Figure of Merit

Figure of merit, or FOM, is the headline number used to grade image intensifier tubes: the tube's measured signal-to-noise ratio multiplied by its limiting resolution in line pairs per millimeter. Signal-to-noise ratio captures how cleanly the tube amplifies — how much scintillation, the sparkling noise grain, appears in the image at low light. Resolution captures fine detail. A modern high-end tube might pair a signal-to-noise ratio around 30 with resolution of 72 line pairs per millimeter, giving a FOM above 2100; export control thresholds are written in these terms.

FOM is useful because it compresses two genuinely different failure modes into one number: a noisy but sharp tube and a clean but soft tube can score identically while looking very different in the field. That compression is also where the marketing misuse begins. Because FOM multiplies the two figures, vendors can advertise a high score built on an extreme value of one factor and a mediocre value of the other, and the number says nothing about halo around bright lights, photocathode sensitivity, or cosmetic blemishes.

The informed reading treats FOM as a floor, not a verdict: compare the signal-to-noise and resolution components separately, then weigh halo, sensitivity, and tube cosmetics, none of which the number captures.