TARDIS scanner

The TARDIS scanner, also known as the video screen, (COMIC: The Zombies) was the main method for the occupants of the vessel to observe the outside environment. The appearance and specifications of the scanner system varied significantly in the course of the Doctor's travels.

Basic principles and specifications
Most of the time, the scanner system appeared to be a simple optical relay whereby a screen in the console room displayed images from a camera mounted in the outer shell of the TARDIS (indicated on one occasion to be in the light atop the police box). (TV: Enlightenment)

The scanner screen could also display other images and data, like photographs and recorded images or videos. (TV: The Edge of Destruction) On several occasions other powerful individuals were able to commandeer the display screen to display information of their own choosing (most frequently, to allow themselves to communicate with the Doctor). (TV: The Armageddon Factor, The Three Doctors, Resurrection of the Daleks, The Keeper of Traken, Victory of the Daleks)

On these occasions the scanner broadcast sound as well as vision, which was not always the case when it was used to view the external environment.

The scanner screen variously appeared as a small monitor attached to the ceiling of the console room, (TV: An Unearthly Child, Doctor Who) a circular screen in one of the roundels ,(TV: The Curse of Peladon, The Time Monster) as a larger screen concealed behind shutters, (TV: The Masque of Mandragora) embedded in the wall itself, (COMIC: Emperor of the Daleks!) and as a smaller monitor incorporated into the main control console. (TV: Rose, The Snowmen, Deep Breath, et al.)

During his first incarnation, the Doctor labelled the scanner with a black marker pen. (AUDIO: 1963)

During the Doctor's sojourn in E-Space, the scanner system suffered from a serious flaw in that, rather than being an optical relay as previously indicated, it displayed an image of the space at the co-ordinates occupied by the ship.

The absolute value of the co-ordinates was utilised, causing problems in an environment like E-Space with negative co-ordinates (the scanner showed images of Gallifrey while the ship was on Alzarius, the two planets having the same co-ordinates in different universes). The key component in the system was the image translator — inserting an image translator from the local environment rectified the fault. (TV: Full Circle)

When Jo Grant brought a Micro Servo Robot into the TARDIS, it connected itself to the computer and began transmitting adverts. One of these adverts had scanner display adverts. (AUDIO: Pop-Up)

River Song made the scanner work while the TARDIS was cloaked, which, according to the Eleventh Doctor, wasn't normally possible. (TV: The Impossible Astronaut)

Other uses
The Second Doctor used a time scanner to have a glimpse of the future destination of the TARDIS. (TV: The Moonbase)