I am currently studying the textbook Handbook of Modern Sensors - Physics, Designs, and Applications, fifth edition, by Jacob Fraden.
In this textbook, the author states the following:
All sensors may be of two kinds: passive and active. A passive sensor does not need any additional energy source. It generates an electric signal in response to an external stimulus. That is, the input stimulus energy is converted by the sensor into the output signal. The examples are a thermocouple, a photodiode, and a piezoelectric sensor. Many sensors are direct sensors as we defined them earlier.
In researching this further, I came across this Quora question, in which user Giulio Moro claims the following:
An active sensor would be like your garage door sensors. One side has a light and the other a sensor. A beam fires across the area when the doors closing to ensure it's clear. If the light beam is broken, the door stops, and then opens wide.
A passive sensor would be like a metal detector. It creates a field, and when anything crosses it, it makes a sound based on the type of obstruction. A low pitch for say a plastic bottle or piece of wood, and a high pitch for a metal object.
There are two aspects of this definition of passive sensor that slightly confused with:
Firstly, what is meant by an "additional" energy source? And analogously, if there is an "additional" energy source, then what would the base/default energy source?
I think that this second confusion likely stems from the first, and clarification of the definitions of an "additional" and base/default energy source will probably clear this up, but I want to ask it anyway just in case. From a physics standpoint, I'm not totally convinced of this definition of passive sensor. In particular, I'm struggling to come to terms with this idea that a sensor can function without an energy source (although, again, the text does say additional energy source). The example of the metal detector also seems unconvincing, since my understanding is that metal detectors are certainly connected to some energy source (say, at airports or such)?
I would greatly appreciate it if people would please take the time to clarify this.
EDIT: After reviewing the answers (thank you all), one thing is obvious: There is a terribly large amount of contradiction with regards to what sensors qualify as active or passive.