Insulator disc

Insulator 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)

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)