Image translator

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

When the Fourth 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. (TV: 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. (TV: Warriors' Gate)