Luminosity discharger

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The luminosity discharger 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)

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 red Supreme Dalek of the New Dalek Empire was distinguished by having three luminiosity dischargers. (TV: The Stolen Earth/Journey's End)

Size

Early Daleks had small, bulb-shaped luminosity dischargers. (TV: The Daleks et al.) The Supreme Dalek 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)

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)

Imperial Daleks seen in the Shoreditch Incident had golden luminosity dischargers. (TV: Remembrance of the Daleks)