Adjustable resolution optical sensor

An optical sensor and a method of operating an optical sensor. The optical sensor includes a plurality of unit cells, a plurality of groups of optical detectors, an amplifier and a plurality of selection circuits. Each of the unit cells includes at least first and second pixels, and each group of optical detectors is associated with a respective one of the unit cells. Each group of detectors includes at least first and second detectors to generate first and second electric signals representing the intensity of light incident on, respectively, the first and second pixels of the associated unit cell. Each of the selection circuits is associated with a respective one of the groups of detectors for transmitting to the amplifier the electric signals from the associated group of detectors. Also, each selection circuit has first and second modes for conducting to the amplifier, respectively, either the electric signal from only the first detector of the associated group of detectors, or the electric signals from both the first and second detectors of that associated group of detectors. By switching the selection circuit between the first and second modes, the sensor may operate at either a course resolution or a fine resolution. Thus, by combining electronic switching and summing of detector elements, a single array of detectors can function in both course and fine resolution modes.

BACKGROUND OF THE INVENTION 
This invention generally relates to optical sensors; and more specifically 
to staring optical sensors having adjustable resolutions. 
A staring optical sensor is usually a two-dimensional focal plane mosaic of 
photodetectors each of which generates an electric current representing 
the intensity of light incident on a given area of observation, referred 
to as a pixel, and representing one unit of resolution. Typically, the 
area behind each photodetector is used for electronic circuitry to amplify 
and multiplex the detector signal. 
The spatial resolution of a staring optical sensor is typically determined 
by the size and spacing of the detectors on the focal plane of the sensor, 
in conjunction with the focal length of the sensor optical system. In many 
circumstances, it is desirable to provide the sensor with the ability to 
operate at both fine and coarse resolutions. The sensor may be used to 
observe a wide field of view at a coarse spatial resolution to detect a 
target of interest, and then, once the target is detected, the sensor may 
be operated to track the target using a narrower field of view with finer 
spatial resolution. For instance, many space borne applications involve 
observing the entire earth with a wide field of view for a target, and 
then tracking the target with a narrower field of view and a finer 
resolution. 
In many situations, including typical space borne applications, all of the 
sensor components that affect spatial resolutions are fixed and cannot be 
changed after launch. The requirement, to simultaneously observe a wide 
field (coarse resolution) and a narrow field (fine resolution), is 
normally satisfied either by providing either a single wide field sensor 
with fine resolution globally, or by providing two sensors, one with 
global coverage and coarse resolution, and a second, pointable sensor with 
a narrow field of view and fine resolution. Optical zoom techniques 
usually can not simultaneously offer wide and narrow field performance and 
present reliability and costs concerns. 
Both of the above-mentioned approaches have disadvantages. On the one hand, 
the traditional single sensor approach, while offering some design and 
cost advantages, nevertheless requires a relatively large number of 
inefficiently utilized detector channels, thus requiring a wide bandwidth 
data link to handle the resulting large quantity of data. The data 
processor needed to handle this data must also be oversized. On the other 
hand, the dual sensor approach is burdened by the added weight and costs 
of a pointing mechanism and of two separate focal planes, optical systems 
and signal processors. 
SUMMARY OF THE INVENTION 
An object of this invention is to improve optical sensors. 
Another object of the present invention is to provide a single optical 
sensor capable of a wide field of view coverage with coarse resolution 
coupled with the ability to command local areas of a focal plane to 
operate with finer spatial resolution. 
A further object of this invention is to construct an optical sensor from a 
multitude of unit cells, each of which can be operated with either a fine 
or a coarse resolution. 
Another object of this invention is to separate an optical sensor into a 
multitude of unit cells, to generate two different optical signals from 
each cell representing two different areas of the cell, and to process 
selectively either one or both of those optical signals to operate the 
sensor with, respectively, either a fine or coarse resolution. 
An object of the present invention is to operate a single array of optical 
detectors in both coarse and fine resolution modes by combining electronic 
switching and summing of the detector elements with multiple mirror toggle 
displacements. 
These and other objectives are attained with an optical sensor and a method 
of operating an optical sensor. The optical sensor includes a plurality of 
unit cells, a plurality of groups of optical detectors, and amplifier and 
a plurality of selection circuits. Each of the unit cells includes at 
least first and second pixels, and each group of optical detectors is 
associated with a respective one of the unit cells and includes at least 
first and second detectors. The first and second detectors of each group 
of detectors generates first and second electric signals representing the 
intensity of light incident on, respectively, the first and second pixels 
of the associated unit cell. Each of the selection circuits is associated 
with a respective one of the groups of optical detectors for transmitting 
to the amplifier the electric signals from the associated group of optical 
detectors. Also, each selection circuit has first and second modes for 
conducting to the amplifier, respectively, either the electric signal from 
only the first optical detector of the associated group of optical 
detectors, or the electric signals from both the first and second optical 
detectors of that associated group of detectors. 
Preferably, each of the selection circuits includes an output line, first 
and second input lines and a switch. The output line is connected to the 
amplifier. The first input line is electrically connected in series 
between the output line and the first detector of the associated group of 
detectors, and the second input line is electrically connected in series 
between the output line and the second detector of the associated group of 
detectors. In this way, the first and second input lines transmit electric 
signals to the output line from, respectively, the first and second 
detectors. The switch is located in the second input line to control the 
transmission of electric signals therethrough, and the switch includes 
first and second states, respectively, to transmit, or to prevent the 
transmission of, the electric signal to the amplifier from the second 
detector of the associated group of detectors. 
The optical sensor may operate at either a coarse resolution or a fine 
resolution. To operate the sensor at a coarse resolution, the switches of 
the sensor are operated in their second states so that the output of each 
amplifier of the sensor represents the light signal incident on the 
entirety of the associated unit cell. To operate the sensor at a fine 
resolution, the switches of the sensor are operated in their first states 
so that the output of each amplifier of the sensor represents the light 
signal incident on only the first pixel of the associated unit cell. Thus, 
by combining electronic switching and summing of detector elements, a 
single array of detectors can function in both course and fine resolution 
modes. 
Further benefits and advantages of the invention will become apparent from 
a consideration of the following detailed description given with reference 
to the accompanying drawings, which specify and show preferred embodiments 
of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a group of cells of an optical sensor, and the detectors 
and electronic circuitry associated with one of those cells. In 
particular, FIG. 1 shows cells 10, 12, 14, and 16, detectors, 20 and 22 
circuit 24 and amplifier 26. The embodiment of circuit 24 illustrated in 
FIG. 1 includes output line 30, first and second input lines, 32 and 34, 
switch 36, control means 40 and ground connection 42. As shown in FIG. 1, 
detectors 20 and 22, circuit 24 and amplifier 26 are associated with cell 
10. Similar detectors and electronic circuitry are associated with each of 
the other cells 12, 14 and 16; however, for the sake of simplicity, these 
additional detectors and electronic circuitry are not shown in FIG. 1. 
Cell 10 includes at least first and second pixels 10a and 10b, and each of 
detectors 20 and 22 is associated with a respective one of the pixels 10a 
and 10b for generating an electric signal representing the intensity of 
light incident on that one pixel. Circuit 24 is connect to detectors 20 
and 22 and to amplifier 26 to conduct electric signals from those 
detectors to the amplifier; and circuit 24 has first and second modes for 
conducting to amplifier 26 either only the signal from detector 20, or 
both signals from detector 20 and detector 22. More specifically, in the 
first mode, circuit 24 conducts the electric signal from only first 
detector 20 to amplifier 26; and in the second mode, circuit 24 conducts 
the electric signals from both detectors 20 and 22 to amplifier 26. 
Amplifier 26 receives the electric signals from circuit 24 and amplifies 
the received signals. 
In operation, an image, or a portion of an image, is incident on each of 
the cells 10, 12, 14 and 16. Each of the detectors 20 and 22 associated 
with each of the cells generates an electric signal representing the 
intensity of light incident of the cell pixel associated with the 
detector. For instance, detector 20 generates a signal representing the 
intensity of light incident on pixel 10a of cell 10; and corresponding 
detectors generate signals representing the intensity of light incident on 
each of the pixels 12a, 14a, and 16a of cells 12, 14, 16. Likewise, 
detector 22 generates a signal representing the intensity of light 
incident on pixel 10b of cell 10; and corresponding detectors generate 
signals representing the intensity of light incident on each of the pixels 
12b, 14b, and 16b of cells 12, 14, and 16. 
To operate a particular unit cell at a fine resolution, the circuit 24 
associated with that unit cell is operated in its first mode, so that only 
the signal from one of the pixels of the unit cell is conducted to the 
associated amplifier 26. To operate a particular unit cell at a course 
resolution, the circuit 24 associated with that unit cell is operated in 
its second mode, so that the signals from both pixels of the unit cell are 
conducted to the associated amplifier 26. 
The detectors associated with cells 10, 12, 14 and 16 are located in a 
common plane, referred to as an image or focal plane. In addition, as 
depicted in FIG. 1, each of the unit cells 10, 12, 14 and 16 has a square 
shape and forms two pixels. The first pixel of each unit cell has a square 
shape, approximately one-quarter the size of the unit cell; and the second 
pixel of each unit cell is substantially the remainder of the unit 
cell--that is the area of the unit cell outside the first pixel of the 
cell. Pixels 10a and 12a are located in the upper right hand corner of 
their respective unit cells, and pixels 14a and 16a are located in the 
upper left hand corner of their respective unit cells. 
As will be understood by those of ordinary skill in the art, it is not 
necessary that unit the cells have square shapes. In addition, each unit 
cell may form more than two pixels; and the pixels of the unit cell may 
have shapes and sizes other than as shown in FIG. 1. For example, each 
unit cell may have four pixels, each of which is a square, arranged in a 
two-by-two array. As another example, each unit cell may form two, three 
or more rectangular shaped pixels. Also, in practice, the optical 
detectors associated with each unit cell are located closely adjacent to 
each other, and the area of each detector forms and defines one of the 
pixels of the unit cell. 
As previously mentioned, circuit 24 conducts electric signals from 
detectors 20 and 22 to amplifier 26; and this circuit is operated so that, 
in a first mode, only the electric signal from detector 20 is transmitted 
to amplifier 26, while in a second mode, the electric signals from both 
detectors 20 and 22 are transmitted to amplifier 26. Preferably, in this 
second mode, the electric signals from detectors 20 and 22 are summed, at 
summing junction 44 and this summed signal is applied to amplifier 26. 
More specifically, each of the input lines 32 and 34 of circuit 24 is 
electrically connected in series between output line 30, and a respective 
one of the optical detectors 20 and 22 for conducting an electric signal 
from that one detector to amplifier 26. As shown in FIG. 1, input line 32 
is connected to detector 20, and input line 34 is connected to detector 
22. Switch 36 is located in input line 34 for controlling the transmission 
of electric signals therethrough, and this switch has first and second 
states. In the first state, switch 36 prevents the electric signal from 
detector 22 from being transmitted through the switch and to amplifier 26; 
and in the second state, switch 36 conducts the electric signal from 
detector 22 to amplifier 26. 
Thus, depending on whether switch 36 in the first state or the second 
state, the amplifier output represents the light falling on either a 
small, high resolution detector 20, or the equivalent of a large coarse 
resolution detector--that is, the combination of detectors 20 and 22. In 
addition, preferably, when switch 36 is in the second state, the switch 
electrically connects input line 34 to ground connection 42 and, as a 
result, brings the electric voltage level of optical detector 22 to a 
ground level. 
FIG. 1 schematically represents switch 36 as a mechanical switch, moveable 
between two positions in which the switch 36 is connected to contacts. 
Preferably, however, switch 36 is an electronic switch and the switch is 
changed between the above-mentioned first and second states by the 
application of gate currents. With this preferred embodiment of circuit 
24, control means 40 is provided to control the state of switch 36, and in 
particular, to apply the requisite gate currents to the switch to change 
the switch between its first and second states in the desired manner. This 
control means 40, for example, may apply first and second current or 
voltage levels to switch 36 to cause the switch to assume its first or 
second states respectively. 
FIG. 2 shows in greater detail the focal plane circuitry associated with 
each unit cell. Detectors 20 and 22 are connected to input lines 32 and 
34, respectively, and switch 36 is located in line 34. When switch is in 
the second state, current from both photodetectors 20 and 24 are conducted 
to summing junction 44, at which these two currents are summed, and this 
summed current is thence conducted to preamplifier 26. Preamplifier 26 
amplifies the current conducted to it and feeds its output to multiplexer 
46. Multiplexer 46 has parallel input branches from other cell units, and 
a control circuit to control or identify the output signals conducted off 
the multiplexer. 
The circuit of FIG. 2 is designed for the use of photo-voltaic detectors. 
If photo-conductive detectors are used, exterior current source 20a and 
22a (shown in phantom) are included. One or more filters (not shown) may 
be included in circuit 24, either to process the electric signals 
conducted through the circuit or to select particular frequencies or 
frequency bands for transmission through the circuit. Also, photodetectors 
20 and 22, switch 36, amplifier 26 and multiplexer may be standard, 
conventional elements. 
Preferably, cells 10, 12, 14 and 16 are part of a larger two-dimensional 
array of similar or identical cells that form an optical sensor, 
schematically illustrated at 50 in FIG. 3. Each cell of sensor 50 is 
provided with a pair of detectors analogous to detectors 20 and 22 of FIG. 
1, and with an electronic circuitry analogous to circuit 24 and amplifiers 
26 of FIG. 1. Sensor 50 may operate at either a coarse resolution of a 
fine resolution. To operate sensor 50 at a coarse resolution, the switches 
36 of the sensor are operated in their second states so that the output of 
each amplifier represents the light signal incident on the entirety of the 
associated unit cell. To operate sensor 50 at a fine resolution, the 
switches 36 of the are operated in their first states so that the output 
of each amplifier 26 of the sensor represents the light signal incident on 
only the first pixel of the associated unit cell. 
When set for the coarse resolution mode, the detector array 50 provides one 
hundred percent coverage of the image falling on it without any blank 
regions. In the fine resolution mode, however, the array output represents 
the light signal falling on the small detectors only. In addition, 
preferably, the large inactive detector is grounded and acts as a guard 
ring to define electrically the optically active detector. 
Preferable, when sensor 50 operates in the fine resolution mode, the image 
is moved relative to the sensor, or alternatively the sensor is moved 
relative to the image, so that each portion of each unit cell is stepped 
across one of the first detectors of array 50. Any suitable means and any 
suitable procedure may be used to move the image relative to sensor 50 to 
achieve this result. 
For example, with reference to FIGS. 4 and 5, mirror 52 may be used to step 
or toggle the image on sensor 50. More specifically, each unit cell of 
sensor 50 may be divided into a plurality of sections, referred to as 
pixels or virtual pixels; and mirror 52 may be used to step a plurality of 
cell sections, one at a time, over the first detector 20 of one of the 
unit cells. In this way, mirror 52 is toggled in a pattern to create an 
array of virtual detectors surrounding each physical detector. 
With particular reference to FIG. 5, each unit cell may be divided into 
four sections, identified as 54a, 54b, 54c and 54d. Section 54a is the 
same as pixel 12a of the unit cell; and sections 54b, 54c and 54d are 
formed by separating pixel 12b of the unit cell into three equal-size 
square areas. Mirror 52 may be used to step four cell sections vertically 
over each detector 20; and, for instance, sections 54a and 54b of the cell 
referenced at 56, and then sections 54a and 54b of the cell referenced at 
60 may be stepped over detector 20 of cell 56. Thus, by combining 
electronic switching and summing of detector elements with multiple mirror 
toggle displacements, a single array of detectors can function in both 
coarse and fine resolution modes. 
While it is apparent that the invention herein disclosed is well calculated 
to fulfill the objects previously stated, it will be appreciated that 
numerous modifications and embodiments may be devised by those skilled in 
the art, and it is intended that the appended claims cover all such 
modifications and embodiments as fall within the true spirit and scope of 
the present invention.