Monocular NVGs:
The definition of a monocular is a single eye unit that has no magnification. These are what we call the Swiss army knife of night vision equipment. A quality monocular is the most versatile of all the night time devices. Their compact size and lightweight make them ideal for head mounting. Many of the higher generation monoculars can be attached to rifle scopes and spotting scopes and can also be mounted directly onto a weapon. Many of these same higher generation monoculars can be mounted onto a weapon in front of a red dot sight that is night vision compatible, such as certain models of Eotech's( i bet this is the reason why army went for lots of pvs 14s). Most of these devices can also be adapted to a camera for photography with the use of a camera adapter. These versatile optics are often small and light enough to fit in a shirt pocket.
There are both pros and cons of the single eye of a monocular verses both eyes of a goggle. The pros are that you can switch back and forth between your eyes when you get tired and that your unaided eye will maintain it's night adaptation and some of your peripheral vision. The US Military feels that this gives the user better situational awareness. The cons to this single eye is that it doesn't feel as natural and will take some time to get used to. With a monocular the user needs to get used to keeping both eyes open to have the best experience.
To sum it up monoculars will give you the most versatility as well as the most accessories to choose from. They are a great choice for those who want a product that will have the ability to do a lot of multi-tasking. The best monocular right now is the
PVS-14 (gen 3+)
Night vision Goggles (single tube biocular or dual tube nvg) :
The great thing about these types of devices is that they feel very natural when wearing them and there is very little learning curve to get used to them. There are two types of goggles; a two eye-piece that views through a single image tube(pvs7 type) and a two eye-piece that views through dual image tubes(dual tubes), also known as stereo-vision. Like monoculars, a goggle is a great navigation device. Dual tube goggles offer "stereo vision" meaning each eye sees it's own slightly different image rather than the same image being shared with both eyes. This stereo-vision gives the dual tube goggles even better depth perception allowing for improved navigation ability. The down side of a goggle is more weight and less versatility than a quality monocular. Goggles tend to give a bit better depth perception than a monocular and a dual tube goggle(pvs21 types) gives even better depth perception than a single tube goggle(pvs 7 types) because each eye sees a slightly different image which allows for better judgment of distances for example while wearing a PVS-7 and you’ll notice the image is identical, It’s not true stereoscopic vision. This better depth perception does help with navigation, weather you are walking through a field or navigating your boat around the rocks, it will help. The disadvantage is increased size and weight and less versatility. When wearing a goggle in a head mounted position the extra weight tends to cause the unit to ride down below your eyes when walking, and you'll tend to get more tired with the extra weight as well. Goggles do not have the option to be mounted on rifles or adapted to rifle scopes and camera adaptation is limited also.
Goggles really excel at shorter duration navigation and stationary observation activities. However, if you will be doing more time consuming and physical activities that require constant head mounted hands free use, than the lighter monocular is probably the better choice.
So how many tubes is best for you? This is a tough question. It all depends on your priorities and budget. What will you be using NVGs for? If you are training recreationally on a square range, then all you need is a PVS-14. If you have the disposable income to upgrade to dual tubes, then by all means get the best you can afford. There’s a lot of aftermarket support for single- and dual-tube night vision nowadays. So it’s easy to get what you want and set it up to suit your needs. Quad-tube devices are super expensive. While the ANVIS-10/GPNVG 18 is priced similarly to the PVS-31, they’re both hard to find and require specialized knowledge
The price of night vision also varies with what types of tubes are inside. Those are at the heart of the system. A PVS-14 with a single, un-filmed, white-phosphor tube costs more than a dual-green-phosphor-tube Gen2+ model. So more tubes does not mean a better setup.
Will talk about Generations and working of image intensifiers some other day, i hope everyone will have a healthy discussion over this instead of rants flying here and there