There is a certain logic that is always in place for lighting in film and video. All light that is on the subjects and objects in a given frame are supposed to be inspired by something, as this is true in real life. What this means is that all of this light must come from sources that are supposedly in the story space. This is never actually true in film and video production as all light comes from produced or engineered sources, placed by the Gaffers under director of the Director of Photography. The goal, then, is to make the entire light look as though it could come from a logical place in that artificial location, even though it could not have in real world settings. Night scenes, which in real life are too dark to see anything with a film or video image, are often bathed in a certain ‘night’ light. This light is often used to suggest the presence of the moon, even though it does not serve to reason that a bright moonlight is present in ninety percent of night scenes where there are not streetlights. To do this effectively you just have to make a few adjustments to regular lights so that they will have the appearance of moonlight.
The first, and most important, aspect of altering a light so it appears as moonlight is to apply a blue gel. You may want to apply a half strength as this will give enough of a blue tint to suggest moonlight but will not be enough to make it look like a neon colored light. This may make the light look too sharp blue anyway, which means you may want to put a frosting filter in front of the light as well. If this does not work or is not available you can also try a silk, but there is no definitive answer for how to get the correct intensity. Light most things with video lighting, you will have to try a few things to see if it gets exactly the look you want for your specific environment. You may want to use stronger gels when lighting through a window.
Light Type and Position
You are going to try to use a relatively powerful light, but if you do not have large HMIs available you can just use a daylight balanced light. If it is a small location you may be able to use something as small as a 600 watt light, but for outdoor locations that give you more room you will want to shoot for a 1K+. You are going to back the light up as much as you can and then pedestal it up feet above the subject’s head. It does not have to be so high as to suggest a full top down direction of light, but just a little from above will be enough for the audience to suspend disbelief. You then angle the light down onto them so that is will gleam on their front or back.
You are likely not going to use this blue moonlight directly on someone’s face, especially since this may be bright enough to adjust their eye movement. Moonlight would never do that in the real world, so you do not want them squinting here. You will just hit the side of their face, backlight the back of their head, or hit the area around them. You may even want to adjust it so it works as a standard fill light, like what is used in interview setups.