Electron Multiplication (Gen 2 and Gen 3)

Generation 2

In the late 1960s, Gen 2 technology brought a major breakthrough in night vision with the development of the microchannel plate (MCP). Additionally, the photocathode was further refined to the S-25 cathode and produced much higher photo response.

MCP
MCP

It was the introduction of the MCP that made Gen 2 unique. The MCP begins with two dissimilar pieces of glass. A large tube of solid glass (core) is placed within a tubular sleeve of glass (clad). The two glasses are then heated together and stretched to form a very small diameter glass fiber. The fibers are ultimately compressed together to form a bundle of glass fibers called a boule. The boule is then sliced at an angle to obtain thin discs. Further chemical processing removes only the core glass, thus creating the channels within the MCP. During the tube operation, the electrons travel into the channels and, as they strike the channel walls, they produce secondary electron emissions which create several hundred electrons, multiplying with each strike.

The other advancement with Gen 2 was the reduction in overall size and weight of both the tube module and the power supply. It allowed for compact head mounted night vision devices.

Generation 3

Developed in the mid-1970s and placed into production during the 1980s, Gen 3 was an advance in photocathode technology. Gen 3 tubes use gallium arsenide (GaAs) photocathodes. This increases the tube’s sensitivity dramatically and particularly in the near-IR. The increased sensitivity improved system performance under low-light conditions.

Disassembled PNVM-14
Disassembled PNVM-14

However, the highly reactive GaAs photocathode could be easily degraded by the inherent chemical interactions that take place within a tube under normal operation. Most of the chemical reactions take place within the MCP due to the electron interactions with the walls of the MCP channels. Thus, to overcome the degrading effects of the photocathode, a thin metal-oxide coating was added to the input side of the MCP. This coating, more commonly known as an ion barrier film, not only prevented premature degradation of the photocathode but also enhanced the tube life by many times that of the Gen 2 tubes. Today’s Gen 3 tubes have a lifetime of at least 15.000h.

Gen 2 and Gen 3 tube manufacturers have made continuous improvements through the years to increase the signal-to-noise ratio within each respective technology. Continuous improvements have been made within MCP manufacturing so as to improve the overall resolution also.

Gen 3 Tube design
Gen 3 Tube design

There has been considerable effort expended in developing a Gen 3 tube without the ion barrier film. The effort proved successful, but the manufacturing costs were to high compared to the performance improvements. For a brief period of time, the Gen 3 tube without the ion barrier film was termed Gen 4. This terminology, however, was rescinded shortly after it was announced, though some resellers of night-vision tubes still use the nomenclature. Today L3 is the only company manufacturing these “Filmless” tubes. L3 calls them infinity tubes.

Continuous developments

One area that has contributed to the improvements is the advancement of the miniature high-voltage power supply. Early developments with the power supply included protection circuits to automatically control the output brightness of the tube under changing input light conditions. These effects, known as automatic brightness control (ABC) and bright source protection (BSP), were directed at protecting both the image tube from highlight exposure and the user’s eyes from excessive brightness. The ABC automatically reduces voltage to the microchannel plate to keep the image intensifier output brightness within optimal limits and to protect the tube. This effect can be seen during rapid changes from low-light to high-light conditions when the image gets brighter and then quickly returns to a consistent level. The BSP reduces voltage to the photocathode rather than the microchannel plate. The BSP protects the image tube from damage and enhances its lifetime.

The scene resolution can be degraded under high-light conditions. Advancements in miniaturized power supplies include the addition of autogating circuits. These circuits control the way the photocathode is operated under changing input light conditions. Autogating allows the image tube to be used under higher input lighting with much less degradation of the image quality.

Autogating turns off the photocathode voltage for brief periods of time, the effect is not visible to the human eye. The cathode voltage is constantly oscillating, but the image appears as if it were continuous. The autogating circuit reduces the time the voltage is on during each oscillation but keeps the peak voltage level up. By controlling the application of voltage in this manner, the resolution quality remains high. In effect, the autogating feature tricks the device into thinking it is always in a low-light environment, which is the optimal environment for maximum efficiency and clarity for the image-intensifier tube. While the most obvious effect of autogating for the user may be improved resolution in high-light conditions, its original purpose was to help extend the lifetime of the tube, a benefit which is most realized with thin-film or filmless tubes.

As opposed to the gradual lifetime decay seen in tubes without an ion barrier film, the Gen 3 tube with gating and the use of a thin film improves tube life and performance far more than any other image-intensifier technology. Typical reliability is well in excess of 15,000 hours without noticeable degradation. This change in durability is a significant accomplishment when considering the much shorter lifetimes of Gen 0, Gen 1 and Gen 2 tubes.