Image translator

An image translator is a TARDIS component that enables the TARDIS' view screen to display whatever is outside of the ship.

When the Doctor entered E-Space, the image translator showed pictures of Gallifrey. This was because Gallifrey and Alzarius, the planet the Doctor was really on, occupied the same points in space, only the image translator was set to positive coordinates. Adric later took an E-Space translator with negative coordinates and gave it to the Doctor for use in E-Space. (DW: Full Circle

After leaving the Gateway, the TARDIS screen, now with the negative image translator, showed blackness, indicating the Doctor had returned to N-Space. (DW: Warrior's Gate)