NIGHT VISION DEVICE BASICS

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Categories: Night Vision

A Night vision device is an optoelectronic device that allows visible images to be produced under a low light illumination condition.

The image produced is typically monochrome (e.g., shades of green, as green was the easiest colour to look at for prolonged periods in the dark), or black and white. Night vision devices may be passive, relying solely on ambient light, or may be active, using an IR (infrared) illuminator to better visualize the environment.

The term usually refers to a complete unit, including an image intensifier tube, a protective and generally water-resistant housing, and some type of mounting system. Many NVDs also include optical components such as a sacrificial window, demist shields, telescopic lenses, or mirrors.

HISTORY OF NIGHT VISION DEVICES 

Night vision devices were developed for military application and first used in the World War II. They came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several “generations” of night vision equipment with performance increasing and price decreasing. Consequently, they are available for a wide range of applications, e.g., for gunners, drivers, and aviators.

OPERATING PRINCIPLE OF NIGHT VISION DEVICES

Operating principles are standard for all Night Vision Devises:

  1. The objective lens captures the light (most often emitted by various celestial bodies) that falls on multiple objects and is reflected from them. The quality of the future image directly depends on the number of collected light rays.
  2. The collected beams are focused in the next stage and enter the image intensifier tube (IIT), the primary design element of any NVDs responsible for light amplification. The image intensifier tube may refer to one of the existing generations (I-III). Depending on this, it will have its design features and unique capabilities.
  3. In the converter, photons of light are converted into a stream of electrons, which will be used to carry out further actions. Initially, it is relatively weak.
  4. To enhance the flow, electrons are subjected to a particular effect, which increases the speed of their movement. This automatically leads to a multiple increase in the number of particles.
  5. The luminescent anode is the next obstacle to the flow of electrons (already amplified). It is affected by an electric charge of low power, which leads to knocking out photons from the total mass of particles. The latter become strengthened many times over and are suitable for further manipulations.
  6. From the amplified photons, a stream is formed that is much more powerful than the original one. It is fed into the eyepiece, transforming into an image visible to people. It becomes brighter, more contrast, more apparent, and more detailed, which the user needs.
GENERATIONS OF NIGHT VISION DEVICES

Generation 1 (GEN I)

First generation passive devices, introduced during the Vietnam War and patented by the US Army, rely on ambient light instead of an infrared light source. Using an S-20 photocathode, their image intensifiers produce a light amplification of around 1,000× but are quite bulky and require moonlight to function properly.

Generation 2 (GEN II)

Second generation devices feature an improved image-intensifier tube utilizing micro-channel plate (MCP) with an S-25 photocathode, resulting in a much brighter image, especially around the edges of the lens. This leads to increased illumination in low ambient light environments, such as moonless nights. Light amplification is around 20,000×. Also improved were image resolution and reliability.

Generation 3 (GEN III)

Third generation night vision systems maintain the MCP from Gen II, but now use a photocathode made with gallium arsenide, which further improves image resolution. In addition, the MCP is coated with an ion barrier film for increased tube life. However, the ion barrier causes fewer electrons to pass through, diminishing the improvement expected from the Gallium arsenide photocathode. Because of the ion barrier, the “halo” effect around bright spots or light sources is larger, too. The light amplification is also improved to around 30,000–50,000×. Power consumption is higher than GEN II tubes.

The technology has been further improved later. First addition is an automatic gated power supply system that regulates the photocathode voltage, allowing the NVD to instantaneously adapt to changing light conditions. The second change is a removed or greatly thinned ion barrier, which decreases the number of electrons that are usually rejected by the Standard GEN III MCP, hence resulting in less image noise and the ability to operate with a luminous sensitivity at 2,850 K of only 700, compared to operating with a luminous sensitivity of at least 1,800 for GEN III image intensifiers. The disadvantage to a thin or removed ion barrier is the overall decrease in tube life from a theoretical 20,000 hrs mean time to failure (MTTF) for Gen III type, to 15,000 hrs MTTF for the improved GEN III type. However, this is largely negated by the low number of image intensifier tubes that reach 15,000 hrs of operation before replacement.

AUTO-GATING FUNCTION OF NIGHT VISION DEVICES

The Auto-Gating Function (ATG function) was designed to improve the BSP feature to be faster and to keep the best resolution and contrast at all times. It is particularly suitable for Aviator’s Night Vision goggles, operations in urban areas or for special operations. ATG is a unique feature that operates constantly, electronically reducing the “duty cycle” of the photocathode voltage by very rapidly switching the voltage on and off. This maintains the optimum performance of the image intensifier tube, continuously revealing mission critical details, safeguarding the image intensifier tube from additional damage and protecting the user from temporary blindness.

The benefits of ATG can easily be seen not only during day-night-day transitions, but also under dynamic lighting conditions when rapidly changing from low light to high light conditions (above 1 lx), such as sudden illumination of dark room. A typical advantage of ATG is best felt when using a weapon sight which experiences a flame burst during shooting. ATG would reduce the temporary blindness that a standard BSP tube would introduce, allowing them to continuously maintain “eyes on target”.

ATG provides added safety for pilots when flying at low altitudes, and especially during takeoffs and landings. Pilots operating with night vision goggles are constantly subjected to dynamic light conditions when artificial light sources, such as from cities, interfere with their navigation by producing large halos that obstruct their field of view.

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