Automatic brightness control for x-ray image intensifier system

Claim 2

An x-ray image intensifier system with automatic brightness control comprising: a. An x-ray image intensifier adapted to convert an x-ray image to an optical image; b. an x-ray tube including an anode and an electron emissive filament, said tube being located to project x-radiation through a subject to create an image that is intercepted by the intensifier; c. a source of AC electric power; d. an electric power control means connected in a circuit with said source of AC power and said filament; a video camera located to receive the optical image from said intensifier and adapted to produce video signals representative of brightness variations in the optical image; f. a video signal peak detector means adapted to select video signal peaks above a certain amplitude, said peak detector means including a diode means through which said selected video voltage signals are conducted and a first capacitor which is connected to the diode means for being charged to substantially the voltage level of the peak video signals that correspond with a peak brightness region in the optical image; g. a reference voltage source and means comparing said peak voltage on said capacitor with the voltage of said reference source to thereby produce an error signal; and h. means responsive to said error signal controlling the electric power control means to adjust the x-ray tube filament current and reduce the error signal.

Claim 3

The invention set forth in claim 1 including: a. means responsive to horizontal and vertical synchronizing pulses from said video camera for producing delayed horizontal and vertical gate pulses of predetermined duration following occurrence of each synchronizing pulse; b. means adapted to receive and add said delayed pulses and produce an output gate pulse having a duration corresponding with the video camera scanning a predetermined window area of said optical image; and c. a semiconductor switch means connected to conduct said video signals to said peak detector and adapted to be turned on by said output gate pulses whereby to permit detection of peak video signals only when said window area is being scanned.

Claim 4

The invention set forth in claim 1 wherein the means that are responsive to the error signal include: a. transistor means connected to receive said error signal and being variably conductive in dependence on the amplitude of the error signal; b. a DC source; c. a second capacitor; d. a first charging circuit for said second capacitor including a resistor connected between one side of the DC source and a terminal of the transistor means and a diode connected from said terminal to said second capacitor; e. a second charging circuit for said second capacitor comprising a resistor connected thereto and to the one side of the DC source: f. A unijunction transistor oscillator means having base terminals and an emitter terminal, the latter being connected to said capacitor, and adapted to render the unijunction transistor conductive normally when the capacitor is charged to the triggering level of the unijunction transistor; g. Means supplying pulses that are synchronized with each half-cycle of the AC power line to the base terminals of the unijunction transistor whereby to permit discharge of any residual charge on the second capacitor between synchronizing pulses after the unijunction is triggered; h. Variations in the conductivity of said transistor means due to said error signal varying the charging time of said second capacitor and causiNg the triggering time to said unijunction transistor and the output pulses therefrom to occur earlier or later during consecutive half-cycles; i. A controlled rectifier having an anode, a cathode, and a gate electrode; j. A full-wave bridge rectifier; k. a filament transformer and an AC power transformer in a circuit with the bridge rectifier and the anode and cathode of the controlled rectifier; and l. the output pulses of said unijunction transistor being applied to the gate terminal of the controlled rectifier to control its conduction angle and thereby control the power to the filament transformer depending on the time at which the unijunction pulses occur during each half-cycle.

Claim 5

The invention set forth in claim 3 including: a. a resistor in parallel with said first capacitor and having an RC time constant substantially equal to two half-cycles of the AC power source, whereby the detected peak video signal voltage will be maintained long enough for two consecutive unijunction output pulses to occur at substantially the same time in any two consecutive half-cycle periods.

Claim 6

The invention set forth in claim 1 including: a. a resistor in series between said diode and said first capacitor in said peak detector means; and b. a normally closed switch shunting said resistor, opening of said switch causing voltage on said first capacitor to be below the peak signal value whereby to increase the error signal and the current through the filament of the x-ray tube.

Claim 7

An x-ray image intensifier with automatic brightness control comprising: a. an x-ray image intensifier adapted to convert an x-ray image to an optical image; b. a video camera means located to receive the optical image from the image intensifier and adapted to produce video signals representative of brightness variations in the image; c. a television monitor that is driven by the video signals and displays the image; d. an x-ray tube having a filament whose input power controls the intensity of the x-rays from the tube that are projected through a subject to the intensifier; e. an AC power circuit supplying said filament; f. a controlled rectifier having an anode, a cathode, and a gate electrode, the anode and cathode being in the AC power circuit of the filament and the conduction angle thereof governing the power to the filament; g. means producing gating signals which occur at a time during each half AC cycle that depends on the peak brightness in a predetermined window area of the optical image, said gating signals being applied to said gate electrode to thereby control the conduction angle and the filament power; h. a video amplifier means having an input terminal receiving the composite signal from the camera means and an output terminal on which only video signals appear; i. a video signal peak detector and a semiconductor switch connecting the output terminal of the video amplifier to said detector; j. means producing a gating signal that exists while said video camera is scanning said window area, said gating signal producing means being controlled by horizontal and vertical synchronizing pulses from said video camera means, and said gating signal turning on said semiconductor switch; k. said video signal peak detector comprising a diode supplied through the semiconductor switch, a capacitor in series therewith and a resistor in parallel therewith, the time constant of the resistor and capacitor being substantially equal to two half-cycles of said AC source; l. a source of reference voltage and means comprising the peak video signal on the capacitor with the reference voltage and producing an error voltage; and m. said error voltage controlling the means that produces the gating signals that are applied to the gate electrode of the controlled rectifier to thereby control its conduction angle and power to the filament.

Claim 8

An x-ray image intensifier with automatic brightness control comprising: a. an x-ray image intensifier adapted to convert an x-ray image to an optical image; b. a video camera means located to receive the optical image from the image intensifier and adapted to produce video signals representative of brightness variations in the image; c. a television monitor that is driven by the video signals and displays the image; d. an x-ray tube having a filament whose input power controls the intensity of the x-rays from the tube that are projected through a subject to the intensifier; e. an AC power circuit supplying said filament; f. a controlled rectifier having an anode, a cathode, and a gate electrode, the anode and cathode being in the AC power circuit of the filament and the conduction angle thereof governing the power to the filament; g. means producing gating signals which occur at a time during each half AC cycle that depends on the peak brightness in a predetermined window area of the optical image, said gating signals being applied to said gate electrode to thereby control the conduction angle and the filament power; h. a video amplifier means having an input terminal receiving the composite signal from the camera means and an output terminal on which only video signals appear; i. a video signal peak detector and a semiconductor switch connecting the output terminal of the video amplifier to said detector; j. means producing a gating signal that exists while said video camera is scanning said window area, said gating signal producing means being controlled by horizontal and vertical synchronizing pulses from said video camera means, and said gating signal turning on said semiconductor switch; k. said video signal peak detector comprising a diode supplied through the semiconductor switch, a capacitor in series therewith and a resistor in parallel therewith, the time constant of the resistor and capacitor being substantially equal to two half-cycles of said AC source; l. a source of reference voltage and means comprising the peak video signal on the capacitor with the reference voltage and producing an error voltage; and m. said error voltage controlling the means that produces the gating signals that are applied to the gate electrode of the controlled rectifier to thereby control its conduction angle and power to the filament.