From The Stargate Omnipedia
Device installed into the S.G.C. control room. The palm scanner prevents unauthorized personnel from opening or closing the Iris, as well as any other species without palms, such as Re'tu, to use the device.
Some time after installation the palm scanner was upgraded to illuminate when activated.
Show and Tell - Following an incident with a Re'tu, Carter recommends the installation of her design for a device which will prevent mishandling of the iris controls.
Redemption, Part 2 - Carter uses the palm scanner to close the iris and is subsequently electrocuted by an electromagnetic discharge.
Fallout - When the Kelownans contact Earth's Stargate Hammond instructs his subordinates to open the iris with the palm scanner.