Automatic exposure and gain control for a sensor using video feedback

An apparatus for viewing dimly illuminated and rapidly changing scenes includes a sensor assembly which generates a video signal. The apparatus further includes a controller which receives the video signal for comparison to a desired average video value whereupon the controller adjusts at least one adjustable variable contained within the sensor assembly. The sensor assembly includes a lens which has an adjustable iris and an image intensification tube which includes a photocathode and a micro-channel plate. The photocathode has an adjustable photocathode gate pulsewidth and the micro-channel plate has an adjustable micro-channel plate gain. A strobe is also provided to illuminate the scene viewed by the sensor assembly wherein the strobe is connected to a processor which is also connected to the controller. The controller determines what the ambient light level is to be at the next illuminated or non-illuminated scene and adjusts the adjustable variables accordingly.

TECHNICAL FIELD 
The invention herein resides generally in the art of image sensors that 
generate a video output. More particularly, the present invention relates 
to image sensors employed in low ambient light areas in which rapidly 
changing scenes are viewed. Specifically, the present invention relates to 
an image sensor which employs a lens coupled to an image intensification 
tube wherein the image sensor has at least one automatically adjusted 
variable for enhancing the video output. 
BACKGROUND ART 
Image sensors and in particular video sensors with image intensification 
tubes are employed in low light areas to monitor scenes with minimal or no 
illumination. For example, such image sensors may be used for night-time 
surveillance cameras in high crime areas, for hand-held video camcorders 
which operate without aid of attached illumination devices or in 
conjunction with aerial drones which view a scene to confirm the location 
thereof. These sensors produce an image of a scene with enhanced 
brightness without introducing spurious brightness variations or noise 
therein. 
Unfortunately, these aforementioned sensors are ineffective where a bright 
contrasting light enters the scene or where rapidly changing scenery is 
viewed. A bright light in a previously dimly-lit scene causes video images 
to smear and become distorted. Rapidly changing scenes, which occur with 
drone-mounted sensors, are incapable of quickly adjusting to the varying 
light levels viewed by the sensor. In other words, contrasting artificial 
light sources tend to integrate and obscure images without providing any 
additional features in the image. This problem is further exacerbated in 
drones which employ temporary illumination to confirm their in-flight 
position. 
One attempt to overcome this problem is to perform an integration operation 
on the current supplied to the image intensification tube by using only a 
part of each image to set operational characteristics. However, this 
approach employs only the image intensification tube in the compensation 
loop and does not consider the operational parameters of the tube or other 
components within the sensor. Moreover, each component within a sensor 
must be individually calibrated for its particular operating 
characteristics to obtain a useable output. Of course, this adds cost and 
production time to the manufacture of the sensor. Additionally, the output 
of known sensors may be adversely affected by degradation of a single 
component therein. 
Based upon the foregoing it is apparent that there is a need in the art for 
a sensor which automatically maintains the desired operational parameters 
in video imagery while the sensor is viewing dimly lit and/or rapidly 
changing scenes. Moreover, there is a need in the art for a sensor which 
is self-adjusting to view scenes that are periodically illuminated. 
DISCLOSURE OF INVENTION 
In light of the foregoing, it is a first aspect of the present invention to 
provide an automatic exposure and gain control for an image intensified 
sensor using digital video feedback. 
Another aspect of the present invention is to provide an apparatus which 
automatically maintains the desired signal to noise ratio and contrast in 
video imagery while scene characteristics are rapidly changing in a video 
sensor's field of view. 
Still a further aspect of the present invention, as set forth above, is to 
compensate for variations in components contained within the sensor. 
Still yet another aspect of the present invention, as set forth above, is 
to provide circuitry which optimizes the viewing of scenes with minimal 
ambient light. 
An additional aspect of the present invention, as set forth above, is to 
provide a sensor assembly with adjustable variables that optimize the 
brightness level of a video output. 
Still yet another aspect of the present invention, as set forth above, is 
to provide the sensor assembly with a lens which has an adjustable iris. 
Yet a further aspect of the present invention, as set forth above, is to 
provide the sensor assembly with an image intensification tube that 
includes a photocathode which has an adjustable photocathode pulsewidth 
and a micro-channel plate which has an adjustable micro-channel plate 
gain. 
Still a further aspect of the present invention is to provide a strobe 
which illuminates the scene and where adjustments are made by the sensor 
assembly for viewing both the illuminated and non-illuminated scenes. 
The foregoing and other aspects of the invention which should become 
apparent as the detailed description proceeds are achieved by an automatic 
exposure and gain control for a sensor using video feedback, comprising: a 
sensor assembly viewing a scene and generating a video signal, a sensor 
having at least one adjustable variable; and a controller receiving the 
video signal and deranging an average video value which is compared to a 
desired average video value, the controller adjusting the at least one 
adjustable variable to equalize the average video value with the desired 
average video value. 
Another aspect of the invention which shall become apparent as obtained by 
an apparatus for enhancing a scene viewed by an image capturing device, 
comprising: means for capturing an image having a lens coupled to an image 
intensifier tube, a lens having an adjustable iris and the image 
intensifier tube having at least one adjustable variable, the capturing 
means generating a video signal of the image; and means for controlling 
the capturing means receiving the video signal for generating an average 
video value which is compared to a desired average video value, the 
controlling means adjusting one of the adjustable iris and the at least 
one adjustable variable to equalize the average video value with the 
desired average video value. 
Other aspects of the invention which will become apparent herein are 
obtained by a method for maintaining a desired contrast in video imagery, 
comprising the steps of: acquiring a plurality of video images of a scene 
at a first predetermined rate with a sensor having at least one adjustable 
variable; determining an average video level from the plurality of video 
images and a compiled video history; adjusting the at least one adjustable 
variable according to the difference between the average video level and a 
desired average video level to generate a video signal with a desired 
contrast level; and compiling the video history from the plurality of 
video images and the corresponding adjustments made thereto.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings and more particularly FIGS. 1 and 2, it can 
be seen that an apparatus for enhancing a scene viewed by an image 
capturing device is designated generally by the numeral 10. As shown, a 
scene 12 is viewed by the apparatus 10, wherein the scene is typically 
exposed to minimal light and is not easily discernable by the human eye. 
Moreover, the scene 12 may include sharply contrasting bright lights that 
tend to saturate other features therein. The apparatus 10 may be employed 
on a stationary fixed platform or upon a moving platform, such as a drone. 
The apparatus 10 includes a sensor assembly 14 which captures video images 
at a rate of about 60 hertz. Included in the sensor assembly 14 is a lens 
16 which has an adjustable iris. In the preferred embodiment, the lens 16 
is a 50 mm video camera with a variable f-stop. It will be appreciated 
that other lenses could be employed. An image intensification tube 18 is 
coupled to the lens 16 for the purpose of brightening the image captured 
by the lens 16. As is well known in the art, the lens 16 captures photons 
of light for amplification by the image intensification tube 18. 
As seen in FIG. 2, the image intensification tube 18 includes a 
photocathode 20 which is adjusted by a photocathode gate pulsewidth 22 
that is variable between about positive 40 volts to about negative 800 
volts. The photocathode 20 functions as a shutter that opens and closes 
based on the value of the photocathode gate pulsewidth 22 to convert the 
photons viewed by the lens 16 to electrons. Therefore, any available light 
within the image may be increased or decreased by adjusting the 
photocathode gate pulsewidth 22. A micro-channel plate 24 is coupled to 
the photocathode 20 for the purpose of multiplying the electrons received 
thereby and further amplifying the ambient light contained within the 
scene 12. The micro-channel plate 24 receives an adjustable micro-channel 
plate gain 26, which ranges from about negative 550 volts to about 
negative 950 volts, to variably adjust the number of electrons generated. 
The micro-channel plate 24 generates a micro-channel plate output signal 
28 whose use is described later in the specification. A phosphor screen 30 
is coupled to the micro-channel plate 24 for the purpose of converting the 
multiplicity of electrons to a multiplicity of photons. The phosphor 
screen 30 includes an output ground connection 32. A fiber optic coupler 
34 is coupled to the phosphor screen 30 and functions to funnel the 
multiplicity of photons to a charge coupled device (CCD) 36. A sensor 
circuit 40 receives the photons from the CCD 36 and converts the image 
from the image intensification tube 18 to a digital image with all of the 
necessary signal processing characteristics required for sensing the video 
image. In particular, the sensor circuit 40 generates a horizontal 
synchronization pulse 42, a vertical synchronization pulse 44 and a video 
signal 46. As those skilled in the art will appreciate, the horizontal 
synchronization pulse 42 is the beginning of a video line and a vertical 
synchronization pulse 44 indicates the top of the video field. The video 
signal 46 may be an analog or digital signal that is ultimately viewed by 
the end user or connected sensing device. The connected sensing device may 
be a video monitor or a device which compares the image to a stored image 
to confirm or correlate the position of the sensor. 
A controller 50 receives the signals 42-46 and derives an average video 
value of the video frames captured by the sensor assembly 14. The 
controller 50 includes a power regulation circuit 52 and an automatic 
exposure circuit 54. The controller 50, as will be described in detail 
hereinbelow, generates the photocathode gate pulsewidth 22 and the 
micro-channel plate gain 26. The controller 50 also receives the phosphor 
screen ground 32 and the micro-channel plate output 28. The controller 50 
also generates an iris control signal 56 that controls the opening and 
closure of the iris 16. A power supply 58 is connected to the controller 
50 and in particular the power regulation circuit 52 to provide the 
necessary power voltages and operational voltages for the controller 50. 
The controller 50 may be implemented on a Xilinx XC4005, a Xilinx XC4010 
or like device. 
A processor 60, through the controller 50, receives the signals 42-46 and 
synchronizes operation of the controller 50 with a strobe 62. As those 
skilled in the art will appreciate, the strobe 62 may be of a xenon type 
which illuminates the scene at a predetermined rate of about 10 hertz. The 
strobe 62 is sequenced by a flash signal 64 from the processor 60 which 
also generates an illumination gate signal 66 so that the controller 50 
can adjust the variables within the sensor assembly 14 whenever the strobe 
62 illuminates the scene 12. The strobe 62 may be employed in high speed 
guidance applications which normally limit the time permitted for video 
integration. Extended integration time, also referred to as long 
photocathode pulsewidth, ultimately results in image smearing. By 
periodically illuminating the scene 12 with the strobe 62 the problem with 
image smearing is eliminated. 
Generally, the sensor assembly 14 views the scene 12 and generates the 
video signal 46 which is received by the controller 50. The controller 50 
derives an average video value which is compared to a desired average 
video value with the difference therebetween being used to adjust at least 
one of the adjustable variables such as the photocathode gate pulsewidth 
22, the micro-channel plate gain 26 or the iris control signal 56. These 
adjustments increase or decrease the amount of ambient light included with 
the image captured by the sensor assembly 14 to optimize the viewing 
thereof. In other words, the controller 50 equalizes the average video 
value with the desired average video value. 
Referring now to FIG. 3, a detailed explanation of the operation of the 
controller 50 is presented. The controller 50 includes a control logic 
circuit 70 which determines when the strobe 62 is going to flash. In 
particular, the control logic circuit 70 receives the synchronization 
pulses 42 and 44 and the illumination gate signal 66 for the purpose of 
generating a control logic output signal 74. Various circuits within the 
controller 50 use the output signal 74 for the purpose of signaling when 
the strobe 62 is about to flash and for coordinating when the variables 
are to be adjusted for a new frame of video. 
The controller 50 receives the video signal 46 through a low pass filter 78 
to eliminate false variations in the image. In particular, thermal noise 
from the image intensification tube 18 and the non-image signal variations 
generated by interference within the sensor assembly 14 are eliminated by 
the filter 78. Those skilled in the art will appreciate that the 
components within the sensor assembly 14 are especially susceptible to 
variations due to the limited amount of ambient light received by the 
sensor assembly 14. The low pass filter 78 generates a filtered video 
signal 80 which is received by a comparator 82 for comparison to a video 
set point signal 76 which is pre-programmed into the controller 50 and 
which sets the desired average video value for optimum viewing of the 
scene 12. The comparator 82 generates a difference signal 84 between the 
video set point signal 76 and the filtered video signal 80 that is 
received by a state machine 86. 
The state machine 86 includes a non-linear mapping circuit 88 which 
averages the difference signal 84 for the entire field of video and 
generates a corresponding output signal 90 that is split and received by 
an illuminated estimator 92 and a non-illuminated estimator 94. Based on 
the ambient light levels, the illuminated estimator 92 generates an 
illuminated output signal 96 that is an estimate of what the light level 
of the scene 12 will be when the strobe 62 flashes based upon previous 
frames of video and strobe flashes. Likewise, the non-illuminated 
estimator 94 generates a non-illuminated output signal 96 that is an 
estimate of the ambient light level of the scene 12 based upon previous 
non-illuminated frames of videos. Both the signals 96 and 98 are received 
by a multiplexer 100 which combines the signals to generate a state 
machine output signal 102 that is received by a control law circuit 104. 
The control law circuit 104 comprises a photocathode control circuit 106, a 
micro-channel plate control circuit 108 and an iris control circuit 110. 
The control law circuit 104 determines which adjustable components within 
the sensor assembly 14 are to be adjusted according to the light levels of 
the scene 12 to optimize the brightness and contrast of the video signal 
46. In the preferred embodiment, the photocathode control 106, the 
micro-channel plate control 108 and the iris control 110 receive the state 
variable output signal 102 such that the micro-channel plate gain 26 is 
maintained at a nominal low level as a first requirement. This is done to 
minimize the video noise contribution by keeping the gain for the 
micro-channel plate 24 as low as possible for as long as possible. Next, 
the control law circuit 104 adjusts the iris control circuit 110 between 
its minimum and maximum values. Finally, the photocathode control circuit 
106 adjusts the photocathode pulsewidth over its range of values. After 
the iris control circuit 110 and the photocathode control circuit 106 have 
exhausted their ranges of values to optimize viewing of the scene 12, the 
control law circuit 104 adjusts the micro-channel plate gain 26 to a next 
minimal value and then readjusts the iris and the photocathode gate 
pulsewidth as described above. Depending on the particular application, 
the hierarchy of the control law circuit 104 may be re-configured to 
adjust the variable components within the sensor assembly 14 in another 
manner. For example, the iris control 110 may be maintained at a 
predetermined level and will not be adjusted until the other variable 
ranges are exhausted. 
The photocathode control circuit 106 is connected to a variable pulsewidth 
generator 112 which generates the photocathode gate pulsewidth 22. The 
micro-channel plate control 108 is connected to a register 114 which in 
turn generates the micro channel plate gain 26. The register 114 functions 
to update the micro-channel plate gain 26 at predetermined intervals with 
respect to the vertical and horizontal synchronization pulses 42 and 44, 
respectively. The iris control circuit 110 is connected to a servo-motor 
116 which generates the iris control signal 56. The servo-motor 116 
functions to open and close the iris 16 as required by the control law 
circuit 104. 
Reference is now made to FIG. 4 which provides an overall operational flow 
200 of the apparatus 10. At step 202, the apparatus 10 is initialized and 
all of the adjustable variables (the photocathode gate pulsewidth 22, the 
micro-channel plate gain 26 and the iris control 56) are set to nominal 
values. At step 204, the sensor assembly 14 begins viewing the scene 12 at 
an imaging rate of about 60 hertz. Likewise, at step 206 the strobe 62 is 
triggered by the processor 60 to flash at a frequency of about 10 hertz. 
At step 208, the controller 50 determines the average video level of the 
video signal 46 and based upon this information, at step 210 adjusts the 
photocathode gate pulsewidth 22, the micro-channel plate gain 26 and the 
iris control signal 56 to optimize the light level in the next frame of 
video. At step 212, the controller 50 compiles a video history which 
provides a basis for making future adjustments to the variable devices 
within the sensor assembly 14. At step 214, the non-illuminated video 
scene is passed along and provided as the video output signal 46. 
At step 216, the controller 50 determines whether the next scene to be 
viewed by the sensor assembly 14 is to be illuminated by the strobe 62. If 
the next scene is not to be illuminated, the controller 50 returns to step 
208 and steps 210 and 212 are repeated. However, if at step 216 it is 
determined that the next scene is to be illuminated, the controller 
proceeds to step 218, whereupon a prediction is made of the photocathode 
gate pulsewidth 22, the micro-channel plate gain 26 and the iris control 
signal 56. At step 220, the strobe 62 is flashed and an illuminated 
average video level is determined at step 222. Accordingly, at step 224, 
the controller 50 adjusts the photocathode pulsewidth 22, the 
micro-channel plate gain 26 and the iris control 56 to prepare for the 
next occurrence of a strobe flash. At step 226, a video history of the 
illuminated scenes is compiled and is employed in conjunction with the 
predicted values derived in step 218 at the next occurrence of the strobe 
flash at step 220. The compiled video history is also provided to the 
video output step 214 to provide a complete output video signal 46 that 
includes illuminated and non-illuminated scenes. 
It is apparent then from the above description of the operation of the 
apparatus 10 that the problems associated with previous low-light video 
sensors have been overcome. In particular, the apparatus 10 is insensitive 
to variations in optical component characteristics from various units. In 
other words, the apparatus 10 can compensate for variations in its optical 
and electrical components over a period of time as these components are 
subject to wear and degradation. The apparatus 10 is also advantageous in 
that it optimizes the signal-to-noise ratio by maximizing the photocathode 
pulsewidth before increasing the micro-channel plate gain. In production, 
this minimizes production alignment procedures required for the high 
voltage power supply circuit included within the controller 50. The 
apparatus 10 also provides greater stability for when the sensor assembly 
is required to view rapidly changing scenes. 
Thus, it can be seen that the objects of the invention have been satisfied 
by the structure presented above. It should be apparent to those skilled 
in the art that the objects of the present invention could be practiced 
with any type of lens and image intensification tube. 
While the preferred embodiment of the invention has been presented and 
described in detail, it will be understood that the invention is not 
limited thereto or thereby. Accordingly, for an appreciation of the true 
scope and breadth of the invention, reference should be made to the 
following claims.