Ultraviolet

Only some things answer the light. Whites, certain pigments and a short list of materials glow, and everything else falls to near black, so the frame sorts the scene into two categories on its own.

The result looks lit from within. A fluorescing surface emits its own light rather than reflecting yours, which is why it reads differently from anything a coloured lamp can do.

Ultraviolet sits just past the blue end of what the eye can see. A fluorescent material absorbs it and re-emits the energy as visible light, usually violet, cyan or a sharp green.

Optical brighteners in laundry detergent are why white clothing lights up so hard under UV. Teeth, tonic water, highlighter ink and white paper do the same thing for the same reason.

Everything without a fluorescent component goes dark, because there is no visible light in the room to reflect. Skin stays dim and slightly mottled, and unpainted surfaces disappear.

Use it for clubs, forensic scenes, body paint and any shot where you want a small number of things to be the only things visible.

Skip it in a scene that needs detail everywhere. Most of the frame will be black, and adding a fill light removes the effect entirely.

Neon Practicals are visible signs inside the shot, throwing saturated coloured light onto the subject and spilling onto wet surfaces. The light is ordinary light that happens to be coloured.

The check is whether surfaces glow on their own. Materials emitting violet and cyan against near black mean Ultraviolet. Coloured light landing on ordinary surfaces, with the source in frame, means Neon Practicals.

Nothing in the scene fluoresces, so the model returns a dim purple photograph. Give it something that answers the light: white fabric, body paint, printed patterns, teeth.

The other failure is even exposure across the frame. Ban white light and even exposure, and say everything without fluorescence falls to near black.