The Honeywell SDP8406 is an NPN silicon phototransistor designed for side-looking applications. Encased in a clear plastic package, the phototransistor is positioned to receive radiation from the side, offering a 50-degree acceptance angle. It's mechanically and spectrally matched to Honeywell’s SEP8506 and SEP8706 infrared emitting diodes, making it ideal for optical sensing and detection systems. The SDP8406 is commonly used in proximity sensors, optical switches, and light-activated circuits across various industrial and consumer applications.