Automatic reticle brightness control circuit means in night vision image intensifiers

An automatic reticle brightness control circuit means for maintaining the brightness of a reticle commensurate with the brightness of an object image incident upon the cathode of an image intensifier tube under varying input light conditions, the means comprising:

• a screen power supply oscillator and voltage multiplier for providing a direct current bias voltage on the screen of said image intensifier tube;
• a screen current sensing circuit having on one side thereof a common node connected to the secondary winding of said screen power supply oscillator and having another side connected to a ground potential;
• an automatic brightness control circuit having an input connected to said common node and an output therefrom;
• an n-p-n transistor base controlled MCP OSC circuit for providing a variable feedback direct current voltage at a feedback terminal in response to a signal from the output of said automatic brightness control circuit applied to the base of said n-p-n transistor;
• a feedback circuit having an input connected to said variable feedback direct current voltage at said feedback terminal and an output therefrom providing a voltage in direct relation to the value of said variable feedback direct current voltage;
• a reticle LED direct current voltage source for providing bias voltage to a reticle LED;
• and
• a reticle brightness control circuit, said reticle brightness control circuit having two common inputs and an output therefrom for controlling the bias voltage to said reticle LED wherein a first of said common inputs is connected to said common node of said screen current sensing circuit and a second of said common inputs is connected to the output of said feedback circuit whereby as screen current varies proportionally to the object image input light and said screen current flows through said current sensing circuit the voltage at said common node decreases as screen current increases and causes the first of said common inputs to said reticle brightness control circuit to vary the bias voltage across said reticle LED directly with the increase in screen current and whereby as the screen current further increases past an automatic brightness control level said automatic brightness control circuit base controls said n-p-n transistor base controlled MCP OSC circuit to change said variable feedback direct current voltage inversely with increasing screen current in which said variable feedback direct current voltage source controls said second of said common inputs to said reticle brightness control circuit for additionally increasing current flow through said reticle LED directly with increasing screen current to keep the reticle visible to an observer under varying light conditions of said object image.