LuxCoreRender Materials Null

Null is a special material that does not have a visible surface. Null can be used in combination with a volume, making the object act as a container for the volume.
Null and the Mix Material
Instead of mixing a material with null to make it semi-transparent, the transparency parameter of the material should be used, because it is much faster.
Null and Volumes
The null material can also be used with volumes to create a "volume-only" material. This is because LuxCoreRender uses the regular material settings to define the surface of the material, and the medium settings to define how light is filtered as it moves through the material. Since null defines a transparent surface, the only thing that appears is the color filtering through the object volume. The result is a mesh that looks like smoke or vapor. Null and a volume can also be used to emit light.