Which light source is described as a compact source iodide used in followspots?

The selection of "CSI" as the correct answer relates to its specific designation as a compact source iodide light source commonly utilized in followspots. Followspots are specialized lighting fixtures used in theatre and live performances to create focused beams of light that highlight performers or particular areas of the stage.

Compact source iodide (CSI) lamps are known for their intense brightness and energy efficiency, making them ideal for followspot applications where a strong, concentrated light beam is essential for visibility and dramatic effect. These lamps are compact in size, which allows for ease of handling and positioning in followspot setups. Their color temperature and brightness can also be controlled effectively, providing versatility in performance lighting.

Other options, while they pertain to lighting or stage operations, do not specifically define a compact source iodide used in followspots. For example, a dimmer would adjust light levels, a cue light serves to signal performers or crew, and a douser is a device used to block or reduce light. These functionalities, while important in stage lighting, do not relate directly to the characteristics of a CSI light source in followspot usage.