Image Tube Incorporating a Brightness-Dependent Power Supply

The invention relates to a noctoviser comprising an image intensifier tube and a cascade high-voltage generator.

For single-stage as well as multi-stage noctovisers comprising an image intensifier tube it is desirable that the luminescence of the screen facing the viewer or a television camera tube remains constant even though the illumination level of the scene changes.

A coarse adaptation can be achieved by using series resistors in the current circuit of the power supplies. However, this has the drawback that the apparatus is switched off at higher illumination levels.

Comparatively expensive control circuits have also been proposed in which the luminescence of the relevant screen is measured via special photosensors or the current of the tubes is used as a criterion.

The invention has for its object to provide a brightness-dependent power supply for a noctoviser in which the drawback of switching-off is avoided and the costs inherent to the control circuits are substantially reduced.

This problem is solved according to the invention in that the intensifier tube is connected to the high-voltage generator output via a parallel connection of a resistor and an avalanche diode. In multi-stage noctovisers each tube is connected to the high-voltage output of each cascade of the high-voltage generator via such a parallel connection.

One embodiment according to the invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 shows a circuit arrangement of a preferred embodiment according to the invention, and

FIG. 2 shows a voltage diagram relating to this preferred embodiment.

One the basis of the fact that for an n-stage arrangement of image intensifier tubes, always also including image-converting tubes, the following formula is applicable for constant luminescence on the last screen:

in which

I.sub.1 is the current in the first tube,

B is the illumination level on the scene, and

U is the acceleration voltage per tube, so for a two-stage arrangement:

and because I.sub.1 .about. B.sub.scene, ##EQU1##

As is shown in FIG. 1, each of the outputs of the high-voltage generators 1 and 2 which are successively connected in cascade has connected thereto a parallel connection of a resistor 3 and an avalanche diode 4 (5 and 6, respectively), via which the high voltage reaches the tubes 7 and 8 which are also successively connected. If necessary, a resistor 9, 10 can be connected in series with the diodes 4 and 6, respectively. The desired voltage can be adapted by selection of these resistances. Avalanche diodes are now commercially available in step-wise decreasing values, also between 10 kV . . . 15 kV. The value thereof corresponds to the known housings for high-voltage diodes. In the apparatus the resistors are encapsulated together with the diodes and the cascades used for generating the high voltage.

As appears from the diagram shown in FIG. 2, the theoretically most favourable variation of U.sub.0 can be very well approximated. The diagram shows the variation of the high voltage U.sub.n and of the voltage U.sub.4, U.sub.6 of the two avalanche diodes 4 and 6, respectively, as a function of the illumination level on the scene (B.sub.scene). The voltage part U'.sub.1 is determined by the resistors 3 and 5. U.sub.B represents the transitional region of the avalanche diodes, whilst the voltage part U".sub.1 is determined by the resistors 9 and 10.