Follow spotlights for manual tracking of actors or groups on stage. They have a double lens optic with zoom for the continuous adjustment of the light cone and the edge sharpness.
The iris, a circular adjustable aperture, allows the size of the light circle to be changed quickly.
Some tracking lights have integrated color changers, other models can be fitted with color filters or color changing magazines.
Inexpensive trackers use halogen lamps, the better trackers are equipped with HMI or MSR discharge lamps for higher light output and less hot housings.