I had a discussion with AJ about sensors and then went to the Wikipedia page. The description includes a nice definition (a sensor is a device which receives and responds to a signal) but does not elegantly describe the types of sensors (just a laundry list of different devices).
I think there are only two classes of sensors:
1) Contact or Force sensors, which I call touch sensors.
2) Electromagnetic sensors, which I loosely call light sensors.
All sensors fall into these categories (see postscripts):
Touch sensors: Anything I hear from you is basically your voice-box pushing a bunch of air to hit my ear drum. This is just like when you sit in a jacuzzi and feel the water jets. You'd say the jets seem like they are pushing against you; that is exactly what sound is doing to our ears. Time-of-flight sonars work in a similar way. Chemical sensors and taste sensors (like your tongue) are sensing bits of the object scattered in the medium. These fragments actually push or scrape against receptors. Gravitational and magnetic sensors (like a compass), again, are due to a force being applied directly onto your sensor.
Electromagnetic sensors: These, like photodetectors or heat/temperature sensors, generate a signal because different types of radiation influence the sensor. In this sense (pun intended) light sensors are transducers: they just convert electromagnetic energy falling on them into an electric reading.
Postscript 1: So whats an image? An image is just a matrix of values that store the responses of one or more sensors. You can have an "image" of sonar sensors or of a group of chemical sensors. If the sensor is on-off in response, the image is binary. The powerful concepts of images and video have nothing particularly to do with light.
If you keep running with this idea, then you'll realize that the retina is not the only thing generating an image in your head. Your tongue is also an array of sensors and it produces an "image" of the food you are eating. Actually its creates a video, because it measures images of taste responses over time. You can't display this video, because your eyes wouldn't make sense of it. But what your optic nerve cannot understand your chorda tympani (your taste nerve) absolutely can.
Postscript 2: Where do gyroscopes fit in? Gyros don't measure anything - they stay the same despite what is happening around them. If you are philosophical, you might say they measure inertia. I put them in the category of touch sensors as a sort of "null set" of that class because they resist all forces and contact.
Postscript 3: Yes, what are magnets really? There is a current loop definition which says that magnets are electric currents that circle around in an object, creating a magnetic field. But in the end, even if the source of the force is electromagnetic, your sensor detects the field by being pulled by it. I'd call that a touch sensor.
I think there are only two classes of sensors:
1) Contact or Force sensors, which I call touch sensors.
2) Electromagnetic sensors, which I loosely call light sensors.
All sensors fall into these categories (see postscripts):
Touch sensors: Anything I hear from you is basically your voice-box pushing a bunch of air to hit my ear drum. This is just like when you sit in a jacuzzi and feel the water jets. You'd say the jets seem like they are pushing against you; that is exactly what sound is doing to our ears. Time-of-flight sonars work in a similar way. Chemical sensors and taste sensors (like your tongue) are sensing bits of the object scattered in the medium. These fragments actually push or scrape against receptors. Gravitational and magnetic sensors (like a compass), again, are due to a force being applied directly onto your sensor.
Electromagnetic sensors: These, like photodetectors or heat/temperature sensors, generate a signal because different types of radiation influence the sensor. In this sense (pun intended) light sensors are transducers: they just convert electromagnetic energy falling on them into an electric reading.
Postscript 1: So whats an image? An image is just a matrix of values that store the responses of one or more sensors. You can have an "image" of sonar sensors or of a group of chemical sensors. If the sensor is on-off in response, the image is binary. The powerful concepts of images and video have nothing particularly to do with light.
If you keep running with this idea, then you'll realize that the retina is not the only thing generating an image in your head. Your tongue is also an array of sensors and it produces an "image" of the food you are eating. Actually its creates a video, because it measures images of taste responses over time. You can't display this video, because your eyes wouldn't make sense of it. But what your optic nerve cannot understand your chorda tympani (your taste nerve) absolutely can.
Postscript 2: Where do gyroscopes fit in? Gyros don't measure anything - they stay the same despite what is happening around them. If you are philosophical, you might say they measure inertia. I put them in the category of touch sensors as a sort of "null set" of that class because they resist all forces and contact.
Postscript 3: Yes, what are magnets really? There is a current loop definition which says that magnets are electric currents that circle around in an object, creating a magnetic field. But in the end, even if the source of the force is electromagnetic, your sensor detects the field by being pulled by it. I'd call that a touch sensor.
