Can anybody tell me what a thin filmed tube does in respect to the halo. Is the halo more transparent or smaller diameter in a thin filmed tube? I realize the closer the MCP is to the photo cathode the smaller diameter but does it also affect transparency? Iv‘e seen pics of so called thin filmed and still see halos...so I‘m wondering. Thanks everyone!
Essentially, it does nothing. Gen 2 prox focus has no film at all and it has a halo too. If anything, it makes it more prominent, because more electrons can get to the MCP and there‘s no film to reduce the halo.
The only modern technology that doesn‘t have a Halo is electrostatic inversion in both Gen2 and Gen3. These tubes have no halo due to the anode cone. It also creates an ion barrier. You can‘t buy Gen3 ESI tubes to my knowledge and Gen2 ESI tubes are very hard to get new. MX-9644 is a good example. David.
The size of the perceived Halo is a function of the distance the MCP is from the photocathode. The closer the MCP is to the photocathode the smaller the Halo, while the farther the MCP is from the photocathode, the larger the Halo.
The film is a membrane of Aluminum Oxide which covers the input of the MCP. This film is also known as an ion barrier as it protects the photocathode from being poisoned by positively charged ions that form when photoelectrons hit air molecules. If this film did not exists, the Gen II and Gen III photocathode would not last very long due to the poisoning by the ions.
Since Gen II and Gen III photocathodes are proximity focused, they are set as close to the MCP as humanly possible. Reducing the thickness of the ion barrier aka film enables the manufacturer to bring the photocathode and MCP even closer to each other than a standard thickness film.
FYI – The MX-9644 does have an ion barrier or film in its Gen II section, otherwise its photocathode would not last very long. The MX-9644/UV is actually a Gen I tube mechanically and optically bonded to a Gen II tube.
Trivia: Autogated tubes with high Halo values make a lot less (whining) noise than autogated tubes with small Halo values.
Ed
REPLIED: February 07 21:56:35, 2010 ( 00: AM PST )
When learning how to build up Gen II image intensifiers, I learned more about the Gen II MX-9916/UV used in the PVS-5 than I did the MX-9644/UV used in the PVS-4. At that time I understood the Module of the MX-9644 to be two separate modules, one Gen I and one Gen II that had been mechanically and optically bonded. NV Forum Member: cj7hawk, sent me some info in a document written by Illes Csorba that detailed the internal structure of the MX-9644 Module. I looked it up and David’s info sent to me concurred with page 36 of Illes Csorba’s book Image Tubes. So in fact the MX-9644 was a Gen I photocathode, i.e. same fiber optic window, photocathode and roughly same photocathode voltage and same distance to the next component, which if Gen I would be the screen but in the MX-9644 is the Gen II’s MCP then screen. Hybrid seems to come to mind as the MX-9644 has the best of both worlds. As such, the anode cone and the MCPs distance from the photocathode keeps the positive ions away from the photocathode, thereby not requiring the need for an ion barrier. Additionally, since the Halo value is a function of the distance between the photocathode and MCP, the MX-9644’s photocathode is on the order of a thousand time farther away from the MCP, therefore a Halo is not visible as it would be to large to see. Thanks David, Ed
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