How Night Vision Works

How night vision works: Everything you always wanted to know about night vision including low-light imaging techniques, thermal imaging and near-infrared illumination.

Introduction

The ability of human beings to see in the dark is limited. Even those with excellent eyesight, under conditions of low light (i.e. at night), can still only see objects within a few meters of themselves, and even then they will only be able to discern the outlines of the objects. 

For example, if a person were walking down a pathway at night, they would be able to see the path, but they would not be able to see much detail about the plants on either side of the path.

Humans have been able to extend their night vision for hundreds, if not thousands of years. The first well-known use was in ancient Egypt where guards on a watch used tallow and oil lamps to provide some light around their post. 

The Egyptians found that by filling these lamps with oil and animal fat, the smoke would fill the area around them and hide them from any enemies. A similar method was used in the Middle Ages in castles where oil lanterns were hung from the ceilings in order to keep intruders at bay.

Up until World War II, most human beings could not see in the dark very well at all. The development of practical night vision technology did not start until the 1930s when the first infrared detectors were developed for military use because of its ability to detect heat sources, such as gun barrels, engine exhausts and warm bodies.

The first practical, usable night vision was developed by the US Army Air Corps just before World War II started. 

It consisted of an image intensifier tube that used a photocathode and an electron multiplier to convert photons into electrons. This electron stream was amplified to produce an image on a fluorescent screen. The image was green or blue, depending on ambient lighting conditions. 

The screen was viewed through a simple magnifier. This system was first used in 1939 in the Bristol Blenheim bombers of the RAF. These devices were used during the Battle of Britain to aid the pilots in their night raids. 

When it was discovered that the Germans were using infrared technology to target British bombers, the British developed a new type of detector, known as the Mk III Airborne Intercept (AI), to counter this threat.

After World War II, infrared technology was developed for military purposes and for scientific experiments. In the late 1960s, night vision military applications were declassified and made available to civilians. 

The first commercial night vision devices became available in the early 1970s. This technology was introduced to the consumer market by a company called Pulsar. 

Pulsar began as an aerospace company but eventually branched out into civilian applications. They made night vision devices and sold them to the public through advertisements in popular magazines such as Popular Mechanics, Popular Science, and Guns and Ammo. 

Pulsar eventually sold out to Raytheon Company, a large defense contractor that is still involved today in the production of night vision devices.

How do we see in the dark?

How do we see in the dark?

Humans have two primary ways of seeing in the dark; firstly by using their eyes, and secondly by using a device to amplify the amount of light that enters their eyes.

Either way, the first step is for the light from an object to be detected by the eye or by a device such as a camera.

Secondly, this information must be processed by the brain in order for it to be useful. Lastly, the object must be detected by an individual’s senses (i.e. sight or hearing).

The process of vision begins when light enters the eye through the cornea, which is the outermost layer of the eye. 

Light then passes through the pupil, which is a hole in the middle of the eye that is controlled by muscles that can expand or contract to control the amount of light that enters. 

Light then passes through layers of tissue (known as the lens) and onto the retina, which is a very thin tissue at the back of the eye that contains photoreceptors called rods and cones. 

Rods are responsible for peripheral vision and are active in dim light, while cones are responsible for central vision and are active under well-lit conditions. Cones contain color receptors that are sensitive to red, green or blue light.

The human eye can detect a single photon of light, but it takes 10–14 photons to excite a rod or cone cell to produce an electrical response that can be detected by the brain and translated into an image. 

This means that in low-light conditions, very little incoming light is detected by rods or cones and so visual acuity is poor. In such conditions, we rely primarily on our ability to detect motion and use peripheral vision.

The degree of sensitivity (i.e. how much light a person needs to see) varies widely from person to person depending on their age and general health.

It also depends on what type of photoreceptors they have remaining in their retina has rods and cones deteriorate with age and exposure to bright light (as occurs with photographers). 

It is rare for a healthy person over 50 years old to have any useful visual acuity after dark because their rods and cones have deteriorated due to old age or exposure to bright sunlight during their lifetime.

Image Intensifier

Image Intensifier

Image intensifiers produce a visible image from a low-level invisible one. In the case of night vision, this is done by using a special photocathode that produces an electron image of the scene being viewed. The electron light image is then amplified by a vacuum tube until it is bright enough to view.

How does an Image intensifier work? 

Image intensifiers, also known as night vision devices (NVDs), amplify existing light by converting photons into electrons via a photocathode and an electron gun before they are amplified using an electron multiplier or microchannel plate (also known as a Photomultiplier Tube or PMT). 

The electron stream is passed through an objective lens which focuses it onto a phosphor screen on which it is converted back into photons creating an image that appears green on black with low gain NVDs or black with high gain NVDs

Electron Multiplying CCD (EMCCD) 

Electron Multiplying CCD (EMCCD)

An EMCCD is a more advanced version of an image intensifier tube (IIT) that produces better images at higher frame rates than IITs can produce alone at any given gain factor. 

The main advantage of EMCCDs over IITs is that the EMCCD can amplify the signal electronically instead of optically, which allows the amplification to be changed on a frame by frame basis, allowing for a greater dynamic range and much lower noise. 

The electron multiplication process in an EMCCD is analogous to that in a photomultiplier tube (PMT), although an EMCCD gives a higher gain. 

The EMCCD typically runs at a lower voltage than a PMT but has a higher gain and is much less sensitive to magnetic field interference.

How EMCCD Works?

The basic mechanism of an EMCCD is the same as that of an image intensifier tube. 

“The image intensifier is a vacuum-tube device that uses cathode ray technology to convert photons into electrons.” (Nii, 2012) Cathode ray technology uses the focusing properties of charged particles (electrons) to form an image. 

“In a cathode ray tube (CRT), electrons are attracted by a magnet and focused into a tight beam by a metal plate known as the deflection plate.” (Image Intensifier, 2007) The deflection plate is designed to deflect the electrons in a way that forms an image on the phosphor screen. 

In an image intensifier tube, the phosphor screen is replaced by an optical lens and the electron beam is deflected several times before reaching the photodetector. 

The photodetector converts the electron beam photons into electrical signals that are then amplified and converted back to photons, creating an image.

Night vision goggles

Night vision goggles

Night vision goggles allow you to see in the dark by amplifying the light that is available. They do this by using an image intensifier tube that multiplies the available light to produce a visible picture. They also have a sensitivity control that allows you to choose how much light is “amplified”. This control is called the gain.

How do night vision goggles work? 

An image intensifier tube consists of two or more vacuum tubes that are sealed with a special gas. They are referred to as a cathode ray tube (CRT). 

Inside the CRT is a small amount of gas that is ionized by an electron gun on the side of the tube. 

The electrons in the beam from the gun pass through the gas in the tube as they travel to the screen, ionizing the gas in their path. 

This produces electrons and positively charged ions. Due to the high voltage difference between the electron gun and the screen, the electrons stream towards the screen, but are blocked by an electrical field that surrounds it. 

The charged ions, however, are not blocked by this field and continue towards the screen, creating an image of what is happening outside the tube. 

The ions are attracted to a phosphor coating on the inside of the CRT, which glows when struck by the ions. The glowing phosphor then produces a visible picture on the inside of the CRT that shows what is going on outside of it.

The sensitivity control, or gain control, allows you to choose how much light to let into the tube, thus allowing you to adjust how bright you want your picture to be. 

In order for your picture to be seen properly, it must be adjusted so that it is not too bright or too dim. If it is too bright, you will get washed out colors and excessive noise in your image; if it is too dim, you may not be able to find your target, or you may not be able to see it clearly enough to use your weapon against it. 

The proper setting will allow you to clearly see your target while still being able to watch out for threats and keep track of your team members’ locations and positions. 

This is usually done at night during a mission rehearsal before a mission takes place. 

The mission commander will set up an area where he can adjust the gain so that he can determine what setting will work best for a given area or situation.

Why does everything look green through night vision goggles?

When you look through night vision goggles, everything will appear green. This is because google’s detector is sensitive to the green part of the light spectrum, but not sensitive enough to see red. Our eyes, on the other hand, are sensitive enough to see both red and green.

Additionally Read: Best Night Vision Binoculars

Thermal Imaging

Thermal Imaging

Thermal imagers detect infrared radiation, which is different from visible light. The picture that is produced by thermal imaging looks different because it picks up objects based on their temperature, not their color. 

The hotter something is, the brighter it will show up in the picture. Cold things will show up as being black or very dark gray. Hot things will show up as being white or very bright gray. 

To the naked eye, most objects appear to be black or gray, but when viewed through thermal imaging they may look lighter or darker than they do to the naked eye. 

This allows a thermal imager to pick up on differences in temperature that are too small for the naked eye to see.

Additionally Read: Best Night Vision Rifle Scopes

How does a thermal picture sensor work? 

Thermal imagers work by using microbolometer arrays to detect infrared radiation. These arrays contain thousands of tiny elements called “thermocouples”. 

A thermocouple is a device that changes its electrical resistance based on the temperature of its surroundings. When infrared radiation strikes one of the elements in the array, it warms the thermocouple causing its electrical resistance to drop.

This change in resistance is then recorded by a special circuit and used to produce an image of what is going on around the thermocouple. 

The microbolometer array is actually composed of two thermocouples laid next to each other with an insulating layer separating them and a metal layer covering them.

One of the thermocouples is used to measure the ambient temperature around the imager while the other is used to measure the temperature of whatever is being viewed. 

The microbolometer array then compares these two temperatures and uses this information to produce an image of what is being viewed.

Most thermal imagers have a resolution of around 80×60 pixels in size, which allows them to produce pictures with a resolution of around 320×240 pixels. 

Some newer models are capable of producing images with a resolution of 640×480 pixels, which is twice as good as most other models. 

The reason for this increased resolution is that these imagers use microbolometer arrays that have four times as many thermocouples as older models and they can produce a full-color image that is in 4:1:1 ratio, which means that every pixel has information about its value in red, green, and blue. 

This allows them to produce an image in full color with an increased level of detail and accuracy.