US 3558809 A
Description (OCR text may contain errors)
United States Patent  Inventor Takao Aoki Kanagawa-ken, Japan  Appl. No. 748,714  Filed July 30, 1968  Patented Jan. 26, 1971  Assignee Sony Corporation Tokyo, Japan a corporation of Japan  Priority Aug. 1, 1967 J p  v 42/49445  AUTOMATIC DARK CURRENT CONTROL SYSTEM FOR PICKUP TUBES EMPLOYIN G A LIGHT INHIBITING STRIP MOUNTED ON THE PICKUP TUBE FACE PLATE 4 Claims, 4 Drawing Figs.
 US. Cl 178/5.4, r 178/72  Int. Cl H04n 5/44, H04n 5/3 8  FieldofSearch l78/7.2E, 7.2A,5.4;315/l0  References Cited UNITED STATES PATENTS 3,102,924 8/1963 Legler l78/7.2E 3,126,447 3/1964 Bendell 178/7.2A 3,206,547 8/1965 Leitich et a1. l78/7.2E 3,407,267 10/1968 Smith l78/6.8
Primary Examiner-Robert L. Grifiin Assistant Examiner-Donald E. Stout Attorneys-Albert C. Johnston, Robert E. lsner, Lewis H.
Eslinger and Alvin Sinderbrand ABSTRACT: An automatic dark current control system for a pickup tube including an opaque region provided on the pickup tube face plate, means for detecting an output signal corresponding to the opaque region, and means for controlling a target voltage with the detected signal.
C DE'FL PCT/0N AUTOMATIC DARK CURRENT CONTROL SYSTEM FOR PICKUP TUBES EMPLOYING A LIGHT INHIBITING STRIP MOUNTED ON THE PICKUP TUBE FACE PLATE This invention relates to an automatic dark current control system for pickup tubes, and more particularly to a system for holding a dark current of a pickup tube substantially constant at all times with purely electronic and simple circuit means.
Conventional television cameras encounter a difficulty in the regulation of the dark current of the pickup tube. Especially, color television cameras employing a plurality of pickup tubes has a drawback such that the white balance of signals is lost unless the dark currents of the pickup tubes are always controlled to be constant. The dark current greatly varice with the temperature of the pickup tube and hence does not become constant until the temperature of the pickup tube reaches a predetermined value after the initiation of its operation. In addition to this, the dark currents of the pickup tubes vary in different manners, so that even if the white balance is maintained in the steady state of the pickup tube with its temperature being held constant, it is difiicult to obtain complete white balance until the pickup tube reaches the steady state after the start of its operation, Further, there is the possibility that the dark current varies due to the variations in the ambient temperature to destroy the white balance.
In view of such a fact, this invention has for its object the elimination of the aforementioned defect by providing a lighttransmission inhibiting region on the target of the pickup tube at one selected area, detecting the level of the portion of the video output signal corresponding to the light-transmission inhibiting region and controlling the target voltage to hold the level substantially constant with the detected output.
Accordingly, it is one object of this invention to provide a system for controlling the dark current of a pickup tube to be constant.
It is another object of this invention to provide a color television camera of excellent white balance.
It is still another object of this invention to provide a color television camera in which the dark currents of a plurality of pickup tubes are uniform to provide for good white balance.
Other objects, features and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic diagram illustrating a conventional type of color television camera device;
FIG. 2 is a block diagram showing one example of an automatic dark current control system for a pickup tube produced according to this invention;
FIG. 3 is a plan view illustrating, by way of example, the front of the pickup tube exemplified in FIG. 2; and
FIG. 4 is a waveform diagram showing one example of a video output signal.
In FIG. 1 there is illustrated a prior art color television camera of the type employing three pickup tubes. With such a color television camera, the light from an object 1 to be televised is separated or broken up into red, green and blue components through an optical system 2 and these color components are applied toglass faceplates 4R, 4G and 4B of pickup tubes 3R, 3G and 38, through which the object 1 is focused into an image on targets 5R, 5G and 5B of the pickup tubes 3R, 3G and 38. Video signals emanating from the targets SR, 56 and 5B are respectively fed to output terminals 7R, 7G and 78 through video amplifiers 6R, 6G and 6B. In this case, the pickup tubes 3R, 3G and 38 may be, for example, vidicon tubes.
However, where the quantity of the incident light is constant, the video signals obtained at the output terminals 7R, 7G and 7B are caused to vary by dark currents of the pickup tubes 3R, 3G and 38 (output currents of the pickup tubes appearing in the case of no light being directed to the glass faceplates 4R, 4G and 4B) and this leads to the lowering of the white balance. Further, the dark currents of the pickup tubes vary in different manners to render the outputs of the pickup tubes different from one another and hence provide lowered white balance.
FIGS. 2 to 4 illustrate an automatic dark current control system of this invention for the elimination of the drawbacks experienced in the prior art.
In FIG. 2 a video signal 9 obtained from a target 5 of an image pickup tube 3 is applied to an output terminal 7 through a video amplifier 6. A deflection unit 11 of the pickup tube 3 is supplied with horizontal and vertical deflecting signals from a deflection circuit 12.
In accordance with this invention a light-transmission inhibiting region or opaque region is formed on the target 5 at one selected area. It is preferred to located the opaque region at a position corresponding to the ineffective video signal period following the blanking signal period, of the video signal. For this purpose, a mask is disposed on the effective area 4a of the glass faceplate 4 at one end thereof relative to the horizontal scanning direction in a manner to extend in the vertical scanning direction as illustrated in FIG. 3, thus providing the light-transmission inhibiting region 8.
Further, one portion of the output of the video amplifier 6 is applied to a detector circuit 10 to detect that portion of the video signal which corresponds to the light-transmission inhibiting region 8. In other words, a dark current is detected. The detector circuit 10 may take that form of, for example, a sampling circuit and is supplied with pulses obtained from the horizontal synchronizing signal by the deflection circuit 12 at times corresponding to the light-transmission inhibiting region 8.
The dark current thus obtained is fed to a comparator circuit 13, which compares a reference voltage E from a reference voltage source with a dark current level E The output of the comparator circuit 13 is applied to a target voltage control circuit 14, which controls the target voltage by feedback to maintain the dark current level of the pickup tube 3 substantially constant.
With such an arrangement as described in the foregoing, there is detected from the target 5 a video output signal such as depicted in FIG. 4 which contains a horizontal flyback period T a dark current period T corresponding to the lighttransmission inhibiting region 8 and a video signal period T in such an order as shown. The signal of the dark current period T,,, that is, the dark current, is detected from the detector circuit l0, and if the dark current level E is, for example, greater than-the reference voltage E, the control circuit 14 is controlled by the output of the comparator circuit 13 to cause a decrease in the target voltage. In this manner, the dark current is held substantially constant.
Namely, in the case where the dark current varies due to the temperature change of the pickup tube 3 until the pickup tube reaches its steady state after the start of its operation or due to the change of the ambient temperature, the variation of the dark current is detected and the target voltage is controlled correspondingly, ensuring to maintain the dark current substantially constant. Consequently, if the amount of the light from the object to be transmitted is constant, a video output signal of constant black level can be obtained from the output terminal 7. With the present invention being applied to the color television camera device employing a plurality of pickup tubes as depicted in FIG. I, one adjustment of the white balance enables the avoidance of the deterioration of the white balance resulting from the temperature change of the pickup tubes. Further, the camera device can be rendered operative immediately upon turning on the power source switch. In addition, if the dark current period T is provided following the flyback period T,,, that is, the light-transmission inhibiting region 8 is located in the ineffective area of the picture, the region 8 does not exert any influence upon the reproduced picture. While the dark current period T,, is rendered to appear following the horizontal flyback period T it may be positioned subsequent to the vertical flyback period.
It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.
1. An automatic dark current control system for pickup tubes comprising means for providing a light-transmission inhibiting region on a target of the pickup tube at one selected area, means for detecting the level of the portion of a video output signal produced by scanning the light-transmission inhibiting region, and means for controlling the target voltage with the detected portion of the video output signal produced by scanning the light-inhibiting region to hold the said level substantially constant.
2. 2. An automatic dark current control system for pickup tubes as claimed in claim 1 including a deflection means of the pickup tube in which the detecting means includes a sampling circuit operated in synchronization with a deflection signal applied to the deflection means.
3. An automatic dark current control system for pickup tubes as claimed in claim 1 in which the light-transmission inhibiting region is scanned during an ineffective video signal period.
4. An automatic dark current control system for pickup tubes as claimed in claim 1 including a plurality of pickup tubes respectively for the different color images.