Iris

The iris is a metal covering that prevents "unauthorized travelers" from exiting the Stargate onto Earth. It dialates and contracts to cover or expose the central part of the stargate where the wormhole connects. It is placed 3 micrometers from the event horizon, thus preventing full object materialization. Friendly travellers are identified by a G.D.O. code.

Earth's iris
The iris on Earth was designed by Stargate Command (SGC) to prevent unwanted travelers from emerging through the Stargate's wormhole. It appears to have 20 overlapping blades that swing outwards to form a near-circular opening, and then close again afterwards. This is similar to the way a multi-bladed leaf shutter works in a camera. Other structures that are similar in appearance are iris diaphragms in cameras and the iris of the human eye. When closed, the iris forms a barrier less than 3 μm from the wormhole's event horizon, thereby preventing most forms of matter from properly reintegrating. Objects coming through the wormhole are destroyed instead, accompanied by a ringing thud against the iris. Radio signals are able to penetrate, however, allowing SG teams to request the opening of the iris using their GDOs.

The original iris was installed in the pilot episode, "Children of the Gods". It was first seen when Jack O'Neill and his team returned from Abydos. General George Hammond described it as Earth's "insurance against any more surprises," referring to Apophis' attack earlier in the episode. The original iris, made of titanium, was destroyed when a black hole's gravitational pull was transmitted through the Stargate. It was replaced with a new iris made of a trinium alloy.

The iris is controlled from either the control room palm scanner, or the manual control down in the Gate room on the left wall. The palm scanners were installed as a security measure after a Reetou, an invisible alien, came through the Stargate and overrode the computer system, which opened the iris. The manual control appears to have been added sometime after the episode"The Fifth Race" where it was stated that because the SGC's systems were down, they had lost control of the iris. Since the iris was installed by the SGC, Earth's Stargate is one of the 'Gates known with such protection,' and the only 'Gate known with a mechanical way of quickly alternating between preventing and allowing travel.'

Breaching the Iris
Although it provides the Earth Stargate with a greater degree of protection than any of the other Gates (with the exception of Atlantis), the iris is not infallible, and several enemies of SG-1 have devised ways around it.

The System Lord Sokar devised a means to breach the iris by firing a particle beam accelerator through the wormhole, the subatomic particles being small enough to reintegrate in the space between the iris and the event horizon (three micrometers), thus heating the iris up with the intent of eventually heating the naquadah and causing a thermonuclear explosion. The SGC used this technique themselves in a later episode to penetrate a "natural iris" that had formed over Edora(P5C-768)'s Stargate after it had been hit by an asteroid and buried while still active. Major Samantha Carter constructed her own particle accelerator and used it to save O'Neill.

The Tollan possessed technology that allowed them to walk through solid matter, which also allowed them to reintegrate their particles through the iris, appearing as though the iris was losing integrity.

Anubis fired an Ancient energy weapon into the Earth Stargate (the Antarctic gate) which caused it to heat up and cause the unstable naquadah to overload and explode. It is noted that, although it did not completely deflect the Ancient energy weapon's discharge, it dissipated the energy enough to significantly prolong the time until the naquadah would explode.

Also, when Major General Bauer (then commander of the SGC) detonated a naquadah-enhanced nuclear warhead on a planet that the SGC was connected to through a Stargate, radiation was transmitted through the Stargate. Closing the iris temporarily decreased the radiation in the gate room, however it would be a matter of time before the iris melted and the radiation transmitted freely.

Some aliens managed to breach the Iris and establish a foothold on the base by mimicking themselves as SG-6. They took over the base and started plans to take over the world. Thanks to their encounters with the Goa'uld Teal'c and Carter were immune to their efforts and Carter managed to escape the base and get help as well as kill the aliens that escaped the base. Most of the remaning aliens escaped through the Stargate and the rest self-destructed and in response an imperceptable tone was started to be played when the SG teams came through the Gate to cause any of the aliens to revert back to their true forms and be stopped.

Replicators once breached the iris by remotely hacking into the control system through an incoming wormhole and stormed the SGC, trying to get out of the base to infest Earth. But each individual block was broken down to its basic elements. The basic concept of the weapon was derived from Colonel O'Neill's replicator Blaster. Anubis (in Redemption) and Ba'al (in several episodes) are seen communicating with the SGC via the use of a hologram, which projects user's image into the gate room despite the iris being closed. The nanoscopic space between the shutters of the iris are enough for the holographic material to reintegrate.

In 2008 the Wraith took over the Midway Space Station and sent the IDC of the station causing the SGC to open the Iris. The Wraith sent some kind of stun bomb through knocking everyone out and infiltrating the base. Thankfully Teal'c and Ronon Dex killed them all before they could get to Earth's surface and the Midway Station was destroyed by a self-destruct.

The Asgard are capable of shutting the entire base down and opening the Iris. The Asgard used this once to go to Earth because they had no ship available. After this, the Asgard were probably given an IDC