A security screen was a type of force field that surrounded the Tantalus Penal Colony. It prevented communications between the colony, which was situated on planet Tantalus V, and nearby spacecraft. The security screen could be deactivated with a control that was in the office of the colony's director. Furthermore, the words "security screen" were visible on a wall of the colony's main power chamber, from where the force field could also be turned off.
While Captain James T. Kirk was visiting Tantalus colony in 2266, the security screen blocked contact between a communicator he had with him and the USS Enterprise, which was orbiting the planet. After Doctor Tristan Adams (the colony's director at that time) informed Kirk that the security screen was in effect, Adams temporarily turned off the force field, permitting Kirk to contact the Enterprise.
Later, shortly after Spock beamed into the colony, he deactivated the security screen from the main power chamber. He then contacted the Enterprise, with a communicator of his own, to report that the security screen had been eliminated. (TOS: "Dagger of the Mind")