Camera tube having photocathode absorptive of shorter wavelength and filter absorptive of longer wavelength light

The optical cross-talk in a camera tube is reduced by the provision of a filter element which overlaps the entire target. The filter may be arranged in front of the photosensitive target and have a spectral transmission which is adapted to the spectral sensitivity of the target. The filter may also be arranged behind the target and preferably constructed so that it is also low-reflective for incoming light. In order to prevent excessive transverse conduction, such a filter may have a mosaic structure consisting of mutually insulated areas. Optical cross-talk may also be reduced by mounting the target on a support which is separately arranged in the camera tube and which has a small thickness.

BACKGROUND OF THE INVENTION 
The invention relates to a camera tube comprising an entrance window, a 
photo-sensitive target which is arranged opposite the entrance window, an 
electron gun for generating an electron beam for scanning the target, and 
means for reducing optical cross-talk in the target. 
A camera tube of this kind is known, for example, from British Pat. No. 
1,067,186. A camera tube described in this Patent Specification has an 
anti-halo window for reducing optical cross-talk in the target. Because 
the major part of the light which is reflected by the target lands outside 
the target after reflection from the entrance surface of the anti-halo 
window (due to increased lateral displacement of the light,) an anti-halo 
window of this kind results in a substantial reduction of the optical 
cross-talk. In many cases, particularly in camera tubes with increased 
red-sensitivity, the effect of the anti-halo window, however, is not 
completely adequate. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a camera tube in which the 
optical cross-talk in the target is reduced to a greater extent than in 
tube of known construction. To this end, a camera tube of the kind set 
forth includes the means for reducing the optical cross-talk in the target 
which overlaps at least substantially the entire target surface and 
reduces either the intensity or the degree of lateral displacement of 
light which is subject to lateral displacement due to reflections at this 
area. 
In a preferred embodiment of the camera tube in accordance with the 
invention, the cross-talk reducing means comprises a spectrally selective 
absorbing filter which is arranged, viewed relative to the incoming light, 
in front of the target and which may be included, for example, in an 
anti-halo window. A further preferred embodiment of a camera tube in 
accordance with the invention comprises an absorption filter which is 
arranged, viewed relative to the incoming light, behind the 
photo-sensitive layer of the target. This filter need not be spectrally 
sensitive. 
A further embodiment of a camera tube in accordance with the invention 
comprises an interference filter with suitable spectral transmission. This 
filter is arranged between the entrance window and the target at a 
distance from the target which is small relative to the dimension of the 
picture elements in the target. 
In another preferred embodiment of a camera tube in accordance with the 
invention, optical cross-talk is reduced by providing the target on a very 
thin support which is separately mounted in the camera tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The camera tube shown in FIG. 1 comprises an envelope having an entrance 
window 2, a cylindrical tube 4 and a tube base 6 with pins 8 and a pumping 
stem 10. Provided in the envelope is an electron gun having a cathode 12 
with a filament 14, a control grid 16, a first anode 18, an output anode 
20 and a mesh electrode 22. A photo-sensitive target 24 is preferably 
arranged on the entrance window and in this embodiment comprises a 
photo-conductive layer of lead monoxide. A signal electrode (not shown) is 
arranged on a side of the target 24 facing the entrance window 2. Camera 
tubes of this type are usually provided with an anti-halo window 26. With 
this arrangement, as a result of the combined large thickness of the 
entrance window and the anti-halo window, a substantial portion of the 
light reflected from the target is incident, after subsequent reflection 
from the entrance face 28 of the anti-halo window, outside the actual 
target due to the large lateral displacement. This light is thus prevented 
from disturbing the image. Even though a substantial improvement is thus 
obtained, in many cases disturbing optical cross-talk still occurs, inter 
alia, due to the fact that the thickness of this additional window may not 
be large enough so that a part 34 of a light beam 30 can still land on the 
target after multiple reflections. Part of the incident beam 30 is thus 
reflected from the target, resulting in a beam 32 which is subsequently 
reflected from the entrance face 28 resulting in beam 34 which strikes the 
target. In addition, a part 38 of the light 36 transmitted by the target 
can still be intercepted by the target after reflection from the mesh 
electrode 22, while a part 40 can also be intercepted by the target after 
reflection from elsewhere in the camera tube, for example, from one of the 
electrodes of the gun. For the sake of brevity, hereinafter the disturbing 
light which originates from light initially reflected by the target will 
be referred to as optical cross-talk by reflected light, while disturbing 
light resulting from light initially transmitted by the target will be 
referred to as optical cross-talk by transmitted light. The invention 
provides shielding of the target against one of these two types of 
cross-talk separately, or both types simultaneously. The cross-talk by 
reflected light could be reduced by making the anti-halo window slightly 
absorbing, but the sensitivity of the camera tube would then be reduced. 
This is often considered unacceptable. 
A substantial reduction of the flare is achieved without undesirable loss 
of sensistivity in accordance with the invention by arranging, between the 
face of incidence 28 and the target 24, an absorption filter having an 
absorption which increases from substantially 0% to approximately 100% as 
the wavelength of the light increases between approximately 0.6 .mu.m and 
0.7 .mu.m. It is known that within the visible spectral region, light of 
short wavelengths is absorbed to a high degree by a lead monoxide layer. 
Thus, no additional absorption should be introduced for this light. For 
light with long wavelengths, however, the absorption of such a layer is 
substantially lower and more of this light will be transmitted as well as 
reflected. For this spectral region, notably in camera tubes of increased 
red sensitivity, additional absorption will result in a substantial 
reduction of the optical cross-talk. In camera tubes comprising an 
anti-halo window, it is efficient to include the absorbing materials, 
adapted to the spectral properties of the target, in the glass of this 
window. Good results have been obtained by means of a mixture of rare 
earth metals such as, for example, Tm, Nd, Er and Ho. It is to be noted 
that the spectral sensitivity of this filter requires adaptation to 
different types of photo-sensitive layers. A camera tube of the kind set 
forth usually has an interference filter in the form of a dichroic mirror 
which is normally provided on the surface 28. This filter is added to 
adapt the spectral distribution of the light incident on the target to the 
eye sensitivity curve. An absorption filter in a camera tube in accordance 
with the invention combines the reduction of the optical cross-talk with 
the adaptation of the spectral sensitivity. Then, in comparison with a 
camera tube having a dichroic mirror, the sensitivity of the camera tube 
need not be less. In camera tubes without an anti-halo window, the 
absorbing materials may be provided in the glass of the entrance window. 
In a camera tube with a dichroic filter on the entrance surface of the 
window, the optical cross-talk by reflected light is rather intensified 
relative to a camera tube without such a filter. This is because a filter 
of this kind either transmits light of a given wavelength or reflects this 
light, but does not absorb it. Consequently, a comparatively large part of 
the light reflected by the target will be subsequently reflected by this 
filter and return to the target. In an embodiment of a camera tube in 
accordance with the invention, this drawback is eliminated by arranging 
the dichroic filter as near to the target as possible rather than on the 
entrance surface of the camera tube. 
FIG. 2 shows an entrance section of a camera tube comprising a dichroic 
filter 50 which is arranged on an inner surface of an entrance window 2. 
In a filter of this kind severe requirements are imposed on the thickness 
of the layers which determine the wavelength, because this thickness 
amounts to an odd number of half wavelengths in interference filters of 
this kind, so that the mounting of the filter in a fused tube requires 
complex precautions for ataining uniform thickness. In camera tubes in 
which the connection of the entrance window and the cylindrical tube does 
not require heating of these parts to the softening temperature of the 
glass, this drawback is eliminated because the filter can be provided on 
the flat entrance window before it is secured to the cylindrical tube. The 
filter is preferably arranged directly on the entrance window, followed by 
the deposition of a signal electrode 56 of tin oxide and/or indium oxide 
and a photo-sensitive layer 54. According to this sequence, the 
photo-sensitive layer is protected by the signal electrode against any 
detrimental effects of the filter material. If desired, an additional 
separating layer 52 can be provided between the signal electrode and the 
filter for similar reasons. Even though this construction does not reduce 
reflections, the adverse offects thereof on the picture quality will be 
much smaller, because the reflections involve a much smaller lateral 
displacement. From this point of view it is advantageous to use no 
separating layer or a separating layer which is as thin as possible. 
In an embodiment of a camera tube shown in FIG. 3, the target 24 is 
arranged on a separate support 57 which is formed, for example, by a plate 
of mica or a glass foil having a thickness of, for example, from 2 to 50 
.mu.m. The support with the target, mounted in a ring 58, is arranged in 
the tube envelope opposite the mesh electrode 22. Again no reduction of 
the reflection initially occurs, but because the support is very thin, the 
lateral displacement is small, so that disturbing optical cross-talk is 
avoided. The distance between the entrance window and the support may be 
arbitrarily small, provided that no contact is made at any area. In order 
to prevent light, which is reflected from the target and subsequently from 
the entrance window, from having a disturbing effect on the picture, the 
distance between the support 57 and the window 2 is preferably increased 
to 5 to 10 mm. A dichroic mirror can then also be arranged without 
objection on the inner or outer surface of the entrance window. 
The embodiments described thus far have a common aspect in that initially 
the detrimental effects of optical cross-talk by reflected light are 
counteracted. Because the said filters are also effective against light 
which has been transmitted twice or more, a given reduction in the flare 
by transmitted light will also occur. 
In a preferred embodiment shown in FIG. 4 there is provided a filter which 
is active particularly for transmitted light. This camera tube comprises a 
filter 60 which is arranged on the inner side on the target 24. No 
requirements as regards the spectral sensitivity need be imposed on a 
filter so arranged. This filter is preferably constructed so that all 
light is absorbed. Excessive lateral conduction and adverse influencing of 
the photo-sensitive layer should be avoided. A filter of this kind may be 
formed, for example, by a layer of soot consisting of carbon. 
Alternatively, a filter consisting of a vapour-deposited layer of a noble 
metal such as silver has also been found to function satisfactorily. In 
order to minimize the lateral conduction of a filter of this kind, it is 
advantageous to impart only a limited thickness to the layer or to deposit 
it via a mask which can, for example, be the mesh electrode. The sealing 
of a filter thus formed will usually not be 100%, but a substantial 
reduction of optical cross-talk, notably by transmitted light, will thus 
certainly be achieved. In the case of a transmission of, for example, 20%, 
secondary incidence, after reflection from the mesh electrode or elsewhere 
in the camera tube, causes only a negligible part of the light initially 
transmitted by the target to be transmitted again. An additional advantage 
of such a filter is that it has been found a reduction of reflection also 
occurs for incident light, so that a reduction is also obtained of the 
flare by reflected light. When a separating layer is added between the 
target and the filter in order to prevent mutual influence, it should be 
ensured that this intermediate layer does not cause additional reflection 
of light incident from the entrance side of the camera tube.