This dimensionally transcendental circuit controls the exterior shell's Cloaking Device and Camouflage Unit. The Camouflage Unit allows the shape, colour, mass and texture of the Shell to be altered to blend in with its surroundings. This feature is designed to help prevent changes to history. Chameleon Circuits were developed soon after the Gallifreyans began travelling in time.
This was done because lost timeships were creating legends and mythologies of monsters and gods among the lesser species. The Chameleon Microcircuits can be accessed by removing a roundel in the access tunnel near Control Room Number 2.
Even a detailed scan of the atoms of the exo-shell will reveal its molecular structure to be consistent with its disguise. The Perception Filter further augments the Chameleon Circuit's disguise. The choice of disguises is nearly infinite (trees, police boxes, chairs, an entire lake, an Egyptian Shabti, or even ionized gas) An Ionic Column disguise comes as one of the factory presets for a TARDIS. If a TARDIS is disguised as an object that is too small for a standard sized door then (once the key is inserted) a door will appear hanging in mid-air outside the capsule.
Usually the operator chooses the disguise. The Type 40 has an early version of the Chameleon Circuit, and to create a new disguise the operator must use a retractable keypad that extends out of console when a switch under the console is thrown. This keypad allows new disguises to be created manually with machine code. Pre-used disguises are automatically accessible. Type 40 TARDISes make a whooshing noise when their chameleon circuit alters the exo-shell.
All Type 58 and later timeships (as well as TARDISes with the Mark IV upgrade) are designed to automatically scan the target area right before materialization and automatically select a disguise that will hide it. Since every piece of matter in the universe casts an image in the Vortex, a TARDIS can scan this image. During the first nano-second of materialization the TARDIS creates a 12 dimensional data map of everything within a thousand mile radius and determines which outer shell would best blend in with its environment. It then imprints this image on its exo-shell (or its infrastructure), allowing it to precisely duplicate the appearance of the nearby object. It is possible to use the manual override codes to select a different disguise. If a TARDIS is abandoned by its operator it will deactivate most of its systems, and alter its appearance as necessary to avoid attracting attention. One operator of a Type 40 augmented his Chameleon Circuit so that it would operate automatically, but the TARDIS's choice of disguise tended to be inappropriate.
This automatic feature of a Mark IV Chameleon Circuit can be overridden by the operator if desired. And the change implemented after materialization. If a TARDIS has materialized around an object, the Chameleon Circuit can be set to automatically imitate that object. It should be noted that this will only be a loose copy of the object, and that (without complex and careful programming) the disguise will lack any true functionality. It is possible to Time Map the internal dimensions onto the external dimensions. This will increase the Exo-Plasmic Shell to the size of a small moon or planet. However it will limit the capabilities of some of the Defence Mechanisms.
Every time the Chameleon Circuit creates a new exo-shell to disguise the TARDIS a duplicate of the shell is stored in the Shell Room. It should be noted that even if the Chameleon Circuit suffers a complete system breakdown the Shell Room will still continue to collect new unused shells every time the TARDIS materializes.
The shell of a fully operational TARDIS has a very slight vibration that can be detected with the right equipment. However if the Chameleon circuit is faulty, the outer Plasmic Shell might produce an audible hum and an easily detectable vibration. Faulty chameleon circuits have also been known to leak chronon radiation out of any "windows" that are part of the chameleon disguise. This takes the form of white light to a lesser species. Removal of the Visual Stabilization Circuit's fusion plate leaves a TARDIS invisible. Not only does this cause an enormous drain on the timeship's power reserves, but it makes finding the keyhole and the Dimensional Gateway difficult.
It is possible to disable the Exo-Shell's invulnerability so that it is only as strong as the object it is currently disguised as. This is not recommended, since the endo-shell traditionally has only modest structural integrity. Under such circumstances a collision or attack with enough kinetic energy could penetrate the Control Room.
Older TARDISes (such as the Type 40) were prone to system breakdowns of their Chameleon Circuits, which would freeze the exterior appearance. They required regular Chameleon Conversions to combat this tendency. This involves hotbinding the fragment-links and superseding the binary coding with the right block-transfer equations. Care should be taken when performing a Chameleon Conversion. Errors in data fed into a TARDIS during the conversion will cause a dimensional spiral leading to the shrinkage of the exo-shell. Sonic projectors can be used to create a temporary zone of stasis to halt the spiral, but the only way to reverse the spiral is to correct the errors in the original data. It is also possible for a failed conversion to cause the TARDIS to try to mimic a living organism with the exoshell. Under such circumstances that Artron Mainframe would become completely devoted to maintaining this complex dynamic disguise and operator would have to find a way to remind the ship that it was something more then just a simple organic life-form. For this reason, it is recommended the operator consult the TARDIS Manual before attempting any repairs. These problems were fixed with the introduction of the Type 65. Despite all the advances of the Type 65, some types of TARDIS produced after the Type 40 (specifically the models popular at the time of the assassination of President Eldhind) had much more limited Chameleon Circuits than the Type 40.
If left in the same shape for several years a TARDIS could suffer from chameleonic fluctuation, which causes the exterior to slowly alter its exterior shape and colour over time. Each alteration will be minor but the cumulative effect could be quite noticeable. A breakdown between the Basal Programming and the Autonomous Elements will cause a TARDIS to go into Chameleonic Spasm, leading to wild fluctuations in the appearance and size of the exterior shell.
The following color code is used:
- Black: For information from the TV Series, including Dimensions in Time, and 1996 TV Movie.
- Blue: For information from the Novels and Audios including Target, Virgin, BCC, and Big Finish.
- Green: For information from 'licensed' reference sources such as the Technical Manual, Doctor Who Magazine, and the Role Playing Games.
- Red: For information from unofficial sources -The Faction Paradox series, behind the scenes interviews, author's speculation, and popular fan belief.
The TARDIS Technical Index is copyright Will B Swift.