Device having a camera tube

A device having a camera tube of the vidicon type. During flybacks of the deflection, the beam current and the cathode potential is increased so as to achieve the so-called anti-comet tail effect. In order to ensure that the beam lands on the radiation sensitive layer perpendicularly everywhere during flybacks, as well as during scanning of the target, the electron beam is focused at the deflection point of the deflection coils during the flyback.

The invention relates to a device having a camera tube of the vidicon type 
comprising an electron gun for generating an electron beam, deflection 
coils for deflecting the electron beam over a radiation-sensitive layer, 
means for focusing the electron beam and means for directing the deflected 
electron beam perpendicularly onto the radiation-sensitive layer. In such 
a device the cathode potential of the electron gun during flybacks of the 
deflection is increased with respect to the radiation-sensitive layer 
while simultaneously increasing the current strength of the electron beam. 
Such a device is disclosed in U.S. Pat. No. 3,548,250. 
In a vidicon, a radiation-sensitive layer, for example a photoconductive 
layer, or a pyroelectric layer, is scanned by means of a beam of slow 
electrons. The electron beam should impinge perpendicularly on the 
radiation-sensitive layer. For that purpose, a vidicon is provided with an 
electron lens, e.g. a collimation lens which directs the deflected beam 
parallel to the axis of the tube. This electron lens generally comprises, 
inter alia, a gauze-like electrode immediately in front of the 
radiation-sensitive layer. 
The above U.S. patent states that the potential differences on the 
radiation-sensitive layer which result from the image projected thereon, 
are always reduced again to the cathode potential by the scanning electron 
beam. 
Sometimes, however, these potential differences are so large--due to local 
excessive exposure to light--that the electron beam is not capable of 
doing this. Moving dots of high light intensity may then produce so-called 
comet tails in the displayed picture. According to the U.S. patent this 
disadvantage is avoided by reducing excessively large potential 
differences on the radiation-sensitive layer to an acceptable level by the 
electron beam during flybacks of the deflection. Regions having potential 
differences which are not too large are not affected. This is done by 
increasing the cathode potential by, for example, 5 Volts, so that all 
potential differences above 5 V are reduced to 5 V. This is done with a 
greatly increased beam current and preferably a defocused beam. Since this 
occurs during flybacks, the picture signal is not influenced. 
It has been found to be of great importance to reduce the too large 
potential differences to an accurately determined level. Variations of 
this level prove to be visible in the displayed picture. Moving dots of 
high light intensity, for example, do show comet tails in some parts of 
the picture and do not show these in other parts. The occurrence and 
disappearance of the comet tails in the displayed picture is annoying to 
the viewer. This disadvantage proves to occur of course in the device 
disclosed in the aforementioned United States patent. 
It is the object of the invention to provide a device of the kind mentioned 
in the preamble in which excessive potential differences are reduced to an 
accurately determined level. 
For that purpose, according to the invention, the electron beam is focused 
substantially in the deflection point of the deflection coils during 
flybacks. 
The invention is based on the following understanding. As already stated, 
the electron beam, during flybacks, should preferably be strongly 
defocused at the area of the radiation-sensitive layer. This is done by 
focusing it at a different point. By choosing for this point the 
deflection point of the deflection coils, not only does the electron beam 
land perpendicularly on the radiation-sensitive layer during the sweeps, 
but also does this during the flybacks, because then all electrons of the 
defocused beam pass through the deflection point, and the collimation lens 
is constructed so that the electron paths are directed parallel to the 
axis of the tube and hence perpendicularly to the radiation-sensitive 
layer.

The tube shown in the FIGURE comprises a glass envelope 1 having a face 
plate 2. The face plate 2 is provided with a photoconductive layer 3 on a 
transparent conductive signal plate 15. The photoconductive layer 3 is 
scanned by an electron beam which is generated by an electron gun 4 and is 
deflected by deflection coils. 5. The electron beam is focused by a 
focusing lens comprising the electrodes 6, 7 and 8 and, also when it is 
deflected, impinges on the photoconductive layer 3 perpendicularly under 
the influence of the collimation lens comprising the electrode 9 on the 
inner wall of the tube and the gauze-like electrode 10 immediately in 
front of the photoconductive layer 3. See, for example, the deflected 
electron beam 16. 
During the sweeps of the deflection which generally occurs according to the 
known television frame, the electron beam is focused on the layer 3 to a 
spot which is as small as possible. This beam is denoted by 11. As stated 
previously, the electron beam is strongly defocused at the area of the 
layer 3 during the flybacks, in which the potential of the cathode 12 of 
the electron gun is also increased, for example, by 5 V and the beam 
current is strongly increased. Until now the defocusing has been carried 
out by focusing the electron beam at a point immediately adjacent the 
electron gun, as appears from U.S. Pat. No. 3,548,250. According to the 
invention, however, the electron beam is focused by the focusing lens 
(6,7,8) at the deflection point 13 of the deflection coils 5. The 
defocused beam is shown at 14. Although the electron beam 14 impinges on 
the layer 3 with a wide spot, all electrons nevertheless originate from 
the deflection point 13 and land on the layer 3 perpendicularly due to the 
influence of the collimation lens 9, 10.