Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-07-27
2003-07-29
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S088000, C345S089000
Reexamination Certificate
active
06600471
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to MOS imagers, and more particularly relates to techniques for increasing the dynamic range of a MOS imager.
Conventionally, MOS imagers are characterized by a linear voltage-to-light response, or transfer function; that is, the imager output voltage is approximately linearly related to light incident on the imager. Specifically, the output voltage transfer function is linearly proportional to the intensity of the light incident on the imager. This linear transfer function can be characterized by a dynamic range, given as the ratio of the highest detectable illumination intensity of the imager to the lowest detectable illumination intensity of the imager. It is well understood that the dynamic range of the transfer function sets the overall dynamic range of the imager. If the dynamic range of a scene exceeds the dynamic range of an imager, portions of the scene will saturate the imager and appear either completely black or completely white. This can be problematic for imaging large dynamic range scenes, such as outdoor scenes.
Conventionally, a MOS imager pixel includes a phototransistor or photodiode as a light detecting element. In operation, e.g., the pixel photodiode is first reset with a reset voltage that places an electronic charge across the capacitance associated with the diode. Electronic charge produced by the photodiode when exposed to illumination then causes charge of the diode capacitance to dissipate in proportion to the incident illumination intensity. At the end of an exposure period, the change in diode capacitance charge is detected and the photodiode is reset. The amount of light detected by the photodiode is computed as the difference between the reset voltage and the voltage corresponding to the final capacitance charge. The illumination intensity that causes the photodiode capacitance charge to be completely dissipated prior to the end of the exposure period, thereby saturating the pixel, sets the upper end of the pixel dynamic range, while thermally generated photodiode charge and other noise factors set the lower end of the pixel dynamic range.
A variety of techniques have been proposed for expanding the dynamic range of a MOS imager. In one particularly effective technique, the voltage-to-light transfer function of the imager is modified to be a nonlinear function of illumination intensity, with transfer function slope increasing linearly as a function of illumination intensity. This transfer function modification is typically implemented as a photodiode capacitance charge control function within a CMOS imager pixel.
Specifically, in this technique, over the course of an exposure period a control voltage is applied to the photodiode capacitance to control charge dissipation from the capacitance. The charge control voltage is typically decreased from the starting pixel reset voltage value to, e.g., electrical ground, with each control voltage value at a given time during the exposure period setting the maximum charge dissipation of the photodiode. This control voltage decrease acts to increase the photodiode charge dissipation capability, whereby the pixel can accommodate a higher illumination intensity before saturating, and the dynamic range of the pixel is thusly increased. This charge dissipation control overrides the conventional linear voltage-to-charge transfer function of the pixel to produce a nonlinear transfer function, generally referred to as a compressed transfer function, and a correspondingly expanded dynamic range of the pixel and the imager.
Theoretically, the charge dissipation control voltage applied to a pixel photodiode is preferably continuously adjusted over the course of an exposure period. This enables the production of almost any desired transfer function compression characteristic. For many applications, this theoretical condition is not practical, however. Conventional MOS imagers include an array of pixel columns and rows and typically do not include pixel memory. Therefore, at the end of an exposure period each row of pixel values must be immediately read out. But in general, only one row of pixel values can be read out at a time. To accommodate this condition, the exposure periods of the pixel rows are typically staggered in a time sequence corresponding to the sequential pixel row read out. As a result, the desired pixel charge control voltage waveform must also be applied to the pixel rows in a staggered sequence; the same control voltage waveform is applied to every pixel row but is staggered in time between rows.
As a practical matter, given, e.g., a conventional VGA imager including 480 pixel rows, it would be difficult to deliver 480 continuous-time control voltage waveforms to the imager array or to generate 480 delayed versions of a single continuous-time waveform. It has been found that the approximation of a continuous-time control voltage waveform by a discrete-time, or stepped, control voltage waveform addresses this timing concern while enabling more flexibility and ease in control voltage generation and sequential delivery to a pixel array. In this technique, a desired continuous-time control voltage waveform, or transfer function compression curve, is approximated by voltage steps. This results in a finite number, e.g., eight, of distinct control voltage levels to be applied in a discrete manner to a pixel over the course of that pixel's exposure period. Conventionally, the prescribed discrete-time analog control voltages are generated off-chip from the imager array and then delivered to each pixel row on-chip in a staggered sequence controlled by, e.g., a digital controller. It has been found that this scenario enables good pixel control as well as timing control and additionally provides the ability to modify the transfer function compression characteristic.
It has been recognized that the discrete analog voltages produced to impose imager transfer function control preferably are regulated to be precise and noise free, and preferably are maintained free of glitches, where a “glitch” is here defined as a rapid excursion, or spike, in the voltage. Without such regulation, the desired compression function could be distorted, with the resulting images including noise or appearing unnatural. Regulation of the control voltages is particularly important as the voltages are switched from one pixel row to the next. Specifically, when a given control voltage is applied from one pixel row to the next, a voltage excursion, or glitch, is produced due to an inherent row switching capacitance. Such an excursion in a voltage source could cause rows of pixels already connected to that voltage to dissipate charge or accumulate charge in a manner not consistent with the desired transfer function.
For this reason, conventional imager configurations typically employ external, off-chip analog control voltage sources having output voltages that are bypassed by large and external bypass capacitors. The bypass capacitors are employed to eliminate glitches due to row switching capacitance as the charge control voltages are sequentially applied to rows of an imager. It has been understood that without the use of such bypass capacitors, a staggered-row control voltage application scenario does meet most imager performance requirements.
The typical row capacitance of a VGA imager is on the order of about 5 pF. In order to effectively reduce switching glitches from this row capacitance by an order of magnitude or more, bypass capacitors for each analog control voltage source are required to each be at least about 50 pF, and more typically are provided as an external capacitor or about 0.1 &mgr;F for each external analog voltage generation circuit. This external control voltage generation circuit configuration adds to imager system power consumption, complexity, cost, and overall imager system extent, and influences other imager performance factors. But practical implementation of a compressive MOS imager transfer function characteristic has heretofore required accommodation o
Fife Keith G.
Lee Hae-Seung
Sodini Charles G.
Hjerpe Richard
Lesperance Jean
Lober Theresa A.
SMaL Camera Technologies, Inc.
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