Which of the following is NOT a type of sensing method used by Photo Detectors?

The correct answer identifies that "Reflective beam" is not classified as a distinct sensing method used by photo detectors. In the context of photo detection, the methods typically referenced include transmitted beam, light diffuse, and retro-reflective.

Transmitted beam sensing involves a light source and a detector placed on opposite sides of an area; when an object interrupts the beam, it is detected. This approach is effective in applications like safety light curtains. Light diffuse sensing operates by emitting light that scatters off an object and returns to the sensor, allowing it to detect objects based on the intensity of the reflected light. Retro-reflective sensing uses a reflective surface to bounce light back to the source, which helps in detecting when an object is present between the sensor and the reflective surface.

By distinguishing reflective beam as not commonly recognized as a standard method, it sheds light on the nuances of how photo detectors function across various applications.