Figure of Merit

A useful summary of signal-to-noise ratio and center resolution—and one of the most overworked numbers in the night-vision business.

What FOM Is

Figure of Merit, usually shortened to FOM, is a numerical rating used to summarize two important image-intensifier specifications:

FOM = Signal-to-Noise Ratio × Center Resolution

Signal-to-noise ratio describes how well useful image information stands above image noise.

Center resolution describes how finely the tube can separate detail near the center of the image under the applicable test method.

FOM combines those two measurements into one convenient number.

It is not independently measured. It is calculated.

"FOM is multiplication, not magic. If you want to understand the tube, look at the two numbers somebody multiplied."

Calculating FOM

Example:

32 SNR × 72 lp/mm = 2,304 FOM

Another example:

28 SNR × 64 lp/mm = 1,792 FOM

When the data sheet lists SNR and resolution but does not display FOM, multiply the actual measured values.

Do not calculate FOM using:

• A minimum SNR and an actual resolution
• An actual SNR and a minimum resolution
• Values from two different tubes
• Marketing claims that do not appear on the matching data sheet
• High-light resolution unless the manufacturer specifically defines that calculation

Use values measured for the same individual tube under the applicable test and reporting method.

Actual FOM Versus Minimum FOM

A system may be advertised by a minimum FOM tier. That number is a performance floor, not necessarily the tube's exact measurement.

For example, a system classified as minimum 2,000 FOM should meet or exceed that stated minimum under the supplier's applicable requirements. The individual tube's actual FOM may be higher.

The matching tube sheet is where the actual measurements should be verified.

Always separate:

Do not advertise a minimum-FOM system as though every tube has the same exact FOM. It does not.

Same FOM, Different Tubes

Different SNR and resolution combinations can produce the same FOM.

Tube A:

32 SNR × 72 lp/mm = 2,304 FOM

Tube B:

36 SNR × 64 lp/mm = 2,304 FOM

Both tubes have the same calculated FOM.

• Tube A has higher center resolution.
• Tube B has higher signal-to-noise ratio.

Tube A may hold an advantage when the scene provides enough light and contrast to use its additional measured resolution. Tube B may maintain a cleaner image as illumination becomes severely limited.

The same FOM does not mean identical performance.

What FOM Tells You

FOM provides a useful first look at the tube's combined SNR and resolution performance.

It can help:

• Sort tubes into broad performance categories
• Compare tubes tested under compatible methods
• Confirm that a tube meets a stated minimum tier
• Identify major performance differences
• Summarize two important data-sheet measurements
• Provide a starting point for further evaluation

L3Harris describes FOM as a general performance metric rather than a precise predictor of detection, recognition, and identification distance.

A higher FOM generally represents more combined SNR and resolution. It does not guarantee a fixed percentage increase in field performance.

What FOM Leaves Out

FOM includes exactly two measurements. It does not include:

• Equivalent Background Illumination
• Halo
• Gain
• Photocathode response
• Fixed-pattern noise
• Spots or cosmetic blemishes
• Screen uniformity
• Edge distortion
• Autogating behavior
• Optical quality
• Housing quality
• Assembly and alignment
• Tube matching in a binocular
• Current condition or remaining service life

A high-FOM tube can still have an objectionable spot, large halo, distracting pattern, poor screen uniformity, or characteristics that do not fit the application.

Why a Higher FOM May Not Look Better

A higher-FOM tube may not look obviously better in every comparison.

Possible reasons include:

FOM does not stop any of these factors from mattering.

Field Scenarios

Comparing FOM Correctly

Use this process:

1. Confirm each data sheet matches the tube serial number.
2. Identify actual measurements versus minimum requirements.
3. Verify SNR and center resolution.
4. Calculate FOM yourself.
5. Confirm both tubes used compatible test and reporting methods.
6. Compare the individual SNR and resolution values.
7. Review EBI, halo, gain, photocathode response, and cosmetics.
8. Evaluate the complete systems under matched conditions.

Do not compare a verified actual FOM against a vague advertised minimum and pretend they are the same kind of number.

Common Myths

Lesson Takeaway

FOM is calculated by multiplying SNR by center resolution. It is a useful way to summarize two important tube characteristics, but it cannot tell you how those characteristics are divided or describe everything else that affects the image.

Use FOM as a starting point. Then read the rest of the sheet.