To put it as simply as possible, most thermal binocular scopes, goggles, monoculars and binoculars are electro-optical devices that amplify whatever existing light there is and send it onto your eyes.
You actually don’t look directly through the optics of a thermal binocular or other night vision device as you do on normal optical binoculars (when used for thermal binocular known as night glasses), you actually look at an amplified electronic image on a phosphor screen. The little light that there is available enters the thermal binocular glasses through an objective lens and strikes a photo cathode.
This cathode has high energy charge from the power supply which accelerates across a vacuum inside the intensifier and strikes a phosphor screen (similar to a TV screen) and is then focused. The eyepiece then magnifies the image for your eyes to see.
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