How does night vision goggles work?
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There are two types of NVG. One is a signal booster of sorts that amplifies visible light, so naturally cannot work in complete darkness.
The other types emits light on wavelengths invisible to humans such as infra-red, and the sensor recieves this light and converts it into visible wavelengths for display inside the goggles.
The Wikipedia article [https://en.wikipedia.org/wiki/Night-vision_device](https://en.wikipedia.org/wiki/Night-vision_device) has all the answers you need.
Basically it combines amplification of low-intensity visible light sources with an infrared camera to create an image in near total darkness.
I recently bought a smartphone with an infrared camera, and it works quite well as a night-vision device when viewed in grayscale in a totally dark room with some warm sources (like a mini-refrigerator). The warm sources produce invisible infrared energy that illuminates the surroundings, and it’s all picked up by the infrared camera.
Another thing of note with NVGs is why it uses Green as the colour to display – primarily because the human eye sees green better than any other colour in the spectrum, and can therefore make out more detail in the monochromatic image that NVGs display.
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