The incident occurred in the emergency room area of the hospital at 250 S. Grand Ave. about 12:45 a.m., a Los Angeles County Fire Department dispatch supervisor said.
When firefighters arrived, two hospital employees and a civilian complained of eye irritation, but all declined treatment, the dispatch supervisor said.
Firefighters evacuated the emergency room while hospital maintenance crews searched for the source of the explosion, he said.
The emergency room was expected to reopen sometime before 3 a.m., the dispatch supervisor said.
A lighting ballast serves to deliver the initial electrical impulse that sets off the discharge of the fluorescent gases that create light.