Hard Light
Hard Light Surfaces are a puzzle element of Portal 2. They are solid slats of light produced by special emitters, and can be redirected through portals (just like Thermal Discouragement Beam and excursion funnels). They can be used as floors or ceilings, block LOS from turrets or stop flying objects in mid-air, like cubes.
They can be painted with any kind of gel, however only repulsion and propulsion gel change anything. Portal shots and lasers can pass through hard light bridges or walls, which is why conversion gel and reflection gel don't work. Painting one side of the surface affects both sides.
Bug:In single-player, gel on a hard light surface will not save. No official singleplayer puzzles used this feature, so Valve never made code to save it. [todo tested in ?]
Tip:Hard Light Bridges can be detected with a trigger. Set the "Everything" flag and filter it to detect projected_wall_entity, the actual entity representing the wall itself.Creation
Hard light surfaces are emitted by a single prop_wall_projector. Create a 'dirty' wall projector by adding a keyvalue skin with a value of 1 (Disable "Smart Edit" to do this).
External links
Example map with a VMF included.