You've most likely seen them in a theatre or
auditorium, or a
movie that takes place in a
theatre or auditorium. At the back of the auditorium, someone is pivoting a large bright
spotlight to
follow an
actor or
performer across the
stage. Because of the follow spot's size and need for
mobility, the operator usually controls the size, shape, intensity, and
color of the
light independent of the
lighting booth.
Before electric bulbs, follow spots used a burning flame of oxygen and hydrogen applied to rotating cylinder of lime-- putting the actor onstage in the limelight.