In the simplest way,
Image Intensification works by collecting and amplifying tiny amounts of light, including the Near IR (NIR) portion of the electromagnetic spectrum.
Analog night vision devices are ran by Image Intensifier Tubes (sometimes written as IIT or I2 tubes) which collect and amplify the light spoken of above. This is achieved via a collection of components inside of the tube itself, which are the Photocathode (PC), Microchannel Plate (MCP), and Phosphor Screens.
To understand these devices a bit more in depth, we will cover them below.
What is a Photocathode?
In the simplest terms, a photocathode is a surface that is engineered to convert Photons (visible and NIR light) into electrons. This is coated with a photosensitive material, such as S-25 Multialkali (Photonis and Gen 2+) and Gallium Arsenide/Gallium Arsenide Phosphide, which convert the photons via the Photoelectric Effect.
What is a Microchannel Plate?
Only found in Gen 2 and Gen 3 I2 tubes, a Microchannel Plate (MCP) is a specially fabricated plate that amplifies electron signal. A MCP has several million independent channels in array, and each channel works as an independent electron multiplier via cascading secondary emissions when an electron strikes the wall of the channel. This causes the secondary electrons to be released, and then accelerated by a high voltage burst applied to both ends of the MCP that create an electrical field. These secondary electrons will also then strike the channel surface, and create their own secondary electron emissions, which will continue to cascade, leaving you with a large output of electrons at the other end. These MCPs are usually coated in a dielectric aluminum oxide film called "Ion Barrier Film," which extend the life of the I2 tubes.
What is a Phosphor Screen?
A Phosphor Screen is a multi-layered screen containing a thin aluminum layer, a phosphor, and a support layer. Electrons slam into the aluminum layer of the Phosphor Screen and pass through to the phosphor. This excites the phosphor and causes it to emit photons. The aluminum film must be thick enough to reflect the photons from the excited phosphor through the phosphor screen rather than be wasted by reflecting back into the tube, but also must be thin enough to allow more than enough of the multiplied accelerated electrons.
The quick and dirty
- Light and NIR light first enter the objective lens and is focused and directed to the glass input window of the I2 tube.
- The photons entering the glass input react with the Photocathode, which converts the available light into electrons.
- Those electrons then hit the electrode on the input side of the Microchannel Plate, and then are accelerated into the glass microchannels by high voltage bursts, this causes electron multiplication by the tens of thousands.
- These electrons maintain their position in relation to the channel that they passed through, which keeps the output image in electrical energy the same as the photonic input image.
- The multiplied electrons then hit the thin aluminum oxide layer on the Phosphor Screen, pass through, and then are converted into photons to output an image through a fiber-optic twist, which is then presented to the optical lens, and then the end users eye.
Well wait, Fiber Optic Twists?
That is correct, most fiber optic imaging is done through fused fiber optics. Fiber optic inverters can 'flip/twist' an image by 180 degrees. This technique is used in Night Vision as it reduces traditional optical path length in order to provide the most compact and lightweight option. This is needed as the image becomes flipped during the image intensification process. This can introduce what is called "S-distortion." This is usually very small and almost unnoticeable by the human eye.
And Ion Barrier Film? What is that?
An Ion Barrier Film is an ultra thin dielectric film made of aluminum oxide, that is applied to the input side of the MCP on Generation 3 I2 tubes. This is designed to prevent ions generated in the MCP during operation from migrating back towards the photocathode, and damaging its activation layer (for GaAs/GaAsP, this would commonly be a set of two cesium oxides. Cesium Peroxide and Cesium Superoxide.) This can affect Signal to Noise ratio (electron scattering), as well as Photocathode Response and affect how tubes work in the darkest of environments.
Ion Barrier Film used to come in multiple thicknesses. The thicker the film, the longer the I2 tube's lifespan. This technology has had a lot of research and development through the years, and only continues to get better as time moves on.
Thin filmed tubes are the new normal for 3rd Generation I2 tubes, while L3 has developed a 'filmless' technology, where they have been able to increase the life of the Photocathode, while benefiting from the increased Photocathode Response, and increased signal to noise.
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