You may be looking for the Bane's image translator.

An image translator was a communications technology that enabled the display of images from the exterior of a ship. TARDISes utilised these components and were based on coordinates.

Even in the darkest of areas, the image translator was able to enhance the available light enough to allow images to be seen. (PROSE: White Darkness)

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 image translator from the Terradon Starliner 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)

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