What is the gain of image intensifier tube?

In night vision technology, image intensifier tubes play a key role in enhancing the visibility of low-light environments. These tubes contain an input photocathode that converts photons into electron flow, which is then accelerated through a vacuum-sealed tube and eventually amplified to a brighter image output. The output image is brighter than the input image due to the amplification of electrons, which is commonly referred to as gain or gain factor. Gain is a critical parameter of image intensifier tubes that measures the strength of the electron flow and determines the brightness of the output image.
There are two units of gain measurement for tubes.The first is that (SI) unit is cd·m−2·lx−1, i.e.candelas per meter squared per lux.The second is the unit is Fl/Fc (foot-lamberts per foot-candle).The difference between the two measurements is actually pi or about 3.142x. This means that a gain of 10,000 cd/m2/lx is the same as 31.42 Fl/Fc. Some night vision vendors use different units to advertise gain in their marketing, so as consumers we need to be careful to recognize this when purchasing!

The tube gain (which is also referred to as brightness gain or luminance gain) is the magnitude, or number of times the image intensifier amplifies the light input. 
In practice, gain is expressed as a ratio between the output and input light level of the tube, usually in units of decibels (dB). The greater the gain, the brighter the output image relative to the input image. However, high gain also increases the likelihood of image distortion and noise, which can result in a grainy image with reduced clarity and resolution. Therefore, gain must be optimized according to the specific requirements of the night vision application, such as distance, lighting conditions, and target identification.
To control gain, image intensifier tubes often include a gain control function, either manually or automatically. Manual gain control allows the user to adjust the gain level according to the changing environment and operator preference. On the other hand, automatic gain control (AGC) adjusts the gain level based on the ambient light level to ensure a consistent level of brightness and contrast, regardless of the changing lighting conditions.

Our common image intensifier tubes usually have two gain adjustment methods: automatic gain (Automatic Brightness Control) and manual gain.
Automatic gain (Automatic Brightness Control) is commonly found in image intensifier tubes of models such as MX10160 dual contact sheet/mx10130, which can automatically reduce the voltage of the microchannel plate to keep the brightness of the image intensifier within the optimal range and protect the tube. The image gets brighter when changing quickly from low-light conditions to bright-light conditions, then suddenly dims to a constant level after a short delay.
Manual gain is common in image intensifier tubes such as MX11769/three wires/MX10160 three-contacts. Users can manually adjust the gain according to different environments.
In addition to the gain of the image intensifier tube, I also need to consider the system gain. The gain of the image intensifier tube is also affected by the lens. The lenses, beam splitters and filters of the night vision goggle optical system all reduce the system gain.

Overall, gain is a critical factor in the performance of image intensifier tubes and the overall effectiveness of night vision technology. The optimal gain level depends on the specific requirements of the application and must balance the need for brightness and clarity with image resolution and distortion. By understanding the role of gain in image intensifier tubes, users can effectively control and optimize the night vision image quality for their specific needs.