A perception camouflage matrix was used to make things invisiable.
Clyde is touching the perception camouflage matrix that is hiding Commander Kaagh's space pod. (TV: The Last SontaranThe Last Sontaran)
The things were still touchable, but couldn't be seen. Commander Kaagh used such a perception camouflage matrix to hide his space pod. He also could activate a perception camouflage matrix to hide himself by pressing on his Sontaran arm control. (TV: The Last SontaranThe Last Sontaran)