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Night Levels provide a convenient way to talk about different ranges of darkness. These are listed below.
Night Level 0 - 1 Lux to 3.4 Lux - Between Full Moon and the Dark Limit of Civil Twilight under a clear sky. Including Astronomical and Nautical twilight under a full moon. After twilight but before night. However, it's too dark for eyes and too bright for night vision. Here, special night vision capabilities such as autogating are required.
Night Level 1 40 to 1000 mLux Full moon. This is Gen1 territory, and is where your typical ex-Russian cheap scope performs beautifully, with a bright and useful image. You can see better with your eyes, but if you've just come out from a bright room, even Gen1 looks good under these conditions. This is in the mesopic range of vision, and you'll still see colors here, though some colors are more difficult to see than others ( excepting color lights )
Night Level 2 10 to 40 mLux Half moon. Gen1 still works well here, though some cheaper models may start to suffer. You'll still see better with your eyes once you dark-adapt, but Gen1 provides more detailed image in the center of your vision, where your cones are. This is still mesopic vision ( color and monochrome ) but forget trying to make out much color. Gen1 provides benefits here, especially if you're shooting.
Night Level 3 2 to 10 mLux Some moon. Gen1 pretty much fails here, but Gen2, even the older stuff, looks great! This takes you all the way to Scotopic vision, so forget color your cones might still be helping but your peripheral vision is better than your direct vision here. You need a system gain of several hundred or higher to see anything useful. It's really dark. Only some Gen1 even provides a useful image here though Cascade systems work very well. This is the region of color NV for military use, and some better digital struggles, but will provide an image. It's that awesome space where cheap NV that's not Gen1 still looks awesome, and works just as well.
Night Level 4 700 uLux to 2 mLux Starlight It's really dark now, starlight. Lots of snow? It's really bright, but Gen1 isn't going to do much. However at the other end, if it's all dead vegetation around you, then even Gen2 is going to struggle, but will still provide a useful image. At this point, you're struggling to see anything with your eyes, and you need Gen2 or Cascade minimum.
Night Level 5 70 uLux to 700 uLux Overcast Starlight We're not talking overcast near the city now, which is more like NL3 We're talking dark and stormy nights, like the opening lines to a bad story. This is a "can't see your hand in front of your face night and if you can see you're hand, it's still NL4. This is Gen3 and Super-Second-Generation (eg, xd4, xr5 ) territory and it's about as dark as it gets. Not much in the way of digital technology handles this, and vegetation stands in start contrast to dark tree trunks. Most cheaper nods begin to dissolve into noise around here, and Gen2 is barely functional, resembling a TV that's poorly tuned at the lower light levels.
Night Level 6 Below 70 uLux This covers indoor regions, caves and other areas where it's so dark, illumination is often needed. Unless you're Mil-Pol-Gov, you probably won't encounter this often. This is being able to see someone inside an open door under starlight, or walking into a building with only starlight coming in from outside. It's dark, even with night vision and you won't see anything of shape or size no matter how well adapted your eyes are. And forget eating carrots, because you won't be able to see them. Pilots probably won't encounter this sort of light level, but people on the ground will. This is Steven King's territory. Tread carefully. It's also very seldom referenced by documentation, because it's just so dark.
All Night Levels can be abbreviated to NL. As an example, Night Level 4 could just be written as NL4. In the context of night vision, it's pretty well understood. Beyond this, there are some notable exceptions to Night Levels - 100 uLux is a standard point for measuring S/N ration for a tube. This is at the base of NL5.