How much of a difference does the Light Interference Filter (LIF) make on the AN/PVS 7 type goggles?

As good as none...

It's only filtering out a couple wavelengths of certain lasers, so the difference is minimal, but like wearing sunglasses, anything you are filtering out is giving you less to work with.  Unless  you think lasers (and the lasers used fall into the range used by the LIFs) have a likelyhood of being used around you and may damage your goggles, I wouldn't bother with them.  I've never flown with a set with the LIFs installed...it's never really been a concern.

IIRC it is a light interference filter, not  a laser.
It blocks mostly lower wavelengths of light. It doesn't bloack infrared at all.

It basically alows you to see better in higher lit situations.

IIRC it is a light interference filter, not a laser. It blocks mostly lower wavelengths of light. It doesn't bloack infrared at all. It basically alows you to see better in higher lit situations.

Nope, it`s a Laser interference filter, it blocks the YAG targeting lasers, among others, Wildbill would know.

I was also under the impression that Laser Interference Filters were intended to mitigate tube damage in environments where such devices might be present.

Since TM 11-5855-306-10 references IR for the filter, I had assumed that the wavelength for which the 5855-01-379-1410 LIF was considered effective would be in the ~840 nM range?

i believe i was reading another post where victor said these filters were regulated by the DOD and civilians could not get them...

(If im wrong hopefully he will corect me) but i THINK thats what i read...

I read in a recent edition of preventive maintnace monthly that the lif also protects from blight flashes like tracers and bright muzzle flashes as well as lasers. is that true?

Nope, it is a Light Interference Filter.  Lasers are amplified "Light"  I went into the ANVIS operators manual and cut this directly from the manual.  This should clear it up.

LIGHT INTERFERENCE FILTER. This is a light protection filter
for the binocular. Use of this filter will result in a slight reduction in
system gain.

With that said, the LIF is placed on the goggles for the purpose of protecting the tube from laser burns.

As for the tracers and such, tracer light doesn't hurt the goggles, I would spend up to six hours a night flying over baghdad and that city was lit up worse than most U.S. cities and the flame tower south of the green zone was way worse than any tracer in the air, not a LIF function, more a job of ABC.  

Quoted: I read in a recent edition of preventive maintnace monthly that the lif also protects from blight flashes like tracers and bright muzzle flashes as well as lasers. is that true?

Bright source Protection is what protects the goggles during bright flashes.  The LIFs only filter out a couple wavelengths of energy used by certain lasers, so it doesn't really do anything to protect against general bright flashes.

As for the tracers and such, tracer light doesn't hurt the goggles, I would spend up to six hours a night flying over baghdad and that city was lit up worse than most U.S. cities and the flame tower south of the green zone was way worse than any tracer in the air, not a LIF function, more a job of ABC.

Actually, ABC was protecting you....not the goggles.  BSP is the function protecting the tubes.  Speaking of bright lights in Iraq...what about Abu Ghraib.  I could nearly see that place as I took off from Al Asad.

Quoted: I was also under the impression that Laser Interference Filters were intended to mitigate tube damage in environments where such devices might be present. Since TM 11-5855-306-10 references IR for the filter, I had assumed that the wavelength for which the 5855-01-379-1410 LIF was considered effective would be in the ~840 nM range?

I have an LIF and an IR laser-I'll test to see if putting the LIF in front of the beam and report back if you guys are interested.

From a practical civvy standpoint, a LIF doesn't do much for you than a Sacrificial Lense but cost more and pass a little less light. Not sure what I was thinking when I bought it-had it not been a good price I'd have passed.

Quoted: I have an LIF and an IR laser-I'll test to see if putting the LIF in front of the beam and report back if you guys are interested. From a practical civvy standpoint, a LIF doesn't do much for you than a Sacrificial Lense but cost more and pass a little less light. Not sure what I was thinking when I bought it-had it not been a good price I'd have passed.

I'd be curious to see what your tests show as compared to the ones that I have already performed with mutiple sources of both IR and visible wavelengths.  I pretty much run the LIF as a sac lens that allows simultaneous installation of the day cover.

Ok. I'm back.

Putting an LIF on your NVD to stop/reduce IR light is about twice as effective as used saran wrap. I couldn't tell any different with my stuff, YMMV.

But, that LIF is a great insurance policy if you ever walk around in the woods much. I walk into branches all the time, and hit them on my mountain bike even more frequently when out ghosting around at night in my local nature preserve/county park.

Wildbill 846, that is incorrect, BSP is used to protect the tubes, BSP works by shutting the goggles down.  To shut goggles down with BSP, you literally have to take them out into the daylight for about 90 seconds.  ABC is what regulates bright lights by adjusting the gain.  The newer tubes are much better with about doing this without causing NVG bloom.

I found Taji to be the worst, all those bright white lights pointed outward on the approach end of the runway.  I hated BIAP taxiway lights because they were blue and with the minus blue coating on the goggles, you couldn't see them on approach and had to look under the goggles to find them.  if you weren't paying attention, you would miss the taxiway and that was embarrassing.

height=8

Quoted: Wildbill 846, that is incorrect, BSP is used to protect the tubes, BSP works by shutting the goggles down.  To shut goggles down with BSP, you literally have to take them out into the daylight for about 90 seconds.  ABC is what regulates bright lights by adjusting the gain.  The newer tubes are much better with about doing this without causing NVG bloom. I found Taji to be the worst, all those bright white lights pointed outward on the approach end of the runway.  I hated BIAP taxiway lights because they were blue and with the minus blue coating on the goggles, you couldn't see them on approach and had to look under the goggles to find them.  if you weren't paying attention, you would miss the taxiway and that was embarrassing.

Sorry, Sniper-66...I stand by everything I've said...Both BSP and ABC are parts of the gain control.  BSP regulates voltage across the photocathode when it is subject to a high current load due to a bright light.  This protects the photocathode which is the sensitive part of the NVGs that needs the protection.  ABC adjusts voltage after the MCP so that the phosphor screen maintains a constant brightness.  As this is past the main parts of the tube....I don't see how you can claim it protects the goggles, unless you think the phosphor screen is the part being damaged.  Service life of NVGs however are really limited by the photocathode, hence the need for BSP and the aluminum oxide barrier film.

In short: BSP protects NVGs, ABC protects the viewer.
 
When a bright light is introduced, both circuits kick in, but they are protecting different things.  Sorry, but I teach this stuff for The Navy and Marine Corps, so I'm a goggle geek...and if you don't believe me, I can show you the page on the NVG manual.

PS
I used to have a helluva time finding the fuel pits at Balad for the same reason...their damn LED blue lights.  I've been noticing that at civilian airfields around the states now too when they upgrade to LED lighting.  Pure blue light and invisible on the pipes.




Bill

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Also, what have you pilot types done in the past with cockpit lighting? I know there was some studies that we're being conducted while at the DOD putting NV CP lighting in many flight stations. Has that ever happened? Don't mean to hijack this thread! Vic

Vic,

Today aircraft are equipped with NVG compatible lighting as to not interfere with the goggles, usually green LED type lighting.  We have, as sniper-66 said, the MBF-minus blue filter on aviation goggles.  The name is a little deceiving as it actually filters all visible light up to orange or lower red depending on whether its class A or B (sometimes with a notch to allow a certain wavelength of green through for use of a HUD), but it makes use of that light possible inside the cockpit.  Civilian aircraft such as Lifeflight and Law enforcement helicopters are now getting fit with these "NVG kits" as well.  The FAA actually has to certifiy aircraft for NVG use just as MAWTS-1 has to do it for the Navy and Marine Corps.

Now, with some of the older aircraft, they don't always replace the entire instrument panel, but have flip-over or velcro-on filters for incompatible lights.

Now, I'm only old enough to have flown on AN/AVS-6s but I love the stories from guys who flew the full face 5s.  As a full face design there was no look-under capability to see the instruments un-aided, so they had to read the instruments using the NVGs.  This meant focusing one tube for instruments, and the other for infinity to look outside.  Imagine one eye always being out of focus everywhere you looked.

Technically, you are correct on the BSP, yes, because of it's function, it will lower resolution, but as you will note in the statement below from the 23&P manual, you will see that it will not function in the conditions initially stated, dark night with tracers, that is still a function of ABC.  Now, flying at dusk when chicken wire is present and then flying into the sunset glow, that is when BSP would start kicking in.  ABC doesn't have the ability to shut the NVG down, BSP does.  Therefore, the conditions have to be so significant to damage the tube, meaning light enough that you can see with the naked eye, before BSP does it's primary job, can't have BSP running around unchecked, shutting goggles down while flying.  I give you the fact that BSP will reduce voltage to the photocathode, but if it's unwarranted, ABC will kick up the gain to make up for it if the BSP is wrong. Take a pair of ANVIS outside in the daylight, turn them on, put a watch to them and I will bet you a cold beer that in 90 seconds, or thereabouts, they shut off, that is BSP.  By the way, I also teach it for the Army as an IP and I have been a goggle maintainer since PVS-5s, I was one of the ones in the early days with a saw cutting the full face goggles down. Somewhere, I still have a set of day filters for the PVS-5s, unless I gave them to Ed Wilcox.  My unit was one of the first to put PVS-5s on GM-6 mounts.    

23&P Manual:  Under highlight conditions, the ABC automatically reduces the voltages to the MCP to keep the image intensifier’s brightness within a set limit. The effect of this function may be seen when rapidly changing from low light to highlight condition; the image gets brighter and then, after a momentary delay, suddenly dims slightly to a
constant level. The BSP function reduces the voltage to the photocathode when the goggles are exposed to bright light sources. The BSP feature protects the image intensifier from damage and enhances its life; however, it also has the effect of lowering resolution. Therefore, under bright light conditions when the goggles would not normally be used, the image produced is not sharp.

As for Balad, they got that fixed.  When I left in September, they had just got done building the C-130 revetments up in spooky world and new FARP pads with amber lighting.  Much easier now for mortar gunners to properly zero in on you at night now!

Victor, everything now has the blue/green lighting. In the hawk, we have the ability to turn the cargo compartment overhead lights and the cockpit flood light from white to blue.  In the Huey days, when we had red lights, they would build blue filters that you would flip over certain lights, the others were shut off and a blue external light would be shined onto the guages until they started getting some converted over to blue/green backlighting.  We used to have what we called a donkey dick, it was a blue light on the end of a flexible wand that you would stretch over whatever you wanted to see, do your work and then shut it off.  The Air Force was even worse, they invented this weird cylume light strand that they would put all around the instrument panel, didn't work very well, but that was all they had, poor bastards.

Sniper, we'll probably just continue to disagree....no need to continue this thread hi-jacking we committed.

Do either of you have autogated tubes in your ANVIS? If they are OMNI VI they are, and would probably make a slight whining noise, I think it reduces the power on the photocathode end to protect the tube, that might explain the difference in performance you are experiencing, they might have done away with the BSP on the gated goggles because they don`t need it. I don`t think the BSP is dynamic, I think it just shuts the power off to the goggles like a photocell. it`s external and wired in between the switch and the tube , thats how it works on my old PVS-5C`s. on the pvs-14, the BSP sensor is right beside the IR LED in the housing.

I just looked in the PVS-14 manual, and it does say "light interference filter" I always thought it was a laser interference filter because that`s what it does, strange.

OK, hijack over, I'll initiate it as I think it will bring to light a lot of information that the lay person doesn't normally get.  I will call it BSP vs. ABC.  Wildbill, I'm counting on us disagreeing as that is how we will get the information out. Meet me there and we can get rolling, I'm betting that I will even learn something.

LIF blocks green and blue lasers. It let's red and infrared pass through. I believe it is for better resolution, in higher lit situations.

height=8

Quoted: LIF blocks green and blue lasers. It let's red and infrared pass through. I believe it is for better resolution, in higher lit situations.

Wrong.  It's to protect the tubes.  There are certain wavelengths that it stops, but that data is sensitive.  It actually reduces the image quality.

Quoted:  It actually reduces the image quality.

Not from my experience. In an urban environment, image quality is improved.

2. NVG LASER Hardening. LASER protection for the NVGs themselves is available by using
the Light Interference Filter (LIF). The LIF is an out-of-band optical filter designed to protect
NVG tubes from LASER damage. The LIF does not provide any additional protection for the
human eye. By design, LIFs reduce the amount of light entering the NVG tubes. The HMX-1
assessment of the LIF concluded that NVG performance might be reduced during low light level
and low target/terrain contrast conditions. During conditions of high target/terrain contrast,
regardless of light level, NVG performance was not degraded by the LIF use. The LIF will only
mount to those NVGs modified with a LIF collar over the objective lens assembly. The LIF
collar and mounting instructions was distributed to the fleet as part of the complete AN/AVS-9R
retrofit. All AN/AVS-9R delivered include the LIFs in individual cases.
3. Types of LIF Protection. Original LIFs, have evenly spaced lugs or notches around the
outside. New LIFs have a lug missing on each side, 180 degrees from each other, and provide
protection against more LASER threat wavelengths. LIF countermeasure information (LASER,
wavelength, and optical densities) is outlined in CMC Washington DC//APW//220004Z AUG 94
(SECRET) and is to be incorporated into helicopter TACMANS (Vol. II)

Quoted: 2. NVG LASER Hardening. LASER protection for the NVGs themselves is available by using the Light Interference Filter (LIF). The LIF is an out-of-band optical filter designed to protect NVG tubes from LASER damage. The LIF does not provide any additional protection for the human eye. By design, LIFs reduce the amount of light entering the NVG tubes. The HMX-1 assessment of the LIF concluded that NVG performance might be reduced during low light level and low target/terrain contrast conditions. During conditions of high target/terrain contrast, regardless of light level, NVG performance was not degraded by the LIF use. The LIF will only mount to those NVGs modified with a LIF collar over the objective lens assembly. The LIF collar and mounting instructions was distributed to the fleet as part of the complete AN/AVS-9R retrofit. All AN/AVS-9R delivered include the LIFs in individual cases. 3. Types of LIF Protection. Original LIFs, have evenly spaced lugs or notches around the outside. New LIFs have a lug missing on each side, 180 degrees from each other, and provide protection against more LASER threat wavelengths. LIF countermeasure information (LASER, wavelength, and optical densities) is outlined in CMC Washington DC//APW//220004Z AUG 94 (SECRET) and is to be incorporated into helicopter TACMANS (Vol. II)

If it's laser damage, it must be visible lasers.

I have a LIF and experimented with different lasers. The LIF blocked mostly visible lasers and did nothing, as far as I can tell, for red and  infrared lasers.


When I use an LIF  in a urban environment with lots of lights, the lights themselves seem to interfere less with the image.

Just thought of something:

LIF's by design CANNOT reduce IR light to the tube. If they did, the IR illuminator on the goggles wouldn't work, and IR lasers used by the soldier wearing the goggles wouldn't be visible.

Quoted: Just thought of something: LIF's by design CANNOT reduce IR light to the tube. If they did, the IR illuminator on the goggles wouldn't work, and IR lasers used by the soldier wearing the goggles wouldn't be visible.

And therefore, they are not designed to block IR Lasers.

The clue is in the name. Light Interference Filter. Bright lights interfere with Nods.
With the exeption of the PVS 14, all other nods don't have variable gain. So in order to use them in higher lit situations, a filter is needed. What kind of filter? One that blocks visible light, while letting red and infrared through.

From my amateur tests, I discovered that the LIF blocks most visible lasers; especially green. Hense the green coating on the lens.  Are green lasers widly used in the military?

Green just happens to be a large part of the wavelength emitted my incandesent lights.

For those that have a LIF, look at a bulding or house with a porch light on. Then try it with the filter on. You'll see what I mean.

height=8

Quoted: Just thought of something: LIF's by design CANNOT reduce IR light to the tube. If they did, the IR illuminator on the goggles wouldn't work, and IR lasers used by the soldier wearing the goggles wouldn't be visible.

Keep in mind, the Near IR spectrum includes wavelengths from over .700 microns up to 3.0 (the NVGs being sensitive to slightly over .9 microns).  An IR laser/designator does not not need to transmit over all of those wavelengths, but can be on one particular wavelength.  Just throwing that out there.

And therefore, they are not designed to block IR Lasers. The clue is in the name. Light Interference Filter. Bright lights interfere with Nods. With the exeption of the PVS 14, all other nods don't have variable gain. So in order to use them in higher lit situations, a filter is needed. What kind of filter? One that blocks visible light, while letting red and infrared through. From my amateur tests, I discovered that the LIF blocks most visible lasers; especially green. Hense the green coating on the lens. Are green lasers widly used in the military?

What ever effect you are experiencing with visible light is incidental to the real purpose of the LIF, which is to block very specific frequency`s of LASER light. lasers emit in a very narrow band, so the LIF is a "notch" filter, it might let 840nm and 880nm pass, but block 860nm.(just an example, I don`t know what frequency`s it blocks) a 500mw laser is no joke, it could take out an nv tube in an instant,.....Hmmm?

1paintball has got the idea...

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Quoted: height=8 Quoted: Just thought of something: LIF's by design CANNOT reduce IR light to the tube. If they did, the IR illuminator on the goggles wouldn't work, and IR lasers used by the soldier wearing the goggles wouldn't be visible. And therefore, they are not designed to block IR Lasers. The clue is in the name. Light Interference Filter. Bright lights interfere with Nods. With the exeption of the PVS 14, all other nods don't have variable gain. So in order to use them in higher lit situations, a filter is needed. What kind of filter? One that blocks visible light, while letting red and infrared through. From my amateur tests, I discovered that the LIF blocks most visible lasers; especially green. Hense the green coating on the lens.  Are green lasers widly used in the military? Green just happens to be a large part of the wavelength emitted my incandesent lights. For those that have a LIF, look at a bulding or house with a porch light on. Then try it with the filter on. You'll see what I mean.

Well, tell me then why AN/AVS sets use LIFs.  They are already equipped with minus blue filters which filter out visible light in the 665nm and below range.  You may want to refer to my post on laser hardening straight from the USMC NVG manual.

Also your keen sense to pick out the word light must mean visible light must apply to lasers too then as a laser is Light Amplication by Stimulated Emission of Radiation.  But we all know there are IR lasers despite the use of the word "light".  Funny thing with terms here....most people refer to illumination when talking goggles, but since NVGs (especially GEN III) are more sensitive to Near IR than vis light (of which there is much more abundant energy in the night sky than visible light) the proper term is irradiation, but old habits die hard.

LIFs are designed to block out Nd:YAG laser range finders used in tanks and other fire control systems that operate at about 1064nm. These lasers are usualy very powerful (several hundred millwatts or even a few watts) and are on the very bottom of 3rd Gen's spectrum.

mrf2

Quoted: And therefore, they are not designed to block IR Lasers. The clue is in the name. Light Interference Filter. Bright lights interfere with Nods. With the exeption of the PVS 14, all other nods don't have variable gain. So in order to use them in higher lit situations, a filter is needed. What kind of filter? One that blocks visible light, while letting red and infrared through. From my amateur tests, I discovered that the LIF blocks most visible lasers; especially green. Hense the green coating on the lens.  Are green lasers widly used in the military? Green just happens to be a large part of the wavelength emitted my incandesent lights. For those that have a LIF, look at a bulding or house with a porch light on. Then try it with the filter on. You'll see what I mean.

Bubba, you are doing a lot of speculation on the LIF that is incorrect.  The reason the LIF is called "light" is because lasers are amplified light, nothing else.  The material that is used in the LIF is designed to negate the effectes of amplified light, it is not designed to "filter" normal light to reduce any effects on the NV.  We are also issued laser glasses that use the same material as the LIF, so I have a lot of time behind this glass and can tell you that it does nothing to filter normal light.  The coating on NV lenses is called minus blue because blue is not a common light during night, so that coating was placed there so that blue lighting can be used on instrument guages so they won't impact night flying, nothing to do with green lasers.  I'm away from home on duty, when I get back home, I will try to get some pictures that compare these issues.