*Click on any word LINK to jump to our GLOSSARY of TERMS BLOG ENTRY

FOM, or Figure of Merit, is the standard “At-A-Glance” figure by which tube performance is measured. FOM is 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). 

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. 

What does that mean for tubes with a RES rating of 72, or even 84? It means the tube is actually better than your eye. The tube can resolve the image, but your eye cannot!  

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. 

But wait, if the tube in my bino or mono has a CENTER RES higher than 72, doesn’t that also mean the FOM figure is artificially inflated, in a sense?

EXACTLY. 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. But that won’t necessarily mean the tubes will be priced accordingly. Many retailers WILDLY inflate the prices of tube with slightly higher FOM levels, whether that increase is something you can really see and use or not.   

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. Or, another way to say it is, the actual increase in performance one is likely to notice between a 2376 tube and say, a 2650 tube, simply isn’t worth the additional expense. 

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. Therefore, our primary tubes offerings are GEN2+ tubes at roughly ~1800 FOM, offering roughly 90% of the performance of even the best similarly spec’d GEN3 tubes at roughly half the cost. Nevertheless, we do offer GEN3 tubes up to roughly 2376 FOM as an upgrade for those who desire them.