FOM 1800 Gen 2+ Image Intensifier Tubes: Higher SNR or Higher Resolution?

What Does FOM 1800 Mean?

FOM, or Figure of Merit, combines two image intensifier specifications: limiting resolution and signal-to-noise ratio.

FOM = Resolution (lp/mm) × SNR

Resolution describes the tube's ability to distinguish fine detail. SNR describes the useful signal relative to noise. Because FOM is their product, two tubes can share the same FOM while offering different combinations of these characteristics.

A matching FOM does not mean an identical image. For this comparison, a higher resolution value is paired with a lower SNR. This is a mathematical relationship at a fixed FOM, not a claim that improving resolution always reduces SNR.

FOM 1800: Comparing 64 and 68 lp/mm

Assume both tubes have an exact FOM of 1800. Dividing FOM by resolution gives the corresponding SNR:

• Tube A: 1800 ÷ 64 = approximately 28.13 SNR.
• Tube B: 1800 ÷ 68 = approximately 26.47 SNR.

These are calculated examples, not measured specifications for individual products. SNR values are rounded. If “FOM 1800” denotes a minimum grade or a rounded rating, use the actual tube test sheet instead of deriving SNR from that label.

Tube B has the higher limiting resolution, but that alone does not make it the better choice for every application. Tube A offers a different balance, with more SNR at the same FOM.

Why We Would Favor Higher SNR in Low Light

The difficult question in a very dark scene is often whether the image remains clean enough to interpret. Fine detail becomes less useful when noise makes that detail difficult to distinguish.

Under comparable conditions, a higher SNR generally supports a cleaner image and better visibility of low-contrast features. MKU also identifies SNR as an important indicator of image intensifier performance under low illumination.

That is why, for the example above, we would generally lean toward 64 lp/mm with an SNR of about 28.1 rather than 68 lp/mm with an SNR of about 26.5 when low-light observation is the priority.

This is a selection preference, not a measured performance result for these two hypothetical tubes.

Does SNR always matter more than resolution? No. SNR helps establish a usable image in low light, while resolution contributes to the fine structure that image can reveal. Both matter, and their practical value depends on the scene and the complete viewing system.

Does 68 lp/mm Look 6.25% Sharper Than 64 lp/mm?

The numerical increase from 64 to 68 lp/mm is:

(68 − 64) ÷ 64 × 100 = 6.25%

However, a 6.25% increase in limiting resolution does not translate directly into a 6.25% increase in perceived sharpness.

Once the tube is installed in a night vision device, the objective lens, image intensifier, eyepiece and observer all contribute to the final viewing experience. Optical design and modulation transfer function (MTF), which describes how contrast is transferred at different levels of detail, also influence the image.

A tube's center limiting resolution does not fully describe the complete device's edge image quality. The practical difference between 64 and 68 lp/mm therefore needs to be considered in the actual system and lighting conditions.

How We Would Choose a FOM 1800 Gen 2+ Tube

Start with the conditions in which the device will be used, then compare the individual specifications behind the FOM rating.

• For low-light observation: We would generally prioritize the higher SNR when other important characteristics are similar.
• For fine-detail observation with more available light: Higher resolution may deserve greater weight, especially if the rest of the optical system can make use of it.
• For the final decision: Compare individual tube test sheets and, where possible, view both tubes in equivalent devices under the same conditions.

FOM is a useful starting point. Understanding the balance between SNR and resolution makes it more useful for selecting a tube that fits your application.

Frequently Asked Questions