Why do we need Night Vision? So we can see that night is the basic answer. Still, when you consider 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 obvious - because we don't have a tapetum lucidum. This tapetum lucidum, which translates from Latin as 'a bright tapestry', is a layer of tissue in the eye of many vertebrates 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 can't 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.
But why do we want to be able to see at night?
Well, we no longer need to hunt our food, so night vision doesn't aid us there. However, it does provide a distinct advantage in several other areas, including espionage, surveillance, warfare, crime prevention, search and rescue, navigation and even nature trekking.
So how does it work?
There are two types of night vision; thermal imaging and image enhancement. You'll 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 the 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, but 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 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 best (and most expensive) night vision tech can enable you to see a person standing still in complete darkness over 200 yards away.
Why is Night Vision green?
Night vision is rendered in green because the makers of night vision technology assume you'll be monitoring the output for extended periods of time. 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.
Night Vision Goggles
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.
Night Vision Spy Cameras
A more affordable option for seeing in the dark could be a night vision camera. Night vision cameras not only let you see what's going on in the dark, but they also let you record the action. Ideal for evidence gathering and for discouraging antisocial or illegal behaviour.