/ - 'rection cannot be Carried out. Accordingly, as one
DARK CURRENT- CORRECTION CIRCUIT IN- . countermeasure, the target voltage of the camera tube
TWO-TUBE COLOR TELEVISION CAMERA' is lowered at the time of a temperature rise thereby to
DArtrDnnvn Tuc ... suppress the increase in the dark current, and, at the
BACKGROUND .OF THE INVENTION 5 same time, to suppress the increase in the shading.
This invention relates generally to a dark current In a two-tube color camera, however, if the above
correction circuit in a two-tube type color television mentioned target voltage control is carried out in only
camera. More particularly, the invention relates to a the camera tube for chrominance signals, there will
dark current correction circuit which, in a color'televi- arise the problem of difference between the sensitivi
sion camera of two-tube type having a camera tube for 10 ties of the two camera tubes for chrominance signals
chrominance signals and a camera tube for luminance and for luminance signals,
signals, eliminates the effect of dark current variations ...
due to temperature variations due to temperature vari- SUMMARY OF THE INVENT ION
ations of the two camera tubes. Accordingly, it is a general object of the present
In general, camera tubes of the vidicon class such as 15 invention to provide a novel and useful dark current
vidicons, Plumbicons, and Newbieons, are accompa- correction circuit in a two-tube color television camera
nied by the problems of much dark current and large in which the above described difficulties and problems
variation of dark current occurring particularly when have been overcome.
there are temperature variations. A specific object of the invention is to provide a dark In a color television camera, if the setup (black level 20 current correction circuit in a two-tube color camera as a reference) of a chrominance signal fluctuates, the having a camera tube for luminance signals and a camWhite balance will be disturbed, and therefore it is era tube for chrominance signals, which dark current necessary to prevent such fluctuation of the setup. correction system, while having a dark current detecWhen there is a temperature variation, the dark current tion circuit and a dark current compensation circuit in of the camera tube also varies, and for this reason, the 25 the chrominance signals system of the camera, is prosetup fluctuates; Accordingly, for the purpose of sub- vided in the luminance signal system with a dark curstantially preventing variation of the setup even when rent correction circuit of a novel and simple circuit the dark current of the camera tube varies, a dark organization which does not rely on the conventional current detection circuit and dark current compensa- dark current detection circuit and dark current comtion circuit have heretofore been used. 30 pensation circuit.
In such a circuit organization, a masking plate is Another object of the invention is to provide a dark
placed at one part of the image pickup surface of the current correction circuit in a two-tube color camera
camera tube, and the dark current variation is compen- which system is capable of correcting dark current
sated or corrected for by subjecting the signal of that variation accompanying variation of temperature of the
part corresponding to this plate (that is, the dark cur- 35 camera tubes and, moreover, is capable of equalizing
rent part) to sampling by the dark current detection the sensitivities of the camera tube for chrominance
circuit and subjecting this to subtraction with the image signals and the camera tube for luminance signals,
pickup output signal by means of the dark current Further objects and features of the invention will be
compensation circuit. apparent from the following detailed description when
In the case of a monochrome (black-and-white) tele- 40 read in conjunction with the accompanying drawing, vision camera, the problem of the above described DDti:r ... no TMc nwiimwr' adverse effect due to fluctuation of the setup does not BRIEF DESCRIPTION OF THE DRAWING arise, but in a color television camera, the above de- In the drawing, the single FIGURE is a block diagram scribed countermeasure becomes necessary. In the showing the essential organization of one embodiment case of a two-tube type color camera, it is necessary to 45 of a dark current correction circuit according to the provide the above mentioned dark current detection present invention in a two-tube color television camcircuit and dark current compensation circuit in the era.
signal system of the camera tube for chrominance sig- Hptaii en npsrRTPTinw
nals. For this reason, there arises the necessity of set- ut 1 A1LbL» UE5.CKlf l ION
ting the black level also with respect to the signals of 50 In the circuit illustrated in the FIGURE, light from an
the camera tube for luminance signals. object 11 to be picked up by the camera passes through
Accordingly, the solution would seem to be the pro- a camera lens system 12 and a half mirror (or prism)
vision of a dark current detection circuit, a dark cur- for optical path division. One portion of the light thus
rent Compensation circuit, etc., similar to those of the divided passes through a stripe filter 14 and reaches the chrominance signal system, also for the signal system of 55 image pickup surface of a camera tube 15 for chromi
the camera tube for luminance signals. This measure, nance signals, while the remainder portion of the light
however, would give rise to a complication of the cir- is projected on the image pickup surface of a camera
cuit organization and an increase in the cost of produc- tube 16 for luminance signals. A masking plate 17 for
tion of the camera. :; dark current detection is provided at the front face of
On one hand, when there is a temperature rise in the 60 the pickup surface of the camera tube 15.
image pickup surface of a camerd tube, the dark cur- A target voltage is supplied by way of a resistor 22 to
rent increases over the entire image pickup surface, the target of the camera tube 15 for chrominance sig
and, at the same time the static shading (that is, a con- nals from a target voltage supply circuit 21 comprising
dition wherein the'dark'current is not'uniform over the a resistor 18, a thermistorl9, and a variable resistor 20.
entire image pickup surface but is large particularly in 65 The thermistor 19 is a heat-sensitive resistance element
the end parts) increases. For this reason, the dark cur- of which resistance value varies greatly with variation
rent detection circuit detects'also the resulting increase in temperature, and which is provided in the vicinity of
in this shading, thereby an accurate dark current Cor- the face plate of the camera tube 15, for example, at a