Imager having multiple storage locations for each pixel sensor

An imager includes an array of pixel sensors. Each pixel sensor indicates at least two different primary color components of an image. For each pixel sensor, at least two storage locations are located in the array and store the indications from the pixel sensor.

BACKGROUND

The invention relates to an imager.

Referring toFIG. 1, a typical digital camera12uses an imager18to electrically capture an optical image11. To accomplish this, the imager18typically includes an array13(seeFIG. 2) of photon sensing, pixel sensors20. During an integration interval, each pixel sensor20typically measures the intensity of a portion, or pixel, of a representation of the image11that is focused (by optics of the camera12) onto the array13. At the expiration of the integration interval, each sensor20typically indicates (via an analog signal, such as a voltage, for example) an intensity of light of the pixel.

The camera12typically processes the indications from the pixel sensors20to form a frame of digital data (which digitally represents the captured image) and transfers the frame (via a serial bus15, for example) to a computer14for processing. For video, the camera12may capture several optical images and furnish several frames of data, each of which indicates one of the captured images. The computer14may then use the frames to recreate the captured video on a display9.

Referring toFIG. 2, the sensors20are typically arranged in rows and columns. In this manner, after the expiration of the integration interval, row24and column22decoders (of the imager18) may begin selectively, electrically selecting the sensors20to retrieve the indications from the sensors20.

The decoders22and24route the selected indications to signal conditioning circuitry26which might include, for example, analog-to-digital converters (ADCs) and circuitry to compensate for noise that is introduced by the sensors20. The signal conditioning circuitry26may furnish data signals to an output interface28which includes circuitry for interfacing the imager18to other circuitry of the camera12. A control unit30may coordinate the above-described activities of the imager18.

Typically, a multi-color mosaic filter (not shown) covers the array13and configures each sensor20to sense the intensity of a primary color component of the associated pixel. For example, for the set of red, green and blue primary colors, the filter may cause each sensor20to sense either a red, green or blue primary color component of the pixel. Thus, with this arrangement, each sensor20only senses one primary color component of the pixel. However, for true color imaging, it may be desirable to obtain the other primary color components of the pixel.

Typically, the missing primary color components are obtained through interpolation. Using interpolation, the missing primary color components for a given pixel are derived by interpolating color components from the nearest pixels (adjacent, for example), or “nearest neighbors.” For example, if a given sensor20senses a green component of a pixel, the red primary color component for that pixel may be interpolated from the information provided by neighboring sensors20that sense red primary color components. However, the interpolation only provides estimates (and not the actual values) of the missing color components.

Thus, there is a continuing need for an imager that provides actual primary color components of each pixel.

SUMMARY

In one embodiment, an imager includes an array of pixel sensors. Each pixel sensor indicates at least two different primary color components of an image. For each pixel sensor, at least two storage locations are located in the array, and these locations store the indications from the pixel sensor.

In another embodiment, a camera includes an array of pixel sensors, a programmable color filter, a controller and storage locations. Each pixel sensor indicates at least two primary color components of the image, and the programmable color filter substantially covers the array. The controller programs the color filter to cause the pixel sensors to sense the primary color components one at a time. For each pixel sensor, at least two of the storage locations are located in the array, and these locations store the indications from the pixel sensor.

DETAILED DESCRIPTION

Referring toFIG. 3, an embodiment50of a digital imager in accordance with the invention includes an imaging semiconductor chip54which has a light sensing, imaging surface51. The surface51is located in a focal plane onto which images to be captured are focused and includes an array60(seeFIG. 4) of pixel sensing units68, each of which senses an intensity of light that strikes a portion, or pixel, of the surface51. The surface51is covered by an electrically programmable color filter52(a color filter available from ColorLink, Inc., of Boulder, Colo., for example) which is electrically configured by the chip54to allow primary color components (primary color components from the set of red, green and blue primary colors, as examples) from the image to individually strike the surface51in succession and be detected by the pixel sensing units68, as described below.

For this to occur, in some embodiments, the filter52(as described below) is configured by the chip54to successively cycle through three primary color modes. In each mode, the filter52allows a different one of the primary color components to pass through the filter52and strike the surface51. As described below, each mode has a duration that is sufficient to permit each pixel sensing unit68to sense the corresponding color component of the associated pixel, and after sensing the color component, each pixel sensing unit68locally stores a color value that is indicative of the sensed primary color component. As described below, because the color values are stored locally in the array60, the array60, in some embodiments, is only scanned after all primary color components are captured.

After the color filter52cycles through all three modes, the array60is scanned to retrieve the stored color values (for all three color components). Referring toFIG. 4, each scanning of the array60may include successively scanning the rows of the array60, one at a time. For example, row63and column61decoders may begin the scan by electrically selecting a group of the pixel sensing units68and one of the primary colors. Once selected, the pixel sensing unit68transfers the stored value (that corresponds to the selected color) to signal conditioning circuitry62. The circuitry62may, for example, filter noise from the values before transferring the values to an output interface64. The output interface64may include buffers for temporarily storing the color values and circuitry to interface the imager50to external circuitry (other components of a digital camera, for example). The chip54might also include, for example, a control unit66which has circuitry such as state machines and timers to control the scanning and data flow through the chip54. The control unit66is coupled to selection lines69for selecting the stored color values in the array60, and the control unit66also interacts with the color filter52to control the modes of the filter52.

The advantages of storing the color values locally in the array60may include one or more of the following: The primary color components of a video image may be captured before the array is scanned; true color pixel values may be retrieved from the array; and the values may be formatted into digital words on the imager.

Referring toFIG. 5, in some embodiments, the color values for each pixel sensing unit68may be represented by analog voltages that are stored in analog storage units70(units70a,70band70c, as examples). Each storage unit70stores a different color value. For example, if the pixel sensing unit68stores values for the primary colors of red, green and blue, the storage unit70amay store the red color value, the storage unit70bmay store the green color value, and the storage unit70cmay store the blue color value. The storage units70may include, for example, capacitors.

Each pixel sensing unit68may include a photosensitive element72, such as a photosensitive diode, that provides an output signal (a current, for example) that indicates the intensity of light that is sensed for the pixel. Because just one element72may be used to detect all three primary color components, the pixel sensing unit68may include multiplexing circuitry (described below) to route the signal from a sampling node91(an output terminal of the photosensitive element72) to the appropriate storage unit70based on the mode of the color filter52(i.e., based on which primary color component is being sensed by the array60).

The multiplexing circuitry includes switches to route the sensed color values to the appropriate storage unit70. In this manner, in some embodiments (where the primary color components of red, green and blue are being captured, for example), when the color filter52is allowing the red color component to pass through, a switch73a(which is controlled by a signal called SAMPR) closes to route the output signal from the sampling node91to the storage unit70a. Similarly, when the color filter52is allowing the green color component to pass through, a switch73b(controlled by a signal called SAMPG) closes to route the output signal from the node91to the storage unit70b. Lastly, when the color filter52is allowing the blue color component to pass through, a switch73c(controlled by a signal called SAMPB) closes to route the signal from the node91to the storage unit70c. When one of the switches70a,70bor70cis closed, the other two switches are open. Therefore, the output signal from the photosensitive element72is routed to only one of the storage units70based on the primary color component being sensed by the array60.

The decoders61and63(seeFIG. 4) select a particular color value that is stored by the pixel sensing unit68. In this manner, the decoders61and63activate an appropriate select signal (described below) to select both the pixel sensing unit68and one of the primary colors. When this occurs, the multiplexing circuitry of the pixel sensing unit68transfers an indication of the selected color value from the corresponding storage unit70to an analog-to-digital converter (ADC)74. The ADC74might be, for example, a serial ADC and may be shared by more than one pixel sensing unit68. For example, in some embodiments, the ADC74may be a multi-channel bit serial (MCBS) ADC as described in David X. D. Yang, Boyd Fowler & Abbas el Gamal,A Nyquist Rate Pixel Level ADC for CMOS Image Sensors, The Custom Integrated Circuit Conference, May 1998, p. 237–40. In some embodiments, the ADC74converts the indication (an analog value, for example) from the storage unit70into digital bits that are serially communicated to the signal conditioning circuitry62(seeFIG. 4) via a bit line76.

The multiplexing circuitry of the pixel sensing unit68also routes the indications from the storage units70to the ADC74. For example, the multiplexing circuitry may include switches75(switches75a,75band75c, as examples). When the decoders61and63assert (drive high, for example) a select signal (called WLR) to initiate retrieval of the red color value, the switch75acloses to couple the storage unit70ato the ADC74. Similarly, when the decoders61and63assert a signal (called WLG) to initiate retrieval of the green color value, the switch75bcloses to couple the storage unit70bto the ADC74. Lastly, when the decoders61and63assert a select signal (called WLB) to initiate retrieval of the blue color value from the unit70c, a switch75ccloses to couple the storage unit70cto the ADC74.

Referring toFIGS. 7,8,9,10,11and12, as an example, to capture a multi-color image, the control unit66might first assert (drive high, for example) the SAMPR signal (at time T0) to close the switch73aand deassert (drive low, for example) the SAMPG and SAMPB signals. The control unit66keeps the SAMPR signal asserted for an integration interval (called TI1) from time Toto time T1during which the signal (a current signal, for example) from the node91is integrated by the storage unit70ato form the stored red color value. Next, the control unit66deasserts the SAMPR signal at time T1and asserts the SAMPG signal during another integration interval (called TI2) from time T1to time T2to store the green color value in the storage element70bin a similar manner. The control unit66then deasserts the SAMPG signal and asserts the SAMPB signal during another integration interval (called TI3) from time T2to time T3to store the blue color value in the storage unit70c. After all three integration intervals expire, the decoders61and63may then assert, as examples, the WLR signal (from time T4to time T5), the WLG signal (from time T5to time T6) and the WLB signal (from time T6to time T7) to selectively couple the associated storage units70to the bit line76.

Referring toFIG. 13, in some embodiments of the pixel sensing unit68, the photosensitive element72includes a photosensitive diode90that is exposed at the surface51(seeFIG. 3) to sense light. The cathode of the diode90is coupled to ground, and the anode of the diode90is coupled to the sampling node91. An n-channel, metal-oxide-semiconductor field-effect-transistor92has its drain-source path coupled between the sampling node91and a positive voltage supply level (called VCC). The gate of the transistor92is driven by a reset signal (called RESET) which is briefly asserted (driven high, for example) by the control unit66near the beginning of each integration interval TI1, TI2and TI3. In this manner, referring toFIG. 6, the control unit66pulses high the RESET signal at the beginning of each integration interval to cause the transistor92to conduct and pull the voltage level of the sampling node91near the Vcc positive voltage supply level. As a result, this brief pulse93causes a predetermined initialization value to be stored in the storage unit70.

In some embodiments, the storage unit70might include a capacitor that is formed from an n-channel, metal-oxide-semiconductor field-effect-transistor82. Each switch73(switches73a,73band73c, as examples) might include an n-channel, metal-oxide-semiconductor field-effect-transistor80that has its drain-source path coupled between the associated storage unit70and the sampling node91. The drain of the transistor80forms a storage node79for, as an example, storing a voltage indicative of the light intensity of a specific color value. The gate of the transistor80is driven by the associated SAMPR, SAMPG or SAMPB signal.

Alternatively, the switch73may include an n-channel, metal-oxide-semiconductor field-effect-transistor that is configured as a source follower. In this manner, the source of the transistor is coupled to the storage node79, the gate of the transistor is coupled to the sampling node91, and the drain of the transistor receives a voltage (in place of the SAMPR signal) that turns the transistor either on or off.

Each switch75might include an n-channel, metal-oxide-semiconductor field-effect-transistor84that has its drain-source path coupled between the VCCpositive voltage supply level and the bit line76. The gate of the transistor84is coupled to the storage node79. As a result of this arrangement, when the initialization value is stored in the associated storage unit70, the storage node79has a positive voltage which places the transistor84in a linear conduction mode. As a result of this arrangement, during the integration interval, the voltage of the sampling node79is indicated by the source of the transistor84.

The switch75may also include an n-channel, metal-oxide-semiconductor field-effect-transistor86which has its drain-source path coupled in series between the source of the transistor84and the bit line76. The gate of the transistor86is driven by the associated WLR, WLG or WLB signal which causes the transistor86to either conduct or not conduct.

In other embodiments, the pixel sensing unit may include digital storage units. For example, referring toFIG. 14, a pixel sensing unit100includes digital storage units102that may, for example, replace the analog storage units70of the pixel sensing unit68. In this manner, each digital storage unit102stores a digital value that represents a color value for the pixel. In the pixel sensing unit100, the sampling node91is coupled to an input terminal of a sample and hold (SH) circuit78. When the control unit66asserts (drives high, for example) a signal (called SAMPLE), the SH circuit78integrates and temporarily stores the signal provided by the sampling node91. When the control unit66deasserts (drives low, for example) the SAMPLE signal, the SH circuit78transfers the resultant integrated signal to an ADC103(a serial ADC, for example). In some embodiments, the S/H circuit78may include a switch and a storage unit that are arranged in similar manner as the switch73and storage unit70. The SAMPLE signal is used in place of the SAMPR, SAMPG and SAMPB signals and may be represented, in some embodiments, as a result of a logical ORing of these signals.

The digital output signal that is furnished by the ADC103is routed to one of the digital storage units102. To accomplish this, each switch101(switches101a,101band10c, as examples) is coupled to the output terminal of the ADC103and to a different one of the digital storage units102. As an example, for the case where the red, green and blue primary color components are being sensed by the array60, at the end of the integration interval for the red primary color, the control unit66asserts a signal (called SWR1) to close the switch101ato couple the ADC103to the digital storage unit102a. After a predetermined interval of time elapses (eight clock cycles, for example), the control unit66deasserts the SWR1signal to decouple the ADC103from the storage unit102a. The other switches101band101care coupled in a similar manner and are exclusively activated by either the SWG1or SWB1signals in a similar manner after the expiration of the green and blue color integration intervals, respectively.

In some embodiments, for purposes of retrieving a color value from the pixel sensing unit100and furnishing that value to the bit line76, the decoders61and63assert one of three select signals: an SWR2signal (to select the storage unit102a), an SWG2signal (to select the storage unit102b) and an SWB2signal (to select storage unit102c). The SWR2, SWG2and SWB2signals activate different switches104, each of which is coupled between its associated digital storage unit102and the bit line76.

In other embodiments, the pixel sensing unit may have fewer storage units than the number of primary colors being sensed. For example, referring toFIG. 15, in some embodiments, a pixel sensing unit108may be used in place of the units68or100. The pixel sensing unit108is identical to the pixel sensing unit100except that the unit108does not include one of the digital storage units102, such as the digital storage unit102c, for example. Unlike the arrangement of the unit100, one of the switches101(the switch101cactivated by the SWB1signal, for example) is coupled between the output terminal of the ADC103and the bit line76.

Due to this arrangement, an indication of the blue primary color component, for example, may be transferred directly from the ADC103to the bit line76without requiring temporary storage. As a result, the pixel sensing unit108may consume less semiconductor die area.

In some embodiments, a pixel sensing unit may be arranged similarly to the pixel sensing unit108except that this pixel sensing unit does not include the switch10c. In this manner, the decoders61and63retrieve the stored color value from either the storage unit102aor102bbefore the ADC103processes the third color value (a blue color value, for example). In this manner, the control unit66may activate the appropriate switch101aor101bto store the third color value in the storage unit102aor102bfrom which the stored color value has already been retrieved.

Referring toFIG. 16, in some embodiments, the imager50may be part of a digital camera110. Besides the imager50, the camera110may include optics160which form an image on the imaging surface51(seeFIG. 4) of the imager50. In some embodiments, the camera110includes a scaling unit166that may, for example, scale up or down the resolution of a transmitted image before communicating it to a bus120(a serial bus, for example) that is used for communication with, for example, a computer. The camera110may also include a compression unit168and a bus interface unit170to interact with the bus120. To coordinate activities of these units, the camera110may include a microprocessor162.