Moonlight Gel

Real moonlight is too dim to shoot and nearly colourless. The blue gel is the convention that replaced it, and audiences have read it as night for so long that it no longer needs explaining.

It also keeps the night legible. A scene lit only by the light that would truly be there is a black frame. A blue key from one side gives the audience shapes to read while still saying the sun is down.

One hard source from a high side angle, gelled blue, with very little fill. Direction is what sells it. Moonlight comes from one place, so the far side of a face stays dark.

Depth comes from letting the background fall away. A blue key on the subject with near black behind them reads as night. Blue on everything reads as a blue room.

The gel can go past moonlight and become a colour in its own right. A saturated blue or red wash that no window or lamp could explain stops pretending to be light and becomes part of the design.

Use it on exteriors after dark, on a bedroom lit through a window, and on any night that still has to show a face.

Move away from it when the scene has practical light of its own. A street with sodium lamps or a room with a television already has a colour, and a blue key will argue with it.

A cool blue grade moves the whole frame toward blue after the fact, evenly, everywhere. A moonlight gel is blue light with a direction, arriving from one side and leaving the other side dark.

The check is the shadows. Under a gel, the unlit side of a face goes black. Under a grade, it goes blue.

Dario Argento and Luciano Tovoli light Suspiria (1977) with saturated red, blue and green gels that no window or lamp in the scene could account for, and the colour becomes the world rather than the weather. Mandy (2018) works the same way, washing whole sequences in red and blue until the light stops behaving like light.

Flat blue fill comes back, which reads as a colour wash rather than as moonlight. Ask for a single hard source from one side and deep shadow on the other.

The second failure is a night that has gone completely dark. Say the frame stays legible and name what the key falls on, such as one side of the face and the edge of a shoulder.