Eye lens

The eye lens was the lens or eye of the Dalek casing. 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)

Variants
Before the Last Great Time War, the eye lens of the Dalek eyestalk appeared as a white circle, (TV: The Daleks) with later Daleks sporting a black "pupil" in the middle. (TV: Day of the Daleks) 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) 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)

Uniquely, the Black Dalek overseer 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 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)