FOM, or Figure of Merit, is the standard “At-A-Glance” figure by which tube performance is measured. FOM is calculated derived from two sets of numbers on your Intensifier Tube Data Sheet (also known as “Spec Sheets”). To determine the FOM of a particular tube, locate the RES (Center Resolution) figure and the SNR (Signal To Noise Ratio). Multiplying these two figures together will give you the FOM. (example: SNR:25 X RES:72 = 1800 FOM).
So what do these figures represent?
SNR is easy. It’s a ratio of how much real signal your eye gets versus “noise” created by the tube’s processing itself. Anything above 25 is very good, anything above 30 is phenomenal.
But what about RES? Center Resolution is measured in Line Pair Per Millimeter. The industry has a standard graph for measurement of Tube performance, usually with the aid of a HOFFMAN machine. You may have seen through-the-tube images of a weird-looking chart, almost like an android version of the letters chart they make you read with one eye closed when testing your vision. Well, that’s exactly what it is, and those lines you see in the chart are the “line pair” we referenced above. RES is the number of pairs you can make out at a given distance without the pairs blurring into each other. But there’s a catch here; if you have perfect 20/20 vision, the highest number of line pairs your naked eye will be able to see at that same distance is about 62-68. Very few people have natural vision good enough to resolve anything above 70 line pairs per mm.
So what does that mean for tubes with a RES rating of 72, or even 84? It means the tube is better than your eye. The tube can resolve the image, but your eye cannot!
So why are there tubes with 81, 84, or even higher RES? While somewhat rare, those tubes are best used, and often manufactured specifically for, CLIP-ON devices to be used in conjunction with a magnified optic like an LPVO. Since the LPVO will essentially be magnifying the output image of the tube, the increased resolution is useful. This also means that, if we’re talking about Helmet-Borne monoculars or binos, a tube with a very high RES also has an artificially inflated FOM. For example a tube with RES:81 and SNR:32 for a FOM:2592, will not actually perform any better in any way you can see, than a tube with RES:72 and SNR:32 for a FOM:2304.
You may have seen the FOM figure of “2376” several times, especially with reference to military nightvision. That’s because the Dept of Defense and their Defense Consultants figured out the 2376 FOM is functionally about as good as a tube is going to get in the real world. Nevertheless, most dealers charge a VERY high premium for high-FOM tubes. If you pay $3500 for an L3Harris Tube with a FOM of 1800, and $3575 for the same tube with 2000 FOM , but you can expect to pay $4500 for one with 2200 FOM, and even as high as $5500 for a FOM of 2750. For some users, the juice is worth the squeeze, but for most, it isn’t.
What this all means in the end, and what is often the painful truth people only discover after spending enormous amounts of money chasing ultra-high-spec tubes, is that the real returns diminish so rapidly after 2000 FOM (assuming an SNR of 32 or more), it’s like falling off a cliff. As discussed elsewhere, we at Aeontac focus on providing our customers with MAXIMUM VALUE for money. So, while we do offer some high-spec GEN3 tubes, our focus is GEN2+ tubes 1600 – 1800+ FOM.
