There are two primary forms of night vision. Both forms work on a similar principle, they simply target different parts of the ‘light spectrum’, one type focuses on light in the visible spectrum (mostly) and the other focuses on the invisible spectrum (Thermal). However, for this blog we’re going to focus on what most would call ‘night vision’, i.e. image enhancement.
Image enhancement night vision is the classic ‘green’ image that you will see in films, television, video games and was first invented in 1929 for anti-aircraft defence in Britain.
Night vision today has evolved into much smaller and efficient solutions than the original devices created almost 90 years ago. Today, image enhancement night vision utilised a technology known as ‘Image Intensifier Tubes’.
Image Intensifier Tubes
An important bit of information to note is that, whilst your eyes can’t see anything when it’s dark, there is still a very small amount of light. Your eyes just aren’t sensitive enough to detect it. The image intensifier tube is what does all of the hard work that your eyes aren’t sensitive enough to see and allows you to see in the darkness.
How does an intensifier tube work?
As the very small amount of light (photons) enters the night vision optic it hits the first in an array of “lenses” (called plates). The first plate that the photons hit is a Photocathode. This negatively charged plate, when struck with light, emits electrons (electricity). Very simply, the small dimly lit image that hits the night vision device is converted directly to electricity.
This electron image then hits the next plate within the device: a ‘micro-channel plate’. This is a plate of thousands of little parallel tubes, like cutting open a box of drinking straws. As an electron enters one of these tubes, it causes more electrons to be emitted from the walls of the tube, which in turn induces more electrons. By the time the original electron exits the micro-channel plate, it has caused a chain reaction of thousands of electrons to exit also. This happens for each electron that is emitted by the first plate in the individual area of the original light source. Thus amplifying the original image.
So now, the device has an amplified electron image and needs to convert this ‘electric image’ back into photons (light) that our eyes can detect.
This is where the Phosphor screen comes in. This screen when struck by the, now abundant, electrons emits its own photons from the parts of the screen where it’s hit. This reproduces the ‘low light’ image from the start, but much brighter.
When you are looking ‘through’ night vision devices, you are actually looking at a small glowing screen that is often magnified and focused by some optics for your eyes.
The simplest and cheapest phosphor screen to use in Night Vision Devices is a combination of zinc sulphide and copper, which emits a green glow.
This is why night vision has the distinct green colour and has the added benefit of being the colour shade that our eyes are most sensitive too!