Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2002-05-23
2004-01-13
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C342S135000, C342S139000, C342S145000, C356S005080
Reexamination Certificate
active
06678039
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to processing data acquired from three-dimensional imaging systems, and more particularly to enhancing the dynamic range associated with data acquired from imaging systems using low-power sequential analog-to-digital conversion, distributed in space and time, including such systems implemented in CMOS.
BACKGROUND OF THE INVENTION
Imaging sensors are used in a variety of applications, including cameras, video, radar systems, and instrumentation. In generally, such sensors rely upon detection of electromagnetic (EM) energy, for example emitted energy that is reflected off of a target object and then detected.
Various techniques are used to detect EM energy, varying upon the application, the energy wavelength, and the desired speed of data acquisition. For example, in radar systems, EM waves are generally focused by a maze of waveguides and detected within a cavity using a diode that is sensitive to the frequency of interest.
In light and infrared (IR) imaging systems, EM waves that are reflected from a target object are collected by photodiode detectors in the form of a small photodiode current. This current can then be used to charge a relatively large capacitor over a long integration time period. After integration time ends, the signal voltage developed across the capacitor will eventually be read out, for example via sequential charge transfer as in charge coupled devices (CCDs). The signal voltage representing target objects farther away will be lower in general that the signal voltage representing nearby target objects.
An especially useful IR-type sensing application with which the present invention may (but need not) be practiced will now be described for background purposes. One form of IR-type sensing is so-called three-dimensional sensing and is described in U.S. Pat. No. 6,323,942 entitled “CMOS-Compatible Three-Dimensional Image Sensor IC” (2001), assigned to assignee herein. The '942 patent discloses the use of light emissions and measurements of partial energy reflected from a target a distance Z away to determine distance between such a sensor system and the target.
FIG. 1
depicts a generic IR image sensing system
10
such as that described in the '942 patent, a system that can determine distance Z between system
10
and a target object
20
. Much of system
10
is implemented on a CMOS IC
30
that includes an array
40
of pixel detectors
50
(e.g., photodiodes), and dedicated electronics
60
preferably associated with each pixel detector. An optical energy emitter
70
, e.g., a LED or laser diode, emits energy via a lens
80
, some of which energy is reflected from the target object
20
and can be detected by at least some pixel detectors in array
40
. (Emitter
70
may in fact be implemented off-IC
30
.) Every pixel detector within the array captures the partial energy of the light being reflected by a point on the target's surface and thus captures the distance from the pixel detector to such point.
IC
30
includes a microprocessor or microcontroller unit
90
, memory
100
(which preferably includes random access memory or RAM and read-only memory or ROM memory, and various input/output (I/O) and interface circuits, collectively
110
. Microprocessor
90
controls operation of the energy emitter
70
and of the various electronic circuits within IC
30
. Using various signal processing techniques, the time-of-flight (TOF) for optical energy to travel from system
10
to a point on target object
20
and be at least partially reflected back via an optional lens
120
to a pixel detector
50
within array
40
can be determined. This determination is often termed TOF acquisition. Since the speed of light is known, the distance Z associated with a given time measurement can be determined, e.g., perhaps time t
1
is associated with a distance Z
1
, whereas a longer time t
3
is associated with a more remote distance Z
3
, etc. One can construct a three-dimensional image of a target or scene by combining the data collected from every pixel in the array. Various raw data (DATA) can of course be exported off-IC for further and perhaps more extensive signal processing.
Within detector array
40
, the measurement of incoming light energy reaching a given pixel detector is known as brightness acquisition. Various techniques for TOF acquisition and/or brightness acquisition useable in multi-dimensional image sensing exist. A very practical problem encountered with sensing systems, including those described above, is that the peak power of the light energy that is detected may vary by several orders of magnitude, e.g., representing information from a very dim surface point to representing information from a very bright surface point of the target object.
In imaging devices such as above-described, the ratio between the highest and lowest measurable EM energy is limited by the lowest detectable energy in the EM wave, and by the saturation voltage across the integration capacitor. The simultaneous detection of very dim and strong sources of light using the same mechanism is generally performed using two techniques, namely automatic gain control (AGC) and over-sampling.
On one hand, AGC techniques employ an automatic gain control preamplifier that adjusts amplifier gain level so as to keep the amplified photodiode signal within a predefined range. The readout data includes both the amplifier output and the gain value, and can be interpreted as the mantissa and exponent of the desired output signal. In various CCD device applications, AGC techniques have been developed to cope with dynamic ranges of about 35 dB.
On the other hand, over-sampling techniques include comparing the amplified signal with a pre-defined threshold, and resetting the signal and generating a pulse when the threshold is attained. Such generated pulses form a continuous stream of bits that can be coded onto digital words representing the amplified photodiode signal. This second technique is analogous to a class of over-sampling analog-to-digital converters (ADCs) known as sigma-delta (or delta-sigma) converters.
Acquisition of information detected by the pixel array
40
in
FIG. 1
may be performed in two phases: a first phase directed to brightness acquisition, and a second phase directed to TOF acquisition.
In a first (brightness acquisition) phase, incoming pulses of light energy are captured by photodiodes or pixel detectors
50
within array
40
, which detectors translate the photon energy into detector current. The detector current from each pixel can be integrated over a variable amount of time to create an output signal voltage pulse. Eventually the integrated voltage signal level reaches a given threshold, at which time the integration period ends and a logic pulse is generated for use in incrementing a logic counter. At the end of acquisition, the logic counter holds a logic state uniquely representing the total number of received logic pulses. The brightness of light at a given pixel is proportional to such state. This first phase is performed simultaneously and independently in a matrix array of N×M points of acquisition or pixels.
In a second (TOF acquisition) phase, the time delay between the energy pulse emitted by emitter
70
and the target-reflected received pulse detector within array
40
is automatically matched to a normalized value. The signal voltage associated with such value will be a measure of the TOF, which measure can be stored in the very same logic counter noted described above.
The logic counter-held digital content for each pixel in the array may be accessed sequentially or randomly, and the overall image detected by the array can subsequently be decoded and stored in local random access memories (RAMs), e.g., associated with memory
100
in FIG.
1
. The RAM contents can then be uploaded to a personal computer or other device using standard communication links, e.g., wireless links, wired links, etc.
As noted above, reflected incoming energy may represent a very bright region of a
Buczinski Stephen C.
Canesta, Inc.
Dorsey & Whitney LLP
Kaufman Michael A.
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