Stargate shield

A Stargate shield is a shield that fulfills the same role as the metal Iris for Stargate Command. The most famous example of a shielded Stargate is the one in Atlantis and on Starbase Atlantis.

The Atlantis shield
Unlike the Iris protecting the Stargate on Earth, the shield on the Atlantis Stargate is translucent; the suppressed Unstable vortex and the impact of objects prevented from rematerializing can clearly be seen. The Atlantis shield has yet to be breached by anything because it is placed so close to the event horizon that it blocks even subatomic particles, unlike Earth's iris which can only block objects larger than an atom. However, a "back-door" previously coded into the shield's control system by Dr. Rodney McKay allowed him to remotely deactivate Atlantis' gate shield from an off world location, preventing anyone else (including anyone using the shield's control panel in Starbase Atlantis itself) from reactivating it.

GDOs and IDCs are still used by both Atlantis' offworld teams and the Athosians, which usually prevents unfriendlies from entering the starbase via Stargate although that is not always the case. It is unknown what method the original Lanteans used to identify themselves as incoming travelers, but it is possible that the Puddle Jumpers themselves performed this function in some way.

The Atlantis Stargate shield generator is not built into the gate itself, but is located somewhere else in the control room, or elsewhere on Atlantis and Starbase Atlantis.