In the last 5 years, the NV world seems to be caught up with "FOM-Fever" and while FOM does offer some useable information to aid in a purchasing decision, it was actually only developed by government agencies as a way to determine import/exportability of tubes.
The actual definition of Figure of Merit (FOM) is the product of Signal-to-Noise Ratio (commonly expressed as SNR) multiplied by Centre Resolution (in line-pairs per millimeter)⠀⠀⠀⠀⠀⠀
FOM = SNR x Resolution⠀⠀
The reason why FOM doesn't matter, especially when SNR and Res is available is because you can have a tube that has a very high SNR, and low Res, up against a tube that has a low SNR and high Res, and both could have the same FOM. ⠀⠀⠀⠀⠀⠀⠀
Example: Tube 1 has 67lp/mm and 32 SNR; 32 * 76 = 2432 FOM⠀⠀⠀⠀⠀⠀⠀⠀
Tube 2 has 76lp/mm and 23 SNR; 76 * 23 = 2432 FOM⠀⠀⠀⠀⠀⠀⠀⠀
Despite the identical FOM numbers, the first tube will provide a very balanced image, while the 2nd tube will provide a very detailed, but noisy image.
Each data point on the spec sheet provides information on how the tube performance in various lighting scenarios. As an example - SNR becomes important in threshold lighting situations, centre resolution provides information on clarity. EBI will tell you how the unit performs in ultra-low-light situations (eg: astrophotography mostly). That is why we try to stay away from using FOM and ALL of our systems ship with their own individual tube spec sheets showing Gain, EBI , Resolution, SNR, etc. We also email you the tube photo of the actual tube you will receive, in advance of shipment.⠀⠀⠀⠀⠀⠀⠀⠀⠀
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With Photonis Echos, the Resolutions are mostly around the 66-69lp/mm range (although there are outliers from 62 all the way to 74), so if two tubes have roughly the same EBI and Gain, in our opinion, it would be very difficult to tell the difference between a 1800FOM tube and a 1950FOM tube. The difference becomes more apparent when the FOM differential is more than 200 (eg: 1600 vs 1950)⠀⠀⠀⠀⠀⠀⠀⠀⠀
As always, email us with any questions.