An electro-mechanical shutter has been developed for possible use in a new photometric technique. The shutter is of the open-shut type and is used to produce a square wave modulation of a pencil of light with a repetition frequency of 33 c/s. `Rise times' as short as 40 μsec have been achieved. The ratio of light to dark intervals can be continuously varied while the shutter is in operation, over a range corresponding to average transmissions of between 95% and 5%.