Can someone please point me to a resource that can explain exactly what constitutes a subjectively good PVS-14, and how one would go about testing/rating one?  If you only had one PVS-14 with nothing to compare to, how would you know if you had a good one in your hands?

I understand most non night enforcer systems come with a data sheet.  Besides the subjective part, what values objectively result in a good unit?

I've read most of the stuff I could find here, and the info/videos TNVC has published but it's still not clear to me unless one had previous experience, how you would tell if you got your money's worth/a good one.


Thanks much,


-Freq

Firstly, most PVS-14's are OK. There is great and excellent, but if you want just "good" then most will fit this, unless damaged - Here's what to look for.




1) Gain control - make sure it works.


2) Image - Look for any large burns or problems with the image - small specks aren't a problem.


3) Emission points - with the front lens covered with foil, there should be no bright sparky spots anywhere - this is a sign of imminent tube failure.


4) Shading - the circle where the image is should be clear and fit the whole screen. Use with a pinhole lens cap, so you can see the screen edges and check the image covers it all - if it's impinging on one side, this is a worst case scenario and the tube will soon fail.


5)  Brightness - With the lens cap covered with foil, you shouldn't see any image, but you should see speckles with the image brightness turned all the way up. What you shouldn't see is complete darkness. ( note: Unless it's a new supertube, but generally, this means a used-up tube )


6) External damage - Unit should be intact, without any signs of serious damage. The purge screw should be present. It's near the back. Make sure the lenses aren't all scratched up.


7) Comes with other parts - J-arm, etc.


8) Not an amazing bargain. If it's $500, be wary. Make sure you ask enough questions so that you're comfortable it's not stolen. If it is stolen and you genuinely don't know you probably won't get in trouble, but you will lose it the next time it's serviced.


9) Is a real PVS-14 - Have a close look at the pictures on the internet. There are variants, but the upper is always the same ( the main body ).


10) Battery type - Older ones are dual battery - make sure the clips that hold the battery are present on older ones. Newer ones are AA or CR-123 type.


11) The tube - If you can open it, look for signs it's a military tube. If it is, be careful that it's not stolen ( ref: 8 ). Sometimes you can flash a torch in the eyepiece and hold on an angle to see any military markings on the tube - Not all tubes with military markings are stolen, the majority are most likely legitimate, but again, ask questions if you find one.


12) Lasers. Does it come with extra's like lasers. Are they civilian lasers? If not, run from the deal. Civilian lasers will be marked. There is NO private ownership of military lasers in the US. Even if it's bought from a legitimate shop with legitimate paperwork.  You will lose it.


13) The switch. Make sure it works. It should be pull to lock IR on.


14) If you can see it disassembled, make sure the anti-rotation pin is in place - it's the metal pin nearest the flange side on the main body.


15) Pigtail - If you can see it, make sure it's in good condition, and not torn anywhere.


16) The contacts near the mount. Make sure they are there, and clean.


17) Lenses move smoothly and freely.


18) Infinity lock is present. and working. That's the ring behind the ocular.


19) Ocular lock is present and working. That's the ring in front of the ocular. It should be tight up against the ocular.


20) That you can't unscrew the objective lens. ( make sure the retainer is in place ).


21) Thump it around in your hand a few times to make sure nothing is loose. See if it rattles. It should not.


22) Paperwork -these things aren't cheap. Most people keep the paperwork they come with.


23) Reputation - Look for the reputation of the seller. A guy who has an online history of selling stuff for 10 years is probably OK even without paperwork.


24) Battery lights work, IR lights work - make sure the light pipe is visible through the ocular.


25) Tube retaining ring - you can't see if the oculars in place, but make sure it's there and fastened if you can. Generally thumping it back and forth will tell of this is an issue though.




Anyway, they are some thoughts, based on issues people on the forum have and what was needed to fix it - Best yet, put it onto a TS-4348 tester, if you can get one. They are cheap - buy one to take with you when looking. A good PVS-14 should perform in the group above the pass-mark.




Regards


David

Notes added later:

The above is all great info, anything David says can basically be taken as gospel. In the absence of testing equipment there are a couple of astronomical features I like to use to inspect resolution subjectively.

One is the Pleides, aka Subaru, or "the seven sisters" is great for checking out a scopes ability.

https://en.m.wikipedia.org/wiki/Pleiades

This group of stars are bright but very fine in detail.  In my experience, older "thick film" goggles with the standard 64lp/mm resolution can make out the 7  brightest stars that make up the main constellation, but it takes higher resolution and smaller halo to start to differentate the many smaller stars around it, or that one of the main 7 is binary.

Keep in mind that image intensifiers work great in the COLD verses hot (lookup EBI, lower is better) and also better when the air is dry and there is very little humidity; see ir emissivity of water vapor. If you are buying an unknown pvs14 in february you might be in for a surprise when the image is super hazy in your summer season, and even a great pvs14 might look mediocre when the air is super saturated with water.

David,

Thanks for all the detail!  Perhaps I should have said excellent instead of good. Are there values that result in an exceptional image?  Without a known good one to compare to, I'm trying to figure out how to validate a purchase.  I have seen descriptions of the different measurements but no recommendations on what a high performance tube should subjectively look like, and objectively measure out at.

Here are some values:
PR 2292
EBI .8
CR 64
Halo .93
SNR 27.9

-Freq

In my opinion that is objectively an excellent tube. EBI under 1 and fairly high SN. It's good to go.

 
Yep, Valley rat is correct - that's an excellent tube, at least as specs go -

Thanks much all, there are no noticeable spots or burns.

If SNR was higher, would I see a noticeably better image?


-Freq

Are these measurements posted by the manufacture on the tube or is there a machine you use to determine these numbers? I am looking at a couple tubes right now, both are blems. One has some burn lines and one has a dot. The pictures through the tube look pretty decent, but with a camera it can be kinda difficult to show the actual picture quality through the tube.

Anything over 25-26 and you kind of seem to hit a point of diminishing returns.

27+ is very good.  I've looked through some otherwise similarly spec'ed tubes with one being 28 and one being 33 and frankly there isn't much functional difference, and if you mixed up the tubes and didnt tell me, I'm not sure I'd have been able to tell.

As long as the tube is free of tree bark/chicken wire and blem spots, I'd be very happy with those specs.

 
You won't see anything close to what you need to in an image - unless it was taken by an expert specifically to highlight a difference between two tubes, and even then, you can't make a good estimate of the difference.


Hoffman do make testers to give you this information, but they are quite expensive and very few people have them - most of us just use the same basic tester that they use in the field, and the best is gives is a pass/fail mark.


Regards

David

 
If comparing a 21 S/N and a 27 S/N side-by-side, you might notice the difference. Otherwise, once you get above 21, you're more likely to notice other aspects of the image than the actual quality - eg, the relative brightness, how "snowy" the image looked, etc.


I've even seen people shun very high S/N tubes because they had different image characteristics from a much lower performing tube which produced an image they preferred - even though the high-SN tube was clearly superior. ( And the difference was enough for me to spot immediately ).


So I'd suggest that it's really hard to evaluate a tube by looking through it - even when you have a lot of experience.


People like Vic at TNVC who do it with tens of thousands of tubes with known specifications start to develop an ability to measure a tube's real quality by eye - I'm not quite there yet ( I don't look through enough ) and I have to create ad-hoc test conditions to get anything realistic from a tube.


So it really is hard to evaluate a tube with human eyes - The best way I can suggest - go into a very low light situation, and see if the tube is working well - this is when they are pushed to hardest to produce a useful image - eg, in a closed house, with all the curtains drawn, on a starlight only night, with no artificial light anywhere nearby. It's at these times that it's easiest to see the difference in performance between two tubes.


The good news is that conditions like that are kind of rare when using NODs, so generally anything above a S/N of 21:1 is acceptable for general use.


Generally, I work on the idea of something like this.


21+ Acceptable/Good.

24+ Good / V good.

26+ Excellent

28+ Exceptional

30+ Supertubes.

38+ True Supertubes.


It's a rough guide - but it does give an idea of how a big change is S/N is needed to really notice any difference, even under optimal conditions.


Regards

David.

After further inspection, with the IR on up against a white wall, I do see a very faint honeycomb/chicken wire, and notice a few small specs that I didn't see previously (maybe dust?).

How much chicken wire/honey comb/fpn is expected on a new OMNI VII tube?


-Freq

Well you're gonna, thats the one scenario that is going to really force the chicken wire to show up. When you have all that IR splashing back, it tends to show up, people just meant that if you had chicken wire showing up in the image when you are looking at stuff in general, that is somewhat distracting. Also, if you didnt notice the specks before you were staring at a clean, white wall looking for any little flaw, you most likely arent going to notice them when out & about, so its not a huge deal.

OMNI VII tubes are fantastic performers, some of the highest spec'd tubes ever made, its pretty hard to go wrong with one. I have two myself, off at Nightlong Industries for RBHoovers tender ministrations

For general night hunting on foot in the heat, how would you rate the importance of each characteristic in terms of most to least? i.e.

1. EBI
2. Luminous
3. S/N
4. Resolution
5. FOM
6. Halo

I sometimes have trouble seeing lighter colored deer in the higher grass without the IR light.  Is this due to my tube's poor EBI?

My tube:

EBI - 1.77  
Lum - 2542
S/N - 27.6
Res - 64
FOM - 1766.4
Halo - .95

EBI will manifest as more noise during hot weather (in basic terms) There are a few videos on youtube made by a member who took a very active interest in EBI that should show you what it looks like.

The issue about seeing lighter colored deer is one of contrast. Signal to noise and resolution (aka FOM) is the best you will see on a modern spec sheet that might cover how much useful signal you will get in a given light environment. FOM can be equated to the range you can resolve a certain size target at a certain distance (resolution and lack of noise being key factors) although it's not measured in that way that is what it can do for you, which is why it's one of the export controls for US equipment. Several factors play into contrast with specific targets and backgrounds. The reflectivity to light the intensifier sees is important, of note, the IR reflectivity of plants is different in the IR spectrum as seen by Gen III NVGs than your eye, and some Gen II NVGs. This is one of the reasons people are so interested in the Photonis INTENS image tubes as they offer sensitivity over a greater range, perhaps increasing contrast due to more "information" (or bandwidth) put though the image tube.

Another way to look at contrast is Modular Transfer Function (MTF) which is a system measurement, takes into account the glass in the objective and eyepiece lens. There is one for a AN/PVS-14, although I don't have it off hand, and what is interesting is that beyond a certain point you may have more issues caused the device than the tube, so you can only get so much better no mater how super your tube is in regards to MTF.

One last question:

My lp/mm is 64.  How much of a difference would I notice with a 72 lp/mm system?


-Freq