Rotational modulation collimators (or RMCs) are a specialization of the modulation collimator, an imaging device invented by Minoru Oda [fr; de].
Devices of this type create images of high energy X-rays (or other radiations that cast shadows).
Since high energy X-rays are not easily focused, such optics have found applications in various instruments.
RMCs selectively block and unblock X-rays in a way which depends on their incoming direction, converting image information into time variations.
The Small Astronomy Satellite 3, launched in 1975, was one orbiting experiment that used RMCs.