Dark current compensating lens iris control

The iris of the lens of a closed circuit television camera is controlled to maintain an approximately constant magnitude video signal irrespective of dark current generated by the camera tube. The television camera includes an optically black filter covering the initial horizontal scans of the camera. A dark current sample is taken for each video field during a horizontal scan behind the optically black filter. A compensated video signal is generated by substituting the dark current sample for blanking portions of the video signal. The compensated video signal and the dark current sample are passed to a differential integrator the output of which is used to drive a motor which automatically controls the iris of the camera lens to maintain a selected magnitude video signal. Manual control of the iris can be selected by the camera operator; however, a protection circuit couples the automatic iris control to partially close the lens iris if lighting conditions could potentially damage the camera tube while the iris control is in the manual mode.

BACKGROUND OF THE INVENTION 
The present invention relates to closed circuit television cameras 
utilizing silicon diode arrays and, more particularly, to the automatic 
control of the lens iris of such television cameras to maintain an 
approximately constant magnitude video signal regardless of fluctuations 
in the dark current level of the array. 
Silicon diode arrays are commonly used in closed circuit television 
cameras. In such arrays, a charge density pattern is formed by 
photoconduction in response to incident light on the array. The charge 
density pattern which is effectively stored on the surface of the silicon 
is raster scanned by an electron beam to produce an output signal. The 
resulting output signal comprises a video current component proportional 
to the intensity of light illuminating the array and also a leakage 
current component commonly referred to as the dark current of the array. 
The leakage current component is referred to as the dark current since it 
is produced even in the absence of light. For a silicon diode array such 
leakage or dark current approximately doubles for each 10.degree. C. 
increase in the temperature of the array. 
In typical closed circuit television applicatons, e.g., security 
monitoring, it is desirable to have the cameras automatically maintain 
video signal levels of a sufficient amplitude to allow effective 
monitoring. Such video signal levels can be insured by utilizing the video 
signal to control the iris of the closed circuit television cameras. Such 
iris control maintains the video signal amplitude over varying lighting 
conditions for the areas to be observed. For example, poorly lighted areas 
require a more open iris setting than brightly lighted areas. By 
monitoring the magnitude of the video signal generated by the closed 
circuit television camera, the iris can be effectively controlled. 
For a required magnitude of video signal, a first iris setting is required 
in a dimly lit area. When additional light is provided to that area, such 
as by increases in natural sunlighting, the iris must close down to 
maintain the same video amplitude and prevent the monitored picture from 
becoming washed out, thus reducing the effectiveness of the monitored 
picture. Conversely, of course, the iris must open up when the lighting to 
brightly lit areas is reduced. Such iris control is effective in 
controlled environments, such as internal monitoring for a security system 
in an apartment or commercial building. However, when closed circuit 
television cameras are mounted in harsh environments, such as to monitor 
outdoor locations or various industrial processes where temperature 
variations can cause drastic changes in the dark current level of the 
signal generated by the camera, alternate measures must be taken. 
In the past, iris control in such hostile environments has included 
adjusting the video level acceptable for iris control when elevated 
temperatures of the camera are expected; manual operator control of the 
iris of the camera; or building an artificial environment for the camera 
where suitable temperatures can be maintained. Such prior art measures 
tend to make operation of the system inconvenient, complex or, in the case 
of artificial environments, overly expensive. 
SUMMARY OF THE INVENTION 
The shortcomings of the prior art are overcome in accordance with the 
present invention by controlling the lens iris of a closed circuit 
television camera with a video signal which is processed to compensate for 
varying dark current levels generated by the camera tube. The television 
camera is fitted with an optically black filter covering portions of the 
camera tube corresponding to selected horizontal beam scans of the camera. 
The video signal from the television camera is sampled during a horizontal 
beam scan which occurs under the optically black filter for generating a 
signal representative of the dark current. Horizontal and vertical 
blanking portions of the video signal are replaced with the sampled dark 
current to generate a compensated video signal. The compensated video 
signal is then utilized to automatically control the iris of the 
television camera lens. The circuitry for controlling the iris of the 
television camera further comprises circuitry for selectively setting the 
magnitude of the video signal maintained by the automatic iris control 
circuit. 
In accordance with one feature of the present invention, automatic iris 
control can be overridden by manual controls operated at the monitor of 
the associated television camera. When the iris control circuit is in the 
manual mode, camera protection is provided by coupling the automatic iris 
control circuit to partially close the lens iris for defined elevated 
video signal levels. 
In the illustrative embodiment of the invention, the automatic iris control 
comprises a differential integrator responsive to both the dark current 
sample and the compensated video signal to control the lens iris so that 
the video signal generated by the camera is maintained at a selected 
level. 
It is an object of the present invention to provide an improved lens iris 
control circuit which automatically compensates for variations in dark 
current; to provide an improved lens iris control circuit which will 
automatically control the lens iris irrespective of dark current levels 
generated by the associated camera tube and also allow manual control of 
the lens iris; and to provide an iris control circuit which automatically 
compensates for dark current generated by the associated camera tube, 
permits selective manual control of the lens iris and protects the camera 
tube from potentially damaging overexposure by intercoupling the automatic 
and manual controls. 
These as well as other objects and advantages will become more apparent 
from the detailed description of the invention when read with reference to 
the drawings and the appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a closed circuit television camera 100 having a lens system 
102 including a motor driven iris which receives signals on the conductors 
104 to control the iris opening. The camera 100 generates video signals on 
the conductor 106. The video signals on the conductor 106 are passed to 
the black current compensating iris control circuit 108 which generates 
iris control signals on the conductors 104 to control the iris opening of 
the lens 102. 
FIG. 2 illustrates an optically black filter 200 which prevents light from 
striking a selected portion of the camera tube face. Preferably, the 
filter is formed on the face of the camera tube or incorporated into the 
faceplate of the tube; however, any light blocking arrangement is 
satisfactory in accordance with the present invention. The optically black 
filter 200 permits a dark current sample to be taken by the iris control 
circuit 108. 
The basic theory of operation of the present invention is to generate a 
dark current compensated video signal which can be used to control the 
iris opening of the lens 102 of a closed circuit television camera 100 to 
maintain the generation of an approximately constant amplitude video 
signal by the television camera. A portion of the light input to the 
television camera is blocked by the optically black filter 200 shown in 
FIG. 2 so that a dark current sample may be taken on horizontal scans of 
the camera 100 occurring behind the filter. For each field of video, a 
dark current sample is taken and maintained in a storage device during 
that field. 
The video signal is amplified and the horizontal and vertical blanking 
portions of the amplified video signal are replaced with the dark current 
sample to generate a dark current compensated or clamped video signal, 
i.e., the video signal superimposed on the dark current generated by the 
camera tube. The dark current sample is then effectively subtracted from 
the dark current compensated video signal by means of a differential 
integrator. The difference between the two signals is integrated to 
generate a signal representative of the magnitude of the video information 
signal generated by the camera 100. 
For automatic iris control, the integrated signal is passed to a first 
operational amplifier, the output of which is connected to a second 
operational amplifier. The motor which controls the iris of the lens 102 
is connected between the outputs of the first and second amplifiers. The 
first and second amplifiers are also connected to a reference potential 
and interconnected so that a positive change on the input of the first 
amplifier causes a positive change in its output signal and a negative 
change in the output signal of the second amplifier. Conversely, a 
negative change on the input of the first amplifier causes a negative 
change in its output and a positive change in the output of the second 
amplifier. 
As long as the integrated output signal is equal to the reference voltage 
connected to the two operational amplifiers, the iris setting remains 
constant. If the video information signal decreases in amplitude, the 
output signal of the first operational amplifier increases and the output 
signal of the second operational amplifier decreases. These signals are 
applied to the motor which controls the iris of the lens 102 and are poled 
to open the iris thus admitting more light and increasing the magnitude of 
the video information signal to restore the system to equilibrium and 
remove the iris "move" signals from the outputs of the first and second 
operational amplifiers. Conversely, if the video information signal 
increases in amplitude, the output signal of the first operational 
amplifier decreases and the output signal of the second operational 
amplifier increases to close the iris of the lens 102 until equilibrium is 
once again restored. 
For manual control, a switch which passes the integrated video information 
signal to the first operational amplifier is opened and the inputs to the 
first operational amplifier are controlled by manual iris open and close 
signals generated by the operator of the closed circuit television 
monitoring system. A by-pass circuit is connected around the open switch 
from the integrated video information signal to the input of the first 
operational amplifier to protect the camera tube. In the event that the 
integrated video information signal becomes large relative to the 
reference voltage applied to the operational amplifiers, the lens iris is 
partially closed to prevent damage to the camera tube. 
Reference is now made to FIGS. 3A and 3B which together form a detailed 
schematic diagram of the dark current compensating iris control circuit 
108 of FIG. 1. The video signal from the camera 100 is received on the 
conductor 106 and amplified by operational amplifier 201 which is 
connected as a unity gain amplifier and serves as an impedance converting 
butter to isolate the video signal on the conductor 106 from the remainder 
of the iris control circuit. The capacitor 202 together with the switch 
204 and light emitting diode (LED) 206 form a clamp circuit which 
re-establishes the direct current (DC) level of the video signal. The 
switch 204 is operated by the horizontal drive signal received on the 
conductor 207. 
The light emitting diode 206 establishes a positive voltage of 
approximately 1.5 volts on the input to an operational amplifier 208 to 
insure that the operational amplifier 208 does not operate with ground 
potential on its input. The operational amplifier 208 serves as a buffer 
to isolate the clamp circuit from the remainder of the iris control 
circuit. The output of the operational amplifier 208 is the video input 
signal on the conductor 106 with a base DC level of approximately 1.5 
volts. 
To remove the effects of the dark current from the signal on the output of 
the operational amplifier 208, a dark current sample is taken during each 
video field. The sample is taken during the time that the silicon diode 
array is being scanned at a location behind the filter 200 of FIG. 2. The 
filter 200 covers approximately the first 12 horizontal scan lines of the 
camera tube. A sample of the dark current is obtained during the seventh 
horizontal scan of the silicon diode array. The sample is taken by an 
eight-bit shift register 210 which is clocked by the horizontal drive 
signal received on conductor 207 and reset by the vertical drive signal 
received on conductor 211. 
The input signal to the shift register 210 is held high so that a high 
signal or logical "one" is sequentially shifted into the individual stages 
of the shift register 210. The output signal of the eighth stage of the 
shift register is inverted by a NAND gate 212 and combined with the output 
signal of the seventh stage of the shift register by a NAND gate 214. The 
output of the NAND gate 214 is inverted by a NAND gate 216 to drive the 
sampling switch 218. Thus, the shift register 210 is cleared by each 
vertical drive signal and the horizontal drive signals clock the shift 
register to shift a logical one sequentially into the eight stages. Once 
the shift register 210 is filled by the action of the first eight 
horizontal drive pulses, it remains full until cleared by the next 
vertical drive pulse. 
The arrival of the high signal at the seventh stage of the shift register 
210 activates or closes the switch 218 via the NAND gates 214, 216 and the 
arrival of the high signal at the output of the eighth stage of the shift 
register deactivates the switch 218 via the NAND gates 212, 214 and 216. 
The resistor 220 together with the capacitor 222 form a sample and hold 
circuit with a large charging time constant so that intermittent overloads 
of the target surface will not appreciably change the charge on the 
capacitor 222. A Schottky diode 224 and resistor 226 form a fast discharge 
network for the capacitor 222 for overloads which persist for a sufficient 
period of time to change the charge on the capacitor 222. 
A dark current compensated video signal is generated at the output of 
operational amplifier 228 by replacing the horizontal and vertical 
blanking portions of the video signal received from the operational 
amplifier 208 with the sampled dark current stored on the capacitor 222. 
Switches 230 and 232 are driven by a combined blanking signal, i.e., the 
combination of the horizontal and vertical blanking signals, on the 
conductor 233 and the inverse of the combined blanking signal from the 
NAND gate 234, respectively. Thus, during periods of active video 
information, the operational amplifier 228 receives the video signal from 
the operational amplifier 208 through the switch 232. During periods of 
horizontal and vertical blanking, the signal from the operational 
amplifier 208 is interrupted by opening the switch 232 and the sampled 
dark current stored on the capacitor 222 is passed by a unity gain buffer 
amplifier 236 through the switch 230 to the input of the operational 
amplifier 228. Accordingly, the signal on the output of the operational 
amplifier 228 comprises the video signal information imposed on a DC 
voltage level which is equal to the dark current generated by the silicon 
diode array of the television camera tube. 
The dark current compensated video signal on the output of the operational 
amplifier 228 is differentially integrated to control the iris of the lens 
102 of the camera 100. The signal from the operational amplifier 228 is 
passed via conductor 238 to the negative input of an operational amplifier 
240, while the dark current signal on the output of the operational 
amplifier 236 is passed via conductor 241 to the positive input of the 
operational amplifier 240. The operational amplifier 240 is connected as a 
differential integrator so that the signal on its output is effectively 
equal to the integrated value of the difference between the compensated 
video signal on the output of the operational amplifier 228 and the dark 
current signal on the output of the operational amplifier 236. 
Accordingly, the operational amplifier 240 generates a video level signal. 
The resistor network 242, connected to the positive input of the 
operational amplifier 240, can be adjusted to set the video level 
maintained by the iris control circuit 108. 
For automatic control of the iris setting of the lens 102, the output 
signal of the operational amplifier 240 is connected to the positive input 
of operational amplifier 246 through a switch 248. Since the compensated 
video signal from the operational amplifier 228 is received on the 
negative input of the operational amplifier 240, increases in the 
magnitude of the video signal produce decreases in the integrated output 
signal of the operational amplifier 240; and decreases in the magnitude of 
the video signal produce corresponding increases in the integrated output 
signal of the operational amplifier 240. 
The output of the operational amplifier 246 is connected to the "open" 
terminal 247 of the motor 249 which controls the iris of the lens 102 and 
also to the negative input of an operational amplifier 250. The output of 
the operational amplifier 250 is connected to the "close" terminal 251 of 
the motor 249 which controls the iris of the lens 102. The negative input 
of the operational amplifier 246 and the positive input of the operational 
amplifier 250 are connected to a reference voltage generated by a 
resistance network 252. The motor 249 opens the iris of the lens 102 if 
the open terminal 247 is positive relative to the close terminal 251 and 
closes the iris if the close terminal 251 is positive relative to the open 
terminal 247. 
As long as the value of the integrated signal from the operational 
amplifier 240 is equal to the reference voltage applied to the operational 
amplifiers 246, 250, the motor 249 controlling the iris is not activated 
and the iris setting does not change. If the video signal level increases 
resulting in a decrease in the integrated output of the operational 
amplifier 240, the output signal of the operational amplifier 246 is 
decreased which causes an increase in the output signal of the operational 
amplifier 250 to apply a voltage which operates the motor 249 to close the 
iris. The iris closes until the video signal amplitude is reduced to a 
point where the integrated output of the operational amplifier 240 is once 
again equal to the reference voltage applied to the operational amplifiers 
246, 250 so that the iris setting is once again stable. 
Conversely, if the magnitude of the video signal decreases which results in 
an increase in the integrated output signal of the operational amplifier 
240, the operational amplifier 246 has an increase in output signal level, 
while the operational amplifier 250 has a decrease in output signal level. 
This voltage polarity operates the motor 249 to open the iris of the lens 
102. The iris is opened until the video signal level increases to a point 
where the output of the operational amplifier 240 is once again equal to 
the reference voltage applied to the operational amplifiers 246, 250. In 
this manner, the iris setting of the lens 102 is automatically adjusted to 
maintain a magnitude of video signals determined by the setting of the 
resistance network 242. 
The iris can also be controlled manually by operating switch 253 to apply a 
ground signal on auto/manual input terminal 254. In the manual mode the 
switch 248 is opened and manual control switches 256, 258 are closed via 
switch 259. The opening of switch 248 removes the output signal of the 
operational amplifier 240 from the positive input of the operational 
amplifier 246. However, the switch 248 is shunted by a protection circuit 
comprising a series combination of a resistor 260, a diode 262 and a zener 
diode 264. This shunt path provides protection of the television camera 
tube while the iris control circuit is in the manual mode as will be 
described hereafter. 
When the iris control circuit is in the manual mode, the switches 256 and 
258 connect PNP transistors 266, 268 to the positive and negative inputs 
of the operational amplifier 246, respectively. A switch 270 is provided 
to connect ground to either the "close" terminal 272 or the "open" 
terminal 274 to manually control the operation of the iris motor 249 to 
respectively close or open the iris of the lens 102. The switch 270 is 
biased to return to a neutral position, as shown in FIG. 3, when not 
activated by the operator of the monitoring system utilizing the iris 
control circuit. 
The protection circuit comprising the resistor 260, the diode 262 and the 
zener diode 264 protects the camera tube from damage in the event that an 
operator leaves the iris control circuit in the manual mode with the iris 
so fully opened that the incoming light may damage the tube. In that 
event, the protection circuit partially closes the iris if lighting 
conditions are such that the camera tube could be damaged. The integrated 
signal from the operational amplifier 240 is coupled to the positive input 
of the operational amplifier 246 through the protection circuit. If the 
integrated output of the operational amplifier 240 exceeds the reference 
voltage by the sum of the zener voltage of the zener diode 264 and the 
forward voltage of the diode 262, the iris will be partially closed by the 
protection circuit coupling to the automatic portion of the iris control 
circuit. If the iris control circuit is then switched to the automatic 
mode by opening switch 253, the protection circuit is by-passed by the 
operation of the switch 248 and the iris is closed to the full extent 
provided by the automatic operation as previously described. 
While a large variety of circuit components are available and may be 
utilized in the illustrative embodiment of the present invention, typical 
commercially available devices are listed below for the major circuit 
elements. 
______________________________________ 
Operational amplifiers 
CA3160, available from 
201, 208, 228, 236 
Radio Corporation of 
and 240 America 
Operational amplifiers 
.mu.A759, available from 
246 and 250 Fairchild Corporation 
Switches 204, 218, 230 
MC14066B, available from 
232, 248, 256, 258 
Motorola Corporation 
and 259 
Shift Register 210 
MC14015B, available from 
Motorola Corporation 
______________________________________ 
While the form of apparatus herein described constitutes a preferred 
embodiment of the invention, it is to be understood that the invention is 
not limited to this precise form of apparatus and that changes may be made 
therein without departing from the true spirit and scope of the invention 
which is defined in the appended claims.