Specs, specs, specs, specs, specs. A lot of people toss that word around. I'm guilty. It's already confusing enough if you're looking for your first device or just bought one and now you have to figure out specs!? Your head is probably already spinning with all the night vision lingo, housings, mounts, shrouds, helmets, accessories, etc. I know I was confused! I've been asked many times what the specifications (specs) mean, what good ones are, and decided that something needs to be up on the site. So here ya go!
First off, this post is in regards to US made gen 3 image tubes and what you'll find listed on their spec sheets (aka data sheets). These companies have a bizarre love triangle going on. ITT years back became ITT/Exelis. Then Harris/Exelis. Harris recently merged with L3 and the night vision part of Harris (ITT) was sold to Elbit. So right now the two US gen 3 manufacturers are L3Harris and Elbit.
Second, the only way to know the actual specs of a tube is by having the spec sheet that the manufacturer provided for it. Spec sheets are ITAR controlled, so only US Citizens are allowed to see them, and the actual spec sheet with info cannot be openly listed to the public. Spec sheets "usually" come with new tubes that are sold commercially. I say "usually" because I've seen new tubes being sold without them. I personally feel it's a disservice to not provide a spec sheet on a new tube to a customer. Their thought could be that the customer is better off not knowing and will be happier not getting hung up on the specs. There's some truth in that, but knowing the specs, buyers will be able to relate it's performance to the specs, and will be better informed if they decide to buy more night vision. It also greatly helps resell. People want to know the specs.
You might see sellers say they have "mil-spec" tubes. Calling tubes "mil-spec" can be misleading. The tubes the military gets have to meet the minimum specs outlined in their Omnibus (Omni) contract and pass an inspection. That's what you could call the "mil-spec". I'll keep using "mil-spec" just so you know what I'm referring to but I hate using it when it comes to tubes. I believe they're mostly referring to "commercial" tubes as ones with lower specs than "mil-spec" minimums but usually it's marketing spin because folks want to buy "mil-spec". Saying tubes meet Omni VIII "mil-spec" minimums would be the better way to phrase it but it's only useful if they list them or you know what the minimums are.
To give you an idea of the specs that make a tube "mil-spec", and I'll go over these specs below, the current Omni VIII "mil-spec" minimums are 1600 FOM (64 resolution, 25 signal to noise ratio), photocathode sensitivity 2000, gain between 25,000-110,000, halo 1.0, and EBI 3.0. These minimums are average-to-below average for what's available "commercially" although there are tubes with much lower specs. "Commercial" tubes may also have blemishes such as spots in the center of the tube.
The 3 specs thrown around the most are FOM (figure of merit), resolution (lp/mm), and signal to noise ratio (SNR). These specs and all the others are different for every individual tube because of how they're made. I'll put a web page at the bottom that goes into detail about how they're made. It's pretty wild.
FOM (Figure of Merit)
FOM is a calculated number using resolution (res) multiplied by the signal-to-noise ratio (SNR). FOM = Res x SNR. It's probably the most used for how good a tube is. It isn't the be all end all but it's a damn reliable way to determine a tube's general performance / quality. As an example, I'll use the "mil-spec" 1600 FOM. The majority of newer gen 3 tubes will have a minimum res of 64 lp/mm (line pairs per millimeter). Take 1600 and divide it by 64 and you get 25:1 SNR. Nobody lists it as 25:1 because the 1 is implied, so it's just 25. This is the easiest way to determine what the SNR "should" be based on only having the FOM to work with. Tubes can have higher than 64 res but it's the most common. Here's my ranking of gen 3 FOM based on all the tubes I've had and specs I've seen:
Below average: 1500 FOM and under
Average: 1500-1800 FOM
Above average: 1800-2000 FOM
Excellent: 2000-2200 FOM
Awesome: 2200 FOM and higher
FOM (resolution & SNR) are just two of several other important specs provided on the spec sheet. Think of it like horsepower & torque. Knowing how powerful the engine is gives you an idea of a cars performance but you also have to factor in the transmission, suspension, etc. etc. If you're shopping for a car with some balls you might start with 400HP, so you aren't going to have cars with less HP on your list. It's a starting point.
Search "image intensifier specifications" and you'll find a lot of technical mumbo-jumbo engineer speak that doesn't always lay it out in an easy to understand way. Here's my attempt to fix that. These are the important specs you'll find on the spec sheets and what they mean. The only picture I used is halo because it isn't affected by camera settings.
Photocathode Sensitivity (sometimes called response)
How well the tube converts light. The higher the number, the better it does it. An example of a really good number is 2700 and a bad one would be 1400. The tubes I've come across (mine & others) have been in the 2200-2700 range. I recommend higher than 2200.
EBI (Equivalent Background Illumination)
How well the tube can form an image in low light levels. The lower the number the better and the higher the worst. Temperature affects it, so the warmer the worse it is, and the colder, the better. EBI is very important if you'll be using it where/when it's real dark or doing astronomy. Out in the boonies on a no Moon night you'll see how important EBI really is because low EBI forms a better image in very dark conditions. A real good number is .4 and a real bad one 2.0. I recommend under 1.0 EBI. As an example of why I don't care about "mil-spec" when it comes to tubes, is that 3.0 is the minimum for current "mil-spec".
How big the circle of bloom is around lights. Through night vision, lights will have a blooming effect. The lower the better. Under 1.0 is best but the lower the better. It's the difference between a laser dot looking basketball or beach ball (or bigger) size. A large halo will hide details in the image inside the halo. The picture shows street lights with 1.0 halo.
Luminous gain (aka brightness gain)
How many times the tube can multiply light coming in. The sheet will show a gain number in the tens of thousands. A good number would be in the 60,000-70,000 range and a bad one 40,000-60,000. ITT/Harris (pre L3 Harris) sheets usually don't list it as gain but you can find it below EBI (the line will show "2x10-6 fc" as in the top picture. Some companies like Night Vision Devices will include a 2nd inspection sheet that also lists system gain, which is a lower number. It's easier to use the bigger gain number because it's listed on every spec sheet.
Signal to noise ratio (SNR)
The ratio of the signal (image) to the "noise" in the background. The higher, the better. A low SNR will show more static/scintillation. In my experience, SNR is the most reliable gauge of a tube's performance, and why it is used in the FOM calculation. Every new modern gen 3 tube will be at least 64 resolution but can go up to 72 or higher. Most are 64 so you can see how SNR makes a big impact on the FOM. Personally, I wouldn't want lower than 25, and found my best tubes were all over 30.
Resolution (aka LP/MM *line pairs per millimeter*)
How well a tube can make out different stuff that's close together. It's different between the center of the tube and the outside. Sheets always list the center resolution. Lenses and how well you focus them can affect resolution as seen by your eye. With new tubes, 64 is the standard, but higher than 64 is becoming more common. My best tubes are 72 but their other specs are also high. I've had very good tubes with 64 res.
Spec sheets will also list if the tube has any spots. It'll give the size of the spots (black circles) and which zone they are in. These zones are concentric circles called zone 1, 2, and 3. Zone 1 is the center, zone 2 is in between the center and outside, and zone 3 is the outside. Usually, tubes with larger spots will sell for a lower price. Ideally, you want the spot to be in zone 2 or 3 (zone 3 is best), so it's not in the center of the image. Small spots are called peppering, can be common, and aren't listed on the sheet. Tubes can have shading which usually isn't listed either. Shading are certain areas of the tube that are darker than the rest.
How much do these specs matter?
I've heard sellers say that specs don't matter once you go over a certain FOM, that most people can't tell the difference, and blah blah blah reasons that are basically telling you to just accept whatever specs you get because it'll be fine. They're not wrong in that most average tubes in the 1600-1800 FOM range will be fairly close in performance all other specs also being close. But take a second to think about it. Dealers are in the business of selling night vision. Every tube they get from the tube mfg's can't be "high spec". They can expect to get a minimum or average level of specs. These "average" specs are still very good and let you see and do things that you couldn't before without night vision, but the fact is higher specs are better than lower specs. I wouldn't chase small increases in specs but there is a very noticeable difference between average tubes (1500-1800 FOM) versus 2200 and higher FOM.
What you'll use it for.
Having a purpose helps determine what kind of performance you'll want. For example, astronomy, photography, and filming need higher specs and good screen clarity (no spots or shading). Average specs are fine for recreational use unless you want the extra "wow" clarity/performance. If you'll be using it in dangerous settings, higher specs can make the difference in determining if what you are staring at is a person with a gun or a tree / mountain lion or bush / shadow or big ass hole in the ground. There's a bunch more different uses but you get the idea.
We (Nitewalker) live in a very remote rural area that can get extremely dark and I prefer high specs (2400+ FOM). These higher specs came at a higher price which we were willing to pay. They are clearer and give a more usable image than the lower spec tubes we've had/have. We also have a pair of average-to-above average tubes (don't have spec sheets but I wager they are 1600-1900 FOM). These tubes work very well but when it's very dark (no Moon heavy clouds) the 2400+ FOM beats them hands down. Honestly, using an illuminator will solve almost all performance issues with tubes no matter their spec. Use a good illuminator and a low spec tube will work just fine. I prefer using an illuminator as little as possible and just simply enjoy having the best image I can get. You have to decide for yourself if the higher price and sometimes longer wait are worth it to you.
To wrap it up
If you're looking to buy your first night vision device brand new, don't hesitate to talk to multiple dealers and ask questions about the tubes they sell. Most websites don't list much info (or none at all) on the specs. A lot of them won't have tubes on the shelf, which sucks, because they can't tell you for sure what you're going to get. Ones that have tubes in stock will usually let you pick the one(s) you want from the sheets. You can always request that you get certain minimums, and the good dealers will make sure that's what you get. 10 year warranties on tubes has become standard with the bigger sellers (TNVC, JRH Enterprises/Night Vision Devices, N-Vision Optics) and both L3 and ITT usually offer a standard 2 year warranty on new tubes. Weapon sights usually have a shorter warranty.
The phrase "buyer beware" applies big time to night vision. It's usually the case that lower spec tubes sell for less than higher specs but pricing is all over the place, so don't assume that a device that costs more will have higher specs. You need to ask and verify before buying if you care about the specs. If you're looking at used tubes, it's possible to determine the minimum specs if it has a NSN number and/or contract number on the tube. These tubes went to, or were destined for the military, which may or not mean they were acquired legally, so the seller may not give you this info. Don't buy stolen tubes. Besides not being cool, the Government will confiscate them and could criminally charge you if you bought them knowingly. This forum post lists the different minimum specifications per Omnibus contract: https://www.ar15.com/forums/armory/-/18-317705/?
I highly recommend reading this:
"What you will find inside an image tube" by cj7Hawk