Methods and apparatus for providing wide range exposure control for image intensifier cameras

The dynamic range of an image intensifier camera may be extended by using an automatic exposure control system which, in response to imager output (for example, the output of a charge coupled device (CCD) imager included in a solid state image intensifier camera), (a) generates a first control signal that controls the gain of the I.sup.2 Tube itself; (b) generates a second control signal for selectively gating the I.sup.2 Tube on or off; and (c) generates a third control signal for controlling a light attenuator that, according to a preferred embodiment of the invention, is included in the camera. The automatic exposure control system contemplated by the invention has a control range in excess of 10.sup.10 .times.; takes a measurement of the exposure from an imager (again, for example, a CCD sensor) for comparison to a desired setpoint; the measurement used may be a peak, an average or some weighted function and the automatic exposure control logic then adjusts, in order, the I.sup.2 Tube gain, the I.sup.2 Tube gating and the light attenuator (using the aforementioned first, second and third control signals), to adjust for steadily increasing light from the object under observation. In addition to automatic exposure control systems per se, the invention is also directed to camera systems which incorporate such automatic exposure control systems and to methods for realizing the automatic exposure control systems of the type described hereinafter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to imaging systems or cameras (such as 
video cameras), and exposure control systems for such systems/cameras. 
More particularly, the invention relates to image intensifier cameras 
(cameras that includes an Image Intensifier Tube, referred to hereinafter 
as an "I.sup.2 Tube"), and automatic exposure control systems for such 
cameras. 
According to one aspect of the invention, the dynamic range of an image 
intensifier camera may be extended by using an automatic exposure control 
system which, in response to imager output (for example, the output of a 
charge coupled device (CCD) imager included in a solid state image 
intensifier camera), (a) generates a first control signal that controls 
the gain of the I.sup.2 Tube itself; (b) generates a second control signal 
for selectively gating the I.sup.2 Tube on or off; and (c) generates a 
third control signal for controlling a light attenuator that, according to 
a preferred embodiment of the invention, is included in the camera. The 
automatic exposure control system contemplated by the invention has a 
control range in excess of 10.sup.10 .times.. 
According to a further aspect of the invention, the automatic exposure 
control system takes a measurement of the exposure from an imager (again, 
for example, a CCD sensor) and digitizes it for comparison to a desired 
setpoint. The measurement used may be a peak, an average or some weighted 
function. The automatic exposure control logic then adjusts, in order, the 
I.sup.2 Tube gain, the I.sup.2 Tube gating and the light attenuator (using 
the aforementioned first, second and third control signals), to adjust for 
steadily increasing light from the object under observation. 
In addition to automatic exposure control systems per se, the invention is 
also directed to camera systems which incorporate such automatic exposure 
control systems and to methods for realizing the automatic exposure 
control systems of the type described hereinafter. 
2. Brief Description of the Prior Art 
A variety of automatic light control systems for imaging/camera systems are 
well known in the prior art. In addition, a wide variety such systems that 
utilize image intensifiers (such as an I.sup.2 Tube) and/or solid state 
imagers (such as a CCD imager) are also well known. 
These systems have many applications, including television camera systems, 
navigational and tracking systems, etc. For many of these systems it is 
desirable to have a wide dynamic range of performance which permits system 
operation over a wide range of changing light conditions, ranging from 
very low light conditions (e.g., systems which operate at night), to high 
intensity light conditions. 
Several patents are briefly described hereinafter for background purposes 
and to illustrate the state of the prior art. 
Prince et al., in U.S. Pat. No. 4,050,085, describes a mechanical system 
for automatically regulating light input to an electro-optical imaging 
system used in tracking and navigational systems. According to Prince et 
al., light measurements, that include peak, average and weighted 
measurements, are taken and used for controlling an I.sup.2 Tube, a 
vidicon with adjustable gain, a mechanical iris controller and a number of 
amplifiers. 
Gilligan et al., in U.S. Pat. No. 4,202,014, describes a pulse modulated 
automatic light control system for a television camera. 
The use of CCD or solid state imaging devices in place of film in cameras 
is described in Ochi et al., U.S. Pat. No. 4,541,016. A further example of 
a prior art CCD or solid state imaging device is disclosed in U.S. Pat. 
No. 4,851,914, to Pfanhouser et al., which describes a high speed full 
frame imaging CCD camera. 
Woolfolk, in U.S. Pat. No. 4,872,057, describes an exposure control system 
for a TV camera which uses a gated I.sup.2 Tube to enable low light 
television systems to be used over a wide dynamic range of light input. 
Lam et al., in U.S. Pat. No. 4,918,534, describes an image processing 
system which includes an I.sup.2 Tube, an LCD attenuator, a TV camera and 
a brightness control circuit which controls the attenuator. 
Gilligan et al., in U.S. Pat. No. 4,985,773, discloses an exposure control 
system for a TV camera which includes an I.sup.2 Tube controlled using an 
amplitude adjustable, periodically pulsed signal. 
Wijen, in U.S. Pat. No. 5,101,275, describes a video camera with light 
intensity control means including an adjustable light attenuating liquid 
crystal plate. 
Yoshimura et al., in U.S. Pat. No. 5,140,424, describes a solid state video 
camera (CCD based) that includes noise reduction signal processing 
apparatus. 
Seppi et. al., in U.S. Pat. No. 5,168,532 describes a method for improving 
the dynamic range of an X-ray camera which includes an IIT. The video 
signal is digitized for further processing in the control circuitry. 
Alford et al., in U.S. Pat. No. 5,233,428, hereby incorporated by 
reference, discusses (in the "Related Art" section of this reference) the 
use of CCDs in camera images to enable the production of smaller and more 
durable camera imagers; the effort spent on developing exposure control 
systems for effectively controlling imager sensitivity to incident light 
on the imager (that is, controlling the amount of charge a CCD accumulates 
during a field integration period); prior art exposure control systems 
which relied on (i.e., are implemented using) mechanical devices (such as 
a mechanical iris/or mechanical shutter wheel, etc.); relatively large 
vacuum tube type imagers; on chip shuttering options which allow 
accumulated charge in the charge storage wells of a CCD to be dumped 
before the end of a normal integration period, etc. 
In addition to the outlining of systems exemplifying the state of the art, 
the incorporated reference discloses a high performance exposure control 
system for effectively controlling CCD light sensitivity under varying 
light conditions. In particular, the Alford et al. reference teaches 
controlling CCD light sensitivity by electrically controlling a CCD such 
that the CCD discharges the charge resultant from incident light for a 
percentage of a field integration period. In other words, the incorporated 
reference electronically varies the CCD's exposure time. 
The technique proposed by Alford, et al. for controlling exposure in their 
solid state imager, namely electronically controlling CCD exposure time 
directly, has a potentially negative impact on imager tube life (for 
example, I.sup.2 Tube life), which is a major contributor to imager 
failure rates. The failure mechanism of these devices is associated with 
the average current, which is not held constant by Alford at al., and 
similar systems. 
Furthermore, the use of tube current for exposure level detection does not 
lend itself towards tailoring the exposure control loop to various 
applications in which it may be desirable to emphasize, de-emphasize or 
even ignore portions of a scene contributing to a desired exposure level. 
Still further, none of the exemplary prior art discussed hereinbefore 
provides a wide range exposure control mechanism for image intensifier 
cameras, where wide range is defined as a control range in excess of 
10.sup.10 .times.. 
Accordingly, it would be desirable to provide image intensifier cameras 
having a wide dynamic range of performance which permits system operation 
over a wide range of changing light conditions, ranging from very low 
light conditions (e.g., systems which operate at night), to high intensity 
light conditions. 
In particular, it would be desirable to provide methods and apparatus for 
being able to extend the dynamic range of image intensifier cameras to a 
control range in excess of 10.sup.10 .times.. 
Furthermore, it would be desirable to provide methods and apparatus for 
controlling the dynamic range of image intensifier cameras, in particular 
those cameras utilizing I.sup.2 Tubes, which prolong tube life by directly 
controlling the image intensifier tube itself (as opposed to controlling, 
for example, CCD exposure time as performed by Alford et al.). 
Still further, it would be desirable to provide methods and apparatus for 
controlling the dynamic range of image intensifier cameras which allows 
the response of the exposure control loop to be tailored to various 
applications in order, for example, to allow specific parts of the scene 
can be emphasized, de-emphasized or ignored totally as they contribute to 
the desired exposure level. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the invention to provide an image 
intensifier camera having a wide dynamic range of performance which 
permits system operation over a wide range of changing light conditions, 
ranging from very low light conditions (e.g., systems which operate at 
night), to high intensity light conditions. 
In particular, it is an object of the invention to an automatic exposure 
control system (and related methods and apparatus) that is able to extend 
the dynamic range of image intensifier cameras to a control range in 
excess of 10.sup.10 .times.. 
Furthermore, it is an object of the invention to provide methods and 
apparatus for controlling the dynamic range of image intensifier cameras, 
in particular those cameras utilizing I.sup.2 Tubes, which prolong tube 
life. 
Still further, it is an object of the invention to provide methods and 
apparatus for controlling the dynamic range of image intensifier cameras 
which allows the response of the exposure control loop to be tailored to 
various applications. 
According to one aspect of the invention, an image intensifier camera 
system, together with an automatic exposure control system for the camera 
system, are disclosed which includes a number of discretely controllable 
components, each of which is capable of operation over some dynamic range. 
The exposure control system of the camera essentially gangs these together 
to obtain an overall dynamic range which is the sum of each component's 
dynamic range. 
According to one embodiment of the invention, the camera includes a lens 
with a selectively attenuating filter, an I.sup.2 Tube which is adjustable 
for gain through a Micro Channel Plate (MCP) voltage and through gating of 
the I.sup.2 Tube in time, and a CCD image sensor which has adjustable 
gain. The control system takes a measurement of the exposure from the CCD 
sensor for comparison to a desired setpoint. The measurement used may be a 
peak, an average or some weighted function. Exposure control logic then 
adjusts, in order, the I.sup.2 Tube gain, the I.sup.2 Tube gating and the 
light attenuator, to adjust for steadily increasing light from the object 
under observation. 
According to a further aspect of the invention, the dynamic range of an 
image intensifier camera may be extended by using an automatic exposure 
control system which, in response to imager output (for example, the 
output of a charge coupled device (CCD) imager included in a solid state 
image intensifier camera), (a) generates a first control signal that 
controls the gain of the I.sup.2 Tube itself; (b) generates a second 
control signal for selectively gating the I.sup.2 Tube on or off; and (c) 
generates a third control signal for controlling a light attenuator that, 
according to a preferred embodiment of the invention, is included in the 
camera. The automatic exposure control system contemplated by the 
invention has a control range in excess of 10.sup.10 .times.. 
According to yet another aspect of the invention, the automatic exposure 
control system takes a measurement of the exposure from an imager (again, 
for example, a CCD sensor) for comparison to a desired setpoint. Again, 
the measurement used may be a peak, an average or some weighted function. 
The automatic exposure control logic then adjusts, in order, the I.sup.2 
Tube gain, the I.sup.2 Tube gating and the light attenuator (using the 
aforementioned first, second and third control signals), to adjust for 
steadily increasing light from the object under observation. 
In addition to automatic exposure control systems per se, the invention is 
also directed to camera systems which incorporate such automatic exposure 
control systems and to methods for realizing the automatic exposure 
control systems of the type described hereinafter. 
The invention features automatic exposure control systems per se and image 
intensifier cameras including such control systems, having a control range 
in excess of 10.sup.10 .times.. 
Furthermore, the invention features image intensifier cameras having a wide 
dynamic range of performance which permit system operation over a wide 
range of changing light conditions, ranging from very low light conditions 
to high intensity light conditions, while preserving tube life and 
allowing the response of the exposure control loop to be tailored to 
various applications.

DETAILED DESCRIPTION 
Reference should now be made to FIG. 1 which, as indicated hereinbefore, is 
a high level block diagram depicting image intensifier camera (camera 
system 100), including an automatic exposure control system (control 
system 120) of the type contemplated by a preferred embodiment of the 
invention. 
More particularly, exemplary camera system 100 is shown in FIG. 1 to 
include lens 101; I.sup.2 Tube 102, a gateable high voltage power supply 
(power supply 103) to operate I.sup.2 Tube 102; a video camera, shown by 
way of example to be CCD imager 104; prior art camera electronics 105; and 
the aforementioned control system 120 contemplated by the invention. 
An exemplary automatic exposure control system 120 for an image intensifier 
camera, like camera 100, having a control range in excess of 10.sup.10 
.times. (in accordance with the objects of the present invention) will be 
described hereinafter with reference to FIG. 2. 
Camera system 100 depicted in FIG. 1 operates as follows: Light enters lens 
101 and is focused by the lens onto I.sup.2 Tube 102. According to a 
preferred embodiment of the invention, lens 101 includes an integrated 
neutral density filter (not shown) for attenuating light, which may be 
switchably placed in the path of the input light. 
Lens 101 and I.sup.2 Tube 102 are both commercially available devices well 
known by those skilled in the art. 
As will readily be appreciated by those skilled in the art, the functions 
of the preferred lens 101 (including the integrated filter), may be 
performed by other arrangements, such as the combination of a separate 
commercially available lens and a separate light attenuation device which 
could, for example, be switchably placed and removed from a position in 
front of the lens using a selenoid switch. 
As shown in FIG. 1, I.sup.2 Tube 102 has associated therewith gateable 
power supply 103 (also a commercially available device) which is shown 
controlled, along with whatever light attenuation device is used to 
implement the perferred embodiment of the invention, by the aforementioned 
control system 120 via control links 150, 151 and 152. As depicted in the 
illustrative embodiment of the invention shown in FIG. 1, link 150 is used 
to control switching the light attenuator into the light path; and links 
151 and 152 are used to control tube gating and MCP voltage respectively 
as will be described in detail hereinafter. 
Light focused onto I.sup.2 Tube 102 is amplified by the I.sup.2 Tube 102 in 
a manner well known by those skilled in the art. In accordance with one 
aspect of the invention, the gain of I.sup.2 Tube 102 may controlled by 
changing the Micro Channel Plate (MCP) Voltage associated with power 
supply 103, as a function of CCD Imager 104 output as processed by control 
system 120. This too will be explained in detail hereinafter. 
It should be noted that (1) the range of control of the image intensifier 
camera 100 that can be effected by changing the MCP Voltage associated 
with power supply 103 is greater then 10.sup.2 .times.; and that (2) 
I.sup.2 Tube 102 may be turned ON and OFF by gating the Cathode via gated 
power supply 103 to further extend the dynamic range of camera 100. 
According to the illustrative embodiment of the invention being set forth 
with reference to FIG. 1, CCD 104 (the video camera used in a preferred 
embodiment of the invention) produces its' signal by integrating the light 
output from I.sup.2 Tube 102 for the period of one video field. Gating 
I.sup.2 Tube 102 off for part of this period, reduces the signal produced 
by CCD 104. 
It should be noted that camera electronics 105 depicted in FIG. 1 is shown 
for the sake of completeness only and does not constitute a part of the 
automatic control system contemplated by the invention. Those skilled in 
the art will readily appreciate that such electronics are used, for 
example, to make gamma corrections, add synchronization and blanking 
signals, etc., to the video camera (CCD 104) output signal, to form a 
camera 101 composite video output signal as shown in FIG. 1. 
Reference should now be made to FIG. 2 which, as indicated hereinbefore, is 
a detailed block diagram showing exemplary components for implementing 
control system 120 depicted in FIG. 1 and which, in accordance with a 
preferred embodiment of the invention, may be used to realize the objects 
of the invention. 
In particular, FIG. 2 depicts lens 201, I.sup.2 Tube 202, power supply 203, 
CCD imager 204, control system 220 and links 250-252; all of which 
correspond to devices 101-104, control system 120 and links 150-152, 
respectively, described hereinabove with reference to FIG. 1. 
In addition, FIG. 2 depicts the following components: video level output 
detector 275, first amplifier means 276, crossover network 277, second 
amplifier means 278, analog to digital (A/D) converter 279 and exposure 
control logic 280. 
The operating principals of the invention may now be d emonstrat ed with 
reference to FIG. 2. 
The MCP Voltage (for power supply 203) is, according to one aspect of the 
invention, controlled by a straight forward automatic level control 
circuit that includes video level output detector 275 and first amplifier 
means 276 (shown in FIG. 2 as a high gain amplifier). 
The amplitude of the video signal input to control system 220 (via link 280 
from CCD 204 as shown in FIG. 2) is converted to a dc value by detector 
275. This detector may be designed by those of ordinary skill in the art 
to respond to the average, the peak, or some weighted peak function of the 
video signal input via link 280 from CCD 204. 
The dc produced by detector 275 is, according to a further aspect of the 
invention, compared to a reference and the difference is amplified by 
first amplifier means 276. the output of first amplifier means 276, shown 
on link 252, is a first control signal which may be used to directly 
control the intensifier MCP Voltage and therefore its gain. 
The output of detector 275, according to a further aspect of the invention, 
also drives second amplifier means 278 (via link 281); and the output of 
second amplifier means 278 is then digitized in analog to digital (A/D) 
converter 279 (also shown in FIG. 2) for use by exposure control logic 280 
(to be described hereinafter). 
In addition to the detector 275 input, according to a preferred embodiment 
of the invention, second amplifier means 278 is also fed by the MCP 
control voltage (the aforementioned first control signal) through 
crossover network 277, via link 290 as shown in FIG. 2. The function of 
this circuitry is to maintain the signal to A/D converter 279 at a low 
level until the MCP gain has been reduced to a value which produces a 
minimum noise signal. After this MCP control Voltage is reached, second 
amplifier means 278 output is allowed to trace the detected video level. 
This signal (the output of second amplifier means 278 output which is 
allowed to trace the detected video level) is, according to the invention, 
digitized (by device 279) and used to control a gating pulse which turns 
OFF I.sup.2 Tu be 202 for part of a field. 
The length of this gating pulse (also referred to herein as the second 
control signal which is an output of logic 280 that is placed on link 282) 
is, according to one embodiment of the invention, altered by logic 280 
such that the product of the incident light level, times the resultant 
exposure time, produces a CCD 204 signal at a constant level which 
corresponds to A/D converter 279 output at mid scale, as taught in the 
incorporated Alford et al. reference. 
The range of this gating pulse is from 16.7 milliseconds (ON all of the 
time) to less then 1 microsecond (a&gt;10.sup.4 .times.range). 
Finally, according to a preferred embodiment of the invention, a light 
attenuator (assumed for the sake of illustration only to be int egrated 
into lens 201 as explained hereinbefore) may be switched into the input 
optical path to further extend the dynamic range of camera system 100 (of 
FIG. 1) by another factor of 10.sup.4 .times.. The switching could, for 
example, be accomplished by using a selenoid to move the light attenuator 
into the light path (before the I.sup.2 Tube). The selenoid would be under 
the control of a third control signal generated by logic 280, output via 
link 283 shown in FIG. 2. It should be noted that a selenoid switching 
arrangement is not explicitly illustrated in FIG. 2 since such switching 
mechanisms are well known by those of ordinary skill in the art. 
The digital implementation of the gating control function (explained with 
reference to devices 278-280 of FIG. 2), allows the electronic control 
system to be easily combined with the optical attenuator to accomplish the 
aforementioned 10.sup.4 .times. dynamic range extension. 
In particular, according to one embodiment of the invention, when the 
exposure control is at its minimum time, and the signal is still too 
large, the light attenuator is switched in, and simultaneously the 
exposure time is increased by 10.sup.4 .times. to compensate for the 
resultant loss of light. This criteria for switching may be easily 
designed by those skilled in the art into logic 280 and by utilizing this 
technique the very large step change of the light attenuator may be 
smoothed out. Control system 220 then works normally to control the signal 
level. The process is reversed if the attenuator is in place and the light 
level falls. Both of the processes described hereinabove are graphically 
depicted in FIGS. 3A and 3B which, as indicated hereinbefore, depict the 
operation of the gating/attenuator hysterisis features of the system 
depicted in FIG. 2. 
Reference should now be made to FIG. 4 which graphically depicts the 
operation of the automatic light control features of the system depicted 
in FIG. 2. 
In particular (with reference to FIG. 4), it may be seen that as scene 
illumination changes from darkness to light, first the gain of the I.sup.2 
Tube is reduced to a level which produces a quiet picture (one in which 
sparking noise is minimized). Next, the I.sup.2 Tube's On Time is reduced 
to shorten the exposure time of the imager. The combination of tube gain 
change of greater then 10.sup.2 .times. and an exposure time change of 
&gt;10.sup.4 .times. produces a very wide dynamic range in and of itself. 
Finally, as may be seen with reference to FIG. 4, the optical attenuator 
provides a further increase in dynamic range of 10.sup.4 .times.. 
It should be noted and will be appreciated by those skilled in the art that 
(according to the preferred embodiment of the invention), I.sup.2 tube 
gating (as performed by logic 280) can implemented digitally in a manner 
similar to the digital electronic exposure control for CCD's taught in the 
incorporated Alford et al. reference because of the ease of synchronizing 
the pulse with the CCD's charge transfer. If no CCD type shuttering were 
to be used, a relatively straight forward analog approach to gating 
control could be utilized. 
Furthermore, the technique of controlling the exposure by controlling the 
Intensifier Tube directly (as opposed to controlling the CCD exposure 
time) will prolong the life of the tube. The failure mechanism of these 
devices is associated with the average current, which is held constant by 
this method. 
Finally, the use of video for the exposure level detection instead of the 
tube current, allows the response of the exposure control loop to be 
tailored to various applications. That is average, peak, and weighted peak 
detection can be used. Specific parts of the scene can be emphasized, 
de-emphasized or ignored totally as they contribute to the desired 
exposure level. 
What has been described in detail hereinabove are methods and apparatus 
meeting all of the aforestated objectives. As previously indicated, those 
skilled in the art will recognize that the foregoing description has been 
presented for the sake of illustration and description only. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. 
The embodiments and examples set forth herein were presented in order to 
best explain the principles of the instant invention and its practical 
application to thereby enable others skilled in the art to best utilize 
the instant invention in various embodiments and with various 
modifications as are suited to the particular use contemplated. 
It is, therefore, to be understood that the claims appended hereto are 
intended to cover all such modifications and variations which fall within 
the true scope and spirit of the invention.