Active Pixel Sensors (text from [1])
A sensor with an active amplifier within each pixel was proposed [2]. Figure 1 shows the general architecture of an APS array and the principal pixel structure.
The pixels used in these sensors can be divided into three types: photodiodes, photogates and pinned photodiodes [1].
Photodiode APS
The photodiode APS was described by Noble [2] and has been under investigation by Andoh [3]. A novel technique for random access and electronic shuttering with this type of pixel was proposed by Yadid-Pecht [4].The basic photodiode APS employs a photodiode and a readout circuit of three transistors: a photodiode reset transistor (Reset), a row select transistor (RS) and a source-follower transistor (SF). The scheme of this pixel is shown in Figure 2.
Generally, pixel operation can be divided into two main stages, reset and phototransduction.
(a) The reset stage. During this stage, the photodiode capacitance is charged to a reset voltage by turning on the Reset transistor. This reset voltage is read out to one of sample-and-hold (S/H) in a correlated double sampling (CDS) circuit [5]. The CDS circuit, usually located at the bottom of each column, subtracts the signal pixel value from the reset value. Its main purpose is to eliminate fixed pattern noise caused by random variations in the threshold voltage of the reset and pixel amplifier transistors, variations in the photodetector geometry and variations in the dark current [1].
(b) The phototransduction stage. During this stage, the photodiode capacitor is discharged through a constant integration time at a rate approximately proportional to the incident illumination. Therefore, a bright pixel produces a low analogue signal voltage and a background pixel gives a high signal voltage. This voltage is read out to the second S/H of the CDS by enabling the row select transistor of the pixel. The CDS outputs the difference between the reset voltage level and the photovoltage level [1].
Because the readout of all pixels cannot be performed in parallel, a rolling readout technique is applied.
Readout from photodiode APS
All the pixels in each row are reset and read out in parallel, but the different rows are processed sequentially. Figure 3 shows the time dependence of the rolling readout principle.
A given row is accessed only once during the frame time (Tframe). The actual pixel operation sequence is in three steps: the accumulated signal value of the previous frame is read out, the pixel is reset, and the reset value is read out to the CDS. Thus, the CDS circuit actually subtracts the signal pixel value from the reset value of the next frame. Because CDS is not truly correlated without frame memory, the read noise is limited by the reset noise on the photodiode [1]. After the signals and resets of all pixels in the row are read out to S/H, the outputs of all CDS circuits are sequentially read out using X-addressing circuitry, as shown in Figure 2.
Other types are global shutter and fast-reset shutter, but such things are out of scope of this note.
Rolling shutter
In electronic shuttering, each pixel transfers its collected signal into a light-shielded storage region. Not of all CMOS imagers are capable of true global shuttering. Simpler pixel designs, typically with three transistors (3T), can only offer a rolling shutter [6]. Each row will represent the object at a different point of time, and because the object is moving, it will be at different point in space.More sophisticated CMOS devices (4T and 5T pixels) can be designed with global shuttering and exposure control (EC) [6].
Typically, the rows of pixels in the image sensor are reset in sequence, starting at the top of the image and proceeding row by row to the bottom. When this reset process has moved some distance down the image, the readout process begins: rows of pixels are read out in sequence, starting at the top of the image and proceeding row by row to the bottom in exactly the same fashion and at the same speed as the reset process [7].
The time delay between a row being reset and a row being read is the integration time. By varying the amount of time between when the reset sweeps past a row and when the readout of the row takes place, the integration time (hence, the exposure) can be controlled. In the rolling shutter, the integration time can be varied from a single line (reset followed by read in the next line) up to a full frame time (reset reaches the bottom of the image before reading starts at the top) or more [7].
With a Rolling Shutter, only a few rows of pixels are exposed at one time. The camera builds a frame by reading out the most exposed row of pixels, starting exposure of the next unexposed row down in the ROI, then repeating the process on the next most exposed row and continuing until the frame is complete. After the bottom row of the ROI starts its exposure, the process ``rolls'' to the top row of the ROI to begin exposure of the next frame's pixels [8].
The row read-out rate is constant, so the longer the exposure setting, the greater the number of rows being exposed at a given time. Rows are added to the exposed area one at a time. The more time that a row spends being integrated, the greater the electrical charge built up in the row's pixels and the brighter the output pixels will be [8]. As each fully exposed row is read out, another row is added to the set of rows being integrated (see Fig. ).
If there is a requirement of shooting with photoflash, there must be succeed some conditions. The operation of a photoflash with a CMOS imager [7] operating in rolling shutter mode is as follows:
- The integration time of the imager is adjusted so that all the pixels are integrating simultaneously for the duration of the photoflash;
- The reset process progresses through the image row by row until the entire imager is reset;
- The photoflash is fired;
- The imager is read out row by row until the entire imager is read out.
The net exposure in this mode will result from integrating both ambient light and the light from the photoflash. As previously mentioned, to obtain the best image quality, the ambient light level should probably be significantly below the minimum light level at which the photoflash can be used, so that the photoflash contributes a significant portion of the exposure illumination. Depending on the speed at which the reset and readout processes can take place, the minimum exposure time to use with photoflash may be sufficiently long to allow image blur due to camera or subject motion during the exposure. To the extent that the exposure light is provided by the short duration photoflash, this blur will be minimized.
Bibliography
- 1
- Orly Yadid-Pecht.
Active pixel sensor (aps) design - from pixels to systems.
Lectures. - 2
- P. Noble.
Self-scanned image detector arrays.
IEEE Trans. Electron Devices, ED-15:202, 1968. - 3
- J. Yamazaki M Sagawara Y. Fujita K. Mitani Y. Matuzawa K. Miyata F. Andoh, K. Taketoshi and S. Araki.
A 250,000 pixel image sensor with FET amplification at each pixel for high speed television cameras.
IEEE ISSCC, pages 212-213, 1990. - 4
- R. Ginosar O. Yadid-Pecht and Y. Diamand.
A random access photodiode array for intelligent image capture.
IEEE J. Solid-State Circuits, SC-26:1116-1122, 1991. - 5
- J. Hynecek.
Theoretical analysis and optimization of CDS signal processing method for CCD image sensors.
IEEE Trans. Nucl. Sci., vol.39:2497-2507, Nov. 1992. - 6
- DALSA corp.
Electronic shuttering for high speed cmos machine vision applications.
Technical report, DALSA Corporation, Waterloo, Ontario, Canada, 2005. - 7
- David Rohr.
Shutter operations for ccd and cmos image sensors.
Technical report, Kodak, IMAGE SENSOR SOLUTIONS, 2002. - 8
- PixeLink.
Pixelink product documentation.
Technical report, PixeLink, December, 2007.
