What is Moonlight Mode?
Simply put, moonlight mode is any mode of the flashlight with an output less than one lumen. This amount of light is only really useful to dark adjusted eyes, but it has the benefit if keeping your night vision intact, where a blast from some 1,000 lumen pocket rocket will ruin your night vision.
Why would you want a very small amount of light when modern flashlights can put the power of automobile headlights in your pocket? Well, for most people into having flashlights with moonlight mode, there are generally several reasons:
A modern 1xAAA flashlight can do about 60 hours on moonlight mode, and it just gets better from there. Since a 1xAA battery has so much more capacity, these types of lights can potentially give you months' worth of light with a handful of batteries.
A couple factors affect the runtime of a moonlight mode flashlight that any enthusiast should be aware of. Some flashlights feature an electronic switch, which puts a small amount of parasitic drain on the battery, even when the flashlight is turned off. And control ring flashlights such as the Sunwayman V series have an overhead due to the design of the control ring. The overhead is small enough not to have a noticeable effect on the runtime when the light is on high, but on moonlight mode, the control ring overhead cuts the runtime in half at least.
Another factor is PWM versus constant current circuits. PWM gives you much better tint at the cost of efficiency (runtime) and constant current gives you superior runtime with a tint people generally complain about, usually green-ish.
So the best runtime on moonlight mode is going to be a twisty, which has no switch, or a simple click switch--forward or reverse.
With the right setup such as a solar charger, in an emergency, moonlight mode gives you enough light to function with most tasks in virtually unlimited amounts. Pretty much every disaster kit should have an efficient, multi-mode flashlight with a long runtime low mode.
Even with some overhead of the electronic switch, the Sunwayman D40A has a ridiculous runtime because of its 4xAA batteries.
Let's say you are tent camping at a public campground. You wake in the middle of the night needing to use the restroom. You don't need to light up the entire camp site just to find the well defined path to the facilities. In fact, turning on a really bright light when you don't absolutely need to could be considered rude, or at least not very neighborly.
This is why I like to have a compact, moonlight mode "twisty" style, flashlight close by me when I'm camping, such as the L3 Illuminations L10, which has been a champ on camping trips. And the Thrunite T10, which also takes 1xAA battery with a nice, and also features an efficient constant current circuit.
The last time I went camping, my L10 on high with a Sanyo Eneloop AA was the brightest flashlight of anyone camped around us, and I mostly just used the moonlight mode for late night restroom trips.
The moonlight mode is also useful for checking on sleeping children when they are sick. You want to see that they are OK, but you don't want to wake them up by turning on the bedroom light. I also use it to check on the missus when she is sick. I can wake her up to take some medications without blinding her, and she can go right back to sleep.
3. Tactical Applications
Some people use moonlight mode in tactical applications, and some of the newer lights even have low output modes in red and other colors. There are times when it's beneficial to preserve your night vision and also not have your light be seen by others. I'm just a computer programmer and not a commando, but I have talked to military guys on various discussion forums.