Scriabin’s Prometheus includes a stave marked luce: a part for an instrument of light, notated in ordinary notes, each standing for a color from his key-color system. The slow voice follows the harmony’s roots; a faster voice tracks the mystic chord’s transformations.
The premiere in 1911 went unlit for want of an instrument; a 1915 New York performance attempted the part with a purpose-built projector to mixed reviews. Modern performances realize it with stage lighting rigs and, increasingly, full visual systems.
The luce part remains the most concrete artifact of synesthetic ambition in the orchestral repertoire: color as a section of the orchestra, written, not improvised.