Image intensifier tube comprising a chromium-oxide coating

In an image intensifier tube comprising a photosensitive layer an insulating wall portion is coated with a layer of chromium oxide which is deposited in the form of a chromium nitrate layer which is subsequently baked at a temperature of approximately 525.degree. C. A thin, suitably adhesive, uniform and transparent coating layer having a comparatively high resistance value is thus achieved.

BACKGROUND OF THE INVENTION 
The invention relates to an image intensifier tube, comprising a housing 
which is formed by an entrance window, an exit window and an envelope 
portion which consists partly of a translucent, electrically insulating 
material, which housing accomodates an entrance screen and an 
electron-optical system for imaging photoelectrons on an exit screen. 
An image intensifier tube of this kind is known as an X-ray image 
intensifier tube from U.S. Pat. No. 3,026,437 and as a brightness 
intensifier tube from U.S. Pat. No. 4,286,148. In order to prevent local 
discharge phenomena in such tubes, a glass portion of the envelope is 
often coated with a semiconductor material on an inner side. In image 
intensifiers tubes comprising a photosensitive entrance screen, discharge 
phenomena are liable to have an image-disturbing effect because light then 
emitted activates the photosensitive layer, releasing, for example 
photoelectrons which are imaged on the exit screen, together with 
image-carrying photoelectrons, and thus participate in the imaging. 
U.S. Pat. No. 3,026,437 mentions chromium oxide as an example of a coating 
material. Known coating layers have the drawback that the layer is not 
transparent because it consists of, for example green chromium oxide, or 
that the resistance of the layer is comparatively low so that a rather 
large leakage current occurs which substantially increases the power 
required for operating the tube. Moreover, known coating layers have a 
comparatively large thickness and their thickness and structure are not 
very uniform. 
SUMMARY OF THE INVENTION 
It is the object of the invention to mitigate the described drawbacks; to 
achieve this, an image intensifier tube of the kind set forth in 
accordance with the invention is characterized in that at least a part of 
a transparent envelope portion is coated with transparent chromium oxide. 
Because a transparent portion of the envelope is coated with a transparent 
resistive layer in an image intensifier tube in accordance with the 
invention, the transparency is sustained so that via these portions the 
photocathode can be activated for test measurements and the like and a 
resistive layer exhibiting suitable adhesion and a comparatively high 
resistance is achieved. 
In a preferred embodiment the image intensifier tube forms an X-ray image 
intensifier tube comprising a CSi entrance screen, the transparent portion 
of the envelope being situated near the exit screen. A coating layer in 
accordance with the invention enables suitable homogenization of the field 
strength across the surface and a suitably defined, comparatively small 
leakage current and also offers the possibility of external activation of 
the photocathode. 
In a further embodiment, at least a part of the cylindrical housing of a 
brightness intensifier tube is coated with a transparent chromium-oxide 
layer, so that discharging phenomena are again avoided and a reliable, 
comparatively low leakage current is obtained. Because the power supply 
for these tubes is preferably small, a low leakage current is very 
attractive. 
In a preferred embodiment, the translucent chromium-oxide layer is formed 
by depositing a comparatively thin layer of chromium nitrate by brushing, 
spraying or immersion, which layer is subsequently baked at approximately 
520.degree.-530.degree. C. Thus, a thin, suitably adhesive and suitably 
uniform layer of chromium oxide having a comparatively high resistance and 
a comparatively low secondary emission coefficient is obtained, so that 
the risk of local discharges is strongly reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An X-ray image intensifier tube as shown in FIG. 1 comprises an entrance 
window 2, an exit window 4, a cylindrical envelope 6 and an insulating 
ring 7 which together enclose an evacuated space 8. In the space 8 there 
are arranged an entrance screen 10, an exit screen 12, and an 
electron-optical imaging system 14. The entrance screen of the tube forms 
a separate foil, for example of titanium. Even for tubes comprising a 
large entrance window a titanium entrance window need not be thicker than, 
for example approximately 0.2 mm so that only a slight dispersion of an 
X-ray beam to be detected occurs therein. In this case the entrance screen 
comprises a concave support 16, preferably made of aluminium, which may 
also be thin because it does not serve as a vacuum wall. On the support 
there is provided a layer of luminescent material 18 on which there is 
provided a photocathode 22 with an intermediate barrier layer 20. The 
entrance screen constitutes, for example in conjunction with a shielding 
ring 24 which is also shown, a first electrode of the electron-optical 
imaging system 14; this system also includes a focusing electrode 26, a 
first anode 28 and an output anode 30 which is preferably in electrical 
contact with the exit screen. The envelope 6 of the housing has a circular 
cross-section in this case, but it may also have a rectangular shape, 
together with the exit, window, the entrance screen and possibly the exit 
screen and the exit window. The insulating ring 7 in this case consists of 
a translucent material and is coated in accordance with the invention with 
a layer of translucent chromium oxide 32 which is deposited on the inner 
side of the ring wall by the baking of chromium nitrate. A layer of 
chromium oxide thus obtained has a comparatively small thickness and a 
comparatively high resistance. The chromium nitrate is deposited, for 
example by immersion of the ring. 
FIG. 2 shows an image intensifier tube in accordance with the invention in 
the form of a brightness intensifier, comprising a housing 40 which 
includes a, for example fibre optical, entrance window 42, an exit window 
44 and a cylindrical tube wall portion 46. A preferably concave inner side 
48 of the entrance window is provided with a photocathode 50. Opposite the 
photocathode there is arranged a channel intensifier plate 52 having an 
entrance face 54 and an exit face 56. Because the photocathode and the 
channel plate there is arranged an electrode 58 and an electrode 60 which 
is arranged near the entrance face of the channel plate and which is 
preferably integral with a customary input electrode provided on the 
entrance face of the channel plate. Customary photocathodes have an 
electrical conductivity such that they may be considered to form an 
electrode in the electron-optical system. If this is not the case, an 
additional electrode which is transparent to the radiation to be measured 
can be provided. The inner side of the exit window 44 is provided with a 
luminescent layer 62. Via an electrically conductive wall portion 66, the 
photocathode 50 is connected to a terminal 68 and the intermediate 
electrode 58 is connected to a terminal 70. The intermediate electrode 70 
can be adjusted to a positive potential which is comparatively high with 
respect to the photocathode, for example +5 kV, by means of a voltage 
source 72. The input electrode 60, being electrically integral with a 
channel input electrode provided on the channel entrance face 54, 
comprises a terminal 74. Via a voltage source 76, the input electrode can 
be adjusted to a potential which is comparatively low with respect to the 
intermediate electrode, for example +1 kV. Via a voltage source 78 and a 
terminal 80, an output electrode of the channel plate 52 can be adjusted 
to a higher potential with respect to the input electrode and, via a 
voltage source 82 and a terminal 84, the exit window can be adjusted to a 
somewhat higher potential again. In a practical embodiment of a tube 
notably the potentials which are relevent for the imaging of the 
photoelectrons on the channel plate will usually be derived from a common 
source, because any voltage variations then have a proportional effect on 
all potentials, so that the electron-optical setting is substantially less 
sensitive. The tube wall portion 46 in accordance with the invention is 
coated with a layer of transparent chromium oxide, so that a potential is 
achieved which varies uniformly across this portion, the relevant wall 
portion remains translucent and only a small leakage current occurs when 
the potentials are applied.