Night vision technologies and principles

Overview of night vision capabilities

Night vision is the ability to see in low light conditions. It can be achieved both naturally (through scotopic vision) and with the help of night vision devices.

Two key factors are required for effective night vision:

• Sufficient spectral range - the ability to detect infrared or other invisible light.
• Sufficient intensity range - sensitivity to very weak light sources.

The human eye has limited night vision capabilities compared to some animals (e.g., cats, foxes, or rabbits). This is due to the absence of tapetum lucidum, a reflective layer behind the retina that increases sensitivity to light in the dark.

Spectral range

Night vision technologies utilize bands of the electromagnetic spectrum that are invisible to the human eye. Normal human vision is limited to visible light, which covers a narrow part of the spectrum.

The extended spectral range allows night vision devices to detect and utilize radiation in the near-infrared or ultraviolet ranges. This allows you to observe objects in low-light or complete darkness, where the human eye is unable to recognize images.

Please note: Some animals, such as mantis shrimp or trout, have the ability to see in wider ranges of the spectrum, including infrared and ultraviolet radiation. This demonstrates the potential that can be realized with night vision technologies.

Intensity range

Adequate intensity range is the ability to see in very low light levels.

Many animals have better night vision than humans, due to the peculiarities of their eye morphology and anatomy. These features include:

• larger eyeball size
• enlarged lens
• wider optical aperture (pupils can expand to the physical limits of the eyelids)
• predominance of rods over cones or exclusive presence of rods in the retina
• presence of tapetum lucidum, a reflective structure that enhances light sensitivity

Increasing the intensity range in night vision is achieved with the help of technology:

• an electronic optical converter (EOC) - a device that enhances the image in low light
• CCDs with gain multiplication (gain multiplication CCD)
• arrays of photodetectors with very low noise and high sensitivity

It is useful to know that the image intensity of night vision devices directly depends on the sensitivity of photodetectors and the efficiency of signal amplification.

Biological night vision

All photoreceptor cells in the vertebrate eye contain molecules of photoreceptor protein, which consists of a protein (photopsin or rhodopsin) and a retinal molecule. Photopsin is present in the cells responsible for color vision (cones), and rhodopsin is present in the cells responsible for night vision (rods). Retinal is a small molecule that changes its shape when light is absorbed, which leads to a change in the structure of the protein and triggers the physiological process of vision.

After activation, retinal leaves the photoreceptor cell, enters the bloodstream and is transported to the liver, where it is restored. Under bright light conditions, most of the retinal is outside the eye, so it takes up to 45 minutes for the photoreceptors to fully recover their sensitivity to darkness. However, most of the adaptation to darkness occurs within the first 5 minutes.

In the dark, only the rods have sufficient sensitivity to provide vision. The rhodopsin contained in the rods does not respond to long wavelengths of red light. That is why red light is traditionally used to preserve night vision - it activates only the cones without depleting the rhodopsin in the rods.

There is also a hypothesis that starlight, having shorter wavelengths (in the blue-green spectrum), has less of an effect on sensitivity to red light, so using red lighting allows you to maintain the ability to see stars.

Many animals have a tapetum lucidum, a layer of tissue behind the retina that reflects light back through photoreceptors, increasing light sensitivity.

The human pupil dilates in the dark to improve night vision. In some conditions, it can reach a diameter of up to 9 mm, although such dilation is rare without the use of mydriatic agents.

In nocturnal mammals, the rods have a unique structure: the nuclear structure of the cells is inverted - heterochromatin is located in the center of the nucleus, and euchromatin is located on the periphery. In addition, the outer nuclear layer of the retina in these animals is much thicker due to the high density of the rods. The light passes through 8-10 nuclei before reaching the photoreceptor part of the cell, and the inversion of the nuclei creates a lens effect that directs light without scattering. This increases the light sensitivity of the retina by 8-10 times without losing focus.

Night vision technologies

Night vision technologies are divided into three main categories:

• Image enhancement
• Active illumination
• Тепловізійна технологія

Image enhancement

An image intensifier is a device based on a vacuum electron tube, in particular a photomultiplier tube, that allows you to form an image even in very low light conditions (for example, under the light of stars). Such an image can be observed in real time or stored for further analysis.

Important: Although it is often said that light is "amplified," light is not actually amplified. When photons strike the charged photocathode plate, it emits electrons. These electrons pass through a vacuum tube and hit the microchannel plate, which causes the screen to glow in accordance with the incoming light pattern in the range visible to the human eye.

Amplification means that the output image is much brighter than the input image, which allows you to see in the dark. This is what distinguishes passive night vision devices (NVDs), which use natural light, from active ones, which require additional illumination.

Good to know: the principle of operation of an image intensifier is similar to that of a CRT TV, but instead of electron guns, a photocathode is used.

The most common type of image intensifier is the ANVIS (Aviator's Night Vision Imaging System) module, which can be easily integrated into helmet systems. There are also many other models and form factors. Recently, the US Navy announced its intention to purchase a two-color version of ANVIS for use in aircraft cockpits.

Digital night vision technologies

Digital night vision is based on highly sensitive back-illuminated CMOS sensors, also known as sCMOS (scientific CMOS). Such sensors have a sensitivity that can exceed that of the human eye.

Active infrared illumination

Active illumination combines image intensification technology with an infrared source in the near-infrared (NIR) or short-wave infrared (SWIR) range. Examples of such systems are low-light cameras.

Active infrared night vision uses illumination in the spectral range of 700-1,000 nm - just outside the visible spectrum for the human eye. Cameras or CCD sensors sensitive to this radiation generate a monochrome image that is displayed on a conventional display.

Important: Thanks to the ability to use powerful IR illuminators, active night vision systems provide higher image resolution than other night vision technologies.

Such systems are widely used in commercial, residential and public security applications, providing effective video recording in low light conditions. Please note that active IR illumination can be detected by night vision devices, which creates a risk of position disclosure during military operations.

Laser range gated imaging is another type of active night vision. It uses a powerful pulsed light source and synchronizes the pulses with the shutter speed of the camera's detector. There are two modes:

• Single Pulse - the camera captures images in a single pulse;
• Multiple integration - the camera accumulates light from several pulses to form an image.

The advantage: this technology allows not only detecting objects, but also identifying them (target recognition).

Night vision devices

Before the advent of electronic-optical converters (EOCs), the only way to observe at night was with so-called "night binoculars" - optical devices with a large aperture. During World War II, they had a lens diameter of 56 mm or more and provided a magnification of 7-8 times.

Important: The main disadvantages of such devices were their large size and weight, which limited their mobility and ease of use, especially in land operations. At the same time, they were widely used in the navy.

Modern technologies

A modern night vision device (NVD) is a device that contains a hardened cathode ray tube. Such devices are actively used by military units, but recently the technology has become available for civilian use.

Good to know:Enhanced vision systems (EVS) are integrated into the avionics of modern aircraft (e.g., Cirrus or Cessna) to increase pilot situational awareness and prevent accidents. The US Navy has also begun procuring variants of such systems for integration into aviation equipment.

Night vision goggles are a separate type of UAS, which provide a binocular image with a wide field of view and can be used by both infantry and aviation.

The PVS-14 is one of the most widely used monocular night vision devices used in both the military and civilian sectors.

UAS are also used in the automotive industry. Automotive night vision systems use infrared cameras, sometimes in combination with active lighting, to improve driver visibility in darkness or bad weather. Such systems are available as an option in some premium car models.