A palm scanner was installed in the control room after the incident in which Reetou rebels infiltrated the base. The Reetou exist outside of our phase, but to a limited extent they are able to manipulate objects within our phase. When an invisible Reetou was able to use the computer keyboard to override the iris commands and allow Charlie to enter through the stargate, Carter installed a palm scanner to control the iris and wormhole activation and to prevent future security breaches, since Reetou don't have palms.
Cross Reference: Iris, Reetou
Episode Reference: Show and Tell