The design and mechanical details of a rotating‐vane radiant power control for use with a vertical, paraboloidal mirror, arc image furnace are given. Optical measurements show this design to give an attenuation of total radiant power at the image point which is approximately a linear function of vane angle, in the vane angle range 0 to 25°. In addition, the radiant flux profile is left essentially unchanged. The loss in maximum total power incurred by introduction of a specific design of the device, and an example of results obtained by its use in crystal growth experiments are also included.