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Posted: 6/1/2008 3:21:18 AM EST
For those of you that didn't read the LIF thread, a hijack got started about the functions of Bright Source Protection vs. Automatic Brightness Control. The hijack got started when someone asked the question about how NV is affected by bright lights such as tracers.

The big question is how much and when BSP actually works in the protection of the goggles. OK, now that the pool is open, I have to caveat that I am leaving at 0400 in the morning to go back east to pick up an aircraft, so have patience when I don't get back right away. Here is my first statement to get this rolling.

The function of the BSP is to control the photocathode in the event of a massive influx of light, the BSP will shut the power supply to the tube down to prevent permanantly burning the phosphor screen. The function of the ABC is to manage the voltage of the microchannel plate to keep a constant image at a level useable by the human eye.

Soooo.... what really happens in a nv tube when it is hit with a bright light? The pool is open.
Link Posted: 6/1/2008 7:49:04 AM EST
From MAWTS-1 NVG manual (5th edition--I didn't have the 6th lying around) 2-16:

(f) Image Intensifier Tube Automatic Gain Control System. Constant exposure of the image intensifier tube to bright light sources may result in damage to the photocathode or the MCP. To prevent damage to the i2 tubes, the power supply to Generation II and III I2 tubes have been designed with two automatic protection features dersigned to control the gain of the I2 tube.

1: Automatic Brightness Control (ABC) Circuit. The ABC circuitry automatically adjusts MCP voltage to maintain NVG image brightness at a preset output for a full range of ambient illumination levels by controlling the number of electrons that exit the MCP. Sufficient I2 tube gain is available from full moon down to quarter moon illumination. As light levels pass below quarter moon, each subsequent drop in illumination does not result in an increase in I2 tube gain, therefore the NVG image starts to decrease in brightness, contrast and NVG image scintillation increases. The ABC circuit provides a protective function to aircrew by limiting the effect of sudden bright flashes (i.e. forward firing munition, etc.) and by providing an adequate NVG image brightness for low ambient light level oprations missions, albeit a more challenging environment exists under these conditions.

2: Bright Source Protection (BSP). The BSP circuity limits the number of electrons leaving the photocathode by reducing the voltage between the photocathode and the input side of the MCP. This feature automatically activates when high input light levels cause excessive photocathode current flow. Activation of the BSP is elicited when an incompatible light source enters the NVG field of view and the I2 tube shuts down or degains leading to reduced NVG image contrast and detail. The BSP circuit is extremely important because the service life of a second or third generation tube is largely a function of the photocathode service life. Photocathode end of life is primarily caused by ion contamination from the MCP. The higher the light input, the more the ions are generated and the shorter the life expectancy of the tube.



Link Posted: 6/1/2008 2:38:02 PM EST
And that I agree with, I think where the disagreement comes from is when BSP kicks in. I'm saying that BSP doesn't start until after ABC has done everything that it can to minimize the intensity level by dropping it's voltage to the minimum. When the power supply says that is not enough, then the BSP kicks in. The photocathode is only capable of a maximum of one to one emission of electrons from photons. When the BSP starts dropping the voltage on the photocathode, then you start getting a lowering of the number of electrons per photons, hence the dropping of resolution. When the photocathode has dropped it voltage to the lowest level possible and the input is still to much, then that is when BSP says enough is enough and cuts the power to the entire tube, shutting it down.
Link Posted: 6/3/2008 2:36:02 PM EST
Wildbill, no discussion???
Link Posted: 6/4/2008 12:37:08 AM EST
[Last Edit: 6/4/2008 12:40:33 AM EST by wildbill846]
From my understanding they are two separate functions.
ABC tries to maintain a constant brightness on the phosphor screen by regulating MCP electron output. It monitors that side of the tube and adjusts voltage to the MCP to maintain a preset level.
BSP is merely measuring photocathode current level and reduces voltage when it gets too high....it doesn't know or care what ABC is doing. It's function is to limit excessive electron flow to the MCP as that increases ion bombardment of the photocathode. And, as BSP occurs in the front of the tube and ABC towards the rear, I don't see how BSP would wait until ABC has done all it can.....it's already past BSPs influence at that point in the tube. Nor do I believe it is just a shutdown switch....yes, it becomes that at extreme light input levels, but it does have dynamic function before that, as my manual says it takes effect anytime an incompatible light enters the field of view. When we have bright flashes (muzzle flashes, rocket motors, exploding ordnance) I am still under the belief that both functions of automatic gain control are making adjustments.

But it doesn't matter what I think, as I said...I think we will continue to disagree. This is how it was taught to me during my initial training, when I attended the USAF NVG instructors course( which granted...was designed by MAWTS-1), and is part of the course material for the MAWTS-1 certified NITE Lab I taught for. It'll take some engineer from ITT to convince me otherwise.
Link Posted: 6/4/2008 12:30:33 PM EST
The photocathode and the MCP are right next to each other, the photocathode feeds the MCP, the phosphorscreen and the image inverter are behind the MCP, hence why the MCP can bombard the photocathode with ions. I don't think the BSP is merely a switch as someone else said in the other thread, but BSP shutting down any voltage to the photocathode will reduce image quality and why do that as an interim change while ABC still has the ability to work on gain? It wouldn't make sense to reduce image quality if the ABC can reduce gain and make up for it.
I to have taught NV to the military, but also for law enforcement aviation. I constantly work with the NV PM and will see if I can get that Litton engineer for you.
Link Posted: 6/4/2008 2:16:59 PM EST
[Last Edit: 6/4/2008 2:28:30 PM EST by wildbill846]
Ion bombardment is directly related to input light levels. If too much light is exiting the photocathode/entering the MCP, you will damage the photocathode no matter what ABC tries to do. Variations in ambient light level may not cause enough current flow to activate BSP and in those cases, ABC is enough to regulate the output image intensity. However, what brought this up was "bright flashes" and of the one's I've experienced such as .50 muzzle flash, 500 pound bombs, overt flares, etc. My goggles definitely shut down for a moment...to the point of no image at all in some cases, and I believe this was more than just ABC.
Also, I believe BSP is affecting image quality when I fly nightly out of this mixed ops field which maintains the runway lights bright enough for our un-aided flights. Image quality is dimished and I believe the large amount of incompatible lights are enough to activate BSP even though ABC is already reducing the gain.
From my understanding of how they function, it appears BSP has a greater part in protecting the service life of the NVGs and the greater benefit of ABC is realized by the person viewing the image.
Link Posted: 6/5/2008 1:42:20 AM EST
I can't give his name for the usual security reasons, but here is the written response that I got from "my guy": "The I2 has two self-preserving systems, ABC which adjust screen light levels to a constant level and if the amount of light reaching the I2 is more than ABC can control, then BSP cuts the tube power for a period of up to two minutes before allowing power to return to power up the tube again."
Interpret that as you will.
Link Posted: 6/5/2008 3:57:13 AM EST
First off, I don't work for Litton (NGEOS) or ITT.

ABC controls MCP voltage, i.e. gain.

BSP controls PC voltage, i.e. light to electron conversion efficiency.

I have read SPIE paper written by NGEOS and it explained exactly how many volts, etc, but I counldn't locate it. The control loops are closed loop circuits. As for which one works first, I can't really tell you.

ABC lowers the gain if bright light hits it - this is mainly for display purpose from myunderstanding.

BSP on the other hand, lowers the light to electron efficiency. I'm more than sure that there's a certain voltage level that PC works at its optimum, which will generate highest performance.

Obviously, for the highest performance, PC must be at its peak operating quantum efficiency and the gain in MCP must be at its optimum.

If you lower the PC efficiency, the gain must be turned up - just as you lower the CD plaer volume output, you have to turn up the volume in the amplifier. All the components have a noise floor. In PC, it's the dark current (current that PC still gives out while there's absolutely no light).

It can get further in depth, but it just gets more complicated and there'll be more questions than answers. This is why optical system design is done with modeling - working form the front to back, i.e. what do you want to see and how far, then work backward to f/#, aperture size, effecrtive focal langth, coatings, etc.
Link Posted: 6/5/2008 9:16:09 AM EST
Guys, the BSP circuit is external to the tube, it is a photocell. if you look at a PVS-14 it is located right next to the IR LED in the housing. the only circuit dynamically controlling the voltage to the PC is an autogated power supply, if you apply power to an ANVIS tube out of the housing on the bench, it will not shut down! the ABC will dim the screen, but that is all, the tube will sit there and burn up in the bright daylight (don`t ask me how I know this, it`s embarrassing)
Link Posted: 6/5/2008 10:08:01 AM EST
[Last Edit: 6/5/2008 10:20:54 AM EST by sniper-66]
So, are you trying to say there is no BSP on an ANVIS tube? I hope not, you would be dead wrong. The BSP is a part of the power supply that monitors and regulates the voltage of the photocathode. I can show you in the operators and maintenance manuals this fact. As for the tube not shutting down, I can and have demonstrated this functio.n several times. As to why yours didn't shut down, I don't know, but BSP is not a simple "photo cell" I think wildbill and I have one area we can agree to! eh Bill?
The PVS-14 does not have BSP as you pointed out, you are correct in that it has a simple high light cut off circuit.
Link Posted: 6/5/2008 10:28:18 AM EST
[Last Edit: 6/5/2008 10:32:23 AM EST by wildbill846]
Yup, Sniper, I agree there. AN/AVS sets do have BSP internally....and I am not fortunate enough to have Pinnacle tubes at my command, so no autogated power supply on ours. Not being familar with all ground NVG systems, you may be right that BSP on some of those is just an external high end cut-off switch, but I doubt it on high quality GEN III tubes. But I do agree that leaving them on in daylight is not advised and causes damage, even with BSP on the AN/AVS set..it certainly isn't good for it.
Link Posted: 6/5/2008 12:28:37 PM EST
It seems like BSP is kind of a generic term to describe several different power supply designs that accomplished the same thing in different ways, the OMNI II Anvis tube I burned up probably didn`t have the same power supply circuitry as the OMNI IV & V that you guys are using, and it seems like the autogated power supply`s are totally different than them. Also the Litton power supply is probably different than the ITT to avoid patent infringement.so there is probably not a simple answer. Here is a link to a couple of different patents:www.patentstorm.us/patents/5949063/description.html
www.patentstorm.us/patents/5146077.html
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