Recently Rochester Precision Optics released their 3.0 Ultralight lens system. We have been a huge fan of RPO's ultralight lens offerings for some time and we were excited to be one of the first dealers to receive a production batch of these new lenses. Though, most of these are going to an overseas contract to where they are urgently needed, so we apologize that we do not have much available for commercial sale.
We have always had a goal of providing transparent information to the consumer, as the nightvision industry tends to be shrouded in secrecy and disinformation.
Weight:
Immediately, we can see that the 3.0 objective weight is roughly the same, whereas the eyepiece is significantly lighter. For some reason RPO decided the lightening cuts on the body of the objective were not worth the effort. Both remain significantly lighter than standard Mil-spec offerings due to their more advanced optical designs.
Without getting into too much detail as some of this information is controlled, a standard PVS-14 objective lens makes use of 8 individual optical elements. RPO was able to cut that down to 6 elements in the 2.0 lens system by introducing the use of polymer aspherical lens elements. The 3.0 system further reduced that down to 5 elements.
The eyepiece on the other hand is significantly lighter. A standard Mil-spec eyepiece makes use of 3 elements: 2 spherical elements and an asphere. RPO was able to reduce that to 2 elements. Don't worry, the outer element is glass for increased wear resistance.
Another reason why the eyepiece is lighter is the viewing area diameter was decreased from 29mm to 26mm. As stated in one of our previous postings, increasing lens size increases weight by roughly the cube of the increase in diameter or eye relief. The diameter change of roughly 11% would project to a weight decrease of 28%, but paired with the improved use of aspherical elements resulted in a weight reduction of just over 30%. The prediction checked out. It is also notable that the 3.0 eyepiece is shorter, more on that later.
Gain:
RPO 2.0 lens systems always had higher transmission than Mil-spec systems, and therefore produced a slightly better image in low light situations. With the 3.0 system, we wanted to test that. We only tested the objective lens for this as it is the only part that contributes to the tube input, which affects the amount of noise and the usability of image produced.
Test parameters were set such that the same tube was used for all tests, and we tested 5 different 2.0 objective lenses, and 5 different 3.0 objective lenses. Each lens had a gain reading taken 3 times for an average. The reason the gain reading is taken 3 times is because the gain reading can fluctuate based on random scintillations. That is why gain fluctuates more in the low light test where scintillations are more visible.
For low light, we tested at 0.006 mfL which correlates to 2x10-6 fc on a spec sheet
For high light we tested at 0.628 mfL which correlates to 2x10-4 fc on a spec sheet.
Note that a spec sheet only measures tube gain, system gain is always much lower than tube gain as it accounts for many more factors. We are not testing with a low spec tube. We also alternated 2.0 and 3.0 objective lenses in our test sequence in order to ensure our results were not skewed by calibration drift. All tests were conducted with a recently calibrated Hoffman ANV-126A test set.
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2.0 at 0.006 mfL 11003, 11062, 11092 Average: 11114.2 |
3.0 at 0.006 mfL 9745, 9906, 9865 Average: 9867.8 |
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2.0 at 0.628 mfL Average: 5145.9 |
3.0 at 0.628 mfL 4905, 4909, 4911 Average: 4877.7 |
From this, we can see that 3.0 actually has on average 11% less gain in low light and 5% less gain in high light. That sounds bad, but really it's not; it is a trade off. As stated in our previous lens article, RPO sacrificed flare suppression in favour of higher transmission. With 3.0, RPO sacrificed some transmission in exchange for excellent flare suppression.
Please note that we are testing flare suppression in worst case scenarios, and is actually better than shown in this video. The BCO LPMR 4K has an excellent sensor with excellent contrast software. The downside of that is that it picks up the contrast in lens flare more than is perceived by the user. The rings on the 3.0 are significantly easier to see past than the 2.0. The edge streaking by light sources, and concentrated flare points are significantly improved.
Resolution and Contrast:
In conversation with a high level optical engineer with significant experience working in night vision optics, we discussed some other reasons why RPO may have reduced gain output. While on its own, higher gain is better, often times you reach a point where higher gain begins to work against MTF (modulation transfer function) which determines resolution. If bright areas are brighter, but dark areas are also brighter, this can reduce contrast. Whereas if bright areas are brighter but dark areas are made darker through selective filtering, overall gain readouts will be reduced but contrast and resolution are improved.
Testing in this area was inconclusive, but leaned toward the 3.0 system providing marginally better resolution. Using the same tube, resolution was tested using a Hoffman ANV-126A and HVS-126A digital calibration system which removes human subjectivity from resolution measurements. Subjectively, line pairs appeared marginally clearer with the 3.0 system. In our testing, we repeatedly tested a high spec tube for resolution using both the 2.0 and 3.0 complete lens systems. With 2.0, the system was halfway between Group 5 Element 4 and Group 5 Element 5, which correlated to 45 lp/mm and 51 lp/mm respectively. However, the 3.0 system was consistently able to read Group 5 Element 5, correlating to 51 lp/mm. Because both systems were able to read Group 5 Element 5, the test is inconclusive. However because the 2.0 system was not able to consistently read it, it can be loosely inferred that the 3.0 system produces a marginally higher resolution.
This is also why we tell people that tube resolution on a spec sheet is one of the least important specs to pay attention to. There are other vendors that will mislead consumers into believing that high resolution tubes are a must, and will publish "tests" that try to push it as fact when they are obviously using unmatched tubes that differ in other areas, or simply didn't have one of the systems focused properly. It can be quite blatant. That is not to say resolution does not matter, it absolutely does. But prioritize other specs, and prioritize lenses. There is no point in using a 72lp/mm or higher tube when poor quality glass is going to drop your resolution anyway. The lens system is one of the most important factors in determining the performance of a night vision device. Don't use low cost substitutes in this area.
User Experience:
The RPO 3.0 vs 2.0 comparison is full of trade offs, and the user experience highlights this. The 3.0 system produces an absolutely beautiful image.
When building the goggle, it was immediately noticeable that the 3.0 lens system was much shorter than the 2.0 which was roughly similar to a standard Mil-spec system.
This leads to the goggle being shorter than a standard goggle, and to be honest it looks quite novel. It also means that using it in a standard diopter produces sub optimal results, whereas it fits perfectly in a low profile diopter such as the ones we produce.
In the above picture, a goggle is being tested on our Hoffman ANV-126A system. The eyepiece on the left is RPO 3.0, the one on the right is RPO 2.0. Both diopters have been set to zero using a Hoffman diopter scope. This shows how much shorter the goggle can become when using the 3.0 system.
Another thing that is of note, is the zone 3 distortion. There is more zone 3 distortion with the 3.0 system than 2.0 and standard Mil-spec. However the distortion is so far on the periphery that it does not seem to have much affect on the user experience due to how far out it is, and the fact that zones 1 and 2 are clear of that issue. It is also not recommended to mix 3.0 objectives with other eyepiece systems, as the objective and eyepiece are designed to work together. Doing so will result in a more distorted image (we tried).
Because the viewable area is slightly smaller as stated prior, the eye relief is also slightly shorter. It is noticeable to someone that has extensive time behind night vision devices, but may not be noticeable to most users. It is not a significant hindrance and is still very comfortable to use, and produces enough eye relief to be used comfortably with an Avon FM53 gas mask.
Conclusions:
In conclusion, we do not think it is fair to say that one system is definitively better than the other. There are too many tradeoffs.
2.0 has higher gain, less distortion, and better eye relief.
3.0 has better contrast, better resolution, and better weight reduction.
Both are excellent systems, and people with legacy 2.0 systems should not feel the immediate need to upgrade. For customers looking to get into which lens system to go with, we are looking at potentially carrying old stock of the now discontinued 2.0 system to compliment the 3.0 offering to provide more options. In extreme dark conditions such as a forest with full tree canopy, the increased gain of the 2.0 system may be more beneficial. In dynamic lighting situations where flare suppression and contrast are more beneficial, then the 3.0 system may be the system of choice. Each user is different and their needs will be different. We try to provide as much detailed information as possible to help users make informed decisions.
