An infrared sensor device which serves as an performance control device on some Roland synths. (The term "D-Beam" is a Roland trademark.) The D-Beam is usually mounted in the panel facing upwards, and will sense the performer's hand (or other body part) at a height of up to 15" (~40 cm) or so above the device. It provides a control signal proportional to that height, which can be routed to various parameters in the synth. More recent versions actually have two sensors, a left and a right; by careful manipulation of the hand and fingers, the performer can activate one or the other, or make the two output different values.
The D-Beam can be a useful (and dramatic-appearing) alternative controller. However, it can cause problems when a synth containing a D-Beam is mounted in a multi-tier keyboard stand; if the synth is not in the top tier, the D-Beam needs about 18" clearance from the keyboard above it to avoid false triggering.