Throughout my experience in the night vision space, I've come across my fair share of "issues." Now whether these issues actually pose a problem, is ultimately up to the end user and the application these NVDs are being used in. For example, someone trying to shoot paper on a range at night might have a different need when compared to someone flying a multi-million-dollar aircraft at night. Or a police officer using their NVD to spot a felon running away in the woods.
These issues that occur (most of the time) during the manufacturing process of the image intensifier can be categorized into two categories. Either a cosmetic blemish or an operational defect. Now if it wasn't obvious from the names, a cosmetic blemish is just an imperfection in the unit that does not affect the way the unit functions, such as a piece of dust that found its way into the unit when the device was being manufactured. Whereas an operational defect does affect the operation of the unit and can lead to some sort of decay of the image intensifier and render the unit inoperative to an extent if not totally.
In this post, the descriptions of each cosmetic blemish and operational defect will be laid out as simply and straight forward as possible, and therefore not make it a guessing game if one was to ever come across a potential issue. There will even be pictures for you visual learners (like me).
OPERATIONAL DEFECTS
FLASHING/FLICKERING
Intermittent operation of the system causes the image to flicker or flash. It can appear in one or both tubes independently. Pretty straight forward. And there's no question as to why this would be considered an operational defect. No picture necessary.
EDGE GLOW
Edge glow appears on the outer portion of the image. Usually, the culprit for edge glow is an emission point that has grown, but it can also be caused by a defect in the phosphor screen, allowing for light feedback from the photocathode. This defect is fairly easy to diagnose. just cover the objective lens from all light and see if the edge glow is still present. If it is in fact edge glow, then it will still be present.
EMISSION POINT
Emission points are caused when there is a short in the microchannel plate. the main culprit for this operational defect is usually debris that gets into the system during the manufacturing process. These emission points manifest in the form of a pinhole if bright light. The way to test for this defect is the same as testing for edge glow. Simply block all light from entering the objective lens and see if the pinhole is still bright, if so, there is an emission point. Now I know it might be hard to see but in the picture example, the emission point is located in the right tube.
SHADING
Shading occurs on the edge of the image. This defect will occur if the device has a defective vacuum seal within the image intensifier. The defective vacuum seal will eventually cause the photocathode to die, which then causes the shading around the edges. Shading will always start on the edge of the image and will eventually work its way inward. Shading also has distinct edges and will not be faded.
COSMETIC BLEMISHES
IMAGE DISTORTION
More easily detected in high light conditions. Objects appear to wave or bend with head movement. Distortion will not change during the life of the image intensifier.
Fixed Pattern Noise
Fixed pattern noise looks like a hexagonal pattern throughout the image. This pattern will most often be seen with high light levels. This is present in every image intensifier when exposed to high enough light levels.
IMAGE DISPARITY
Image disparity appears when there is a difference in brightness output between the image intensifiers in a dual tube system. Not really applicable when using a monocular system like a PVS14.
CHICKEN WIRE
Chicken wire is a pattern of dark lines that appear within a portion or whole field of view. The pattern is characterized by taking the shape of chicken wire. This results from the construction of the fiber optic inverter.
BRIGHT SPOTS
Bright spots are caused by flaws in the film of the micro channel plate. This can cause a bright spot that is not uniform in shape and goes away when the objective lens is covered to prevent incoming light. if the spot remains, it is an emission point, if it goes away, it is a bright spot.
OUTPUT BRIGHTNESS VARIATION
This condition occurs when there is varying brightness across the viewing area of the goggle. With output brightness variation, there are no distinct lines around the edge of the blemish, not to be confused with shading.
BLACK SPOTS
Probably the most common blemish and the one that most goggle owners have. Appears as a black speck in the viewing area. This is caused by dust or debris between the lenses. This occurs during the manufacturing process and is a very common blemish. The great thing about it, is if you're using your goggles and not just looking at a white wall and dwelling on your spec sheet, it usually goes unnoticed.
Conclusion? Some blemishes might be a big deal to some users for one reason or another. But at the end of the day, the cosmetic blemish is not going to ruin your night vision device, the operational defects will, and hopefully this helps differentiate between the two.
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