Television camera tube with electrostatic focusing and magnetic deflection

A television camera tube with electrostatic focusing and magnetic deflection comprises in an cylindrical envelope a beam current control, a main lens section and a sixth grid in the form of a mesh electrode. The beam current control section includes a cathode, a first grid and a second grid with an electron beam limiting diaphragm in this order. The main lens section includes third, fourth and fifth grids in the form of cylindrical electrodes disposed in this order. Around the cylindrical envelope is mounted a magnetic deflection coil whose length along the envelope axis is 0.18 to 0.40 times the distance from the beam limiting diaphragm to the mesh electrode.

This invention relates to an improvement on a television camera tube with 
electrostatic focusing and magnetic deflection. 
First, the structure and the operation of a conventional television camera 
tube with electrostatic focusing and magnetic deflection will be briefly 
described. FIG. 1 shows in longtitudinal section the schematic structure 
of such a camera tube. 
In FIG. 1, reference numeral 1 is a cylindrical glass envelope. A 
photoconductive target 2 is provided at the front end of the envelope 1, a 
plurality of lead pins 3 are provided through the rear end of the envelope 
1, and high vacuum is maintained in the envelope 1. In this envelope are 
concentrically arranged various electrodes. A cathode 4 emits an electron 
beam and a first and a second grids 5 and 6 serve to control the electron 
current, the converging angle and the cross-sectional area or diameter of 
the electron beam. A small aperture (electron beam limiting diaphragm) 7 
is disposed at the side of the second grid 6 nearer to the photoconductive 
target 2 so as to provide a narrowly defined electron beam. The cathode 4, 
the first grid 5, the second grid 6 and the electron beam limiting 
aperture 7 constitute a beam current control section of the electron gun. 
Third, fourth (middle) and fifth grids 8, 9 and 10 are each in the form of 
a cylindrical electrode and these grids 8, 9 and 10 constitute a main lens 
(focusing lens) section which focuses the diverging electron beam through 
the aperture 7 of the second grid 6 from the beam current control onto the 
surface of the photoconductive target 2 with a small spot. A sixth grid 11 
in the form of a mesh electrode is interposed between the fifth grid 10 
and the photoconductive target 2. The fifth and sixth grids 10 and 11 make 
up a collimation lens for causing the electron beam to always hit the 
photoconductive target perpendicularly. An electromagnetic deflection coil 
12 for deflecting the electron beam is mounted around the envelope 1. In 
this camera tube having such a structure as described above, the electron 
beam emitted from the beam current control section is focused on the 
photoconductive target 2 through the combined function of the focusing 
lens section and the sixth grid or mesh electrode 11 while the beam is 
deflected by the electromagnetic deflection coil 12, whereby a video 
signal is obtained through the scanning of the target 2 by the beam. 
Namely, when an optical image is formed on the surface of the 
photoconductive target 2, there is developed a distribution of potential 
corresponding to the optical image on the surface. Upon incidence of the 
electron beam into the surface, the potential at every point of incidence 
is reduced to about zero volt. At this time, discharge current flows 
through the electrostatic capacitance of the target 2 and this current is 
taken out as a video signal. 
The main lens section and the electromagnetic deflection coil assembly of a 
typical example of a conventional camera tube such as, for example, a 2/3 
inch type camera tube have the following dimensions. The third grid 8 is a 
stepped cylindrical electrode which has interconnected lower and upper 
cylindrical portions 8a and 8b whose inner diameters are different. The 
length of the stepped cylindrical electrode is about 25.4 mm. The inner 
diameter of the lower cylindrical portion 8a is about 7.6 mm and that of 
the upper cylindrical portion 8b about 9.6 mm. The fourth grid 9 is a 
cylindrical electrode, about 12.0 mm long, with its inner diameter of 10.4 
mm. The fifth grid 10 is a stepped cylindrical electrode which has 
interconnected lower and upper cylindrical portions 10a and 10b whose 
inner diameters are different. The length of the stepped cylindrical 
electrode is about 24.4 mm. The inner diameter of the lower cylindrical 
portion 10a is about 11.6 mm and that of the upper cylindrical portion 
about 12.4 mm. The length l.sub.D of the deflection coil 12 in the 
direction of the axis of the envelope 1 (winding width) is about 28.0-30.0 
mm. The total length L of the main lens section, ranging from the electron 
beam limiting diaphragm 7 to the mesh electrode 11, is about 67 mm. The 
distance Z.sub.l from the diaphragm 7 to the middle point of the fourth 
grid 9 in its axial length (hereafter referred to as lens center distance) 
is about 34.4 mm. The distance Z.sub.D from the aperture 7 to the point 
where the magnetic deflection field assumes its maximum value in its 
distribution along the envelope axis (hereafter referred to as deflection 
center distance), is about 37.5 mm. This maximum value is reached at the 
middle point of the deflection coil 12 in the axial direction of the 
envelope. Thus, according to the conventional design, it is customary that 
the deflection center distance Z.sub.D is made equal to the lens center 
distance Z.sub.l or that the former distance Z.sub.D is slightly longer 
than the latter distance Z.sub.l. 
Voltages applied to these electrodes are as follows with the potential at 
the cathode 4 taken as 0 V:-150-0 V to the first grid 5; 200-500 V to the 
second grid 6; 500 V, 60-90 V, 300 V and 500 V respectively to the 
cylindrical electrodes 8, 9, 10 and the mesh electrode 11 for their 
low-voltage operation; 1400 V, 180-210 V, 770 V and 1400 V respectively to 
the cylindrical electrodes 8, 9, 10 and the mesh electrode 11 for their 
high-voltage operation; and 30-80 V to the photoconductive target 2. 
In general, a television camera tube with electrostatic focusing and 
magnetic deflection has an advantage over a television camera tube with 
magnetic focusing and magnetic deflection in that it is small in size, 
light in weight and consumes less electric power, but it also has 
drawbacks of relatively low resolution and of degraded resolution 
especially in the corners of the picture. 
The resolution power is one of the important factors which estimate the 
performance of a camera tube. The resolution of a camera tube depends 
closely on the diameter of the spot of the electron beam on the 
photoconductive target and the smaller is the spot diameter of the 
electron beam, the more improved is the resolution. However, the minimum 
diameter attainable of a focused electron beam depends on the distribution 
of initial velocities of electrons emitted from thermionic cathode (i.e. 
the initial-velocity spread of thermionic emission), the space charge 
effect and the spherical aberration of the focusing lens. Especially, the 
initial-velocity spread of thermionic emission and the spherical 
aberration of the main lens have predominant influence on the spot 
diameter of the beam in the central region of the screen or target. On the 
other hand, the spot diameter in the corners of the picture or the target 
is more affected by the third order geometrical aberration caused in 
deflecting the electron beam than by the previous factors. In order to 
attain a good resolution, therefore, it is necessary both to minimize the 
spread of the electron beam due to the initial-velocity spread of 
thermionic emission and the spherical aberration of the focusing lens to 
decrease the beam spot diameter in the central region of the image screen 
and to minimize the spread of the beam due to the third order geometrical 
aberration to decrease the beam spot diameter in the corner of the 
picture. In the case of an electro-optical system such as the 
electrostatic focusing and magnetic deflection camera tube, however, in 
which the lens region (focusing region) and the magnetic field region 
(deflection region) coexist, the mathematical treatment of the third order 
geometrical aberration is so difficult that the spread of the beam due to 
this aberration cannot be exactly estimated. Therefore, with the 
constitution of the conventional camera tube, the resolution in the 
corners of the picture is not necessarily optimal. 
It is therefore one object of this invention to provide a camera tube with 
electrostatic focusing and magnetic deflection in which the resolution in 
the corners of the picture can be improved without degrading the 
resolution in he central region of the picture. 
This invention has been made on the basis of the fact that the theory of 
the third order geometrical aberration came to be clarified as a result of 
the development of that theory in the electron optics of electrostatic 
focusing and magnetic deflection type. Namely, the relationships between 
various parameters for defining the structure of a camera tube and the 
third order geometrical aberration are calculated through computer 
simulations on the basis of the above theory and the optimal structure for 
a camera tube can be obtained from the above-derived value of the third 
order geometrical aberration. 
A first embodiment of this invention provides a camera tube with 
electrostatic focusing and magnetic deflection having a magnetic 
deflection coil whose length in the direction of the envelope axis is 
0.18-0.40 times the distance from the electron beam limiting diaphragm of 
the beam current control section to the mesh electrode. 
A second embodiment of this invention provides a camera tube with 
electrostatic focusing and magnetic deflection in which the magnetic 
deflection coil is so arranged in the envelope that the distance from the 
beam limiting diaphragm to the middle point of the length of the magnetic 
deflection coil along the envelope axis is greater than 0.56 times the 
distance between the diaphragm and the mesh electrode and smaller than or 
equal to 0.7 times the distance between the diaphragm and the mesh 
electrode. 
A third embodiment of this invention provides a camera tube with 
electrostatic focusing and magnetic deflection in which the distance from 
the electron beam limiting diaphragm to the middle point of the length 
along the envelope axis of the middle one of the three cylindrical 
electrodes is greater than or equal to 0.47 times the distance from the 
diaphragm to paid mesh electrode and smaller than 0.51 times the distance 
between the diaphragm and the mesh electrode. 
With the camera tube as described above, both the focusing lens region and 
the deflection magnetic field region are optimally arranged and therefore 
the spread of the electron beam due to the third order geometrical 
aberration can be suppressed to a great extent, whereby the resolution in 
the corners of the picture can be improved.

The present inventors have derived the third order geometrical aberration 
coefficients in an electron optics of electrostatic focusing and magnetic 
deflection type by further developing the theory of the third order 
geometrical aberration applied to the electron optics of electrostatic 
focusing and magnetic deflection type. Then, by the use of the thus 
derived coefficients the inventors have also calculated through computer 
simulations various third order geometrical aberrations depending on the 
parameters to properly determine the details of the focusing lens and the 
deflection magnetic field, such as, for example, the lengths and the 
diameters of the cylindrical electrodes constituting the focusing lens and 
the voltages to be applied to the electrodes. As a result, the inventors 
have found that among the various third order geometrical aberration 
coefficients the field curvature aberration coefficient has more dominant 
effect on the spread of electron beam due to the third order geometrical 
aberration than the astigmatism aberration coefficient, the coma aberation 
coefficient and the spherical aberration coefficient. 
Detailed description will be made below of the above-mentioned field 
curvature aberration coefficient K.sub.3 is given by the following 
expression (1), provided that it is expressed in the complex coordinate 
system where the envelope axis is taken as z axis, the horizonal 
deflection direction as real axis, and the vertical deflection direction 
as imaginary axis. 
##EQU1## 
In the expression (1), 
##EQU2## 
where z.sub.o : the z coordinate of the (axial point) object (the position 
of the electron beam limiting diaphragm 7), 
z.sub.i : the z coordinate of the image (the position of the mesh electrode 
11), 
a(z): the radius of the paraxial trajectory of an electron emitted with 
zero radius and an inclination of unity of z=z.sub.o, 
b(z): the radius of the paraxial trajectory of an electron emitted with a 
radius of unity and zero inclination, 
.phi.(z): the potential at an arbitrary point on the focusing lens along 
the z axis, 
D(z): the intensity of the horizontal or vertical deflection magnetic field 
at an arbitrary point along the z axis, 
e/m: the ratio of charge to mass of electron (absolute value), 
[']: prime indicating a differentiation with respect to z, and 
j: the imaginary unit. 
The close examination of the above expression (1) has revealed that the 
term including the coefficient a.sub.5 predominates over the other terms 
in the integrand. This means that the field curvature aberration 
coefficient K.sub.3 increases as the product (.phi."/.phi.)D of 
.phi."/.phi. indicating the intensity of the focusing electrostatic field 
and D indicating the intensity of the deflection magnetic field, 
increases. 
The above analysis of the third order geometrical aberration gives a 
conclusion that in order to suppress the spread of the electron beam due 
to the third order geometrical aberration and to improve the resolution in 
the corners of the picture, a camera tube should be fabricated in such a 
manner that the focusing lens region and the deflection magnetic field 
region are separated from each other by as great a distance as possible. 
To do this, there are the three following methods recommended in practice. 
(1) To decrease the length (winding width) of the deflection coil 12 along 
the envelope axis. 
(2) To increase the deflection center distance Z.sub.D. 
(3) To decrease the lens center distance Z.sub.l. 
These methods will now be described in detail respectively. In the 
succeeding description, the total length L of the main lens section, the 
values of the voltages to the respective electrodes and other associated 
conditions are assumed to be the same as in the conventional camera tube. 
First, the above method (1) will be explained. FIG. 2 shows the 
relationship, obtained through computer simulations, between the amount of 
the third order geometrical aberration and the ratio l.sub.D /L of the 
length l.sub.D of the deflection coil along the envelope axis to the total 
length L of the main lens section, when the deflection center distance 
Z.sub.D and the lens center distance Z.sub.l are set the same as in the 
conventional camera tube. In FIG. 2, a curve A-1 corresponds to the 
above-mentioned low voltage operation and a curve B-1 to the 
above-mentioned high voltage operation while cross marks X indicate the 
amounts of the third order geometrical aberration observed in the 
conventional example (l.sub.D /L.perspectiveto.0.42). It is seen from FIG. 
2 that as l.sub.D /L decreases, that is, as the length l.sub.D of the 
deflection coil along the envelope axis decreases, the degree of the third 
order geometrical aberration decreases. However, if the length l.sub.D is 
made too small while the electric constants (e.g. inductance and 
resistance) of the deflection coil are kept substantially constant, then 
the deflecting action of the deflection coil is adversely affected. 
Therefore, the lower limit of the value l.sub.D /L may be about 0.18. In 
this invention, the length l.sub.D is selected such that 
0.18.ltoreq.l.sub.D /L.ltoreq.0.40, so as to improve the third order 
geometrical aberration by more than 5% of that value of the conventional 
example. For example, if the length l.sub.D is reduced to 60% of that of 
the conventional example, that is, if l.sub.D /L is made equal to 0.23, 
then for the low voltage operation the amount of the third order 
geometrical aberration in this embodiment is decreased by 15% of that of 
the conventional example. 
Next, the second method will now be described. FIG. 3 shows the 
relationship, obtained through computer simulations, between the amount of 
the third order geometrical aberration and the ratio Z.sub.D /L of the 
deflection center distance Z.sub.D to the total length L of the main lens 
section when the distance l.sub.D of the deflection coil along the 
envelope axis and the lens center distance Z.sub.l are kept the same as in 
the conventional example. In FIG. 3, curves A-2 and B-2 respectively 
represent the amounts of the third order geometrical aberration for the 
low and high voltage operations while cross marks X give the amount of the 
third order geometrical aberration in the conventional example Z.sub.D 
/L.perspectiveto.0.56). It is apparent from FIG. 3 that as Z.sub.D /L 
increases, that is, as the deflection coil gets nearer to the target, the 
amount of aberration in question decreases. However, when Z.sub.D /L 
exceeds 0.7, the deflection angle becomes large. Accordingly, the angle of 
incidence of the electron beam onto the mesh electrode also becomes large 
and it is therefore difficult to cast the beam perpendicularly onto the 
target. It is also apparent from FIG. 3 that too large a value of Z.sub.D 
/L has little effect on the reduction in the amount of the third order 
geometrical aberration. On the other hand, the value Z.sub.D /L is about 
0.56 in the conventional example, and in this invention the value Z.sub.D 
/L is set to be within a range such that 0.56&lt;Z.sub.D /L&lt;0.70 so that the 
amount of the aberration in question in this invention can be reduced to 
as small a value as about 40% of that in the conventional example. For 
example, if Z.sub.D /L is set equal to 0.64 by shifting the deflection 
coil toward the target, the amount of the aberration in question for the 
low voltage operation in this invention is 51-55% of the corresponding 
amount in the conventional example. Hence, it is possible to improve the 
resolution in the corners of the picture to a considerable extent. 
Finally, the third method will be explained. FIG. 4 shows the relationship, 
obtained through computer simulation, among the ratio Z.sub.l /L of the 
lens center distance Z.sub.l to the total length L of the main lens 
section, the amount of the third order geometrical aberration and the 
magnification of the focusing lens, when the length l.sub.D of the 
deflection coil along the envelope axis and the deflection center distance 
Z.sub.D are rendered the same as in the conventional example. In FIG. 4, 
curves A-3 and B-3 represent the amounts of the third order geometrical 
aberration respectively for the low and high voltage operations, and a 
curve M gives the magnification of the focusing lens (approximately 
proportional to (L-Z.sub.l)/Z.sub.l) while cross marks X indicate the 
corresponding quantities in the conventional example (Z.sub.l 
/L.perspectiveto.0.51). As apparent from FIG. 4, the amount of the third 
order geometrical aberration decreases as the center of the action of the 
focusing lens approaches the beam limiting diaphragm. However, if Z.sub.l 
/L is two small, the magnification of the focusing lens becomes very large 
to increase the spread of the electron beam due to the distribution of the 
initial-velocity spread of thermionic electrons which is the factor to 
determine the resolution in the central area of the picture. The increase 
in the spread of the beam results in the degradation of the resolution in 
the central area of the picture. Usually, the upper limit of the 
magnification is about 1.1. Accordingly, the lower limit of Z.sub.l /L is 
about 0.47. On the other hand, since the value Z.sub.l /L in the 
conventional example is about 0.51, the value Z.sub.l /L in this invention 
is chosen to be within an interval such that 0.47&gt;Z.sub.l /L&lt;0.51. As a 
result, the amount of the third order geometrical aberration can be 
reduced to about 55% of the corresponding amount in the conventional 
example. For example, if Z.sub.l /L is set equal to 0.484 by reducing the 
length of the third grid and by increasing the length of the fifth grid, 
then the amount of this aberration can be reduced to 36% of that in the 
conventional example. 
In the foregoing description, the embodiments wherein the three methods are 
separately used, are detailed. However, it is also possible to further 
improve the resolution in the corners of the picture by the combination of 
the three methods. FIG. 5 illustrates an example of the combination of 
some of the three methods described above, representing the measured 
amplitude response in the high voltage operation of a camera tube 
fabricated for test by the use of the combination of the above second and 
third methods. In FIG. 5, the abscissa indicates Z.sub.D /L and the 
ordinate represents the difference (an arbitrary scale) between the 
resolutions in the central area and the corners of the picture. In the 
figure, a circle o represents the measured difference in the conventional 
camera tube and triangles .DELTA. give the measured differences in a 
camera tube according to this invention. In the case of the conventional 
camera tube, Z.sub.l /L.perspectiveto.0.51 and Z.sub.D 
/L.perspectiveto.0.56, as described above. In this embodiment of the 
present invention, Z.sub.l /L is set equal to 0.50 by shifting the center 
of the action of the focusing lens toward the object (the beam limiting 
diaphragm) while Z.sub.D /L is set equal to 0.6 to 0.69 by shifting the 
deflection center of the deflection coil toward the image (the mesh 
electrode). As apparent from FIG. 5, with the camera tube having such a 
structure as described in this embodiment, the resolution in the corners 
of the picture is much improved so that the uniformity in resolution over 
the picture is also much improved in comparison with the conventional 
camera tube. For example, in the case of an embodiment (the above 
mentioned test tube) with Z.sub.l /L=0.50 and Z.sub.D /L=0.64, the 
difference between the measured resolutions in the central area and 
corners of the picture could be reduced to about one quarter of that in 
the conventional camera tube. 
As described above, according to this invention, the resolution of a camera 
tube with electrostatic focusing and magnetic deflection in the corners of 
the picture can be improved without degrading the resolution in the 
central area of the picture so that the uniformity in resolution over the 
picture can be improved.