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Posted: 1/13/2017 6:56:49 PM EDT
| How important is this? I mean, am i going to notice the difference betwwen 1800 and 2000? |
| All by itself PCR cannot be used as a real means of evaluation. Assuming that the S:N ratios and overall resolution are identical in two units being tested side-by-side (unlikely), then a 200 point bump in PCR might be noticeable in a very dark environment but it won't be a huge difference. I would chase for a better S:N ratio and overall resolution before chasing PCR. If you have too low of a S:N ratio then a high PCR is just blasting more noise at you. Subjective analysis of tubes by a reliable, experienced person is often better than chasing the objectively measured numbers. The numbers don't lie but they don't tell the whole story either. Also, testing procedures have changed for tubes, so if the tubes were not measured using the same conventions then comparing the numbers doesn't mean much, anyways. |
| What two devices are you comparing? What do you plan to do with the NVD once you get it? In what environment do you intend to use it? Some factors make different NVD specifications better suited for different applications. If you are buying new then I recommend utilizing a vendor such as TNVC that has a sales agent that will talk to you and guide you through the selection process. If you are buying used then you need to be comparing units that are similar enough to be comparable. |
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I am of the belief that PR is almost absolutely meaningless. I would disagree with that. With a PCR of zero you essentially have a brick. I have seen a lot of people's numbers and gotten evaluations of their units - all leading me to the opinion that the numbers are very difficult to fully comprehend for even an RKI like myself. I have seen waves of opinions form over the years about which number is most important, and I think that is the fundamental importance. There is no single number that provides a good unit because it is extremely high or low. Ultimately you want the highest PCR possible but the PCR is related to other spec's for the tube in such a fundamental manner that there can be such a thing as too high of a PCR in the real world because the other spec's can't support it. I have looked all over the archives and I have not found a good tube evaluation thread where the facts aren't spread through a stack of pages. If someone more knowledgeable than myself could sit down and write how all of the numbers correlate it would be highly appreciated. I would do it but I'm just not qualified. I understand the categories of evaluation on a data sheet and how they all affect tube performance in general, but I don't understand how they all inter-react. 
I'm betting less than 5 people on the forum truly have a mastery of the data sheets. I can tell a good unit from a bad unit by the data sheet (usually) but it takes a master to be able to read the data sheets and be able to tell a very good unit from a truly exceptional unit. A lot of people think that data sheet evaluation is more art than science. I disagree. I posit that it is more science than art and we just don't understand it as well as we should (and we're all too proud to profess our ignorance). |
| Yeah, that would helpful to have a guide like that. No doubt. I was looking around at a couple of different tube specs. I have a set of F9800 YG's, but saw that there really wasnt a difference between that and the F9800VG. But for PCR. So it got me to thinking. I cant tell the difference between a tube that has a SNR of 27, with a tube that has SNR of 25. Was thinking that perhaps PCR might make a diference, but dont really know enough of how one effects another. |
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I cant tell the difference between a tube that has a SNR of 27, with a tube that has SNR of 25. I would take 2 points on SNR over 200 points on PCR. Resolution is not fixed - it is dynamically tied to the operating environment. If you get blasted with light then you'll lose resolution, but when you are struggling to get enough photons to get an image and you have the gain control turned to maximum then you might notice 2 SNR points (once again, provided you are comparing two otherwise identical units). If you jump 4 points then I think we have a serious jump. But, once again, you'll only see a marked improvement in the usable image when the gain is up, because the noise is in direct proportional to the signal. Somewhere along the line CJ7Hawk gave his personal priority list for spec's and it went something like this: 1) SNR 2) Resolution 3)PCR 4) EBI I would say Halo and Blemishes are very subjective and should be pretty easily judged in importance by the end user. If you need more shock resistance then you need more Halo. If you need to see through point light source image trash better then you need a lower Halo. Blemishes are most annoying when you are staring at a white wall or an urban overcast sky (when your NVD matters the least). Blemishes fade to imperceptibility during the darkest of nights when the noise is turned all the way up and you're panting out of breath and struggling to find the necessary navigational hazards to avoid (you just don't have time to spend crying about image defects when you are busy). I still wouldn't want a great big blemish center-screen because that's where my eye-focus and attention are going to be. In a complicated background on a dark night with the gain up, I cannot see any blemishes on my PVS-14 tubes. They're still there, I just cannot see them... Then we have auto-gating and auto-gating audio noise and their relative importance to you. |
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I'm not competent to generate a guide to selecting tubes but I'm going to do it anyway. Hopefully someone more competent will supersede mine with a better one. From most important to least important, given a general purpose usage (not astronomy, etc.):
1) SNR - The higher the better ALWAYS. You will not see any real improvement in image quality until you are viewing an image in a dark environment that requires a lot of gain. 2) Resolution - The higher the better ALWAYS but don't chase your tail. Aim towards the top end but don't worry about the difference between excellent and the very best because the difference at that point is negligible. 3) PCR - The higher the better GENERALLY but make sure it is well mated to a high SNR and low EBI. Too high of a PCR with too low of an SNR will just blast you with noise. Too low of PCR and you have a brick. 4) EBI - The lower the better I believe in all conditions. 5) Autogating - Yes or NO. ATG stroboscopically cycles one of the layers in the tube to help prevent over saturation of the image. This is generally a good option but some prefer no ATG. Some people state that Non-ATG tubes are tougher. 6) ATG Audio Noise - ATG tubes whine, some more so than others. Some require a stethoscope to hear it. 7) Halo - The higher the better for weapons recoil. The lower the better for everyone else. 8) Blemishes - The fewer, smaller and further from the center of the field of view the better but don't get to bent about small blemishes that are far from the center because they will disappear in a complicated image anyways. 5 through 8 are a little more user preference issues but should be remembered during the selection process. I hope this helps a little and I hope someone that knows better improves upon it because I know it needs improvement (and maybe even some corrections, so please step on my toes). |
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Hi SOT_Solutions -Â
You might want to be careful with that bet, LoL! - there are literally dozens of people on the forum who know a lot about datasheets, and many of them have far more specific knowledge than I do. Many just lurk now, but they are still about.  Also, most of the regular guys here handle data sheets pretty well too. It's just that different people are looking for different things. The big problem with PCR is that it's measured only with a single light - and the 2856K standard leaves much to be desired. There has been movement of the QE of several tubes, with respect to wavelength, and this is critical to take into consideration in comparisons lately if you want the best tubes for a specific purpose. It's complicated by other factors, including the lack of published QE data across multiple tubes now. NVGdude was correct in his general assertion though that it's not really an indicator of anything too much. It will never be zero ( I thought that was kind of funny though ) but even as it drops, if the other tube parameters can be maintained it is possible to keep ahead of higher PCR tubes. A lot of what I mentioned in the past was fairly general - it's intended as a basic guide to make processing complicated numbers a little easier - But applications specific requirements looks at each datasheet differently. Interestingly, it was Dino who found a flaw in my original concepts that took me a year to fully realize. He was pointing out that a particular tube with a high low PCR and low SNR was better to him than a supertube with very high S/N and high PCR, and the guy has seriously good eyes when it comes to looking through tubes. He's a bit like a human Hoffman tester. Over the past year, I've been looking at the flaws in measuring tubes with the 2856K light, and there's some serious flaws inherent to that method - and these flaws come out at night when the tubes are actually in use, and it has a lot to do with how our brain processes the light from tubes at night. Anyway, I thought I'd mention - please don't take some of my past comments as absolute as they were pretty general and were intended to help people who didn't understand datasheets at all to evaluate the metrics a little easier. But as Dino taught me, what the eye sees is still very important. Regards David. |
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But as Dino taught me, what the eye sees is still very important. Thanks for chiming in. To channel Kafka: I agree with you more than you agree with yourself. What the eye sees is beyond "very important". It is the fundamental purpose of the device. If you can't see your intended target with the device to the required level of resolution then the device is worthless for your application. The numbers are all secondary. They are really only there to help someone understand what the image will look like without actually looking through it (besides some possible documentation requirements for a contract, etc.). I would rather get a hand picked tube from someone that I trusted that was competent at picking tubes by sight than choose one myself from the numbers. That having been said, maybe we could eventually launch a conversation about application specific tube parameters. I would be happy to cull the data from the conversation and generate a selection guide from it. I'm competent enough to give casual recommendations in an open forum where those that have more knowledge of the topic can provide some training wheels but I'm not competent enough to generate a truly valuable selection guide on my own. |
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David was very generous and I thank him for it. My method is simple but effective. I have known value tubes I test in my very dark basement.
I test these tubes against unknown value tubes and judge performance. Is it perfect ? No. It does give me a good idea how tubes perform based on what my eyes see. It is simple but because I have a known value of tubes it works well. I also look thru many tubes and that helps As David knows I don't care for filmless tubes. It is not because I don't respect high spec tubes. I found in my unscientific research they suffer from ion strikes which annoyed me. I still think they are great tubes with amazing performance. Just not my cup of tea. I like filmed tubes or Photonis tubes for a clean and crisp image. |
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Do you know what drives the relationship? Quoted:
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I think you will find that S/N is in more or less in direct relation to the photo cathode sensitive and is dependent upon it. (All other things being equal) Do you know what drives the relationship? |
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