The iRobot virtual wall keeps a roomba inside of a room. Because Fizzgig took a tumble down the stairs, I decided to use the virtual wall to give it a better idea of where to stop.
The virtual wall shoots an expanding IR beam across a distance of 3 to 9 feet depending on its setting. However, there is a prism which splits the beam, aims it upwards and reflects it out via a half-dish mirror, so there is a circular protection zone around the virtual wall unit to keep Fizzy from running into it.
The problem is, at least with new batteries, this virtual hole is 3 feet in diameter!
So as a test, I did some low class tests and found that, just like in photography, toilet paper makes a good diffuser.
So I took a couple sheets, folded it over twice on one end to form a little bead, wrapped the bead around the reflector on top and twisted it at the back side.
I then angled the virtual wall unit down the stairs a little, just to move some of the widening beam off the stairs.