Opaque air curtains

Doors are ancient, fiddly and cumbersome inventions. With the exception of automatic doors they require some form of effort to work, and cannot be opened easily with full hands. Automatic doors, however, require a lot of space to be implemented, and due to their cost are usually only added to large doorways. This makes them unsuitable for places such as offices or schools, where space may be restricted.

Air curtains are a perfect substitute - they heat (or cool), do not require a lot of space to implement and separate rooms or areas successfully. However, they're transparent. What's needed is some form of opaque or dark harmless gas that would be passed from the top of a doorway into a grate at the bottom, and cycled back to the top again through the door-frame/wall. This gas would have to be generated in the unit (to replace that lost when one walks through the stream), and would have to diffuse with little effect on the atmosphere.

Something such as the smoke used in discos would work for this, as it would diffuse quickly, and be easy to generate in large enough amounts (however, I'm no chemist, so there may be more suitable substances out there). The movement of the air would give most of the noise-cancelling effects of an ordinary door, and it would even be possible to see if someone had walked through recently, due to the gas displaced.

The gas generation unit would need refilling, but the length of time it will last is dependant on the gas.

The only drawback is locking, and another door would be required for this. As such this solution is probably best suited to internal doors only, such as in an office or school, where the external ones can be locked. (I did think about running chlorine gas through it as a locking mechanism, but thought this a little extreme - and illegal ;-) )

In addition to this ease-of-use, they could be deactivated to provide a doorway, no door-stops or self-closing mechanisms required.