What is Gain?
Gain in a night vision device is a measure of its light enhancement.
As a first step in the image intensification process, the intensifier tube's photocathode converts photons to electrons.
Gain is the measure of multiplication that occurs to these electrons via the image intensifier tube's microchannel plate. Gain is sometimes overlooked because it's relatively consistent across tube models. In other words, gain usually doesn't vary much within certain models like 18UM and 20UA (just 2 examples) of L3Harris unfilmed tubes, or within models of thin-filmed tubes by Elbit or L3Harris (L3H makes thin-filmed tubes too).
From a high level, the microchannel plate ("MCP") within a tube consists of bundled fiber optic channels that bounce and multiply photon converted electrons prior to their output onto the phosphor screen of a device.
Source: Hamamatsu Photonics K.K.
The "how" is probably less important to the end-user than the "why".
Low gain produces a darker (less illuminated) image. On the other hand, gain that is too high is likely to cause additional scintillation (noise).
Very high gain provides additional brightness but may do so at the cost of a cleaner image. To use an imperfect but simple analogy, you could compare NV gain to volume on a home stereo system, where very high gain would be your volume level above redline and a point where clarity suffers due to "loudness". The impact of very high gain can sometimes be demonstrated in a variable gain device like a PVS-14 by turning up its gain to max and then turning it down to ~80-90% for a less noisy image.
Gain has a goldilocks range which will vary amongst tubes and tube manufacturers.
Gain, as its listed on a tube's data sheet is a good performance metric but doesn't necessarily translate to actual brightness to your eyes. Brightness to your eyes is impacted by the lenses used. Therefore, measuring gain from behind the eyepieces (ocular lenses) can be a better holistic measure of gain (aptly referred to as system gain).
Gain is a core tenet of evaluating night vision but sometimes it doesn't get the same attention as other specs. In many ways, this makes sense, as the limited variation of gain within models of tubes tends to make SNR, EBI and other specs much more relevant.
However, evaluating gain can be extremely important. The biggest differences in gain occurs when comparing generations of tubes (Gen 2 to Gen 3).
Sometimes people will compare superb FOM / SNR between between Gen 2 and Gen 3 devices, but this is not an apples to apples comparison. The reason why is usually gain. A tube with high FOM / SNR generally means that a tube has very good clarity in low light. But what if you can't see in low light because you lack light amplification? Does that enhanced clarity matter if you can't see an object due to lack of brightness? The honest answer is it depends on your operating environment.
Everyone has a different operating environment (with varying levels of light). What might be appropriate for one individual and their use case in one environment might not work for another.
Higher gain is one reason why Gen 3 tubes made by L3Harris and Elbit outperform Gen 2 & Gen 2+ tubes (like those made by Photonis) in low light. Another reason for their differing performance is that multialkali based photocathodes, such as those in Photonis Echo tubes, have a different spectral sensitivity than their Gen 3 counterparts whose tubes use gallium arsenide based photocathodes.
As a result, Gen 3 tubes are more sensitive to the near-IR spectrum (sky glow / reflected radiation from the atmosphere, starlight and moonlight).
If you're in the market for a night vision device and want to better understand what might be best for your use case and operating environment, please feel free to contact us.