Insulator disc

Insulator discs or optic discs 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)

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)