Why Higher FOM Is Not Always Better

FOM summarizes two important measurements. The rest of the tube—and the complete night-vision system—still gets a vote.

Figure of Merit is useful. It gives us a quick way to combine signal-to-noise ratio and center resolution into one number. The problem begins when that number is treated as a complete description of a tube.

A higher FOM generally indicates greater combined SNR and resolution under standardized testing. It does not tell you everything about low-light noise, halo, background glow, fixed-pattern noise, screen cosmetics, optics, tube matching, or how the completed device will perform in your environment.

"FOM is the headline. The rest of the tube is the article. If all somebody can tell you is the headline, keep asking questions."

FOM Is Math, Not Magic

FOM = Signal-to-Noise Ratio × Center Resolution

Signal-to-noise ratio describes how well useful scene information stands apart from electronic noise. Center resolution measures the tube's ability to distinguish fine detail near the center of the image under specified test conditions.

FOM combines those measurements into a convenient general performance indicator. L3Harris explains that FOM can help compare overall tube performance under standard conditions, but it does not precisely predict detection, recognition, and identification distance.

A higher FOM is meaningful. It simply is not a complete performance report.

The Same FOM Can Come From Different Tubes

CONCEPTUAL EXAMPLE — NOT MANUFACTURER DATA

Tubes A and B have identical FOM ratings, but they arrive there differently. Tube A has the higher SNR, while Tube B has the higher center resolution. Tube C has the highest FOM, but FOM alone cannot tell us whether it also has the lowest EBI, smallest halo, cleanest screen, best gain behavior, or best match for the intended environment.

Under extremely dark passive conditions, a user may place greater value on strong SNR. For tasks requiring fine detail in adequate ambient light, higher resolution may carry more practical weight. Application decides which tradeoff matters.

Resolution Steps Can Create Large FOM Jumps

Resolution is commonly recorded at defined values produced by standardized test methods. A movement from 64 to 72 lp/mm can cause a noticeable numerical jump in FOM even when SNR barely changes.

Conceptual example:

• 32 SNR × 64 lp/mm = 2,048 FOM
• 32 SNR × 72 lp/mm = 2,304 FOM

The calculated increase is real, but it should not automatically be interpreted as an equal percentage improvement in everything the user sees or does.

"Do not translate a percentage increase in FOM directly into the same percentage increase in identification range, image cleanliness, or overall system usefulness."

What FOM Does Not Include

The following measurements and conditions sit outside the FOM equation:

A tube can possess excellent FOM while also having a characteristic that makes it less desirable for a particular user. That does not make the tube defective. It means the complete specification set must be evaluated in context.

Minimum FOM Is Not Actual FOM

A minimum-FOM designation establishes a performance floor for a product category or contract requirement. It does not necessarily identify the exact measured FOM of the individual tube.

Night Vision Devices states that its products may be classified by minimum FOM levels and that individual systems are supplied with tube specification sheets listing measured characteristics.

Teach yourself to verify:

1. Is the number a minimum requirement or an actual measurement?
2. Does the data sheet match the tube's serial number?
3. Are both SNR and resolution shown?
4. Are EBI, halo, photocathode response, and other reported values visible?
5. Is the document complete and unaltered?
6. Is the manufacturer and tube format identifiable?

"A cropped photograph of one number is not a tube evaluation."

When High FOM Really Matters

High FOM can provide genuine benefits when the rest of the specification set and system quality support it.

Applications may include:

• Demanding movement under very low ambient light
• Professional observation where subtle detail matters
• Rural environments with limited artificial illumination
• Life-safety work requiring the strongest verified performance available
• Long-duration use where image quality and reduced visual workload are priorities
• Carefully matched binocular systems

High FOM should not be minimized merely because it is sometimes marketed poorly. The correct lesson is to verify what produced the number and what accompanies it.

When a Lower-FOM Tube May Be the Better Choice

Red-Flag Sales Language

A Better Decision Process

1. Verify the manufacturer, tube format, serial number, and data sheet.
2. Define the actual application and lighting environment.
3. Read SNR and resolution separately before looking at FOM.
4. Review EBI, halo, gain, photocathode response, FPN, and cosmetics.
5. Inspect the screen using controlled lighting and exposure.
6. Evaluate the housing, optics, controls, and assembly quality.
7. For binoculars, evaluate tube matching.
8. Consider service, documentation, and warranty support.
9. Compare complete systems under the same conditions.
10. Use FOM as a summary—not as the final verdict.

Final Takeaways

• FOM equals SNR multiplied by center resolution.
• Higher FOM generally indicates greater combined performance under standardized conditions.
• FOM does not include several characteristics that affect real-world image quality.
• Minimum FOM and actual measured FOM are not the same thing.
• The best tube is the one whose complete, verified performance fits the application and system.