FOM or the Figure of Merit, is a number calculated by multiplying the Signal-to-noise ratio by the resolution of a night vision image intensifier. FOM number allows quickly estimate general performance level of the specific night vision image intensifier, and consequently the night vision device built around it.
Does it matter?
In recent years, the night vision sector has increasingly emphasized the Figure of Merit (FOM), an evaluative metric initially devised by governmental bodies to regulate the import and export of image intensifier tubes. The FOM is derived by multiplying the Signal-to-Noise Ratio (SNR) with the center Resolution of the image field, expressed in line-pairs per millimeter (lp/mm):
FOM = SNR x Resolution
However, an overreliance on FOM can be somewhat deceptive. For instance, it's possible for one tube to exhibit a high SNR and a low resolution, and another to show just the opposite, yet both could register the same FOM score. Consider two hypothetical tubes: Tube 1 has a resolution of 67lp/mm and an SNR of 32, culminating in a FOM of 2432. Similarly, Tube 2, with a resolution of 76lp/mm and an SNR of 23, also reaches a FOM of 2432. Despite the identical FOM values, Tube 1 would produce a more balanced image, while Tube 2 would offer a more detailed yet noisier image.
We advocate for a more nuanced approach that evaluates each specification on its own merits, as this provides clearer insights into how each tube will perform across different lighting scenarios. SNR is pivotal in dimly lit environments, resolution impacts the sharpness of the image, and Equivalent Background Illumination (EBI) assesses the tube's effectiveness in very low-light conditions, such as in astrophotography. This approach ensures a more comprehensive understanding of each tube's capabilities.