Image intensity degradation tester

In combination, night vision goggles including optical filter means for passing light having wavelengths in the visible spectrum corresponding to that of light from the night sky, and a device for testing the ability of the night vision goggles to pass light of sufficient intensity, said device comprising:

• a light source;
• a first optical filter able to pass light having wavelengths in the visible spectrum near infrared and to block passage of light having shorter wavelengths;
• photosensor means exposed to light from the light source that passes through the first optical filter for producing an electrical control variable whose magnitude changes with the intensity of light passing through the first optical filter;
• feedback control means responsive to the magnitude of the electrical control variable for maintaining substantially constant the intensity of light from the light source;
• and
• an array of neutral density optical filters exposed to light from the first optical filter, each neutral density filter of the array being able to pass light of different intensity than other neutral density optical filters of the array, thereby producing a graduated scale of light intensity, wherein the feedback control means comprises:
• an emitter-coupled differential amplifier including a current bridge carrying current from a power source through the photosensor means;
• cascaded amplifier stages having control current connected to the current bridge;
• means for changing control current in the cascaded amplifier stages in response to changes in magnitude of the electrical control variable of the photosensor means;
• and
• means for changing the voltage drop across the light source in response to control current changes in the cascaded amplifier stages.