Night vision devices let you see in the dark. High-quality night vision goggles and scopes allow you to observe people, animals, and objects at distances of up to 1,000 yards during the night. Monocular cameras designed for night vision capture photos and videos of things that are invisible to the naked eye in darkness. But, how do NVG tubes work?
When it comes to night vision, most of us naturally assume that these products enable us to see at night. However, various night vision technologies serve distinct purposes and possess varying capabilities. Within the broad category of “night vision,” specialists commonly distinguish among three primary types: night vision with image intensifiers, devices for digital night vision, and thermal imaging equipment.
Despite many differences, they all share a common architecture: an optical tube customized for imaging infrared light and an integrated infrared sensor that converts invisible infrared light into visible light, which can be seen and analyzed by the human eye.
How Do NVG Tubes Work?
The older night vision gear utilizes technology for enhancing images through optics and electronics. Optical lenses and a unique electronic vacuum tube work together to seize and boost the visible and infrared light bouncing off nearby objects.
The system’s initial lens, known as the objective lens, captures faint visible light reflected from the subject and some light from the infrared spectrum’s lower part. Similar to all light, this comprises tiny particles termed photons.
These photons move through the objective lens and reach an image-intensifier tube. This distinct electronic vacuum tube is fueled by two small AA or N-cell battery components.
The initial section of the tube is named the photocathode. This part transforms the incoming photons into electrons. As you might recall from your science class, photons, neutrons, and electrons are all very minute particles constituting the elements of an atom. Photons and neutrons join to form the atom’s nucleus—electrons revolve around the nucleus and possess an electrical charge.
These generated electrons move into the subsequent section of the vacuum tube, called the microchannel plate (MCP). The MCP is a petite glass disc with millions of minuscule holes that increase the quantity of electrons, thereby magnifying the electric signal several thousandfold.
As electrons leave the end of the image intensifier tube, they strike a screen coated with phosphors. When hit, these phosphors on the screen light up, forming a bright green image much brighter than the faint light initially entering the objective lens. You observe the phosphor image using an ocular lens, allowing you to focus and, if needed, enlarge the image.
Why doesn’t this traditional night vision display show colors? Why is green used in NVG? This happens by transforming photons into electrons, removing color details from the image, and changing the original colored light into a black-and-white display. Green phosphors were chosen because green is the most convenient color to observe for extended periods in low-light conditions.
Night Vision Image Intensifiers
We are familiar with the image produced by the intensifier tubes we have seen in movies, news, or popular science publications: it is an optic that produces green and black images. Intensifier tubes use reflected ambient infrared radiation, invisible to the human eye, to generate images of the view. Like visible light, this infrared radiation is emitted by the Sun and the stars, illuminating everything—including the Earth and the Moon.
The infrared light generated by the stars reflects from the Moon and forms the ambient infrared illumination used by night vision devices at night. Intensifier-tube night vision devices convert the infrared light reflected from objects to visible images. At the heart of these devices are the intensifier tubes, which are constructed of tubes with a photocathode at one end, an internal anode in the middle, and a phosphor screen on the other. High voltage is applied between the photocathode and the anode to create a strong electrostatic field.
When infrared light strikes the photocathode, electrons are emitted and accelerated by the electric field toward the phosphor screen, which produces a visible image. Since the inception of these devices, their basic principle of operation has remained the same; however, resolution, clarity, and image brightness have improved significantly over the years.
The Generation Gap
Generation 0 is the oldest image intensifier technology, dating to the German army’s first military use during World War II. The operation concept was inspired by the RCA Corporation’s image-converter tubes, developed in the mid-1930s for television use. 0-generation photocathodes, called S-1 cathodes (AgOCs), had very low efficiency, low gain, and short range and produced very dim images on the phosphor screen. To be useful, 0-generation tubes needed powerful external infrared lamps to illuminate the scene.
Since then, this type of night vision evolved from generation 0 to generation 3, improving sensitivity, resolution, image clarity, brightness, and color. However, the main concept of operation remains the same: conversion of reflected ambient infrared light into visible light. Generation 0 technology is considered obsolete and not in production nowadays.
To improve sensitivity, gain, and image brightness, and to reduce reliance on large infrared lamps, a new 1st-generation multi-alkali photocathode design ( employing a sodium-potassium-antimony-cesium “Na-K-Sb-Cs” formula, commonly referred to as an S-20), connecting three intensifier tubes in series, was introduced in the early 1960s. It proved successful in significantly improving sensitivity, gain, and image brightness but made night-vision devices larger and heavier.
Another drawback was that they produced images with a clear, bright center but distorted, darker edges. Additionally, 1st-generation tubes exhibited image blooming, a momentary image washout due to an overexposed phosphor screen.
Today, Generation 1 night vision devices have the same basic design but, thanks to improvements in manufacturing processes, produce images with a resolution of up to 35 lp/mm. This technology is available to consumers and typically is not subject to ITAR export restrictions.
The 2nd-generation night vision technology was born around the late 1960s with the introduction of micro-channel plates (MCP) inside the intensifier tubes. MCPs amplify the number of electrons reaching the phosphor screen thousands of times, increasing the device’s gain. Another significant improvement over 1st-generation tubes was a refinement to S-25 photocathodes.
They also enhanced the sensitivity as well as spectral responses of the devices. The overall increase in sensitivity and gain was enough to obtain bright and clear images with only one intensifier tube. This greatly reduces the weight of NVDs, allowing for headgear-mounted and weapon-mounted configurations. Because they only feature one intensifier tube, they exhibit superior edge-to-edge image clarity and less blooming. Current 2nd-generation devices produce bright and clear images with a resolution of up to 54 lp/mm.
In the mid-1970s, the introduction of gallium arsenide (GaAs/AlGaAs) photocathodes was a major advancement in intensifier tube technology that marked the emergence of 3rd-generation devices. The new tubes had much greater sensitivity, resolution, and signal-to-noise ratios (SNR), which improved detection range and performance in low-light conditions.
However, due to the chemical interaction of gallium arsenide with the MCPs, these tubes degraded easily. To solve this problem, the MCP was insulated by a thin film of metal oxide, an ion barrier, at the price of slightly higher electronic noise and lower SNR, and because of the noise, image detail also suffered.
Despite these drawbacks, the overall performance was much better than that of 2nd-generation devices. In today’s market, one can expect 3rd-generation devices with up to 75 lp/mm resolution and superior sensitivity, image quality, and resolution. These devices are under ITAR restrictions and available only to military and law enforcement.
In a constant quest for better performance, manufacturers tried to overcome the limitations of 3rd-generation devices with an ion barrier film to reduce electronic noise by attempting to develop filmless intensifier tube technology. They succeeded to some degree, and this technology was briefly called the 4th-generation night vision, but the manufacturing costs were excessive compared to performance improvements. This terminology was quickly retracted and called 3rd-generation filmless image intensifiers. Currently recognized classification of intensifier tube devices follows generations 0, 1, 2, and 3.
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