I think I get the concept of a machine sending pulses of light, the light bouncing back to a sensor and measuring that time delay (for speedometers and such). But how does it work with non reflective surfaces? How can the light speed be measured with tiny distances and short travel times? Or do I have it wrong and they’re really just cameras that only measure incoming light?
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There are several kinds of those sensors,
– a small antenna emits a radio signal and calculates the distance to the object by measuring the time it takes for the reflection to come back. (radar, among other frequencies.) If the time suddenly changes, then something got to have moved around, huh?
– a sensor looks for heat. Basically, it’s receptive to IR radiation and measures the intensity of IR around it. If the intensity changes, then there got to be something that is moving around.
– a camera looks at a room and a computer is looking at the picture and compares the past few (or last several hundred, in case the camera has a high frame rate) images to see what changed since the last image. If something large enough changes in the right part of the image, something is obviously moving around.
Complicated sensors use a combination of two or three of these. It’s very common with radar and IR in the same sensor, for example. It keeps down the number of false alarms. Or because it’s more likely to spot something if the sensor looks for two different things at the same time, if you have a facility where false alarms are preferred over not sounding an alarm when appropriate.
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