Before the advent of electric lighting, theaters used heated calcium oxide (quicklime)

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to illuminate their stages.

Quicklime, when heated to a high temperature, emits a brilliant white light. This was achieved by placing quicklime in a special lamp known as a "limelight," which was often fueled by burning oxyhydrogen gas (a mixture of oxygen and hydrogen). The intense light produced by the heated quicklime was used to spotlight actors and performers, making it one of the most effective ways to illuminate the stage before the widespread use of electric lighting.

This is where the phrase 'in the limelight' comes from.