Ah, night vision specs. The topic of many hours of debate, much gnashing of teeth, and needless heartbreak. This is an area with probably more misinformation that anywhere else in the night vision industry, and it almost seems to be perpetuated the most by lurkers in night vision forums and boards who don't even own a unit themselves.
It's only natural that in a hobby where every device comes with an extremely detailed spec sheet, we constantly try to compare our units to someone else's. And of course, with the amount of money we are spending, we want our night vision to have the best specifications possible. But, just because it's natural, doesn't mean it's healthy (or realistic). We've seen plenty of forum posts from users bummed out that the tube they got doesn't seem to meet the super-tube specs the internet told them they need, when in reality, the tube they got is great.
So before we dive deep into this, our official position is this: If the tubes you have do what you need them to do, they're good spec'd tubes.
We unfortunately can't post copies of spec sheets in this article, because they are only allowed to be viewed by US Citizens, and are ITAR controlled. No joke, if you're not a US Citizen, it's actually illegal to even look through Generation 3 night vision. However, the OMNI contract requirements are in the public domain, so we may reference their minimums, or speak in generalities throughout the article.
Mil-Spec vs Commercial Spec Night Vision
The very first thing to understand when shopping for night vision is that everything revolves around military contracts. While growing in popularity, night vision is still an incredibly niche market on the civilian side compared to contract sales, so tubes are often graded in relation to a contract they may qualify for, or otherwise designated as "Commercial" tubes if they fail to meet a spec.
Military procurement tubes are judged by an "Omnibus" (OMNI) contract for their minimum specifications. The current OMNI contract calls for the following specs:
- FOM: 1600+
- SNR: 25+
- LP/MM: 64+
- Photocathode Sensitivity: 2000+
- Luminance Gain: 25,000-110,000
- HALO: <1.0
- EBI: <2.5
There are also certain specifications on where spots can appear, and what size they can be. Anything that may not meet these specs will fallout and be designated as a commercial tube. And some of these specs may vary (especially with spots, for ground grade vs aviation grade). But look at the specs above again. There's nothing impressive about them. In fact, all of those are considered below average for what the internet may say you need. But, those are the tubes that the military is using as a whole (outside of special operations communities). So let's start with our frame of reference there.
Also, it's worth mentioning that it's much easier now than it was even 5 years ago to get ahold of military spec tubes both new from vendors, and on the gray market of actual OMNI contract tubes that have found their way into civilian hands.
FOM (Figure Of Merit)
This one number is probably the source for most of the misinformation on night vision specs as a whole. People seem to be obsessed with having a "High FOM" tube, and are willing to look past other specs in order to get it. Of course, there's nothing wrong with having high FOM, but it doesn't tell the full story.
Figure of Merit is simply the tubes Resolution (or LP/MM) multiplied by it's SNR (Signal to Noise Ratio). LP/MM * SNR=FOM
For example: If you have a tube with a 81 lp/mm, and 30 SNR, your FOM will be 2430, which would be a great tube! But let's say you have a choice between that tube, and one with 72 lp/mm, and a 33 SNR, which give you a 2376 FOM. Despite the FOM number being higher in the first tube, we're going to take the second tube all day (or should we say night) long. SNR tends to be more important than lp/mm, and we would rather take the higher number there.
Figure of Merit is mostly an administrative number to prevent the export of tubes that are too high performing, and it's important not to get too wrapped up in it. Image intensifier specifications are more important to look at individually, rather than multiplied together. There are now tubes coming out with lp/mm in the 80s and 90s, with rumors of tubes with resolution numbers over 100. That may be great for that spec, but if you use it in FOM, it will be very misleading as to the actual performance of the tubes, because while you may have a sky-high FOM, your SNR may be much lower.
SNR (Signal To Noise Ratio)
One of the most important specs on your tube. Signal to Noise Ratio is the relation between how much information the tube is giving you is "true" vs "false". Think of an image intensifier tube like a camera sensor. A tube with a SNR of 30 will give you 30 pixels that accurately represent the image, with one inaccurate one.
Signal to Noise is the most reliable part of the FOM equation in measuring a tubes performance. It's also one of the specs that you will most notice a difference on, even for new users. A vague guideline of SNR specs would be something like:
- 25+ SNR: Average
- 30+ SNR: Good
- 35+ SNR: Great
As night vision image intensifier technology advances, SNR numbers are getting higher and higher. We've seen tubes with SNR number close to 40 nowadays. But don't fall into the technology trap of thinking you're outdated when something better comes out. A tube with a 30+ SNR number is always going to be very good at seeing at night, even if there are better ones out there.
Resolution or LP/MM (line pairs per millimeter)
To keep using the camera analogy, think of this like megapixels. Yes, higher is better, but if you're viewing an image on your phone screen, can you tell if it was taken with a 72 megapixel camera, or an 81 megapixel one? No, of course not.
Night vision spec sheets always list the center resolution of the tube, and the resolution at the edges may be less. Keep in mind, the quality of your glass (your lenses), is going to play a role as well, and so will how dialed in the focus is. In our experience, everything above 64lp/mm looks the same to the eye in a headborne unit, especially 72+. 81+ lp/mm tubes are really desired in magnified rifle clip-on scopes where the extra resolution will be noticeable when using magnification.
EBI (Equivalent Background Illumination)
This is also a frequently misunderstood (or maybe just mis-valued) night vision spec. This can be simplified as how good the tube is at forming an image in low light conditions. Like golf, the lower score the better. The more technical explanation is that the EBI is the amount of light a tube is putting out as default, and so in order for an image to be shown to the user, the incoming light must be greater than what the tube "sees" by default.
An odd analogy: If you're with the girlfriend in Victorias Secret, you are smelling a baselines level of perfume constantly. In order for you to smell an individual perfume, it has to over power that baselines.
EBI has been practically fetishized (hmmm... maybe we can make more Victories Secret analogies), by internet spec geeks. We suspect that this stems from a video from a TNVC employee talking about EBI as the most important spec (a video that is no longer there). We wholeheartedly disagree.
We aren't saying it's unimportant, just that it's seen as much more important than it is. Keep in mind, EBI is really only noticeable in very low-light environments, and a very low EBI is really only necessary for astronomy. Realistically, anything below 2.0 is good, and anything below 1.0 is great (though you probably can't see the difference between a .5 and a 1.5)
Another thing to keep in mind, EBI is also affected by environmental temperature. A warmer night will lead to a higher EBI, and a cooler night will give you a low one. So if you're out hunting in 20* weather, there very likely is no difference between two identical tubes with different EBIs.
HALO
Ever seen that halo around an angels head? Well... that's pretty much what it means in night vision specs too. It's going to be the glow you see when you look at a light source, or an IR laser. In general, under 1.0 is ideal, and considered good. However, for clip-on units that will be used on a rifle, you would typically want a tube with a Halo above 1.0, as the lower numbers are also more sensitive to recoil.
In practice, your halo may not matter to you much. If you exclusively use night vision out in the boonies away from artificial light, you can absolutely get away with higher halo numbers on your spec sheet, and you may only notice it when using a high power laser.
Luminance Gain (or Brightness Gain)
This is the ratio of brightness of the output of the phosphor screen (what you look at), to the input to the photocathode (what receives the light). Out put vs input. In easier terms, it's how many times the light was multiplies before it got to your eyes. The higher the number, the better. A general guide would be something like:
- <50,000 Gain: Bummer
- 50-60,000 Gain: Usable/Acceptable
- 60,000+ Gain: Good
There have been tubes produced on special contracts as high as 110,000 luminance gain. Those guys are probably wearing sunglasses behind their tubes.
Photocathode Sensitivity
This is how well the tube converts photons (incoming light) into electrons (what the tube amplifies). We generally see tubes in the 2000-2700 range. Anything under 2,000 would be a lower performing tube, and 2,700 would be great. We most commonly see tubes around the 2,200 mark.
Spots (Dark Spots or Blemishes)
Spots are little dark spots on your tube. They vary in size and location, and are totally normal to have. Almost all tubes have spots, and they bother some users more than others. An image intensifier tube is broken up into three sections: Zone 1 (the center of the tube), Zone 2 (the middle ring), and Zone 3 (the very outer edge). Spots are measured in micrometers, and will appear on a spec sheet. Most night vision retailers will also show you pictures through a tube before purchase so you can see the spots yourself also. Very very tiny spots are called "peppering", and are not listed on spec sheets. These are only noticeable if you're really looking for them in the right conditions.
There's no way to give a guide on what is good or bad for spots, as every individuals tolerance varies. In general, you want spots as close to the edge of the tube as possible, and not in Zone 1. But it may be a balancing act between the spots you are willing to put up with, and the other specs you can get.
FPN (Fixed Pattern Noise)
This won't appear on a spec sheet, but is something to be aware of as well. FPN, or fixed pattern noise, is noise that shows up no matter what. This is caused by the hexagonal shape in which the microchannel plate is assembled, and is usually the most noticeable in high light conditions. Some tubes show constant FPN, and others very little at all. A very rough rule is that higher SNR tubes may show more FPN. This is also referred to as "chicken-wire".
Why do Night Vision Specifications Vary so much?
In short, because of the way night vision tubes are manufactured. Image intensifier tubes have many delicate processes in their manufacturing and creation, things can vary quite a bit. Tolerances are measured in microns, and any deviations can effect the tubes night vision specifications. For more information on how the magic happens, read our blog: How Night Vision Works.
How much do Night Vision Specs Matter?
That's up to you. Obviously they matter to a degree, and we would all prefer a tube that can see better at night than one that doesn't see as well. But refer back to the latest OMNI contract specs. Is the tube you're shopping for beating that? We almost guarantee it is without even looking at it. Night vision is so advanced today that the difference between tubes and manufacturers is marginal. And if you're reading this, you're likely a first time buyer of night vision. In that case, you likely won't be able to discern a difference between slightly different spec'd tubes anyhow. Experienced night vision users will tell you, what matters most is just enjoying the ability to see at night. Don't get caught up in the spec rat race. If your tubes work for you, they're good tubes.
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