"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 mean?
Equivalent Background Illumination (EBI) is a bit more dynamic than that definition might suggest. Additionally, if you aren't aware of what a photocathode is, this is a difficult explanation to understand (an illustration of an intensifier tube with photocathode is included further down in this article). Finally, EBI's impact on total system performance is pretty misunderstood. Let’s jump in-
As of this writing, "Milspec" EBI has a limit of 2.5 max with a lower number preferred (EBI of 0.0 is possible). This range determines a tube's ability to resolve an object in the darkest light possible.
EBI is affected by temperature, and a warmer environment will increase EBI. As a result, the temperature of the operating environment can be one determining factor for the importance to assign to EBI when considering specs of a given tube.
High EBI can be reminiscent of light pollution above a suburban or urban area at night. The diminished contrast between black and white caused by greater background illumination is also why astronomers / astrophotographers seek tubes with lower EBI numbers (seeking an image with a white star against a darker background). PVS-14s and other monocular variants with low EBI and high SNR are particularly well suited for astronomers / astrophotographers in combination with a telescope. We'll look to explain further on why very high SNR tubes (35+ as of this writing) with very low EBI are particularly challenging to find.
On the other hand, a user wearing helmet mounted NODs isn't usually materially impacted by EBI to the same degree - it doesn't really change the appearance of ones surroundings. For example, if the user is wearing a pair of RNVGs or DTNVS (compared to a PVS-14), viewing one's environment with both eyes provides a better ability to resolve small details in low light. Stereoscopic vision, can help "fill in the gaps" where both eyes looking at the same object from slightly different angles (and experiencing rapidly changing scintillation in each eye) and are better able to resolve small details.
EBI's impact is probably overstated in online communities.
Visually speaking, EBI and SNR are somewhat similar... but different. They both affect visual clarity but EBI is particularly impactful in one environment - near total darkness. In low light, very low SNR (low signal / high noise) includes lots of scintillation (or "tv static"). Very high EBI can resemble a haze reminiscent of a well lit, smokey bar and may include additional noise too.
To go back to the original definition of EBI and simplify it further, it impacts the lowest light level in which one can resolve (identify) an object. With the recent growing interest in night vision and sharing of info on specs in online forums, this happens to be one impact that many seem to understand in theory but misinterpret in practice.
Some people have incorrectly assumed that a device with very low EBI will perform a certain magnitude better as a night vision device. Additionally, some may have read or heard that EBI should be prioritized as the most important spec. In our view, this is wrong. Also keep in mind that night vision technology is constantly improving - what may have been true about tubes just several years ago might no longer be the case.
EBI may be very important for astronomy (and is also important for night flight pilots) but SNR in conjunction with center resolution (figure of merit) has the largest impact on overall intensifier tube clarity and performance. As an aside, current "Milspec" minimums at 64 LP/mm center resolution as well as higher average center resolutions around 72lp/mm+ probably make SNR a more straight forward spec to focus on.
While SNR, EBI, and Gain all impact tube performance in a holistic way, it's common (and admittedly easier) for buyers to evaluate these specs independently. It’s commonplace to go down the list of desired specs (often based on what one may have read online), and have a set minimum, or in the case of EBI, a maximum.
One scenario we've seen play out occasionally is that prospective buyers have set their EBI limit at 1.0 because they may have read or heard that sub 1.0 EBI is ideal. First, what's ideal for one buyer (using the astronomy example) isn't necessary for another. Second, placing hard limits around certain specs ignores that specs often "work together" to tell a story about a tube's performance.
Why is rejecting any tube above 1.0 EBI a flawed strategy?
Rejecting a tube with greater than 1.0 EBI can work against you because it assumes SNR (and gain) is a constant amongst all tubes. If SNR were always about 30 and under, an EBI of less than 1.0 would be a consistantly appropriate target. However, it's certainly not a requirement for a great looking tube. As stated above, it's our view that SNR greatly overshadows EBI in terms of its impact on total tube performance. Again, this is why placing hard limits or goals for specs across the board can work against you in a deceptive way.
Higher SNR values are correlated with higher EBI.
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 probably end up with lower Signal to Noise ratios.
Why is that? Generally, it's because these tubes, on average, are likely have a lower EBI due to lower photocathode sensitivity, and very likely a lower gain and/or signal to noise ratios.
Why does EBI occur?
This may sound complicated but we'll do our best to simplify this:
Equivalent background illumination is caused by thermionic emission by the photocathode. Resulting electrons are over and above the amount of photons (light) that the photocathode converts to electrons. This is another way of saying the tube is producing its own electrons due to heat (rather than those converted from Photons) which causes image interference / noise. Usually, (though not always) a more powerful photocathode produces greater EBI.
Higher photocathode sensitivity is typically one of the main drivers of gain and is also correlated with higher SNR. This is why it's common for the highest spec'd tubes with very high SNR to have high photocathode numbers and but also higher EBI values (~1.3 or higher in unfilmed tubes based on our experience).
To simplify, high spec night vision devices have powerful engines that put out more heat and light which causes some extra glow and noise in the form of EBI. SNR and PC Sensitivity are certainly correlated but high PC sensitivity doesn't always equal high SNR and vice versa. Simply: correlation is not causation and it doesn't always occur. You can have a strong SNR with poor illumination (gain). Having one without the other doesn't translate to a great night vision device (further emphasizing why it's important to avoid falling into the trap of looking for certain specs in isolation of each other).
There can be differences between Elbit Thin-Filmed tubes and L3Harris unfilmed tubes with respect to typical spec ranges, but EBI numbers commonly start to rise above 1.0 at ~35-37 SNR. At that level, and as SNR goes even higher, an EBI below 1.0 starts to become less realistic.
We've seen prospective buyers pass over tubes with 40 SNR and zero spots due to them "checking the box" where they also needed to have an EBI below 1.0. Perhaps they saw a 1.2 or 1.5 EBI number and figured it wasn't good enough. Unfortunately, they skipped a pretty rare tube in a top percentile of performance.
What's the bottom line?
The variables that make SNR high also result in power (signal and ideal gain) and clarity across many light environments, and as a side effect, tend to increase EBI. At the same time, tubes with great SNR and ideal luminous gain can help to offset the effects of a higher EBI.
All other specs being equal, low EBI may result in better clarity and contrast in the very darkest environments. However, even most professional users would be challenged to discern a difference between a pair of 0.7 and 1.3 EBI (0.6 difference, a small, conservative spread) particularly with dual tubes. Many night vision goggle / binocular users often report being unable to tell a difference between tubes with EBI values greater than 0.8 between them (i.e. 0.8 and 1.6) This is particularly true in a set with very high SNR (35+).
It's our opinion that prospective night vision buyers of 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 excellent (~32+ as of this writing), an EBI figure under ~1.7 (still well below the Milspec limit as of this writing) is going to look and perform great for most users.
Today’s most powerful unfilmed tubes are illuminating very dark environments and inching closer to "near pitch black", and clarity gains from low EBI to a helmet mounted user are somewhat diminished. In other words, it typically wouldn't make sense to sacrifice higher SNR (and clarity through a larger range of usable light) for lower EBI. Giving up power and clarity in a greater percentage of light environments to see minimal gains in the darkest one is typically a bad trade off. However, everyone has a different use case.
Rejecting tubes above ~34-35 SNR, to get “below 1.0” EBI may deprive one of a top-tier night vision device. At an SNR of 35+, EBI numbers in the 1.0-1.7 range, for most users (not seeking a device for astronomy purposes), are typically more than acceptable as long as all other specs are good.