Night Vision Specs Explained - EBI

Night Vision Specs Explained - EBI

What is EBI (Equivalent Background Illumination)?

What is EBI and what does high EBI look like?

EBI is often listed on a night vision dealer's FAQ or glossary with a similar definition:

This is the amount of light you see through a night vision device when an image tube is turned on but no light is on the photocathode”.

What does that actually mean? It’s not the most helpful definition and EBI does quite a bit more. Additionally, it’s pretty misunderstood. Let’s jump in-

Currently, Milspec EBI has an acceptable range from 0 to 2.5. The lower the number, the better (i.e. closer to zero). EBI is affected by heat, where a warmer device will exacerbate the effects of EBI.

The temperature of the operating environment can be one determining factor for how much importance to assign to EBI when considering the specs of a given tube. Said differently, someone living in Alaska might not care about EBI at all, whereas someone in south Florida might be more selective about it. But your use case matters more.

Higher EBI is reminiscent of the light pollution associated with cities. The diminished contrast between black and white caused by greater background illumination is why astronomers tend to be fixated on finding a low EBI tube. That’s because EBI diminishes the contrast between a bright star or nebula and its black background. 

On the other hand, background illumination's impact on a helmet mounted user's NODs (also referred to as a headborne system) doesn't materially change the appearance of the wearer's surroundings while navigating on foot.

As of late, EBI's impact has been overstated in online communities. 

Visually speaking, EBI and SNR are the opposite sides of the same coin - they both affect the clarity of the tube but EBI is only impactful in one environment - near total darkness. 

EBI also affects the lowest light level in which you can identify an object. With the release of online educational material on night vision, this happens to be one impact that many seem to understand in theory but misinterpret in practice.  Some people have incorrectly assumed that a low EBI device will automatically perform a certain magnitude better in near total darkness. Additionally, they may have read or heard that EBI is the most important spec. This is wrong.

EBI may be very important for astronomy but SNR has the largest impact on overall night vision (intensifier tube) clarity and performance. And while SNR, EBI, Photocathode Sensitivity, and Luminous Gain work together holistically, it's common (and admittedly easier) for buyers to view these specs independently. It’s fairly typical for buyers to go down their list of desired specs (based on what they've read online), and have a set limit or minimum for each one.

For EBI, that limit is often erronenously less than 1.0 because a blog or a certain popular YouTube video (since removed from YouTube) referenced sub 1.0 as being ideal for EBI.

Why is that flawed thinking?

Trying to select a tube with sub 1.0 EBI can work against you because it assumes SNR is a constant amongst tubes. If SNR were always about 30 and under, yes it might be ideal to search for an EBI of less than 1.0. However, it's certainly not a requirement for a great looking tube. As stated above, SNR greatly overshadows EBI in terms of its impact on total tube performance. Yet interestingly, higher SNR values tend to indirectly elevate EBI as well.

If you sorted through a random batch of tubes, and searched for the lowest EBI numbers (lets hypothetically say 0.2 on average), they'd likely end up as middle of the road night vision devices.

Why is that? It's because these tubes, on average, only have the benefit of low EBI due to weaker photocathodes and low SNR (you need both to have a more than decent night vision device).

 

 

Source: Photonis

  

Why does EBI occur?

This will sound complicated but stick with us:

Equivalent background illumination is caused by thermal electron emission by the photocathode itself. This is over and above the photons (light) that the photocathode converts to electrons (which are multiplied further down the chain). Usually, (though not always) a more powerful photocathode will generate more thermal emission, which leads to additional light and heat cast off by the tube itself.

Greater photocathode (often quoted for spec purposes as "PC" sensitivity / response) almost always leads to higher SNR and is a huge driver of tube performance in and of itself. This is why it's common for the highest spec'd tubes with very high SNR to have EBI values of 1.3 or higher.

To simplify, high spec night vision devices have very powerful engines that put out more heat and light which causes some extra glow and noise. Some put out more than others but the more powerful the device, the more that becomes unavoidable.

EBI numbers generally start to rise above 1.0 at ~35-37 SNR (particularly in L3 filmless tubes). At that level and as SNR goes even higher, EBI below 1.0 starts to become much less realistic and more of a statistical outlier. We've witnessed more than a few people pass over tubes with ~40 SNR and zero spots due to "checking the box" where they NEEDED to have an EBI below 1.0. They saw a 1.2 or 1.5 EBI number and figured it wasn't good enough or "out of spec". Unfortunately for them, they skipped a rare tube in a top percentile of performance. 

What's the bottom line?

The variables that make SNR high also result in power and clarity across every light environment, AND as a side effect, increase EBI. At the same time, tubes with great SNR, photocathode sensitivity, and ideal luminous gain* also help to offset the effects of high EBI.

Conclusion

All other specs being equal, low (good) EBI may result in better clarity and contrast in the very darkest environments. However, with binos, even most professional users would be challenged to discern a difference between a pair of 0.7 and 1.3 EBI (0.6 difference, which is a small, conservative spread). Many night vision goggle / binocular users often report being unable to tell a difference between tubes with EBI values greater than 1.0 between them (i.e. 1.3-2.3) This is particularly true in a set of dual tubes with very high SNR (35+).

Night vision users that wear helmet-mounted NODs (as opposed to a single tube mounted to a telescope) should first and foremost concern themselves with SNR. EBI is important, but as long as SNR is decent, any EBI figure under 2.0 (still well below the Milspec limit as of this writing) is going to look and perform great for most users. 

Today’s most powerful filmless tubes are illuminating objects in near pitch black, so gains from low EBI to a helmet mounted user are diminished to such an extent that it typically wouldn't make any sense to sacrifice lower SNR (and clarity through the entire entire spectrum of usable light) for lower EBI.  You’d be giving up power and clarity in 90-95% of light environments to see minimal gains in the darkest one - and yet high SNR helps there as well, helping to offset loss of clarity and noise. 

TLDR: 

Rejecting tubes above 34 SNR, to get “below 1.0” EBI will to deprive you many top-tier night vision devices with little benefit otherwise (or likely a decrease in performance). At an SNR of 34 and below, EBI numbers in the 1.0-2.0 range for typical users (those not specifically buying a device for astronomy) are typically more than acceptable as long as all other specs are good.

Further reading / visual examples:

 

*As a side note, very high luminous gain in conjunction with lower SNR and Photocathode sensitivity is likely to cause additional noise in a tube  (additional scintillation). This another example of why specs should not be viewed independently of each other.


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