Ben321,
As has been stated by Richnormand, this is normal behaviour of such a Gen 1 image intensifier tube.
The tube does not normally need a continuous high voltage supply to operate. Many Gen 1 NV scopes have a momentary power button that is only pressed when the image starts to dim. It can be kept pressed of course to ensure the brightest possible image. The tube is very low leakage so once charged it will discharge at a rate dictated by the current flow caused by the scene striking its target. If the lens cap is kept on, the charged period can be quite long. This is why you have to be careful when using this Gen 1 technology. People sometimes charge the tube and remove the lens cap in a bright environment before the tube charge has dropped adequately. This is not healthy for the tube and should be avoided.
To clearly illustrate the way the tube operates as a charge store, the Russians made piezo electric powered night vision scopes that utilise a piezo electric crystal that is squeezed by a lever mechanism. The high voltage produced by the crystal is applied across the Gen 1 image intensifier tube and it begins operating. When the image starts to dim a single press of the lever is all that is required to recharge to tube. The interval between charges can be quite long and the instructions explicitly state that thar the lever should not be ‘pumped’.
As a rule, the brighter the observed scene, the shorter the period of operation before a recharge of the tube is needed to maintain the image brightness.
Warning.... exposing a simple Gen1 image intensifier to a bright scene or a scene with a bright object, such as a halogen lamp in the FOV is potentially harmful to the tube and cause a burn that is permanent.
Some users complain of black dots on the phosphor of Gen 1 NV scopes. They cam look like someone has sprinkled pepper over the phosphor ! Gen 1 tubes used in modern Russian jolt NV scopes can date back to The 1960’s ! Their were large stocks of these tubes stored in Russian warehouses from the Cold War era. They continue to be produced but Russian dealers advised me that quality can vary greatly. Gen 1 Tubes are tested and allowed a quality rating. The best have very few, if any, blemishes on the phosphor. The worst have a significant number of black dot blemishes all over the phosphor. These are manufacturing defects that are tolerated as the tubes sill function. It will come as no surprise that the better Gen 1 tubes are the most expensive and the worst tubes are the cheapest. Many budget consumer grade Gen 1 NV scopes either have tubes at the poorer end of the quality range fitted or operate on ‘pot luck’ where a range of qualities are used in production and some are better than others (the tubes are purchased in bulk as unsorted for quality).
The better quality Gen 1 tubes with very few or no defects in the phosphor find their way into the more expensive civilian NV scopes and come with a guarantee of phosphor quality.
I have many Russian NV scopes and saw many identical units rebranded by Western resellers. The NV100 by Moonlight is a classic example. It used ‘bottom of barrel’ poor quality Gen1 tubes and I could buy one for $30 in Moscow Circa 1996. In the UK the same unit was selling as the NV100 for £149 ! Another Russian model that looked very Russian in design (utility rather than pretty) was the Cyclops 1. It was another budget NV scopes that I could buy in 1996 for $30? It was pot luck whether you got a reasonable quality tube so we were allowed to test them at the market and select one we liked
There were better quality NV scopes that contained higher quality (lowers phosphor defect) Gen 1 tubes. An example was the White Night that cost me $100 in 1996. It was well built with a solid metal body and the image quality was excellent on all the units I tested at the Moscow market. The sellers recommended this model for serious Gen 1 night vision work as the the manufacturer selected and used only good quality tubes for them. One of my best Gen 1 NV scopes is nothing pretty but has superb performance for Gen 1 and a large IR illumination that has a ‘zoom’ feature. The unit looks very utilitarian..... almost military
and operates on a Lithium 2CR5 battery which was very uncommon in NV scopes of the time I was in Moscow. It was made in Belarus by a scientific instrument manufacturer and uses the very best quality Gen 1 tube that was available to them. The tube is a lot bigger than those used in common consumer units like the NV100 and Cyclops 1. The powerful IR illumination has an adjustable range control for best illumination of a target at varying distances. The phosphor of the high gain tube is free from blemishes and the produced image in the eyepiece is large compared to other NV scopes. I bought a lot of NV kit from the seller and he got to know what I liked so he bought in the Belarus unit for me. It was around $120 but was worth every penny. He also offered me a Gen 2 military NV sight that was clearly ‘liberated’ from a military platform ! He was asking $1000 in 1996 which was a lot of money back then. I passed on that offer as I did not fancy carrying stolen Russian military Gen2 NV technology through Airport Security
. I could have ended up in very hot water. I could have ended up as a guest at the well known Government Hotel Lubyanka !
https://en.m.wikipedia.org/wiki/Lubyanka_BuildingAs a side note..... I also worked in Kyiv, Ukraine and was offered various NV equipment in the markets. By then I had plenty so was less interested in buying more. A Western chap that I met in a bar offered me Gen2 military NV equipment ! It had been stripped off of Russian Tanks that were being transported through Ukraine on slow moving trains ! Needless to say I did not buy any and made my excuses before putting plenty of distance between myself and this entrepreneurial Western Black marketeer !
Ahhh those were the days, I miss them !
Fraser