Night Vision: Filmed vs. Unfilmed Tubes
When shopping for Generation 3 night vision tubes, you will encounter the terms "filmed" and "unfilmed" (also called "filmless" or "thin-filmed"). These terms refer to the presence or absence of an ion barrier film on the microchannel plate, and the distinction has meaningful implications for tube performance and longevity.
What Is the Ion Barrier Film?
In early Generation 3 tubes, positive ions created during the electron multiplication process inside the microchannel plate (MCP) would travel backward and strike the photocathode, gradually degrading it over time. This ion feedback was one of the primary factors limiting the operational life of Gen 3 tubes.
To address this problem, L3Harris (formerly ITT) developed an aluminum oxide ion barrier film that is deposited on the input surface of the microchannel plate. This thin film blocks positive ions from reaching the photocathode while still allowing electrons to pass through. The result is significantly extended tube life, often rated at 10,000 hours or more.
How the Film Affects Performance
While the ion barrier film successfully protects the photocathode, it comes with a tradeoff. The film acts as a partial barrier to electrons as well as ions, meaning that some of the electrons generated by the photocathode are blocked or scattered by the film before they can enter the MCP channels. This reduces the overall signal reaching the MCP and can result in a measurable reduction in signal-to-noise ratio compared to an equivalent tube without the film.
In practical terms, a filmed tube may appear slightly less bright or slightly grainier in the darkest conditions compared to an unfilmed tube with otherwise identical specifications. The difference is most noticeable in extreme low-light scenarios, such as heavy overcast with no moon.
Unfilmed (Filmless) Tubes
Unfilmed tubes, often designated with an "F" suffix (somewhat confusingly, as in "Filmless"), do not have the ion barrier film on the MCP. This allows the full electron signal to pass from the photocathode to the MCP without attenuation, resulting in higher sensitivity and generally better performance in the darkest conditions.
The tradeoff is tube longevity. Without the protective ion barrier, the photocathode is more susceptible to ion feedback damage, which can lead to a shorter operational life. However, modern MCP manufacturing techniques and improved vacuum sealing have mitigated this concern significantly. Many unfilmed tubes now achieve operational lifespans that, while potentially shorter than filmed equivalents, are still well in excess of what any civilian user is likely to accumulate.
Which Is Better?
For most users, unfilmed tubes are considered the superior option. The improvement in low-light performance is real and noticeable, and the tube life concern is largely theoretical for civilian users who will rarely approach the rated hour limits. This is reflected in the market, where unfilmed Gen 3 tubes typically command a premium over filmed equivalents with similar specifications.
The US military has also transitioned toward thin-film and filmless tube designs in its most recent procurement programs, recognizing the performance benefits.
If you are on a strict budget, a high-quality filmed tube with good specifications will still provide excellent performance. The filmed vs. unfilmed distinction is a refinement, not a fundamental quality divide. A filmed tube with 30 SNR and 72 lp/mm will outperform an unfilmed tube with 25 SNR and 64 lp/mm in most conditions.
For guidance on selecting the right tube for your needs, reach out at [email protected].