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)

Variants
Before the Last Great Time War, the eye lens of the Dalek eyestalk appeared as a white circle as seen on the Dalek War Machines, (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)

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)

Daleks of the Time War and beyond, most prominently, the bronze Daleks, had eyepieces, the lens of 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 be absent, with only black in its place. (TV: The Parting of the Ways)

The Dalek Executioners of the Restoration Empire were distinguished by their red lenses (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)

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

 * 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.