Luminosity discharger

The luminosity discharger, head lamp, or radiation valve was a device, typically appearing in twos, atop the dome of a Dalek's casing. They would light up whenever a Dalek spoke. (PROSE: Prisoner of the Daleks, Engines of War)

Ace used a baseball bat enhanced by the Hand of Omega to destroy the right luminosity discharger of an Imperial Dalek. (TV: Remembrance of the Daleks)

The Special Weapons Dalek notably had neither luminosity dischargers nor an eyepiece. (TV: Remembrance of the Daleks) Instead the glass just below the dome, which also functioned as its vision, would light up when it spoke. (TV: The Witch's Familiar)

The red Supreme Dalek of the New Dalek Empire was distinguished by having three luminiosity dischargers. (TV: The Stolen Earth/Journey's End) And so did its lookalike from the Dalek City on the rebuilt Skaro. (TV: The Magician's Apprentice/The Witch's Familiar)

"Ironside" Daleks deployed by the British Army against the Luftwaffe had covers placed over their luminosity dischargers. These covers were removed by the time they deserted and were subsequently destroyed by the New Dalek Paradigm. (TV: Victory of the Daleks)

Size
Early Daleks had small, bulb-shaped luminosity dischargers. (TV: The Daleks et al.) The Dalek Supreme of the Supreme Council was distinguished by its larger luminosity dischargers. (TV: Planet of the Daleks)

Daleks of the Last Great Time War and beyond, including the New Dalek Paradigm, had large luminosity dischargers of a size which had previously only been sported by the Supreme Council. (TV: Dalek, Victory of the Daleks)

Uniquely, the Black Dalek overseer had exceptionally short luminosity dischargers in comparison to even pre-Time War Daleks. (AUDIO: Planet of the Ogrons)

Colour
As well as size, the Dalek Supreme of the Supreme Council was distinguished by the purple colour of its luminosity dischargers. (TV: Planet of the Daleks)

The Supreme Dalek of the Renegade Daleks during the Imperial-Renegade Dalek Civil War was distinguished by its luminosity dischargers, which were coloured orange. Imperial Daleks seen in the Shoreditch Incident had golden luminosity dischargers. (TV: Remembrance of the Daleks)

Behind the scenes
The Daleks seen in the theatrical film Dr. Who and the Daleks (held to be an invalid source by this Wiki) were the first to have larger luminosity dischargers. Regardless of rank, from the Black Dalek to the Red Dalek and the Blue Daleks, all luminosity dischargers were coloured red.

In Daleks' Invasion Earth 2150 A.D., the size of the luminosity dischargers remained the same, but with different colours; the Gold Dalek had matching gold luminosity dischargers, both the Black Dalek and the Red Dalek had luminosity dischargers coloured red, while the Silver Daleks had blue luminosity dischargers.