These devices projected a hologram around an area up to a hemisphere of 8 meters in diameter such as a camp site or shuttle. This allowed those inside the holographic projection to see outside but individuals outside only saw what the hologenerator projected. The projection ranged from showing nothing to outsiders to appearing as rocks or even a marsh. Furthermore, sonic baffles prevented noises from inside the holographic radius from reaching outside thus heightening the illusion that no one was present in the area being masked. Multiple such generators working in concert with one another allowed for the effect to encompass a larger amount of land.
By the mid-24th century, a new variation of this technology allowed for the creation of holomatter forcefields which mimicked the texture of the object the hologram was copying. An away team or scientist typically keyed their combadges to the frequency of the shield in order to pass through the projection. These were typically used by a first contact team or a scientific expedition conducting anthropological or biological study and even as a tool for covert operations. An aspect of the Treaty of Algeron with the Romulans also meant that these camouflage hologenerators were to be easily pierced through a tricorder scan. (Decipher RPG module: Starfleet Operations Manual)