If you are starting to explore the issue of NVDs/NVGs, then you will need some basis on which to frame your foundational knowledge. This includes the basic principles of how a device works, and the various elements involved. These are also used throughout the industry when presenting units for sale or discussing usage. A bit of warning; the following is a lengthy segment, as the field of night vision is incredibly technical and nuanced. While this is a distilled post, it is no means definitive.
So how do NVGs work?
NVGs are intended to take the ambient light in the environment and amplify it through a process to become more visible. This is achieved by:
- Light (the spectrum of which contains photons) enters the unit through the objective lens, and strikes the Photocathode
- The Photocathode converts these photons to electrons, before letting those electrons pass through to the Micro Chanel Plate
- At the Micro Chanel Plate those magnified electrons are then magnified thousands of times over and accelerated
- The accelerated number of electrons are then passed through a green (or white) phosphor screen that produces a flash of visible light
- The user sees this light emission as an enhanced image of the darker environment – but as green (or white) in color.
This brings us to the (basic) components of NVGs:
Some common terminology includes:
- Green Phosphor: Earlier generations of NVGs commonly utilized technology that modulated the spectrum of light into a “green” image that became synonymous with the night vision industry. Green is known better for its ability to “see” more of the environment in darker settings.
- White Phosphor: Later and more current generations of NVGs transitioned to more current technology that changed the spectrum of light to a “white” (sometimes seen more as a blue) image where the White allots for more detail and easily observed light sources.
- Autogaiting/Autogaining: A common feature to modern-day NVGs, and are crucial for the device’s ability to delineating an image after a sudden or intense emission of visible light (think gun fire and explosions). While the terminology is often interchangeable, the function is the same, and is a small part of the device’s image intensifier tube that allows automatic regulation of the device’s sensitivity by dimming or turning the unit ON/OFF as needed to protect the unit.
- Thin Filmed v Unfilmed Lenses: The image intensifier in an NVG often comes as either a thin filmed device or unfilmed (‘thick’ filmed lenses do exist, although are mainly relegated to older and outdated devices, thus seldom discussed). The largest difference between filmed and unfilmed NVGs are predominantly price, obviously with higher performance (i.e. specs) in unfilmed devices costing easily 50% more than filmed.
- Thin Filmed: In the case of thin filmed devices, the “film” is a very thin layer of aluminum-oxide over the image intensifier that acts as a barrier to harmful ions (and the occasional electron) striking it. This has the advantage of protecting the unit, but at a cost of adding another barrier to light detection. This is not an established rule however, as it is possible to find thin filmed night vision devices with the same (or near same) performance as unfilmed.
- Unfilmed: In contrast, unfilmed night vision devices lack that aluminum-oxide barrier to the image intensifier. Ideally, in an equal world, night vision devices that are unfilmed will have a higher Signal-to-Noise ratio and lower Halo number as they receive more light and have more sensitivity. The largest advantage to having unfilmed night vision is improved performance in the unit under lower light conditions or in environments where ambient light is not present.
- Nitrogen Purging: Part of the professional assembly process for NVGs includes dry nitrogen purging. This is done to clean/seal the internal components of the device and optimize sensitivity. Regular purging of the device can also help ensure dust, moisture, and debris is eliminated from the interior of the device. The frequency of the purging should be based on the environment and frequency of usage, but annually is generally a good recommendation.
- Hoffman ANV-126A Testing: Part of the professional assembly process is to conduct a verification test using an ANV-126A test kit from Hoffman Engineering (seen widely as the leader in night vision testing units). The purpose of the test is to (among other things) verify proper eyepiece diopler focus, and (specifically with dual tube) collimation between the images to ensure the sharpest, and most accurate image possible. If the alignment of optical axis (i.e. the angle between the two tubes) is off, it can cause depth and other perception/focus issues on the part of the user.
Within the night vision market there are (generally) three major manufacturers to image intensifiers, and each has corresponding performance and costs associated to their product. These manufactures include:
- Photonis: One manufacturer of image intensifier tubes is called Photonis. Photonis manufactures and sells “technologies in the field of photo-detection and low light imaging to provide solutions for Defense & Security, Nuclear Safety, Life science and Industrial & Non-Destructive Testing”. Photonis is owned by Exosens who is a European leader in “high-end electro-optical technologies for defense and industrial applications”. Photonis is often considered an inexpensive option, but has lower performance in environments without any ambient light. Because of technology included in the unit, Photonis devices are mostly considered Gen II+ (with the exception of its newest device that Photonis markets as “Gen IV”) as most of their units still utilizes some of that generational legacy technology, but revised using more modern elements.
- Elbit: Elbit Systems of America, LLC is a U.S.-based wholly owned subsidiary of Elbit Systems Ltd., “a leading global source of innovative, technology-based systems for diverse defense and commercial applications”. Elbit Systems Ltd. (the parent company of Elbit Systems) is headquartered in Israel, “developing and supplying a broad portfolio of airborne, land and naval systems and products for defense, homeland security and commercial applications”. Elbit image intensifiers are often seen as having a mid-range price, and performance range. These units will perform better than Photonis in environments without ambient light, but with some limitations in delineating through shadowed areas. Generally, Elbit is where the delineating line between Gen II and Gen III NVGs, with Elbit occupying the latter Gen III and Gen III+.
- L3 Harris: The final and largest manufacturer of image intensifiers is L3 Harris (or colloquially referred to simply as “L3”). L3 alone is a massive US-based technology firm with extensive defense and law enforcement contracts, and produces some of the best NVDs in quality and performance. These devices perform the best in environments with no or little ambient light; however, are almost always at the upper-end price range.
Another aspect of basic terminology for NVGs includes references to tube cosmetics, and can be those centered around the three “zones” of a NVG’s field of view.
Other terms used for discussing tube cosmetics include:
- Spots: Spots happen during the manufacturing process and can range in size and location. Most common causes for spots are bright spots burned out by the factory, broken light fibers, and debris. Spots can also be referred to as ‘artifacts’ or ‘peppering’. The size (larger than .003”) and location is allowable based certain criteria. The categories of tubes specifications and spots are defined as Commercial, Milspec, and Aviation. The specifications for each have both a minimal performance requirement, and volume of permissible blemishes.
- Commercial spec tubes are the broadest in performance and have failed from Milspec level quality at the manufacturer. Commercial-quality tubes can have up to four spots (or less) greater than .003” (with one being allowed in Zone 1).
- Milspec tubes are higher in performance than commercial quality ones. Milspec tubes can have up to four spots (or less) greater than .003” (with none larger than .003” being allowed in Zone 1, and of those no more than three can be in Zone 2 or 3 in one tube).
- Aviation are highest in performance tubes. Can have up to three spots (or less) greater than .003” per tube, but only allowable in Zone 3 (with none larger than .003” being allowed in Zone 1 or 2).
A final word on basic NVG usage also pertains to individual maintenance and care. It is highly recommended (both by the manufacturer and retailers) to never store batteries inside the individual NVG unit. This avoids the possibility of battery leakage or corrosion damaging the internal hardware (i.e. the battery springs or circuit board) as well as leeching of the power source. When cleaning the NVG use a towel or soft bristled brush to remove larger debris, but only a lens-safe or microfiber cloth on any glass surface (the objective or ocular lens).
Black-boxing is a measure the user can take if they suspect the photocathode has been temporarily damage or “burned” after a significant/prolonged light emission. Higher spec tubes are more at risk due to device sensitivity. Black-boxing an NVG unit can resolve temporary burns (often seen as shadows or streaking in the field of view) but cannot resolve permanent burns (i.e. black ‘blems’ or spot). The process of black-boxing is straightforward, place the unit inside a blackened box or the shipping case (so that no light is striking the photocathode) while powered on, and allow the tube to reset and work some of the burn out.
In the next segment, we will review recent models of NVGs, and provide some context to their specs. However, if you would like to return to the Beginner’s Guide main post you can do so here.
Disclaimer: The purpose of this series is strictly informational, much like our COVID Chronicles, body armor, or gear guide, this series is not intended by High Ground to sway or convince the reader that one specific brand of night vision manufacturer is superior to all the others. In the end, this series is intended to provide the reader with a condensed and focused resource—nothing more. It is not to be considered definitive advice nor instruction.
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