One of the most common questions I get asked by folks looking into night vision for the first time is “what are spec sheets, and what do these numbers mean?” Simply put, spec sheets are manufacturer breakdowns of the imaging tube (these are provided by the manufacture themselves at time of production).

There are 5 key metrics that you’ll want to look for in a tube’s spec sheets that will give you a high level understanding of the unit’s quality/performance:

  • Signal to Noise Ratio (SNR)
  • Center Resolution (RES)
  • Figure of Merit (FOM)
  • Equivalent Background Illumination (EBI)
  • Halo

Signal to Noise (SNR) and Center Resolution (RES):

SNR is the most important metric and the ratio of the “Signal : Noise” in the background. As a result, the higher the SNR, the better. Low SNR will show more static.

RES is measured in line pairs per mm (LP/mm). It then becomes the case of ‘the higher the number, the better.’ Common metrics to see are 64 and 72 RES. Though, it’s important to note that the measurement could be higher than listed, but not high enough to cross the threshold of the next RES metric (eg. A 64 RES tube could be as high as 71 RES, but since it doesn’t meet the 72 benchmark, it can’t be listed as such).

Figure of Merit (FOM):

Both SNR and RES are then used to calculate your Figure of Merit (FOM) via the following equation ‘SNR x RES = FOM.’ This is intended to give you a quick high level understanding of the overall tube performance.

Equivalent Background Illumination (EBI):

This can sometimes get blown out of proportion. In short, EBI calculates the amount of light you see through a night vision device when an imaging tube is on, but no light is on the photocathode.

Mil spec limits on EBI is a 2.5 rating. Obviously anything below is preferred. 

For head mounted solutions, EBI is not a real factor as you aren’t able to detect the scintillation. Now if you’re looking to do star gazing/astronomy work I would advise a lower EBI metric. It’s also important to note that higher SNR generally correlates to higher EBI. For most folks it doesn’t make sense to sacrifice higher SNR for lower EBI.

Halo:

Halo is the ability to control bloom when viewing a light source. Somewhere around the 1.0 mark or less (lower the value the better.).