Automatic exposure and gain control for a sensor using video feedback

• US 5,959,668 A
• Filed: 09/26/1996
• Issued: 09/28/1999
• Est. Priority Date: 09/26/1996
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Tactical Defense Systems (Martin Marietta Corporation)
• Original Assignee: Lockheed-Martin Tactical Defense Systems
• Inventors: Cryder, Michael Eugene, Tatko, H. John, Sebok, Dale Robert
• Primary Examiner(s): Garber, Wendy
• Assistant Examiner(s): Wilson, Jacqueline
• Application Number:
  US08/721,064
• Time in Patent Office: 1,097 Days
• Field of Search: 348/224, 348/227, 348/217, 348/216, 348/362, 348/364, 348/370, 348/371, 348/229, 348/336
• US Class Current: 348/229.1
• CPC Class Codes: H04N 23/73 by influencing the exposure time
• CPC Class Codes: H04N 23/75 by influencing optical camera components

Abstract

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.

Claims

1. An automatic exposure and gain control for a sensor using video feedback, comprising:

a sensor assembly viewing a scene and generating a video signal, said sensor having at least two adjustable variables, wherein said sensor assembly comprises a lens for viewing the scene for conversion to an image, said lens having an adjustable iris, wherein said adjustable iris is one of said adjustable variables, an image intensification tube for receiving and brightening said image, wherein said image intensification tube comprises a photocathode for converting said image from a plurality of photons to a plurality of electrons, said photocathode having an adjustable gate pulsewidth that is one of said adjustable variables, a micro-channel plate for multiplying said plurality of electrons, said micro-channel plate having an adjustable micro-channel plate gain that is another of said adjustable variables, and a phophor screen for converting said multiplied plurality of electrons to a mutiplicity of photons;

sensor electronics for receiving said image for conversion to said video signal; and

a controller receiving said video signal and deriving an average video value which is compared to a desired average video value, said controller adjusting one of said at least two adjustable variables to a minimum value and then adjusting the other of said variables prior to readjusting the one of said variables to equalize said average video value with said desired average video value, wherein said controller adjusts said micro-channel plate gain to a minimal value and adjusts said iris and said photocathode gate pulsewidth prior to readjusting said micro-channel plate gain to obtain said desired average video value.

2. The automatic exposure and gain control according to claim 1, further comprising:

a strobe for illuminating the scene at a predetermined rate; and

a processor connected to said strobe and said controller, said processor communicating to said controller when said strobe is to illuminate the scene so that said controller can adjust said at least one adjustable variable accordingly.

3. The automatic exposure and gain control according to claim 2, wherein said sensor electronics comprises:

a fiber optic coupler coupled to said image intensification tube for funneling said multiplicity of photons into a charge-coupled device to generate said video signal.

4. The automatic exposure and gain control according to claim 1, wherein said controller readjusts said micro-channel plate gain and said photocathode gate pulsewidth whenever said strobe is illuminated.

5. 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, said lens having an adjustable iris and said image intensifier tube having at least one adjustable variable, said capturing means generating a video signal of said image;

means for controlling said capturing means receiving said video signal for generating an average video value which is compared to a desired average video value, said controlling means adjusting one of said adjustable iris and said at least one adjustable variable to a minimum value and then adjusting the other of said adjustable iris and said at least one adjustable variable prior to readjusting the one of said variables to equalize said average video value with said desired average video value;

means for illuminating the image at a predetermined rate less than a rate at which said capturing means captures the image;

a processor connected to said illuminating means and said controlling means, said processor communicating to said controlling means when the image is to be illuminated so that said controlling means can adjust at least one of said adjustable iris and said at least one adjustable variable accordingly, wherein said image intensifier tube comprises;

a photocathode for converting said image from a plurality of photons to a plurality of electrons, said photocathode having an adjustable gate pulsewidth that is one of said adjustable variables, said adjustable gate pulsewidth ranging from about positive 40 volts to about negative 800 volts;

a micro-channel plate for multiplying said plurality of electrons, said micro-channel plate having an adjustable micro-channel plate gain that is one of said adjustable variables, said adjustable micro-channel plate gain ranging from about negative 550 volts to about negative 950 volts; and

a phosphor screen for converting said multiplied plurality of electrons to a multiplicity of photons, wherein said controlling means adjusts said micro-channel plate gain to a minimal value and adjusts said iris and said photocathode gate pulsewidth prior to readjusting said micro-channel plate gain to obtain said desired average video value and wherein said controlling means readjusts said adjustable iris and said micro-channel plate gain and said photocathode gate pulsewidth whenever the image is illuminated.

6. The apparatus according to claim 5, further comprising:

a fiber optic coupler coupled to said image intensification tube for funneling said multiplicity of photons into a charge-coupled device to generate said video signal.

7. 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 two adjustable variables;

determining an average video level from said plurality of video images and a compiled video history;

adjusting one of said at least two adjustable to a minimum value;

adjusting the other of said variables prior to readjusting the one of said variables to compensate for the difference between said average video level and a desired average video level to generate a video signal with a desired contrast level;

compiling said video history from said plurality of video images and the corresponding adjustments made thereto;

determining whether a next acquired image of said plurality of images is to be illuminated by a strobe;

predicting a value for and adjusting to said value said other adjustable variable for when said next acquired image is illuminated by said strobe;

flashing said strobe at a second predetermined rate;

acquiring a second plurality of video images from said sensor during said flashing step;

determining a second average video level from said second plurality of video images and a second compiled video history;

readjusting said other adjustable variable according to the difference between said second average video level and said desired average video level to generate said video signal with said desired contrast level;

compiling said second video history from said second plurality of video images and the corresponding adjustments made thereto;

wherein said acquiring steps comprise the steps of;

providing a lens with a first adjustable variable to view said scene for conversion to a video image;

coupling said lens to an image intensification tube which has a second and a third adjustable variable for brightening said video image;

providing said image intensification tube with a photocathode coupled to a micro-channel plate coupled to a phosphor screen, wherein said second adjustable variable is a photocathode gate pulsewidth and wherein said third adjustable variable is a micro-channel plate gain; and

wherein said steps of adjusting and readjusting comprise the steps of;

first adjusting said micro-channel plate gain to a minimal value and then second adjusting said iris and said photocathode gate pulsewidth prior to readjusting said micro-channel plate gain to obtain said desired average video value and wherein said adjustable iris, said micro-channel plate gain and said photocathode gate pulsewidth are readjusted prior to said flashing step.

8. A method for maintaining a desired contrast in video imagery, comprising the steps of:

generating a video signal of a scene;

flashing a light source on the scene at a predetermined rate;

determining an average video level from said video signal;

splitting said average video level into a first and a second average video level;

receiving said first average video level in an illuminated estimator which generates an illuminated output signal based on ambient light levels and an estimated light level when said light source is flashed;

receiving said second average video level in a non-illuminated estimator which generates a non-illuminated output signal;

combining said illuminated output signal and said non-illuminated output signal in a multiplexer to generate an output signal; and

adjusting at least two variables employed in generating said video signal based upon said output signal.

9. The method according to claim 8 further comprising the step of:

adjusting one of said variables to a minimal value and then adjusting the other of said variables prior to readjusting one of said variables to obtain a desired contrast in said video signal.

10. The method according to claim 9 further comprising the step of:

providing a sensor for generating said video signal, said sensor having an image intensification tube with a photocathode coupled to a micro-channel plate coupled to a phosphor screen, wherein said one variable is adjustment of gain of said micro-channel plate.