- An instrument or device which detects and/or measures radiant energy such as infrared, visible, or ultraviolet light. Examples include Crookes radiometers and infrared radiometers.
- A device consisting of an evacuated bulb enclosing four vanes that are suspended, or otherwise allowed to move with the least possible friction. The vanes are arranged like a turnstile and are black on one side, and white or polished on the other. The incident radiant energy, such as that from sunlight, is converted into motion, and the greater the intensity, the faster the motion. Also called Crookes radiometer.