David Cassidy Yells at Fan, You Dumb, Stupid Idiot ... Turn that Light Off!

David Cassidy wore his medical alert bracelet on his sleeve Friday night, going bananas on a fan who ignored his considerable ailments just to get a little video.

The 66-year-old former Partridge Family star was performing at Hudson Valley Theater in Peekskill, NY when a fan shined a cellphone light smack into his cornea.

You gotta watch ... David somehow thinks people know he's had 5 eye surgeries and counting, and issues an ultimatum ... eject the dude or he won't continue.