How do night-vision goggles work?

In adrenaline-pumping action films, night-vision googles are used by highly-trained elite soldiers and spies to let them see in the dark. But how effective are they, and how do they work?

Night-vision goggles certainly work, though they will not do so in complete darkness.

Our eyes detect light from the world around us in the form of light particles known as photons. Inside the eyes, photons are detected by rods that are particularly light-sensitive, and cones that are less light-sensitive but more sensitive to colour and details.

Compared with cats and some other nocturnal animals, human eyes include relatively few rods, which is why we have poor night vision.Yet even in what we would describe as darkness, a certain number of photons will still strike objects around us and be reflected in the direction of our eyes.

Night-vision goggles turn these few photons into more photons via several processes. First the photons are converted into electrons, which are amplified by an electrical field that increases both the quantity and the energy of the electrons. The electrons then strike a plate coated with phosphorus, which absorbs the energy of the electrons and emits sufficient new photons for the eye to detect. Nothing you see through night goggles are actually photons from the outside world.

The phosphorus emits photons with wavelengths corresponding to green colours, so that the view through the goggles appears green. This was deliberately selected because our eyes are most sensitive to green, and we can detect details that we might not if viewed in their natural colours.

If it is absolutely dark, no photons will enter the night-vision goggles, and since the goggles’ processes only amplify existing light, nothing will be visible.