A night vision image (NVI) tube is a key design element of any NVDs. It helps the device convert the light collected by the lens and amplify it many times, achieving better image quality. Several classifications separate this device into different categories. All of them are actively used in the description and analysis of the characteristics of optics. To better understand all the nuances of NVI tubes, you need to take some time and read our guide.

What are NVI tubes?

The NVI or image intensifier tube (IIT) is the primary design element of all devices supporting night vision technology. It is nothing more than a vacuum photoelectronic device responsible for converting radiation invisible to humans into a high-quality image. The latter is greatly enhanced by increasing the brightness and contrast, becoming visible to the human eye. The main task of IIT is forming a stream of electrons from the photons of light coming from the lens. A minimum of time is spent on this process, but its significance for obtaining the final result is as enormous as possible. 

Further, under a particular influence, the number of electrons increases. Photons of light are knocked out of it, forming a multiply amplified stream. Based on it, the final image is formed, on which objects previously outside the field of view of the human eye become visible.

NVI tubes are the primary design element of various night vision devices. The most popular are monoculars, binoculars, scopes, and goggles. Each type of device effectively copes with specific tasks, which allows you to maximize the scope of night optics. From this, we can conclude that NVI tubes enable people to perform various previously impossible work without a bright light source.

Generations NVI tubes

One of the main classifications of all IITs is the division of these structural elements into generations. There is much controversy about the number of such ages and their names. However, according to official terminology, all NVI tubes are divided into five categories. Three of them are essential, and two are intermediate.

Generations image intensifier tubes:

1. Gen I. The most straightforward and least efficient in operation are the first-generation IITs. They appeared many decades ago, but they are sometimes used today. This is done only in those cases when it is necessary to slightly improve visibility and obtain an image in which only the outlines of objects will be visible without the possibility of viewing details. Most night vision devices with Gen I tubes are designed exclusively for simple tasks. This is due to the weak characteristics of the IIT itself and the optics in general (the range is at most 75 meters, and the battery life is only 2-3 hours). Another essential feature of first-generation IIT devices is blurring at the edges of the image. Because of it, it becomes tough to see anything on the display. The negative is also added by the lack of protection from exposure to excessively bright light. The only advantage of such a tube and the devices in which it is installed is its low cost. Thanks to this, Gen I models have become available to everyone.
2. Gen I+. The NVI tubes, which belong to the intermediate generation Gen I+, are a slightly improved version of the Gen I tubes. The critical change is the absence of blurring and other distortions at the edges. Getting rid of this shortcoming can be called a significant breakthrough, which made it possible to improve the quality of the resulting image significantly. This was made possible using a fiber optic plate, absent in predecessor models. Various changes made to IIT have helped Gen I+ devices more than double their light gain. Of the negative, the lack of changes in protecting the handset from exposure to bright light should be highlighted. This made the IIT more vulnerable and less durable.
3. Gen II. The second-generation NVI tubes are classics that remain relevant even after the advent of more advanced models. The creation of Gen II was a real breakthrough in the field of night vision. The change affected all aspects, from the tube design to the values of critical parameters. In addition, IITs were supplemented with a microchannel plate, unique at that time, which opened up new possibilities for improving the quality of the created image. Thanks to this design element of the image intensifier tube, it was possible to increase the brightness of the light at the output several times and eliminate most of the noise. As a result, the picture became more precise and more detailed. In addition, with a second-generation NVI tube, we forgot about blurring at the edges and other defects forever. Some design solutions have also helped to increase the working life of IIT, which automatically increased the life limit of all night vision equipment. The only thing that could not be avoided was the glare that appeared due to the impact of point sources of bright light.
4. Gen II+. Second-generation NVI tubes are good but could be better. Therefore, they have been carefully refined, and a separate category of Gen II + tubes has been obtained from them. These IITs offer slightly improved performance and are on par with Gen II in most critical areas. Because of this, the differences between adjacent generations become significant. One of the main differences is the reduced dimensions. This applies not only to IIT but to the entire device. Also, positive changes affect such essential aspects as energy consumption and life limit. Despite these transformations, Gen II + remains a second-generation model. Therefore, they are inferior to Gen III devices in many ways.
5. Gen III. The most efficient today are third-generation NVI tubes. They do their job well and guarantee high image quality. These design details have excellent performance characteristics, improving all optics' performance. In the manufacturing process of each component, advanced materials are used. This contributes to a multiple increase in reliability and maximizes the working life of NVI tubes. Devices using Gen III tubes are the most versatile. They can be used for almost any purpose. This became possible due to the full-fledged protection of IIT from bright light and other external influences. It opened access to the operation of optics in any lighting conditions. The Gen III generation handset provides a record increase in brightness compared to the original performance. This makes the picture on the display saturated, contrasting, and as straightforward as possible. At the same time, there is practically no noise and many other undesirable defects on it. Of the negative characteristics of this IIT, only their high cost is singled out. It increases the price of the entire structure and makes optics available only to people with good financial capabilities.

Some unofficial classifications indicate the presence of IIT generation Gen III +. These tubes are a slightly improved version of the Gen III models. However, more than their difference is required to classify such IITs separately.

Parameters by which NVI tubes are classified

Dozens of different parameters are used to classify night vision image tubes. Each characterizes the capabilities of a given structural element of various NVDs and allows you to distinguish one IIT from another. All these parameters make it possible to divide NVI tubes into categories with standard features.

Key features:

1. Figure of merit (FOM). This parameter is one of the first to be specified in any documentation with the IIT. It is a measure of the performance of that NVDs design detail. To determine it, use the resolution value, measured in lp/mm, and the signal-to-noise ratio (SNR). These indicators are multiplied by each other and get the final result. The higher it is, the better for optics in most cases of its operation.
2. Gain. An essential characteristic of IIT is luminance gain, or gain. This parameter shows the quantitative difference between the brightness of the light entering and exiting the tube. Naturally, the higher the ratio, the better the image on the optics display will become. Most experts call an acceptable gain of 50-60 thousand times. Higher values will make the picture almost perfect for the human eye.
3. Resolution. This characteristic is one of the key ones, as it shows the device's capabilities to form a high-quality image. This value for IIT is measured in lp/mm. The higher its value, the more precise and detailed the generated image. For different devices, this indicator can vary greatly. However, hands of at least 60-64 lp/mm today are optimal.
4. Black spots (BS). This characteristic shows the number of small dark spots on the tube. They arise from tiny particles of dirt and dust entering the IIT during production. Such defects are on all tubes without exception. However, the smaller the number, the better IIT will work.
5. Equivalent background illumination (EBI). This characteristic shows the background noise of the image intensifier tube. Most often, it occurs due to thermal radiation generated by the photocathode. The higher the IIT heating level, the higher the EBI will become. For modern NVDs, 2.0 is considered a good indicator. Lower values will be an additional plus in the list of characteristics of the handset.
6. Photocathode sensitivity (PS). This characteristic clearly demonstrates IIT's ability to transform the available light into an electronic signal for further amplification. It is measured in mA/lm. The more significant the PS value, the better the light transformation, and the image will improve. Modern devices usually have PS values in the range of 2-2.7 thousand mA/lm.
7. Signal-to-noise ratio (SNR). This is one of the critical indicators for any IIT. It represents the ratio of the proper light signal to the generated noise. Simply put, it shows the capabilities of the tube when operating optics in conditions of minimal illumination. The higher the SNR value, the better the image quality will be. For modern NVDs, this indicator varies from 30 to 40 units. Slightly outdated optics has values of 25-30.
8. Autogated. It is, instead, not a parameter of IIT but its function. Autogated creates additional protection for the handset in case of a sharp increase in light brightness in the area under consideration (for example, due to the inclusion of a powerful searchlight). This makes it possible to avoid loss of resolution and other negative changes.

Different grades of NVI tubes

All NVI tubes are divided into many grades, each with a unique set of characteristics and capabilities for imaging with a certain number of flaws. The higher the IIT class, the more resolution it has and the fewer defects.

Grades of NVI tubes:

1. 2ST. This class of handsets is considered the standard for second-generation IITs. It provides high sensitivity, as well as good resolution. At the same time, it is significantly inferior to more modern Gen II models in terms of performance, which is why it is rarely used in our time. This is due to several areas for improvement that limit the possibilities of optics. 2ST also has one important advantage - the price. It is relatively small, which positively affects the cost of the entire structure.
2. 2CGT/2MS. This Gen II IIT class is an excellent choice for users using NVDs for light to medium tasks. Both tubes are unique. 2CGT is classified as a non-standard class of models. Despite this, they are quite common. 2MS are members of the mil-spec class. What these two IITs have in common is increased performance and improved image quality (compared to the base 2ST). Another important feature of 2CGT and 2MS is the significantly higher cost.
3. 2HP/2HPT. Image intensifier tubes of these two classes are among the high-performance models. They have improved performance compared to other second-generation IITs, resulting in higher image quality. In most respects, 2HP and 2HPT are almost as good as Gen III tubes, often called an intermediate option between the second and third generations. Among the most noticeable advantages of these IITs are very high performance, reliability, and long service life. However, such characteristics and many advantages automatically raise the price. Because of this, 2HP and 2HPT often cost the same as Gen III models.
4. 3ST. The third generation IIT standard is called 3ST tubes. They are far from ideal but still outperform in many respects the very case of the Gen II model. The key difference is the lower number of defects. Thanks to this, the picture is clearer. Also, the difference with the second generation is noticeable in many parameters that affect performance. The main disadvantage is the lack of autogated tubes. Because of this, they are poorly protected from the negative effects of excessively bright light. Also, the disadvantages include the high cost compared with models of previous generations. However, it is smaller than other tubes related to Gen III.
5. 3A. This IIT variant is an improvement over the Gen III standard. The changes have affected many aspects that affect optics' performance and the formed image's clarity. They happened for the better, so the cost of 3A went up. An essential feature of this tube is that its manufacturers always supplement the kit with test sheets with data. They describe the results of the tests carried out for each model. This will allow buyers to see not initially declared but accurate information about the handset.
6. 3P/3AG. These handsets are considered the best option for most users. They are actively used not only by civilians but also by military personnel. This became possible due to the generated image's high performance and excellent quality (almost complete absence of defects). Another advantage of 3P and 3AG is the presence of automation. It creates reliable protection against a sharp increase in the brightness of the light entering the tube. Due to this, tubes of these classes can be used under any lighting conditions (immediately after sunset, late at night, or dawn).
7. 3HS. These IITs belong to the Hand select class, hence their name. These design details are the benchmark of the third generation, as they have even more impressive characteristics than 3P / 3AG. Among them, the minimum signal-to-noise ratio is distinguished, which ensures good picture quality even in shallow light conditions. It remains unchanged even when considering very remote objects from the user. The only negative point is the very high price, which is much higher even than 3P/3AG.

An essential structural element of any night vision device is the NVI tube. She makes it possible to carry out various manipulations with light, leading to its amplification. This positively affects the quality of the generated image and makes visible what was previously hidden in the dark. The presence of an NVI tube in the design of night optics opens up new possibilities for people and maximizes the range of applications for such devices.