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So we can see at night is the basic answer, but when you take into account that nocturnal animals have managed for millions of years without technological help, the correct question is why do humans need night vision? The answer to that question is obviously - because we dont have a tapetum lucidum. This tapetum lucidum, which translates from Latin as bright tapestry is a layer of tissue in the eye of many vertebrates that lies just behind the retina, allowing them to increase the light available to their photoreceptors. Big cats, birds of prey, rodents and even some sea creatures have this capability. We humans are not in possession of such ocular gadgetry, so we cant hunt at night. Fortunately, what we lack in the ability to detect shapes and contrast in low light or pitch dark, we make up for in technological and biological savvy.
Well, we no longer need to hunt our food, so night vision doesnt aid us there. However it does provide a distinct advantage in a number of other areas; including espionage, surveillance, warfare, crime prevention, search and rescue, navigation and even nature trekking.
There are actually two types of night vision; thermal imaging and image enhancement. Youll have seen the former at play on shows like Police, Camera, Action, where a suspect is located hiding in a bush due to him showing up on screen as a glowing red blob. As you might expect, thermal imaging relies on heat to render an image. The more contrast in the heat (think of a man hiding in a garage), the clearer the image. The benefit of thermal imaging is that not only does it enable you to see in the dark, you can also see through, behind and inside things. The second type of night vision technology is image enhancement. Even in total darkness, light exists, but it is invisible to the naked eye. Image enhancement effectively turns up the volume on these hidden forms of light, giving sharper contrast to an otherwise dark image. The very best (and most expensive) night vision tech can enable you to see a person standing still in complete darkness over 200 yards away.
Night vision is rendered in green because the makers of night vision technology assume youll be monitoring the output for extended periods of time and the distinctive green hue of night vision has been proven to be easier on the eye than any other combination of colours. Naturally there needs to be high contrast to render low light visible and green is the easiest colour to pitch contrast against.
These are for seeing in the dark in real time. If you're a soldier in a war zone or pursuing a wild animal at night, you'll need to see what's going on as it happens. That means all of the image processing needs to take space between the lens of the goggle and your eyeball. Quite tricky. It also means it needs to happen extremely quickly. For that reason, night vision goggles are very expensive.
A more affordable option for seeing in the dark could be a night vision camera. Night vision cameras not only let you see whats going on in the dark, they let you record the action too. Ideal for evidence gathering and for discouraging antisocial or illegal behaviour. Image credit - Wiki Commons