Low light level image pick-up tube arrangement

Claim 2

A low light level television pick-up tube arrangement comprising: optical means for focussing radiation received from an observed field; an image pick-up tube for receiving the said focussed radiation and for delivering a video signal, the said pick-up tube comprising image intensifying means arranged between the said optical means and an electron gun from which said video signal is derived by electronically scanning a target, said image intensifying means comprising an input intensifier stage which receives the said focussed radiation; a measuring circuit for generating an error signal representative of the difference between the existing mean value of the said video signal and a predetermined mean reference value; a very high voltage switched supply for supplying a switched voltage to the said input intensifier stage and a control circuit for generating from the said error signal control signals for controlling the said switched supply to provide predetermined variations in the amplitude and duration of the said switched voltage to automatically control the gain of the said intensifying means; and protection means to protect said tube against sudden and too intense illumination, said protection means generating an obturation signal applied to the said control circuit for controlling the said switched supply to provide an instantaneous downward change in the said switched voltage to cancel out the gain of the said intensifier means.

Claim 3

A pick-up tube arrangement according to claim 1, wherein said image intensifying means comprises a plurality of intensifier stages in cascade and a high voltage DC supply to produce separate DC voltages to supply respective ones of the intensifier stages situated downstream of the said input stage, the said error signal being applied to the said DC supply to control the amplitude of at least one of the said DC voltages for regulating the said video signal.

Claim 4

A pick-up tube arrangement according to claim 2, in which the video signal is obtained by a line-by-line scan of a target incorporated in the said electron gun and wherein the said controlled very high switched voltage supply provides a very high voltage switched between a constant lower value and a variable upper value, the said voltage being so switched once upwards and one downwards in the course of each vertical scan, the said upper value being constant during any given vertical scan.

Claim 5

A pick-up arrangement according to claim 3, wherein the said control signals comprise, a rise signal for the very high voltage to rise in the form of a cyclic vertical-scan pulse which is generated during the vertical suppression interval and which has a predetermined delay with respect to the end of the previous vertical scan, and a fall signal for the very high voltage to fall which is likewise in the form of a pulse and whose position can be varied between the said rise pulse and the end of the vertical scan in question, the said rise and fall signals being synchronised with the horizontal suppression signal, the said switched supply causing a change-over from the said fixed value to the said upper value at the rise and vice-versa at the fall, the change-over taking a length of time which is at most equal to the duration of the horizontal suppression signal.

Claim 6

A pick-up tube arrangement according to claim 4, wherein the said control circuit produces an amplitude controlling signal in the form of a pulse whose length can be varied by altering the position of its leading edge and whose leading and trailing edges are respectively synchronised with horizontal suppression pulses, the said leading edge having a fixed delay with respect to the end of the previous vertical scan, the said pulse being produced in the course of the vertical suppression interval and the change in the length of the pulse producing, via the switched supply, a substantially linear response law governing the change in the upper value of the very high switched voltage in the course of successive vertical scans.

Claim 7

A pick-up tube arrangement according to claim 5, wherein the control circuit comprises logic counting and selecting circuits to make the time for which the very high switched voltage is supplied constant within a specified range of variation on the part of the said error signal and variable outside this range, the permitted variation being of the discontinuous type with a power of two geometric progression from one vertical scan to the next.

Claim 8

A pick-up tube arrangement according to claim 5, wherein the control circuit comprises analogue circuits to make the length of time for which the very high switched voltage is applied variable within a specified range of variation on the part of the error signal and constant outside this range, the permitted variation being of the continuous type and being at least equal to one horizontal period.

Claim 9

A pick-up tube arrangement according to claim 5, wherein the said protection means comprises on the one hand, local circuits for measuring the energy received by the target to produce the said obturation signal as soon as a predetermined limiting energy level is reached and on the other hand, external circuits for providing the said obturation signal for a predetermined length of time.

Claim 10

A pick-up tube arrangement according to claim 8, wherein the said local protection circuits include a first threshold comparison circuit receiving a signal representative of the very high DC voltage supplied to the last intensifier stage and a second threshold comparison circuit receiving a signal representative of the DC current supply of the said last stage.

Claim 11

A pick-up tube arrangement according to claim 1, wherein the said protection means include a first threshold comparison circuit receiving a signal representative of the mean current supplied by the said VHV switched supply and a second threshold comparison circuit receiving the said error signal.

Claim 12

A low light level television pick-up tube arrangement comprising: optical means for focussing radiation received from an observed field; and image pick-up tube for receiving the said focussed radiation and for delivering a video signal, the said pick-up tube comprising image intensifying means arranged between the said optical means and an electron gun from which the said video signal is derived by electronically scanning a target, said image intensifying means comprising at least two image intensifier stages connected in cascade; a measuring circuit for generating an error signal representative of the difference between the existing mean value of the said video signal and a predetermined mean reference value; a very high voltage switched supply for supplying a switched voltage to the input intensifier stage receiving the said focussed radiation; a control circuit for generating from the said error signal control signals for controlling the said switched supply to provide predetermined variations in the amplitude and duration of the said switched voltage to automatically control the gain of the said input stage; high voltage DC supply means for supplying the intensifier stages situated downstream of the said input stage respectively with separate DC voltages, said error signal being applied to the said DC supply means to control the amplitude of at least one of the said DC voltages to provide amplitude regulation of the said video signal, and protection means to protect said tube against sudden and too intense illumination, said protection means generating an obturation signal applied to the said control circuit for controlling the said switched supply to provide an instantaneous downward change in the said supply voltage to cancel out the gain of the said intensifier means.

Claim 13

A low light level television pick-up tube arrangement comprising: optical means for focussing radiation received from an observed field; and image pick-up tube for receiving the said focussed radiation and for delivering a video signal, the said pick-up tube comprising image intensifying means arranged between the said optical means and an electron gun from which the said video signal is derived by electronically scanning a target, said image intensifying means comprising at least two image intensifier stages connected in cascade; a measuring circuit for generating an error signal representative of the difference between the existing mean value of the said video signal and a predetermined mean reference value; a very high voltage switched supply for supplying a switched voltage to the input intensifier stage receiving the said focussed radiation; a control circuit for generating from the said error signal control signals for controlling the said switched supply to provide predetermined variations in the amplitude and duration of the said switched voltage to automatically control the gain of the said input stage; high voltage DC supply means for supplying the intensifier stages situated downstream of the said input stage respectively with separate DC voltages, said error signal being applied to the said DC supply means to control the amplitude of at least one of the said DC voltages to provide amplitude regulation of the said video signal, and protection means to protect said tube against sudden and too intense illumination, said protection means generating an obturation signal applied to the said control circuit for controlling the said switched supply to provide an instantaneous downward change in the said supply voltage to cancel out the gain of the said intensifier means.