Civilian usage of interference filters for practical non-military applications.

For many years now, the US military has been using LIFs or Light Interference Filter's to avoid damage to Night Vision Devices from over-exposure to reflected and directed coherent light sources. However since this isn't a discussion of things that the US military would rather you didn't know, I won't add any further information there. Needless to say, it's not difficult to work out but you'll have to work it out yourself.

First a safety message. LIFs will NOT protect your NVD from bright-light damage, most lasers or sources of unexpected illumination. They will not help against rifle-mounted IR lasers at all. They will not protect your NVD from damage due to headlights or flashlights. Anything you can get as a civilian. Unless you're buying from a factory and understand both the technology and lasers well, LIFs CANNOT BE RELIED UPON TO PROVIDE SAFETY EVEN IF THE LIF IS USED CORRECTLY. A pinhole sized defect in the filter is enough to let far too much light through and render it partially or completely ineffective.

However... 

One of the more useful characteristics of a LIF however is that it blocks out certain wavelengths of light. This provides the express benefit of being able to use visibly lighting around night vision equipment without overloading the sensitive tube. This same technique is used in aircraft to allow lighted instruments panels while using NV and can be beneficial to civilians for the same reason.  Afterall, what if you're hunting with NVDs and you drop something? Finding something on the floor with your NVD focus set to infinity is nearly impossible. What you want is something that lets you use a small handheld flashlight with your unassisted eye for up-close work that's not going to damage the image tube of your NVD. 

But you may have heard that military targetting lasers are way up in the non-visible near-infra-red (NIR) and Medium-wave infra-red (MWIR) region? How do you get a LIF to block normal light? This is possible because harmonic frequencies of light that the LIF blocks are present within the visible spectrum. Again, to respect the US military machine, I won't go mentioning this outright - partly because I don't really know what the central bandwidth is.

However it's exceedingly easy to find out what visible frequencies are blocked.

For about US$30, you can get one of these spectrometers. You'll also need a LIF or a similar dichroic bandstop filter you can stick over your NV device. 

EXTERNAL LINK TO SPECTROMETER IMAGE.

Now, a brief diversion into the science of colour for those who aren't familiar with it. White light is made up of colours. You probably new that. But not all light is made up of all colours and some filters only let components of white light through.

Holding up your spectrometer to some sunlight ( DONT EVER POINT DIRECTLY AT THE SUN! ) have a look at the spectrum. Now place your LIF in front of the slot window and repeat. You should see something matching one of the below, with green missing.

You can also confirm your NV response with the LIF.

This information can be directly used to compare the spectral response of your LIF to well known filters designed to provide this protection. Recently produced LIFs show a very strong tendency to this particular laser protection filter.

The spectral response can be see in this below image ( Warning - external link - Image may not be stable ). This shows the transmission and Optical Density of the typical LIF, as manufactured and published by a German company. This LIF is quite close in operation to the US LIF. 

Now you've done that, the question is "how do you use it?"

Below 400nm really isn't useful to us and neither is 1000nm and upwards, however visible lasers around 532nm are blocked and and LEDs with a green output between 500 and 570nm will be attenuated. If you get a deep green LED around this frequency, you can make a torch that won't overly affect your NV device so you can read maps, GPSs and look for detailed stuff up close with your non-NV covered eye.

Finally, if you run a quad-bike or vehicle, you can create an internal "green" set of lighting for the dashboard and inside the vehicle to run with at night so your good eye can see the instruments. That, afterall, is how the pilots who fly planes at night with NV goggles do it.

Useful eh?

Please don't go discussing LIF frequencies on any US forums. They really don't appreciate it and while this is all public domain published information ( By the US military and corporations, and hence is no longer covered by ITAR ) many of the forum operators do not like this matter discussed and will lock threads - possibly even if you link back here.

So use your head and discuss only through PMs. And if you want a LIF? Ebay is still the best place - and remember the spectral images above... If you are exposed to bright light in any other part of the spectrum, the LIF will not help you - ! Light transmission in the 600 to 950nm range is almost 100%.

Coming up still: Selecting the ideal LEDs for civilian use with LIFs.