Eye lens

The eye lens or image intensifier (PROSE: Shadowmind) was the lens or eye of the Dalek casing, equivalent to the humanoid iris. Typically contained within a ball, it was connected to the pivot at the front of the dome via the lens attachment, making up the eyestalk. (PROSE: The Dalek Generation) The Dalek lens, which had telescopic, infrared and X-ray capabilities, was protected from deadly radiation by the insulator discs which adorned the eyestalk. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

History and variance
In what the Time Lords identified as the Daleks' "early period", (PROSE: Dalek Combat Training Manual) the eye lens of the Dalek eyestalk appeared as a white circle or "iris" as seen on the Skaro City Daleks, (TV: The Daleks) the Dalek Earthforce, (TV: The Dalek Invasion of Earth) and the silver Daleks. (TV: The Chase) The Time Lords observed that the silver Daleks following the early Dalek War Machines, 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 text of the Dalek Survival Guide claimed that the eye lens "iris" of the Dalek Prime's Emperor Type 2 casing was "solid white", (PROSE: Dalek Survival Guide) but its own illustration and another source concurred that the lens actually had a black "pupil", (PROSE: Dalek Survival Guide, COMIC: Invasion of the Daleks) and one account showed that the lens was blue. (COMIC: Invasion of the Daleks)

The Dalek Prime's immense Emperor Type 3 casing had a solid white lens like his subordinate Black Dalek Leaders and silver Dalek drones, (TV: The Evil of the Daleks) though a variant of this casing existed with a black pupil. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Later grey Daleks sported a black pupil in the middle of the lens, (TV: Day of the Daleks) as did the modified silver Daleks in the Exxilon Gambit. (TV: Death to the Daleks)

A gold-and-black Dalek Supreme sent to 26th century Spiridon had the unusual feature of a lamp in its eye lens. It held the same communications function as a regular Dalek's luminosity dischargers on the sides of its head, lighting up when it spoke. (TV: Planet of the Daleks)

The lens of the refined Imperial Daleks had a gold pupil, reflecting their livery. The spherical dome of Davros' Emperor Dalek casing lacked an eyepiece; in its place was a silver hexagon with black outlines. (TV: Remembrance of the Daleks)

A Supreme Dalek operating in the 53rd century possessed an organic eye with veins within the eyepiece. (COMIC: Fire and Brimstone)

The bronze Daleks and contemporary models, who would rise to prominence in the Time War and beyond had eyepiece lenses which appeared as a flashing blue light, (TV: Dalek) similar to the artificial eye of the Dalek creator, Davros. (TV: The Stolen Earth) Though circular by default, the shape would mould into a pentagon when a Dalek employed their zoom to scrutinise their targets. (TV: Dalek, Evolution of the Daleks) Should a Dalek's vision be impaired or otherwise inactive, the light would fade, with only blackness in its place; (TV: The Parting of the Ways) when the Human-Dalek Sec exited his casing, the eye lens deactivated. (TV: Dalek in Manhattan)

The Daleks of the Restoration Empire, who followed the Imperial-Renegade Dalek Civil War (PROSE: The Restoration Empire) and preceded the Time War, (PROSE: Exit Strategy) were generally shaped like bronze Daleks and had blue eye lenses, including the Dalek Drones, the Scientist Daleks, the Dalek Time Commander and the Emperor of the Restoration. Dalek Executioners of the Restoration Empire were distinguished by their red lenses whilst the Dalek Prime Strategist, who uniquely used an aged Skaro City Dalek casing, had an older white lens. (PROSE: The Guide to the Dark Times)

The Black Dalek overseer who served during the Time War also had a eyepiece whose lens flashed red. (AUDIO: Planet of the Ogrons)

On the reconstructed Skaro, older silver Daleks had eyepieces refitted with lenses matching those of their bronze counterparts. A grey Renegade Dalek, however, retained its lens. (TV: The Magician's Apprentice)

The Daleks of the New Dalek Paradigm produced by the Progenitor had radically redesigned eyepieces with black rings on the stick, fins on the cone, and the lens being organic rather than robotic, much like a real organic eye. The viewpoint from this eyepiece was similar, except for an opaque orange perimeter with honeycomb circuitry and protruding organic veins. (TV: Victory of the Daleks) If the eyepiece were to be removed from the casing, the orange light would deactivate. (TV: The Wedding of River Song) The Visualiser Eye of Kaalann, which the Daleks used to monitor the universe, appeared as a giant Dalek eyepiece. It could also connect to the eyepieces of every nearby Dalek and could scan for intruders. (GAME: City of the Daleks)

Both the Proto-Daleks constructed by the British and the Germans on behalf of the bronze Daleks' Dalek Project had blue lenses despite the differing designs of their eyestalks. (COMIC: The Dalek Project)

The bronze, scrap casing of the Reconnaissance Dalek had a blue eye lens. (TV: Resolution) The robotic, black Defence Drones derived from this casing also had a blue eye lens as well as a grating section which glowed blue; when the Defence Drones were taken over by the Reconnaissance Dalek's cloned Dalek creatures, these lights changed to red. (TV: Revolution of the Daleks)

Individual cases
By another account, the Great Black Eye belonged to a Dalek which watched, waited, and planned somewhere beyond the Time Vortex and the multiverse, monitoring reality for what seemed like forever. (PROSE: Sometime Never...)

At a zone in the Dalek-Movellan War, the Twelfth Doctor realised that what appeared to be a bronze Dalek was actually sentient oil as it sported the distinctive star shape from Heather's eye in its eye lens. (TV: The Pilot)

Behind the scenes

 * Daleks seen in the animated comedy sketch series were depicted with human-style blue eyes, a quirk which would also be used in The Daft Dimension comics of Doctor Who Magazine.
 * Both The Monsters Are Coming! and Doctor Who at the Proms feature a New Paradigm Supreme Dalek with an eye lens glowing red rather than the usual gold.
 * The online game Dalek Hack offers a choice of six colours for the "Type VIII Dalek" casing: Red, Blue, Orange, White, Black and Yellow. The light of the eye lens is coloured to match the dome with the exception of the Black Dalek, which gives the general blue glow.