U.S. patent application number 12/500953 was filed with the patent office on 2009-10-29 for sensing light and sensing the state of a memory cell.
Invention is credited to Rex K. Hales, Tracy Johancsik, Thomas L. Wolf.
Application Number | 20090268059 12/500953 |
Document ID | / |
Family ID | 38694648 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090268059 |
Kind Code |
A1 |
Hales; Rex K. ; et
al. |
October 29, 2009 |
SENSING LIGHT AND SENSING THE STATE OF A MEMORY CELL
Abstract
A light-to-frequency converter includes a switch (130) connected
in series with a reverse-biased photodiode (120). A node (150) in
the current path through the switch and the photodiode is connected
to the input of a Schmidt trigger (160), whose output controls the
switch. New techniques are provided for motion compensation,
partial readouts, dark current elimination, non-destructive
testing, and sensing the state of a memory cell.
Inventors: |
Hales; Rex K.; (Riverton,
UT) ; Johancsik; Tracy; (Murray, UT) ; Wolf;
Thomas L.; (Salt Lake City, UT) |
Correspondence
Address: |
HAYNES AND BOONE, LLP;IP Section
2323 Victory Avenue, Suite 700
Dallas
TX
75219
US
|
Family ID: |
38694648 |
Appl. No.: |
12/500953 |
Filed: |
July 10, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12327533 |
Dec 3, 2008 |
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12500953 |
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11745929 |
May 8, 2007 |
7476840 |
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12327533 |
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60809703 |
May 30, 2006 |
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60798860 |
May 8, 2006 |
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Current U.S.
Class: |
348/231.99 ;
348/E9.002 |
Current CPC
Class: |
G11C 11/5642 20130101;
G11C 2211/563 20130101; G01J 1/46 20130101; G11C 11/56 20130101;
H03K 3/03 20130101; G11C 2211/5644 20130101 |
Class at
Publication: |
348/231.99 ;
348/E09.002 |
International
Class: |
H04N 5/76 20060101
H04N005/76 |
Claims
1. An imager comprising: a plurality of pixels each of which
comprises a photodetector and a digital storage device for
incorporating data indicative of light detected by the
photodetector; shift control circuitry for controlling the digital
storage devices to form one or more shift registers, at least one
shift register having a plurality of cells each of which comprises
one or more bits of a respective one of the digital storage
devices.
2. The imager of claim 1 wherein each pixel's photodetector is part
of the pixel's light-to-frequency converter, and each pixel's
digital storage device comprises a counter for counting electrical
pulses generated by the pixel's light-to-frequency converter.
3. The imager of claim 1 wherein said shift control circuitry is
operable to form the one or more shift registers as circular shift
registers.
4. A method for obtaining a digital image of a scene with an imager
comprising a plurality of pixels, each pixel comprising a
light-to-frequency converter and a counter for counting pulses
generated by the light-to-frequency converter, the counters forming
one or more shift registers for shifting data out of the counters,
the method comprising: (1) sensing light from the scene by the
pixels; (2) after operation (1), shifting data from the one or more
shift registers and testing the data to determine if additional
image acquisition of the scene is needed; (3) if additional image
acquisition of the scene is needed, then sensing light from the
scene by the pixels.
5. The method of claim 4, wherein in the operation (3) the sensing
starts with the counters containing the data that was present in
the counters at the end of the operation (1).
6. The method of claim 5 wherein each shift register is used as a
circular shift register in the operation (2), the counters
containing the same data at the end of the operation (2) as at the
start of the operation (2).
7. A control circuit for causing an imager to perform the method of
claim 4.
8. A method for obtaining image data with a light-to-frequency
converter, the method comprising: the light-to-frequency converter
generating first electrical pulses indicative of light sensed by
the light-to-frequency converter; a counter counting in a first
direction (up or down) on the first pulses counting in a second
direction (down or up) on second pulses provided to the
counter.
9. The method of claim 8 wherein the second pulses are generated by
a light-to-frequency converter that does not detect light.
10. An imager comprising one or more pixels each of which includes
a light-to-frequency converter and a counter having a first input
connected to an output of the light-to-frequency converter to count
in a first direction (up or down) on pulses from the
light-to-frequency converter; wherein each said counter has a
second input to count in a second direction (down or up) on pulses
at the second input.
11. The imager of claim 10 wherein the second input is connected to
a light-to-frequency converter shielded not to detect light.
12. A method for obtaining a digital image of a scene with an
imager comprising a plurality of pixels, each pixel comprising a
light-to-frequency converter and a counter for counting pulses
generated by the light-to-frequency converter, the method
comprising: (1) loading at least one of the counters with data
other than reset data; and then (2) acquiring a first image with
the imager, with said at least one of the counters counting the
pulses generated by the associated light-to-frequency converter
starting with the data loaded in (1).
13. The method of claim 12 wherein the data in (1) corresponds to a
second image.
14. The method of claim 12 wherein: the light-to-frequency
converters are sensitive to light in a first range of wavelengths
but not in a second range of wavelengths; one of operations (1),
(2) comprises acquiring an image of an object through a light
converter which converts the second range of wavelengths to the
first range of wavelengths; and the other one of operations (1),
(2) comprises acquiring an image of said object without light
conversion from the second range to the first range.
15. The method of claim 12 wherein the first data corresponds to
calibration data.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a division of U.S. patent
application Ser. No. 12/327,533, filed Dec. 3, 2008, which is a
division of U.S. patent application Ser. No. 11/745,929, filed May
8, 2007, now U.S. Pat. No. 7,476,840, issued Jan. 13, 2009, which
claims priority of U.S. provisional application No. 60/809,703,
filed May 30, 2006, and U.S. provisional application No.
60/798,860, filed May 8, 2006, all of which are incorporated herein
by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to sensing light and sensing
the state of a memory cell. Some embodiments relate to
current-to-frequency converters including, but not limited to,
light-to-frequency converters (LTF converters).
[0003] A typical light-to-frequency converter includes a photodiode
that produces an electrical current proportional to the amount of
light (the number of photons) detected by the photodiode per unit
of time. The LTF converter converts the photodiode current to a
sequence of electrical pulses of a frequency proportional to the
current's magnitude. The pulses can be counted to provide a digital
value corresponding to the amount of light per unit of time. See
e.g. U.S. Pat. No. 4,465,562, issued Aug. 28, 1984 to Wicnienski et
al. and incorporated herein by reference.
SUMMARY
[0004] This section summarizes some features of the invention.
Other features are described in the subsequent sections. The
invention is defined by the appended claims which are incorporated
into this section by reference.
[0005] Some embodiments of the present invention include
light-to-frequency converters implemented with simple circuitry. In
some embodiments, for example, an LTF converter includes a switch
connected in series with a reverse-biased photodiode. A node
between the photodiode and the switch is connected to the input of
a Schmidt trigger's input voltage increases. When this voltage
reaches the Schmidt trigger's high trigger voltage, the switch is
opened, and the voltage on the Schmidt trigger's input is
discharged through the photodiode at the rate determined by the
amount of light detected by the photodiode. When the Schmidt
trigger's input reaches the low trigger voltage, the switch is
closed, and the Schmidt trigger input voltage increases again. This
cycle repeats to produce pulses at the Schmidt trigger's output of
a frequency which depends on the amount of light detected by the
photodiode.
[0006] Some embodiments of the present invention provide novel uses
for light-to-frequency converters and other types of photodetector
circuits including prior art converters and converters of the
present invention. For example, novel techniques are provided to
reduce motion blur when taking an image of a moving scene. Some
features take advantage of the light-to-frequency converters'
ability to easily combine different images. Some embodiments
provide circuits for sensing the state of a memory cell.
[0007] The invention is not limited to the features and advantages
described above. Other features are described below. The invention
is defined by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a circuit diagram of a light-to-frequency
converter according to some embodiments of the present
invention.
[0009] FIG. 2 is a timing diagram for a light-to-frequency
converter according to some embodiments of the present
invention.
[0010] FIGS. 3, 4 are circuit diagrams of light-to-frequency
converters according to some embodiments of the present
invention.
[0011] FIGS. 5, 6 are circuit diagrams of current-to-frequency
converters for sensing the state of a memory cell according to some
embodiments of the present invention.
[0012] FIG. 7 is a block diagram of an imager acquiring an image
according to some embodiments of the present invention.
[0013] FIGS. 8-10 a flowcharts of imager operations according to
some embodiments of the present invention.
[0014] FIG. 11 is a block diagram of a light-to-frequency converter
according to some embodiments of the present invention.
[0015] FIG. 12 is a block diagram illustrating an imager operation
according to some embodiments of the present invention.
[0016] FIG. 13 is a flowchart of an imager operation according to
some embodiments of the present invention.
DESCRIPTION OF SOME EMBODIMENTS
[0017] The embodiments described in this section illustrate but do
not limit the invention. The invention is defined by the appended
claims.
Light-to-Frequency Converters
[0018] FIG. 1 is a circuit diagram of a light-to-frequency
converter 110 according to some embodiments of the present
invention. A photodiode 120 has its anode 120P connected to a
reference voltage terminal 124 (e.g. ground) and cathode 120N
connected to a switch 130. (The anode is P type semiconductor
material, and the cathode is N type semiconductor material.) In
this embodiment, the switch is an NMOS transistor whose source is
connected to the photodiode's cathode 120N. The drain of transistor
130 is connected to an output 138 of a circuit 140 to receive a
voltage V1 (e.g. a power supply voltage VCC) which is positive
relative to the voltage on terminal 124. Circuit 140 can be a
voltage source for example.
[0019] Node 150 at the cathode of photodiode 120 and the source of
transistor 130 is connected to the input of a Schmidt trigger
(Schmidt inverter) 160 whose output 170 is the output of converter
110. Output 170 is connected to the gate of transistor 130. Output
170 can also be connected to a counter (not shown in FIG. 1) to
count the pulses generated by the Schmidt trigger.
[0020] In operation, when transistor 130 is on, the photodiode is
reverse biased. The photodiode's intrinsic capacitance is charged
up, and the voltage V150 (FIG. 2) at node 150 increases to reach a
voltage level VtrH which is the high trigger voltage of Schmidt
trigger 160. At this time (time t1), the Schmidt trigger's output
170 (voltage V170 in FIG. 2) becomes low to turn off the transistor
130. Node 150 is discharged through photodiode 120 at the rate
proportional to the light detected by the photodiode. (The term
"proportional" as used herein is broader than "directly
proportional"; thus, the photodiode discharge rate increases with
amount of light detected by the photodiode, but may or may not be
linearly dependent on the amount of light). At a time t2, the
voltage V150 on node 150 hits the low trigger voltage VtrL of
Schmidt trigger 160. Node 170 becomes high to close the switch 130.
Node 150 charges up, and reaches the high trigger voltage VtrH at a
time t3. Node 170 is again driven low, thus completing the first
pulse 210. The process of the time interval t1.about.t3 is repeated
as long as desired.
[0021] The rate of the voltage change on node 150 depends on the RC
delay which is a function of the intrinsic capacitance of the pn
junction of photodiode 120 and of the resistances and capacitances
of the photodiode's cathode 120N and transistor 130. If desired, an
additional capacitor can be connected between node 150 and another
terminal, e.g. terminal 124, and/or an additional resistor can be
added in series with transistor 130 and photodiode 150, to control
the RC delays affecting the node 150 voltage.
[0022] While FIG. 2 pictures the voltage V150 as a linear function
of time, anon-linear profile can be obtained. The difference
VtrH-VtrL can be adjusted as needed, to provide a desired
resolution for example (since the number of pulses increases when
this difference is decreased). Values below 10% of VCC are used in
some embodiments in which the Schmidt trigger is powered by VCC and
ground. In other embodiments, VtrH-VtrL can be 50% of VCC, or can
exceed 90% of VCC. This value can be adjusted for each exposure in
the converter in some embodiments.
[0023] The invention is not limited to a particular type of switch
130. For example, in FIG. 3, the switch is a PMOS transistor. Its
gate is connected to the output of inverter 310 whose input is
connected to the Schmidt trigger's output 170. (Output 170 may or
may not be the output of converter 110; the converter's output can
be the output of inverter 310 or of some other circuitry whose
input is connected to node 170.) Switch 130 can be connected
between photodiode 120 and terminal 170.
[0024] LTF converter 110 may be provided with an electronic
shutter. See FIG. 4. Some conventional imagers use mechanical
shutters which, when closed, shield photodetectors from light. In
an X-ray application, a mechanical shutter can be absent, and the
X-ray source may be turned on and off to act like a shutter. Some
digital cameras use electronic shutters to control the time when
the light detected by the photodetectors is recorded by the camera.
See e.g. U.S. patent publication no. 2004/0002644 A1 (inventors
Seim et al.) published Jan. 1, 2004, incorporated herein by
reference. The converter of FIG. 4 is identical to the converter of
FIG. 3 except that the inverter 310 of FIG. 3 is replaced with NAND
gate 310 whose inputs are the output 170 of Schmidt trigger 160 and
a shutter-closed signal /Sh_C. When /Sh_C is low, the output of
NAND gate 310 is high, and PMOS transistor 130 is off. Node 150 may
discharge if there is enough light detected by photodiode 120, but
node 150 is not charged up, and therefore no pulses are generated
on output 170. When /Sh_C is high, NAND gate 310 acts as an
inverter to provide the same operation as in FIG. 3.
[0025] In some embodiments, NAND gate 310 is replaced with an AND
gate to keep node 150 high when the shutter input is low. The
invention is not limited to any specific electronic shutter logic.
A shutters can also be provided for the converter of FIG. 1.
[0026] The electronic shutters can be used to provide different
shutter operations, possibly closing the shutters of some pixels
(some converters 110) while opening the other pixels' shutters. For
example, a rolling shutter can be implemented.
[0027] The invention is not limited to embodiments described
hereinabove. Some embodiments include a method for
light-to-frequency conversion, the method comprising: (1)
controlling a Schmidt trigger's input with a voltage on a node
(e.g. node 150) located in a current path comprising a switch (e.g.
130) and a photodiode that are connected in series, with the
photodiode being reverse biased at least when the switch is closed;
and (2) during operation (1), controlling the switch with the
Schmidt trigger's output to alternately open and close the switch.
The current path can be the path from terminal 138 to terminal 124
for example. The current paths in FIGS. 1, 3, 4 do not have
capacitors in them, and thus each current path conducts charge
carriers all the way from terminal 138 to terminal 124. Other
embodiments include capacitors in the current path.
[0028] In some embodiments, the switch may be temporarily
controllably disabled from being controlled by the Schmidt
trigger's output. This can be done using the signal /Sh_C for
example.
[0029] Some embodiments provide a light-to-frequency converter
comprising: a current path comprising a photodiode and a switch
connected in series with the photodiode, the current path being for
conducting current through the photodiode and the switch at least
when the photodiode is reverse biased and detects light; and a
Schmidt trigger having an input whose voltage is for being
controlled by a voltage on a node located in the current path, the
Schmidt trigger having an output whose voltage is for controlling
the switch. In some embodiments (e.g. in FIGS. 1, 3, 4), the switch
is connected to the photodiode's cathode, but this is not
necessary. For example, the switch and the photodiode can be
interchanged. In some embodiments, the node is at the photodiode's
cathode, but this is not necessary.
Generation of Electrical Pulses Indicative of State of a Memory
Cell
[0030] A current-to-frequency converter can be used to sense the
state of a memory cell rather than light. In FIG. 5, memory cell
504 is a floating gate transistor connected between the output 138
of circuit 140 and the node 150 at the input of Schmidt trigger
160. The cell's control gate 504CG receives a reading voltage
needed for cell reading. This may be ground or some other voltage,
depending on the cell architecture. The current through the cell
504 is a function of the charge stored on the cell's floating gate
504FG. The drain of cell 504 is connected to output 138 of circuit
140 (possibly a voltage source), and the source is connected to
node 150 at the input of Schmidt trigger 160. Switch 130 is
connected between the node 150 and terminal 124 (which may or may
not be a ground terminal). Switch 130 is an NMOS transistor whose
gate is connected to output 170 of Schmidt trigger 160. (Other
types of switches can also be used as discussed above in connection
with FIGS. 1-4.) A capacitor 508 is connected to node 150, for
example between the node 150 and a ground terminal such as 124, to
adjust the RC delay at node 150. A resistor can be provided in
series with cell 504 and switch 130 (e.g. between the node 150 and
switch 130) if desired.
[0031] The circuit operates in a manner similar to the current to
frequency converters of FIGS. 1, 3, 4. The floating gate voltage
(which is a function of the charge stored on the floating gate and
of the voltage between the control gate and some other part of the
cell) is converted to the current flowing through cell 504 from
terminal 138 to node 150. The electrical pulses on output 170 can
be counted with a counter (not shown) for a predefined duration of
time. The counter thus provides the state of the memory cell. In
some embodiments, if control gate 504CG is grounded or at some
other predefined voltage, no current flows through the cell
regardless of the charge on floating gate 504FG. Therefore, the
counting duration can be controlled by controlling the voltage on
control gate 504CG (and/or on the cell's select gate if the cell
has a select gate) to enable the cell to conduct current only
during the read operation. Alternatively, a shutter-like circuitry
can be used as described above in connection with FIG. 4.
[0032] In some embodiments, cell 504 is a charge trapping cell,
with floating gate 504FG replaced by a charge trapping layer (e.g.
silicon nitride). The invention is not limited to a particular
memory cell type. In some embodiments, cell 504 is a multilevel
cell.
[0033] In some embodiments, multiple memory cells share one or more
of the circuits 160, 130, 140. Thus in FIG. 6, multiple memory
cells 504 are connected to a bitline BL which in turn is connected
to the gate of NMOS transistor 602 whose source is connected to
node 150 and whose drain is connected to output 138 of circuit 140.
FIG. 6 is otherwise identical to FIG. 5. Any one of cells 504 can
be connected to the bitline BL at any given time. The output pulses
on node 170 can be counted by a counter (not shown) to provide the
cell's state. Cells 504 can be volatile or nonvolatile memory.
[0034] Some embodiments of FIGS. 5, 6 are believed to be highly
suitable for multilevel cells because the pulse count can provide
precise differentiation between memory states without requiring
highly precise circuitry.
[0035] The invention is not limited to the embodiments described
above. Some embodiments provide a method for generating electrical
pulses indicative of a state of a memory cell, the method
comprising: controlling a conductivity of a first portion of a
current path (e.g. the portion from terminal 138 to node 150) to
cause the conductivity to reflect the state of the memory cell, the
current path comprising a switch (e.g. 130) connected in series
with the first portion; and alternately opening and closing the
switch in response to a control signal (e.g. the signal on the
Schmidt trigger's output 170); wherein the control signal is
generated from an output of a Schmidt trigger whose input is
controlled by a voltage on a node (e.g. 150) located in the current
path.
[0036] In some embodiments (like in FIGS. 5, 6), the switch is
connected downstream from the first portion in the current flow in
the current path, but this is not necessary.
[0037] In some embodiments (e.g. in FIGS. 5, 6), the node is
between the first portion of the current path and the switch, but
this is not necessary.
Imagers
[0038] Returning now to light-to-frequency conversion, FIG. 7
illustrates an imager 510 having a number of pixels (possibly a two
dimensional array of pixels). Each pixel contains a
light-to-frequency converter 110 and a counter 520. FIG. 7
illustrates a column of pixels. The converters are labeled 110.1
through 110.n, and the corresponding counters are labeled 520.1
through 520.n. The LTF converters 110.i (i=1, . . . , n) can be as
described above in connection with FIGS. 1-4, or can be other
types, including possibly prior art converters. The imager is
controlled by a circuit 534 (e.g. a microprocessor). The imager is
generating a digital image of a scene 530, which may consist of a
single object or be arbitrarily complex. Imager 510 is moving
relative to scene 530. The imager is moving upward in the view of
FIG. 7 as indicated by arrow 540, but other types of motion are
also possible. If the exposure time is long, the image collected by
the imager could be blurred. In convention CCD imagers (Charge
Couple Device imagers), blurring was reduced using "forward motion
compensation". See European patent application EP 1 313 308 A2
published 21 May 2003 and incorporated herein by reference. In CCD
imagers, each pixel stores a charge corresponding to the amount of
light detected by the pixel per unit of time. As the imager moves
up for example (as in FIG. 7), the charges in each column of pixels
are shifted down. Thus, the charge recorded by a CCD pixel from a
portion of scene 530 is shifted to the next CCD pixel, and the next
CCD pixel adds on to that charge by detecting more light from the
same portion of scene 530. Ultimately the charges are shifted out
of the pixel array and digitized to generate a digital image.
[0039] Unlike the CCD imagers, the imager of FIG. 7 shifts the
digital contents of counters 520 rather than analogue charges. The
shifting occurs under the control of the imager's circuit 550. Such
control circuits are well known. The last counter 520.n in the
column has its contents shifted out to a circuit 560 which stores
the image and possibly performs some image processing. One
advantage of this scheme is a large error margin because the
storage and shifting of contents of the digital counters is easy to
perform with less error than the storage and shifting of analogue
charges.
[0040] The imager operation is shown in FIG. 8. The counters 520
are reset at step 604. Then image acquisition occurs at the pixels
at step 610 to store image data in counters 520. At step 620, the
contents of each counter 520.i are shifted to counter 520.i+1,
except for counter 520.n whose contents are shifted out to circuit
560. Counter 520.1 can be reset at this step. Steps 610, 620 can be
repeated as long as desired.
[0041] If shutters are used, they can be open for the duration of
step 610, then closed for the duration of step 620. In some
applications, the imager is moved (in direction 540) by a stepper
motor (not shown) at step 620, but the imager does not move during
the step 610 to improve the image clarity. Such operation is
appropriate for medical X-ray systems, scanning of semiconductor
wafers for defects, and other applications. In other applications,
the imager may move continuously (if installed on an aircraft in a
reconnaissance system, for example). In some embodiments, the
imager does not move, but the scene 530 moves in the direction
opposite to 540. The arrow 540 represents relative motion of the
imager versus the scene and not necessarily absolute motion. Also,
the scene 530 and/or the imager 510 may undergo other motion in
addition to the motion 540. For example, motion 540 may be the
imager motion relative to scene 530, and in addition the scene 530
may include a rotating wheel or other moving objects.
[0042] Step 620 may involve shifting only one or more, but less
than all, of the bits of each counter 520.i (i<n) to the next
counter. For example, the least significant bit may be omitted to
divide or multiply the counter's contents by 2 (this corresponds to
the counter 520.i value being shifted right or left by one bit when
the value is shifted to counter 520.i+1). Counter 520.i+1 then
integrates additional light at the next iteration of step 610, and
the resulting value is again divided or multiplied by 2 when
shifted to counter 520.i+2, and so on. This corresponds to varying
the pixel sensitivity such that each pixel i (containing the LTF
converter 110.i and the counter 520.i) has a lower or greater
sensitivity than pixel i+1. Thus, non-linear integration of light
can be provided. Also, different iterations of step 620 may have
different durations.
[0043] Imager 510 can be used with partial readouts to determine if
the conditions are right to start image acquisition, e.g. to
determine if the lighting level is sufficient or some other
condition has been satisfied. Partial readouts can be performed
regardless of whether or not the scene 530 is moving relative to
the imager. An exemplary process is illustrated in FIG. 9. At step
704, the counters 520 are reset, and a "trial" image acquisition is
performed at step 710. The "trial" image acquisition is performed
simply to determine if conditions are right to start actual image
acquisition. For example, the trial image acquisition can be
performed for a shorter time than the actual image acquisition,
and/or with some particular settings for the diaphragm and exposure
time that are different from the settings for the actual image
acquisition. Then the data from counters 520 are shifted out to
circuit 560 (step 720), with the counters 520.1.about.520.n being
used as a shift register (i.e. each counter's contents are shifted
to the next counter, and the contents of counter 520.n are shifted
to circuit 560). If the image was moving relative to the scene as
in FIG. 7, and the method of FIG. 8 was used for the trial image
acquisition, then some of the counters' data may have been shifted
to circuit 560 at step 710 (note step 620). At any rate, the data
shifted out of the counters are examined at step 730 to determine
if the condition has been satisfied to start the actual image
acquisition. (One possible condition is that at least one of
counters 520 contain some minimal value, and other conditions are
possible.) If not, the procedure returns to step 704 for the next
trial. (In other embodiments, the data are shifted back into the
counters, and the procedure returns to step 710; for example, in
some embodiments, when the data are shifted from each counter to
the next, the contents of counter 520.n are shifted to counter
520.1, so that at the end of step 720 the counters have the same
data as at the end of step 710.) If the "start actual image
acquisition" condition is satisfied, the counters are reset (step
734), and the actual image acquisition is performed (step 740).
Step 734 can be omitted.
[0044] The invention is not limited to using the counters as a
shift register to read or load the counters. The data can be read
or loaded in parallel or in some sequence other than described
above. Imager 510 may have multiple pixel columns, and each column
can be operated on in parallel in the same manner.
[0045] FIG. 10 illustrates a method suitable for long, uncertain
exposure times, e.g. for taking a digital picture of a night sky.
At step 804, counters 520 are reset. At step 810, image acquisition
is performed for some time. At step 820, the counters' contents
representing all or a desired part of the image are read out to
circuit 560. The counter reading operation can be performed using
any of the methods described above in connection with FIG. 9. In
some embodiments, the counters' contents are unchanged (as in the
case of a circular shift, when the counters in each column are used
as a shift register, and the contents of counter 520.n are shifted
into counter 520.1 in each column as described above). At step 830,
the data in circuit 560 are tested to determine if the image
acquisition should be stopped. One possible condition is that at
least one of counters 520 has been filled, i.e. contains a maximum
value, and other conditions are possible. If the image acquisition
should continue (i.e. the data in circuit 560 do not satisfy the
"end image acquisition" condition), the procedure returns to step
810. Thus, the next iteration of step 810 continues image
acquisition starting with the partial image acquired in the
previous iterations. If the condition of step 830 has been
satisfied, the image acquisition ends (step 850).
[0046] In some embodiments, instead of resetting the counters at
step 604, 704 and/or 734, or 804, the counters are preloaded with
some image ("first image"). Then a second image is acquired as
described above. The resulting counter values represent a
composition (a sum) of the first and second images. The first image
may be obtained by the same imager in an earlier operation or may
be generated in some other manner.
[0047] In some embodiments, the counters are preloaded at step 604,
704 and/or 734, or 804 with some data that needed to be added to
the acquired image to perform some image processing operation (e.g.
Fast Fourier Transform). The preloaded data may also incorporate
some calibration data or other unwanted data that must be removed
from the image. Such data may incorporate "dark current", i.e. the
photodiodes' leakage current existing when no light impinges on
them.
[0048] The dark current can also be subtracted using the scheme of
FIG. 11. LFT converter 110D is identical to LFT converter 110 but
is shielded from light. Therefore, the output pulses of converter
110D represent the dark current. Counter 520 has an UP input
receiving pulses from converter 110 to count up on each pulse from
converter 110, as in FIG. 7. In addition, counter 520 has a DN
(DOWN) input receiving pulses from converter 110D, to count down on
each pulse from converter 110D. In some embodiments, the converter
110D receives the same control signals (e.g. the shutter signal
/Sh_C of FIG. 4) as converter 110 to ensure that the converter 110D
generates output pulses under the same conditions as converter 110
(e.g. only when converter 110 is enabled to generate pulses). In
some embodiments, a single converter 110D is provided for a row, a
column, or a whole array of converters 110 and their associated
counters 520.
[0049] In some embodiments, LFT 110D is omitted. Each counter's DN
input receives count-down pulses configured to simulate the dark
current. The count-down pulses are provided to each counter 520
when the corresponding converter 110 is enabled.
[0050] FIGS. 12, 13 illustrate another application making use of
the LTF converter's ability to easily combine different images. The
scheme of FIGS. 12-13 can be used for non-destructive testing. An
object 530 (e.g. a metal pipe) is tested for internal cracks 1210
using X-rays. Imager 510 can be as in FIG. 7. The imager's
photodiodes 110 are not sensitive to X rays but are sensitive to
visible light. A scintillator 1204 is placed between object 530 and
imager 510 to convert the X-rays to visible light. At step 1310
(FIG. 13), a picture is taken with the imager, so the imager's
counters 520 (FIG. 7) obtain counts corresponding to the X-ray
image of object 530. The image may or may not contain externally
visible features 1220. Therefore, it may be difficult to determine
from the image the exact location of cracks 1210 relative to the
visible features. It is therefore desirable to add the features
1220 to the image. This is done by removing the scintillator 1204
and then taking another picture of object 530 by the imager 510
(step 1320), with the object and the imager being in the same
position as in step 1310. The counters are not reset, to add the
image taken at step 1320 to the X-ray image taken at step 1310. The
resulting image will contain both the cracks 1210 and the visible
features 1220.
[0051] Step 1320 can be performed before step 1310 if desired.
[0052] The converters 110 described above in connection with FIGS.
7-12 can be the same as described above in connection with FIGS.
1-4 or can be other types of converters, including prior art
converters.
[0053] The invention is not limited to the embodiments described
above. For example, photodiode 120 (FIGS. 1-4) can be part of a
phototransistor. The invention is not limited to a particular
Schmidt trigger circuit. Any Schmidt triggers described in U.S.
Pat. No. 4,468,562 issued Aug. 28, 1984 to Wicnienski et al. and
U.S. Pat. No. 6,084,456 issued Jul. 4, 2000 to Seol (both
incorporated herein by reference) are believed to be suitable, but
other Schmidt triggers may also be appropriate. Some embodiments of
the invention include CT scanners, airport security systems and
other security systems, and/or other imaging systems.
[0054] Some embodiments include a method for obtaining a digital
image of a scene with an imager comprising a plurality of pixels,
each pixel comprising a photodetector (e.g. 110) and a digital
storage device (e.g. counter 520) for incorporating data indicative
of light detected by the photodetector, the plurality of pixels
comprising at least a pixel PX1 and a pixel PX2 (e.g. the pixels
with LTF converters 110.1, 110.2 in FIG. 7), the method comprising:
(1) for each pixel of one or more of the pixels including at least
the pixel PX1, performing the following operations (a), (b): (a)
sensing light by the pixel; and (b) incorporating information on
the light sensed by the pixel into the pixel's digital storage
device (see step 610 for example); (2) after operation (1),
incorporating information stored in the digital storage device of
the pixel PX1 into the digital storage device of the pixel PX2 (for
example, at step 620, the contents of counter 520.1 are
incorporated into the counter 520.2); (3) when the imager has moved
relative to the scene after a start of the operation (1) (for
example after the start of step 610), performing the operations
(a), (b) for the pixel PX2 (e.g. performing the next iteration of
610 for converter 110.2) to incorporate into the pixel PX2's
digital storage device (e.g. into counter 520.2) the information on
the light detected in the operation (1) by the pixel PX1 and the
information on the light detected in the operation (3) by the pixel
PX2 (for example, the counter 520.2 may have the sum of the count
generated by counter 520.1 in the first iteration of 610 and the
count generated by counter 520.2 in the second iteration of
610).
[0055] In some embodiments, the plurality of pixels comprise a
sequence of pixels PX.1, . . . PX.n with the respective
photodetectors PH.1, PH.2, . . . , PH.n and the respective digital
storage devices DS.1, . . . , DS.n, wherein the pixels PX1, PX2 are
consecutive pixels PX.j, PX.j+1 for some j<n. For example, the
plurality of pixels can be a column or row of pixels (in this
disclosure, the rows and columns are treated interchangeably).
[0056] The invention also provides a control circuit (e.g. 534) for
causing an imager to perform the operations discussed above.
[0057] Some embodiments of the present invention provide an imager
comprising: a plurality of pixels each of which comprises a
photodetector and a digital storage device for incorporating data
indicative of light detected by the photodetector; shift control
circuitry (e.g. 550, alone or in combination with 534) for
controlling the digital storage devices to form one or more shift
registers (e.g. counters 520.1.about.520.n can form a shift
register), at least one shift register having a plurality of cells
(each counter 520.i is a shift register's cell, i.e. storage that
can be shifted to the next cell (or out of the shift register if
i=n) on a clock signal (not shown)). Each cell comprises one or
more bits of a respective one of the digital storage devices (the
cell may be formed by less than all of the counter's bits).
[0058] In some embodiments, the shift registers can be circular,
e.g. the contents of counter 520.n (the last cell) can be shifted
back into counter 520.1 (the first cell).
[0059] Some embodiments include a method for obtaining a digital
image of a scene with an imager comprising a plurality of pixels,
each pixel comprising a light-to-frequency converter and a counter
for counting pulses generated by the light-to-frequency converter,
the counters forming one or more shift registers for shifting data
out of the counters, the method comprising: (1) sensing light from
the scene by the pixels; (2) after operation (1), shifting data
from the one or more shift registers and testing the data to
determine if additional image acquisition of the scene is needed
(e.g. steps 720-730, or 820-830); (3) if additional image
acquisition of the scene is needed, then sensing light from the
scene by the pixels (e.g. step 710 or 810).
[0060] In some embodiments, in the operation (3) the sensing starts
with the counters containing the data that was present in the
counters at the end of the operation (1) (e.g. if the counters are
not reset at step 704).
[0061] Some embodiments include a method for obtaining image data
with a light-to-frequency converter, the method comprising: the
light-to-frequency converter generating first electrical pulses
indicative of light sensed by the light-to-frequency converter; a
counter counting in a first direction (up or down) on the first
pulses counting in a second direction (down or up) on second pulses
provided to the counter. See FIG. 11 for example. In some
embodiments, the second pulses are generated by a
light-to-frequency converter (e.g. 110D) that does not detect
light.
[0062] Some embodiments include a method for obtaining a digital
image of a scene with an imager comprising a plurality of pixels,
each pixel comprising a light-to-frequency converter and a counter
for counting pulses generated by the light-to-frequency converter,
the method comprising: (1) loading at least one of the counters
with data other than reset data; and then (2) acquiring a first
image with the imager, with said at least one of the counters
counting the pulses generated by the associated light-to-frequency
converter starting with the data loaded in (1). For example, the
data in (1) may correspond to a second image, so the first and
second images become combined. In some embodiments, the
light-to-frequency converters are sensitive to light in a first
range of wavelengths (e.g. visible light in FIGS. 12-13) but not in
a second range of wavelengths (e.g. X-rays); one of operations (1),
(2) comprises acquiring an image of an object through a light
converter (e.g. scintillator 1204) which converts the second range
of wavelengths to the first range of wavelengths; and the other one
of operations (1), (2) comprises acquiring an image of said object
without light conversion from the second range to the first range.
In some embodiments, the first data corresponds to calibration
data.
[0063] The invention is not limited to the embodiments described
above. Other embodiments and variations are within the scope of the
invention, as defined by the appended claims.
* * * * *