What type of detector uses a reflector on a wall opposite to the detection unit?

The choice of a reflective beam detector is correct because this type of detector operates by using a light source that beams light toward a reflector positioned on a wall or surface opposite the detection unit. When smoke or other obstructions enter the beam's path, it scatters or interrupts the light, which then causes the alarm system to trigger. This design is effective for covering large areas, as it reduces the number of detectors necessary by utilizing a single emitter and a distant reflector.

In contrast, the other types of detectors serve different functions. For instance, an ionization beam detector relies on changes in ionized air caused by smoke particles to trigger an alarm. A projected beam detector is generally similar to a reflective beam but specifically focuses on a direct light path between the emitter and receiver, rather than using a reflector. A photoelectric beam detector uses light to detect smoke presence but is often not structured around a reflective mechanism, focusing instead on a direct light and receiving setup rather than utilizing a reflection point to gauge smoke presence.

These distinctions highlight why the reflective beam detector is the ideal choice in this context, as it specifically incorporates a reflector across from the detection unit to function optimally.