Question about image intensifier night vision scopes - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: January 07, 2020, 05:38:37 am
content: Sorry this is a bit off topic. I know this section is about thermal imagers, but couldn't find an alternative section for image intensifier night vision scopes, and both are commonly used technologies for night vision, so I'm posting this here.I've heard that if you expose an image intensifier NV scope to too much light, it actually will damage the tube. What I'm not sure about is this. Can this damage occur only when the tube is powered on (internal tube damage such as damage to the viewing screen), as a result of the electron beam being made too strong in regions of the photocathode where the excess exposure is occurring? Or can this damage also occur even when the unit is powered off, simply due to the material of the photocathode being damaged from excessive light exposure?

author: johnelot
author_rank: Contributor
timestamp: January 07, 2020, 08:42:10 am
content: most common damage is when they are turned on and look at too bright a light then it can burn the photocathode , they seem to be sensitive for a little time after switching off to , i have left goggles that i have just stoped using in a bright room with no covers on the lenses and found a temporory image on the unit (thankfully it went away after a little more use .)i have not heard of damage at all when switched off and (cold - not just switched off) but i would not point them at the sun for example as the lens would concentrate the light on the sensor and i would think it might do some harm . , could be worth an experiment on an allready damaged tube though

author: Ultrapurple
author_rank: Super Contributor
timestamp: January 07, 2020, 09:14:11 am
content: It depends very much on which Generation of tube you're looking at and, to a certain extent, the driver arrangements.I believe there's a lot of accumulated knowledge on this sort of thing on the AR15.com, other sports shooting forums and probably military hang-outs - it might be worth looking around some of those.Finally, if you're actually thinking about buying something new, or reasonably recent and still supported, there's no substitute forRTFM.

author: Fraser
author_rank: Super Contributor
timestamp: January 07, 2020, 01:41:11 pm
content: I own Gen 1, Gen 2and Gen 3 Night Vision Scopes and have not damaged any of them as that requires abuse !I do refit lens caps when they are not powered as that is just good practice with all image intensifier based technology. If a image intensifier I put is exposed to a bright scene and switched on, it will not thank you and a burn on the phosphor is possible. Much depends upon the technology used in the image intensifier device and its power supply. Simple Russian night vision scopes use simple power supplies and burn easily. Even professional E2V intensified scopes suffer burn damage if abused. Gated night vision is more robust but should still be treated with respect and good practice.To my knowledge the risks of damage to night vision intensifier modules is during powered operation. I am not aware of damage being caused when not switched on. I am sure an IIT could be damaged if deliberately aimed at the sun via a lens system as many optical equipments can suffer damage through such focussed abuse of the imaging target !A quick Google should answer your question more fully. Manufacturers usually provide guidance on how to use their products as they wish to avoid customer induced dame and the issues of returned damaged units.A quick Google found this fir youhttps://actinblack.com/taking-care-of-your-nvg/You can Google the various manufacturers user manual fir more informationFraser

author: Cat
author_rank: Regular Contributor
timestamp: January 07, 2020, 03:31:40 pm
content: Quote from: Ultrapurple on January 07, 2020, 09:14:11 amAR15 is definitely worth a look. Did some teardowns (of ancient tech you can get in the EU compared to the US) over there and they could always help me.Quote from: Fraser on January 07, 2020, 01:41:11 pmI read something regarding storage of PMTs (Photomultiplier tube) some time ago. Since the basic principle is the same, here are some snippets fromhttps://psec.uchicago.edu/links/Photomultiplier_Handbook.pdf:Page 42:Photomultiplier tubes should be stored in the dark when not in use. Blue and ultraviolet radiation, especially, can cause photochemical changes in the photocathode which result in changes in sensitivity. It is especially important to avoid exposure to intense illumination such as sunlight even when no voltage is applied to the tube. Permanent damage may also result if the tube is exposed to radiation so intense that it causes excessive heating of the photocathode.Page 89:Photomultipliers should be stored in the dark. Storage of tubes in areas where light is incident on the tube results temporarily in a higher than normal dark-current level when the tubes are placed in operation. This increase in dark current is primarily due to phosphorescence of the glass and can persist for about 24 hours. Additionally, storage of tubes designed for operation in the near IR region of the spectrum (above 700 nanometers) in illuminated areas may decrease the “red” sensitivity of the tube.Quote from: Fraser on January 07, 2020, 01:41:11 pmLucky Fraser, Gen. 3 in the EUBut yes, when turned on and looking in a "bright" light there is always the chance to burn a spot or a streak in the phosphor.

author: Fraser
author_rank: Super Contributor
timestamp: January 07, 2020, 05:10:51 pm
content: Cat,I recently got luckyI bought an Electrophysics (Sofradir) Astroscope 9350 for £100 in unknown state but mint physical condition. The recycling seller knew next to nothing about its use. It turned out to have the UK specification Gen 3 IIT module in it and after adding a suitable lens and CCTV camera to the input and output C mounts ports, I tested it. The unit provides an unblemished imageI say “UK specification” as we can buy a Gen 3 IIT from the USA but it has a lower Figure of Merit than permitted in the USA for civilian use, but better than that permitted to be shipped to the EU. I love the performance but, in truth, I have no use for it !Fraser

author: Ben321
author_rank: Frequent Contributor
timestamp: January 07, 2020, 08:11:35 pm
content: Quote from: Fraser on January 07, 2020, 01:41:11 pmTo my understanding Gen1 scopes work like the diagram I made here.When it is turned on, and light hits the photocathode, electrons are freed, and then the anode accelerates the electrons to the display screen. When it is turned off, and light hits the photocathode, electrons are freed, but aren't accelerated. Instead they remain near the photocathode, and then quickly recombine with it. So it would seem that when unpowered, it couldn't be damaged by light exposure. However, I've read some manuals for even gen1 scopes that say to keep their front covered, even when not in use. So I'm not sure exactly what damage mechanism might come into play here. Most storage conditions wouldn't be daylight conditions. A normal room in a house illuminated by a standard 60watt light is not going to have nearly the same amount of light as daylight outside. And I'm not talking about pointing at the sun. I mean diffuse reflections off the ground outside are far brighter than diffuse reflections off even a white wall inside a building. So I'm not sure how storage indoors in indoor ambient light conditions could possibly damage it, yet some manuals for some gen1 nv scopes claim it can cause damage.Is this just them being overly cautious, or is there actual risk of permanent damage to a gen1 nv scope from exposure to indoor ambient light conditions when powered off?

author: Max Planck
author_rank: Regular Contributor
timestamp: January 08, 2020, 07:48:00 pm
content: Long story short.There are gen. 0, gen. 1, gen. 2 and gen. 3 IIT devices. Gen. IV designetion existed for a short period of time but its use ended in 2001 as the technical advances in new IITs was considered insufficient to call it a new generation. Gen. IV today, if any, is just a marketing hype.Gen. 0 was more an infrared to visible light converter than amplifier. The most interesting, IMHO, gen. 0 NVDs were the British WWII systems, first called RG equipment (from red glass). Later named Tabby.http://nighteyebinoculars.net/2017/09/22/ww2-british-military-tabby-night-vision-binoculars/https://www.worthpoint.com/worthopedia/wwii-night-vision-scope-tabby-os-1772387106By the way, if anyone has any information or documents about Tabby WWII combat use, I would be interested.Gen. 1 was basically an improvement of gen. 0, with a better photocathode (S1), electrostatic lenses and at some point cascade configurations with up to 3 IITs in series.Gen. 2 were the first IITs to use MCPs (microchannel plates) for photoelectrons multiplication.Gen. 3 were just like gen. 2, but with a GaAs photocathode.Later came gen. 3 thinfilmed/filmless/and autogated power supply IITs.@Ben321Answering your question, the weakest element of the gen. 1 IIT is its phosphore screen - especially in the old designs without any automatic brighthness control or bright source protecion circuits - just like in old TV sets or oscilloscopes. Memory effect etc.

author: Ben321
author_rank: Frequent Contributor
timestamp: January 08, 2020, 08:18:27 pm
content: Quote from: Max Planck on January 08, 2020, 07:48:00 pmThanks for that info. If the phosphor screen is the weakest part of gen1 scopes, then why do some of them say to keep the lens cap on, even when powered off, in normal ambient light conditions? Are the manufacturers just being overly cautious?Also, which generation did 2 microchannel plates get introduced? I thought that was gen3, but you don't mention that in your description of gen 3.As for gen0, does anybody make those anymore, or can you at least still find old ones on eBay? You mention that gen0 nv scopes are IR to visible light converters. Is that basically the same then as the Find-R-Scope (who's main purpose is viewing IR light sources in a lab, not night vision) which has an image converter tube that sees from 400nm to 1300nm?https://www.findrscope.com/SPD/infrared-viewer-ir-viewer-84499-near-ir-1350nm--80000398-1466100434.jspIronically, that device is not less expensive than gen0 nv scopes. In fact, it's in the price range of gen2 nv scopes (between $1000 and $2000). If that's an older technology, why is it so much more expensive than newer gen1 nv scopes?

author: frogg
author_rank: Regular Contributor
timestamp: January 08, 2020, 09:07:52 pm
content: Quote from: Ben321 on January 08, 2020, 08:18:27 pmIt seems unlikely given that phosphor screens are electron-driven, not photon driven. Not many free electrons in air, I imagine.Without understanding what any particular manufacturer recommends or why (they usually try to educate for the lowest common denominator), keep in mind the following:1. The lens cap of most PMT-driven image intensifiers have a pinhole.2. Keeping the lens cap (with pinhole) on prevents someone from overdriving the PMT or blowing the phosphor screen when turned on in daylight, while still providing an image to the user.The dummy-proofing of the pinhole lens cap is that the user keeps it on all the time. If they turn on their scope accidentally in daylight, they will still see an image. They go, "ooh, nightvision" and then turn it off.If the user doesn't see an image in daylight, they go, "hey, I don't see anything. I'll take the cap off." then "Poof." Stupid? Yes. But that is what humans do. Sucks to be us.If the user leaves the cap on at night time, they go, "hey, I don't see anything. I'll take the cap off." No problemo.Therefore, leave the pinhole lens cap on all the time.The pinhole lens cap serves as the simplest of expert systems.

author: Max Planck
author_rank: Regular Contributor
timestamp: January 08, 2020, 10:15:21 pm
content: Quote from: frogg on January 08, 2020, 09:07:52 pmPhosphors are chemical compounds subject to different chemical reactions. So, not only electron beams.Another possible damage mechanism is thermal overload of the photocathode (for example sunlight or laser beam).It all depends on the exact design of a given IIT. One can trust the manufacturer or experimentQuote from: Ben321 on January 08, 2020, 08:18:27 pmAs I wrote before, MCPs were introduced with gen. 2. IIRC, the latest development are so called 2um MCPs. The diameter of a single microchannel was reduced to 2um, thus increasing the number of channels and allowing for wider FOV while keeping image resolution.Gen. 3 IITs are of course also based on MCPs, in most cases with a protective film added to prevent damage to the GaAs photocathode.Regarding gen. 0, it is possible to find from time to time a Tabby NVD on ebay. After changing the power source, a 6-7 kV battery, many of them are still operational.Regarding the radiation band to which an IIT is sensitive, there is no simple answer. Look for example for a Hamamatsu IITs catalogue.Max

author: frogg
author_rank: Regular Contributor
timestamp: January 09, 2020, 02:49:34 am
content: Quote from: Ben321 on January 08, 2020, 08:18:27 pmI'm not aware of the full breadth of nightvision devices, but I've never heard of any large-scale commercial image intensifier with more than one microchannel plate.AFAIK because of the way that MCPs are fabricated (they're done so in a way similar to pulling candy), unless you were dealing with an exotic or academic design, the geometry and fabrication involved would be....um...difficult.Also note that the anode is typically cone shaped because it focuses and inverts the image. The output image / exit pupil is usually much smaller than the input aperture.

author: Ben321
author_rank: Frequent Contributor
timestamp: January 09, 2020, 07:35:54 am
content: Quote from: frogg on January 08, 2020, 09:07:52 pmImage intensifier tubes are vacuume tubes. They work by photons hitting the photocathode, which releases electrons, which are then accelerated by high voltage electrodes. The fast moving electrons then hit the phosphor screen, just like in a CRT, causing the phosphor screen to emit light. The light from the scene you are observing is not focused onto the screen, but rather onto the photocathode. The phosphor screen is absolutely NOT light driven in an image intensifier tube. It's electron driven in an image intensifier tube.So my question remains, how would exposing the front of an UNPOWERED gen1 scope to normal ambient light conditions (not talking about looking at the sun), cause any damage to any part of the device. Most manufacturers say not to, but I don't understand the physics behind what could cause damage to an unpowered gen1 scope's tube when exposing it to normal ambient light conditions.And it gets even weirder. Some manufacturers say not even to run the scope in normal ambient light conditions with the pin-hole cap in place, for more than 10 minutes. Doesn't the pin-hole cap reduce the total light reaching the tube to a level equivalent of using it at night with the cap off?

author: Max Planck
author_rank: Regular Contributor
timestamp: January 09, 2020, 03:42:44 pm
content: Quote from: Ben321 on January 08, 2020, 08:18:27 pmI'm affraid, you are the first one to propose such design. I wrote about introducing MCPs in gen. 2 systems, not about using 2 MCPs in one.Max

author: frogg
author_rank: Regular Contributor
timestamp: January 09, 2020, 04:31:14 pm
content: Quote from: Ben321 on January 09, 2020, 07:35:54 amAn unpowered image intensifier really only has one component that could be easily damaged due to bright lights: The photocathode.Well, I guess technically speaking, the photocathode is the weakest part of an image intensifier any way you look at it.There is most likely a photodegradation mechanism that causes damage over time - either owing to crystal lattice structure changes that reduce quantum efficiency, or some kind of spontaneous ion implantation. The quality of the vacuum and the chemistry of the photocathode play a huge part.

author: Fraser
author_rank: Super Contributor
timestamp: January 09, 2020, 04:57:46 pm
content: Dual stage MCP IIT’s are common in laboratory Fluorescence experiments. Hamamatsu offer them in their Image Intensifiers. I own both single and dual stage MCP Gen2 IIT’s.My understanding was that Gen1 utilised cascaded tubes for additional gain. In Gen2 they used dual stage MCP IIT’s so it is a Gen2 related performance enhancement that is also available in later generations.The catalogue details the dual stage MCP IIT and its expected gain.https://www.hamamatsu.com/resources/pdf/etd/II_TII0007E.pdfFraser

author: Fraser
author_rank: Super Contributor
timestamp: January 09, 2020, 05:14:23 pm
content: Details of dual stage MCP IIT are shown here......https://www.lambertinstruments.com/technologies-1/category/Image+IntensifiersIt is worth noting that a Dual stage MCP still only contains a single micro channel plate ! It is effectively a single MCP intensifier with a following ‘booster’ stage that does not contain an MCP.Information about image intensifiers is easily found using Google. It is a well understood and very well documented technology.Fraser

author: frogg
author_rank: Regular Contributor
timestamp: January 09, 2020, 05:27:51 pm
content: Quote from: Fraser on January 09, 2020, 04:57:46 pmI always learn something new. Does the Hamamatsu second MCP have its own photocathode, or is the second MCP placed at the output of the first MCP without its own photocathode?My understanding was that "two stage MCP" meant one MCP combined with a conventional image intensifier - hence "2 stage".

author: Fraser
author_rank: Super Contributor
timestamp: January 09, 2020, 05:40:05 pm
content: I attach a picture of single and dual stage MCP IIT’s.The dual stage has a second photocathode but not a second Micro channel plateFrogg, you are correct in your understanding of the terms “Dual Stage MCP”. I was trying to clear up any confusion in the terms used for Ben321 as he may be thinking that a dual stage MCP IIT contains two Micro Channel Plates. Like having two separate MCP IIT modules mounted in cascade. This is not the case of course.Fraser

author: Max Planck
author_rank: Regular Contributor
timestamp: January 09, 2020, 08:21:55 pm
content: I have to correct something I wrote before. There are on the market IITs with double (I mean two) microchannel plates in a single intensifier tube. They are used for example in photon counting applications.Sorryhttps://www.photek.com/pdf/datasheets/detectors/DS001-Image-Intensifier-Datasheet-issue2.pdfMax