How night vision goggles work: from photons to screen

The direct process of converting weak light fluxes into a visible image in night vision devices (NVDs) is based on the amplification of electrons knocked out by photons from the photocathode. Unlike thermal imagers, which detect thermal radiation, night vision goggles work exclusively with reflected light, be it the moon, stars or infrared illumination. The key element here is the electron-optical converter (EOC), where the signal is amplified multiple times, allowing the human eye to see in conditions of almost complete darkness.

If the device does not produce an image or the image is severely distorted, the problem often lies in the depressurization of the vacuum chamber of the image intensifier tube or degradation of the photocathode. Modern models use digital signal processing, but classic analog devices remain the standard for professional use due to their high response speed. Understanding the internal structure of the device allows you to choose the right model for specific tasks, be it perimeter security, hunting or tactical operations.

Physical basis of weak light detection

The basis of the operation of any night vision device is the phenomenon of external photoelectric effect. When a photon of light hits the surface of a photocathode, it knocks out an electron if the energy of the photon exceeds the work function of the material. These electrons form an electronic image that exactly replicates the brightness distribution of the light entering the device. The more photons that hit the photocathode, the more electrons are released, creating a brighter output signal.

The photocathode material is critical for efficient operation. Most often used in modern devices gallium arsenide (GaAs), which has high sensitivity in the near-infrared range. This allows the devices to capture light invisible to the eye, significantly expanding night vision capabilities. Older models could use cesium antimony, but their efficiency was significantly lower, especially in low light conditions.

⚠️ Attention: Direct contact with bright light (headlights, spotlights) on the input lens of a working device can irreversibly damage the photocathode, causing the appearance of “burnt” spots on the image.

It is important to consider that at least a minimal light source is required for the device to operate. In a completely black room, devoid of even infrared radiation, an analog NVG will not work without a built-in IR illuminator. Digital models can amplify the signal more, but the physical limit of photocathode sensitivity remains a fundamental limitation of the technology.

Electro-optical converter device

The heart of the device is image intensifier, which is a sealed vacuum tube. A high voltage is created inside it, which accelerates the knocked out electrons towards the phosphorus screen. When the electrons hit the screen, the energy is converted back into photons of visible light, creating a bright image. This entire process takes fractions of nanoseconds, which ensures that there are no delays when observing moving objects.

To increase image brightness, a microchannel plate (MCP) is used inside the image intensifier tube. This is a special glass disk with millions of microscopic channels located parallel to each other. When an electron enters the channel, it knocks secondary electrons out of the walls, starting an avalanche-like process. As a result, one incoming electron generates thousands of outgoing electrons, providing a gain of up to 50,000 times or more.

• 🔋 Power supply: Provides high voltage (several kilovolts) to accelerate electrons inside the tube.
• 🛡️ Protective glass: Filters ultraviolet radiation and protects internal components from mechanical damage.
• 💡 Phosphor screen: Converts electron flow into a visible glow, usually green.

Why is the image in night vision goggles green?

The phosphor screen is most often coated with P43 or P45 phosphor, which gives a greenish glow. The human eye is most sensitive to the green spectrum and is able to distinguish more shades of this color than any other. This allows the operator to work longer without fatigue and to better see the details of the situation.

The image quality directly depends on the resolution of the MCP and the homogeneity of the photocathode. Defects in the channel structure or uneven application of the sensitive layer lead to the appearance of noise, fixed pattern noise (FPN) and reduced image clarity. Expensive models use additional correction systems to minimize these effects.

Classification of generations of night vision devices

NVD technologies have developed over decades, and today it is customary to divide them into generations (Gen 1, Gen 2, Gen 3, Gen 4). Each subsequent generation offers significant improvements in sensitivity, resolution and service life. Understanding the differences between them is necessary for choosing the right equipment, since the price can differ tens of times.

First generation devices (Gen 1) use electrostatic focusing and do not have a microchannel plate or use simple analogues. They provide a 500-1000x increase in brightness, but have low resolution at the edges of the field of view and require IR illumination under new moon conditions. Third generation (Gen 3) became a standard for the military thanks to the use of a gallium arsenide photocathode and ion barrier film, which significantly increased sensitivity and service life.

Parameter	Gen 1	Gen 2+	Gen 3
Sensitivity (μA/lm)	~200-400	~500-800	~900-1500+
Signal to noise ratio	10-15	20-25	25-35+
Image intensifier resource (hours)	2000	4000-6000	10000-15000
Work without IR illumination	Difficult	Possibly	Confident

There is also an intermediate generation Gen 2+, which incorporates improved manual transmissions and auto electronics, approaching the characteristics of the third generation, but remaining more affordable. Fourth generation (Gen 4) is not formally certified by the US military as a separate class, but designates Gen 3 devices with removed ion barrier film and an automatic power system, which gives better contrast and performance in conditions of sudden changes in light conditions.

Digital technologies versus analog image intensifier tubes

In recent years, digital night vision devices have been taking over the market. Instead of an analog tube, they use a highly sensitive CMOS or CCD matrix, like in video cameras, but with increased sensitivity. The image from the matrix is ​​processed by the processor and displayed on the built-in display (LCD or OLED), which the user views through the eyepiece. This allows you to implement functions that are not available to analogues: video recording, Wi-Fi, zooming without losing the quality of the optics.

The main advantage of “digital” is the absence of fear of bright light. You can safely turn on a digital night vision device during the day, and it will not burn out, while for an analog device this is fatal. In addition, digital devices are often cheaper to manufacture and do not have the export restrictions of Gen 3 technologies. However, they have the significant disadvantage of image latency (lag) and higher power consumption.

• 📹 Data recording: The ability to save photos and videos to a memory card directly during observation.
• 🌗 Operating modes: Switch between black and white, green and sepia mode to reduce eye fatigue.
• 🔍 Digital zoom: Enlarges the image programmatically, although this reduces detail.

⚠️ Attention: Digital NVGs can have a noticeable image delay (up to 50 ms or more), which is critical when shooting at fast-moving targets or in dynamic tactical situations.

Analogue devices still have the advantage in resolution and instantaneous response. For professionals where every millisecond and detail matters, image intensifier remains the only choice. Digital is great for amateur hunters, guarding stationary objects and situations where versatility is important.

Optical system and ergonomics

The quality of the picture depends not only on the electronic unit, but also on the optics. The input lens collects the light, and the output eyepiece focuses the image for the eye. The most important parameter is the lens aperture, determined by the diameter of the input lens and the focal length. The larger the lens diameter, the more light hits the photocathode, which directly affects the detection range of objects.

The ergonomics of night vision goggles are critical since the device is worn on the head. The weight of the device must be perfectly balanced so as not to overload the cervical spine. Modern models use lightweight composite materials and a well-thought-out fastening system (Shroud headband as standard Mil-Spec). The presence of automatic adjustment of interpupillary distance (IPD) allows you to customize the device for a specific user.

An important element is anti-fog protection. If there is a sudden change in temperature (leaving a warm room into the cold), the lenses may become covered with condensation. High-quality models have nitrogen-filled housings and multi-layer coating of optics, which also increases light transmission to 98-99%. The lack of enlightenment can “eat up” up to 40% of the useful light, negating the benefits of an expensive image intensifier.

Operation, care and typical problems

For long service life of the device, you must follow the operating instructions. The main enemy of NVGs is moisture and shock. Even light impacts can disrupt the alignment of the optics or damage the fragile structure of the image intensifier tube. The device should be stored in a dry place, preferably in a sealed case with silica gel. Regularly replacing batteries is also important, as battery oxidation can damage electronics.

A typical problem is the appearance of dark spots or "dead pixels" in the image. In analog devices, this may be defects in the photocathode or dust inside the tube. In digital ones - stuck pixels on the matrix. Minor defects (less than 0.2 mm) are considered acceptable quality standards and will not affect the overall performance of the device.

If the image shakes or floats, check the battery voltage. When the charge is low, the power system cannot provide a stable high voltage for the image intensifier, which leads to unstable operation. It is also worth checking the contacts in the battery compartment - oxidation or poor contact is often the cause of false faults.

FAQ: Frequently asked questions

Can night vision goggles be used during the day?

Analog devices (Gen 1-3) should absolutely not be turned on in bright daylight - this will instantly damage the image intensifier. Digital NVGs are safe for daytime use, since their matrices are protected from overexposure and the image is generated electronically.

Can you see through the window glass with night vision goggles?

Yes, you can see through glass, but with limitations. Glare from IR illumination can be reflected from the glass and illuminate the picture. In addition, ordinary window glass can cut off part of the IR spectrum, reducing the efficiency of the device.

What is the range of night vision goggles?

The range depends on the generation of the device, lighting and object size. For Gen 3, on a moonlit night, human recognition is possible at a distance of 300-400 meters. Detection (a luminous object is visible) is possible at 600+ meters, but without optical zoom it is difficult to see details.

Why do night vision goggles sometimes show rainbow rings?

This is an optical effect associated with the design of the lenses or the features of the image intensifier. In analog devices, the phenomenon of “brain-out” (light flare) from bright points of light is also possible, which looks like blurry halos around light sources.