Who is the Pink Floyd song "Shine On You Crazy Diamond" written about?

The Pink Floyd song "Shine On You Crazy Diamond" is written about Syd Barrett. Syd Barrett was one of the founding members of Pink Floyd but due to his deteriorating mental health and drug use, he became almost unrecognizable to his former bandmates. The song serves as a tribute to Barrett, acknowledging his contributions to the band while addressing his struggles and farewell. The lyrics reflect on the profound impact he had on the band's early years and the sadness surrounding his decline.