Insulator disc

Insulator discs, optic discs or rings were a feature of the Dalek eyestalk whose purpose was to protect the eye lens from deadly radiation. They varied in colour, size and number according to the different types of casing. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe, PROSE: Dalek Combat Training Manual, Dalek Survival Guide)

According to the Dalek Survival Guide, the rings served as physical protection from attack as well as absorbing and assimilating audio data. In effect the rings were the means of hearing for the Daleks, with sounds being converted using holographic quadraphonic sampling so that directional information as well as estimates of distance and audio-pattern recognition, such as voice print identification, could be appended to the data before it was passed to the Dalek creature. (PROSE: Dalek Survival Guide)

History and variance
For a substantial portion of Dalek history, the insulator discs were coloured a pale blue, (PROSE: Dalek Survival Guide) including those of the silver Skaro City Daleks in the Thal-Dalek battle, (TV: The Daleks) the silver Dalek Earthforce including the saucer commander and Supreme Controller, (TV: The Dalek Invasion of Earth) the silver Daleks, their contemporary black Supreme Dalek, (TV: The Chase) and Emperor's Personal Guard, (TV: The Evil of the Daleks) the early grey Daleks from Davros's prototypes (TV: Genesis of the Daleks) to the Dalek-Movellan War, (TV: Destiny of the Daleks) the Gold Dalek, (TV: Day of the Daleks) and the Silver Daleks participating in the Exxilon Gambit. (TV: Death to the Daleks) The gold-and-black Dalek Supreme notably had black insulator discs, (TV: Planet of the Daleks) though the Dalek Survival Guide believed that they were blue. (PROSE: Dalek Survival Guide)

The eyestalks of the Dalek War Machines in the Dalek City were adorned with nine blue insulator discs which increased in size as they met in the middle. (TV: The Daleks) The Time Lords observed that the later silver Daleks, as well as being fitted with solar slats, boasted improvements to the visual systems in the form of a recalibrated eyepiece lens with re-engineered optic discs. (PROSE: Dalek Combat Training Manual)

The eyestalk of the Dalek Prime's Emperor Type 2 casing was supported by red insulator discs. (COMIC: Invasion of the Daleks)

Following the Dalek-Movellan War, the grey Daleks and their Supreme Dalek had white insulator discs, (TV: Resurrection of the Dalek) as did the Necros Daleks created by Davros. (TV: Revelation of the Daleks) Davros's refined Imperial Daleks had gold insulator discs in line with their livery, whilst the now Renegade Daleks and their Supreme Dalek had white insulator discs. (TV: Remembrance of the Daleks) In an dissenting account, the Dalek Survival Guide claimed that both the Necros Daleks and refined Imperial Daleks had blue insulator discs. (PROSE: Dalek Survival Guide)

Bronze Daleks and their contemporaries, from Imperial Guards to Black Daleks to Supreme Daleks, had white insulator discs. (TV: Dalek, The Parting of the Ways, Army of Ghosts, The Stolen Earth)

The original casings of the New Dalek Paradigm had black insulator discs. (TV: Victory of the Daleks)

The bulky, camera-like eyestalks of the Proto-Dalek created by the British and the Germans for the Dalek Project during the First World War had no visible insulator discs. (COMIC: The Dalek Project)

The eyepiece of the reconstructed reconnaissance scout Dalek had no visible insulator discs, (TV: Resolution) nor did the Defence Drones which were derived from it. (TV: Revolution of the Daleks)

Behind the scenes

 * In Dr. Who and the Daleks, the Blue Dalek drones and the Red Dalek had blue insulator discs like the Daleks in the contemporary television series, whilst the Black Dalek had gold insulator discs. In Daleks' Invasion Earth 2150 A.D., all Daleks had blue insulator discs.