into a plurality of sub-exposures for pixels in one or more color channels, with the durations of sub-exposures between the different color channels being different due to the different sensitivities of each color channel. For one color channel (e.g., the least sensitive color channel such as red), the exposure may not be divided or partitioned at all in some examples. This may allow for the least sensitive color channel to accumulate charge over the maximum amount of time possible (based on, e.g., frame rate, user selected exposure duration, etc.), while partitioning the exposure of the other color channels into a plurality of sub-exposures. The sub-exposures may span the same absolute time as the exposure of the least sensitive channel but may effectively expose the respective photodiodes 154 for different durations or amounts of time depending on the color channel sensitivity. Such method may be particularly useful for taking video or still shots of dynamic scenes (e.g., where an object to be imaged is moving). As with pulsed exposure, dividing the exposure into multiple sub-exposures may help make the light that is exposed during the video frames more uniform in order to reduce any stroboscopic effects that may otherwise occur if, for example, the pixels of the image sensor are only exposed for some portion of each frame, while similar to per color channel exposure, the exposure of different pixel color channels is compensated based on the sensitivities of the respective photodiodes 154 to different wavelengths of light. In general, though, pulsed per color channel exposure is not limited to use in any particular situation, and generally may be applied in any video or still imaging application.

[0074] With reference to the pixels 136-1 , 136-2, 136-3, 136-4 shown in Fig. 8 and also with reference to the timing diagram in Fig. 12, one or more control signals may be pulsed (e.g., provided alternatingly as logic high 179 and logic low 178, or alternatingly between any two or more different voltages) to control exposure of the photodiodes 154, with different control signals being provided to pixels 136-1 , 136-2, 136-3, 136-4 in different color channels so that the effective exposure of photodiodes 154 in different color channels have different durations 176t. As above, the control signal may be an anti-blooming signal 171 provided to an anti-blooming gate, a storage gate transfer signal 172 provided to a storage gate 190, a transfer signal 173 provided to a storage gate, generally any control signal, or some combination of these. Also as above, the control signal(s) may selectively cause charge to accumulate during one or more sub-exposures 176 (or sole exposure 176 for the least sensitive color channel) and/or cause charge to be discarded during one or more reset periods 177. [0075] Referring still to Figs. 8 and 12, the first pixel 136-1 may be the most sensitive pixel, with the second pixel 136-2 being less sensitive than the first pixel 136-1 , the third pixel 136-3 being less sensitive than the second pixel 136-2, and the fourth pixel being associated with the least sensitive color channel. Referring to the control signals AB1 171 -1 and SG1 172-1 illustrated in the timing diagram in Fig. 1 1 , the photodiode 154 of the first pixel 136-1 may selectively accumulate charge during a first plurality of sub-exposures 176, with the sub-exposures 176 generally being the period of time when the AB1 signal 171 -1 is provided as logic low 178. After or during each of the first plurality of sub-exposures 176, the charge accumulated in the photodiode 154 may be transferred to the respective storage node 192 through storage gate 190 responsive to a logic high 179 pulse of the control signal SG1 172-1 . The control signals AB2, SG2, AB3, SG3 provided to the second and third pixels 136-2, 136-3 may be similar to those provided to the first pixel 136-1 , except that they may provide for sub-exposures 176 of respectively longer duration. Also, control signals AB4, SG4 may be provided to the fourth pixel 136-4 so that the fourth pixel is only effectively exposed 176 once.

[0076] As one example, the fourth pixel 136-4 may be associated with the red color channel and may be exposed for 28 total milliseconds in a single exposure 176. The third pixel 136-3 may be associated with the blue color channel and may be exposed for a total of 21 milliseconds during seven sub-exposures 176 of 3 milliseconds each. The second pixel 136-2 may be associated with the green color channel and may be exposed for a total of 14 milliseconds during seven sub-exposures 176 of 2 milliseconds each. The first pixel 136-1 may be associated with the clear color channel and may be exposed for a total of 7 milliseconds during seven sub-exposures 176 of 1 millisecond each. Of course, these times are merely given by way of example and there may be different numbers (e.g., 2, 3, 4, 10, 50, 100, 1000, 10,000) of sub-exposures 176 between the different color channels, the sub- exposure 176 durations need not be integer multiples of one another between the different color channels, the durations of sub-exposures 176 within each pixel or color channel need not be the same, and so forth. Also, in some examples, such as image sensors with a traditional Bayer pattern, two or more of the pixels 136-1 , 136-2, 136-3, 136-4 may have identical exposure or sub-exposure durations, and each pixel or each color channel need not necessarily be different from the others.

[0077] As with pulsed exposure, after charge has accumulated and been transferred to the storage node 192 in each respective pixel 136-1 , 136-2, 136-3, 136-4, a transfer signal 173 may be provided to the transfer gate 158 coupled between the storage node 192 and the floating diffusion node 163 for each respective pixel 136-1 , 136-2, 136-3, 136-4. The transfer signal 173 may be asserted as logic high 179 to effectuate the transfer, and may be provided to all of the pixels 136-1 , 136-2, 136-3, 136-4 at substantially the same time in one embodiment, or may be staggered as described above with reference to Figs. 9 and 10. Because all of the charge accumulated in each of the plurality of sub-exposures is collected in the storage node 192, the pixel sums charge in the analog domain, as described above. Conclusion

[0078] The foregoing description has broad application. For example, while examples disclosed herein may focus on particular architectures of image sensors (e.g., photodiode, global shutter, rolling shutter, CMOS sensors, etc.), it should be appreciated that the concepts disclosed herein may equally apply to substantially any other type of image sensor with or without appropriate modifications as would be appreciated by one skilled in the art of image sensors. Moreover, although certain examples have been described with reference to particular color channels or particular exposure times, it will be understood that other color channels and other exposure times are also within the scope of this disclosure and the appended claims. [0079] Furthermore, the various exposure control methods described herein may find application in many different implementations. For example, one or more of the exposure control methods may be implemented in response to a manufacturer setting to, for example, use per color channel exposure control with certain relative effective exposure durations for the different color channels, or to use the pulsed exposure control when a bright light scene is to be imaged. In other examples, the control methods described herein may be implemented in response to a user changing a configurable setting for the image sensor. The user configurable settings may provide various special effects in-sensor in certain embodiments. In still other examples, the exposure control methods may be used and/or modified dynamically based on a preview frame of an image to be taken or on recent images captured by the image sensor. For example, when taking a still shot, the image sensor may be operated in a preview mode to determine the appropriate effective exposure control method to use and the specific parameters to be used (e.g., the length of sub-exposures, the total effective duration for a particular color channel, etc.). As another example, when taking a video with the image sensor, the type of exposure control to be used in a future frame and the parameters of that control may be determined based on analyzing histograms or other characteristics of a current or recent frame. As still another example, any one or more of the effective exposure control methods described herein may be used for interleaved imaging - for example, in interleaved high dynamic range imaging (i-HRD), a single image sensor may substantially simultaneously capture two or more images (for two images, the odd rows may capture an image with a first effective exposure time while the even rows capture an image of the same scene with a second, different effective exposure time). [0080] Also, the various control signals described herein may be provided to the pixels and control circuitry of the pixels in any appropriate manner, including by using multiplexers or other electronic circuits within or outside of the image sensor.

[0081] Accordingly, the discussion of any embodiment is meant only to be exemplary and is not intended to suggest that the scope of the disclosure, including the claims, is limited to these examples.