Passive sensors include which of the following?

• A. Acoustic sensors
• B. Active radar
• C. Laser range finders
• D. Infrared cameras

Answer: (A)

Passive sensing means you don’t emit energy yourself; you only collect energy that comes from the scene. Acoustic sensors fit this because a microphone simply picks up ambient sound and converts it to an electrical signal, with no energy emitted by the sensor. Active radar and laser range finders, in contrast, send out signals (radio waves or laser pulses) and then listen for the returning echoes or reflections, so they are not passive. Infrared cameras detect heat radiated by objects and, while they don’t emit infrared themselves, are typically considered passive imagers too; however, the most straightforward example of a passive sensor among these options is the acoustic sensor.