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In robotics, LIDAR and RADAR are both used for obstacle detection. The data they produce are very similar, but they do have different qualities (LIDAR is more precise but less robust). There are many websites that tell me that the difference between RADAR and LIDAR is that one uses radio waves while the other uses light.

But both of these are electromagnetic waves, so why aren't they the same technology? As far as I'm able to figure out so far, RADAR uses antennae and works on waves no smaller than a millimeter, while LIDAR uses photodetectors of some kind and works in infrared (smaller than a micrometer).

Why can't LIDAR be implemented with traditional antennae? Is the wavelength too short to be detected by current electronics? Or is it due to interference with the sun at higher frequencies? Or maybe higher frequencies don't bounce off objects in quite the same way?

(Btw, my understanding of a "traditional antenna" is that the EM waves induce current in a wire, and the receiver somehow filters out all but a particular wavelength.)

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    $\begingroup$ Are you at all familiar with electronics? With atomic physics? There is a part of me which sees this as too obvious to answer, and another part that wants a semester to cover the setting-up exercises. What kind of answer will help you depends entirely on your level of preparation. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jun 3, 2015 at 21:12
  • $\begingroup$ I have an understanding of basic concepts learned in high school, but not their detailed workings. For example, if you ask me how an antenna works, I'd say that the EM waves induce current in a wire and you listen only for a particular wavelength—somehow. OTOH, I don't know how photodetectors work. I can guess that above a certain frequency the antenna technique breaks down but I don't know why. It could simply be the wavelength is too short, or it could be due to the sun emitting too much of the higher frequencies, or maybe higher frequencies don't bounce off objects in quite the same way... $\endgroup$
    – Neil Traft
    Commented Jun 3, 2015 at 22:59
  • $\begingroup$ With your comment the question is a lot better. Could you edit some of that into the question body. Parts of the question are also already answered around the site, but digging them up could take a while. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jun 3, 2015 at 23:04

2 Answers 2

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Is the wavelength too short to be detected by current electronics?

Yes. visible light has a frequency of around 10$^{15}$ Hz, even microwave radar is only a GHz (10$^9$Hz) so you need electronics a million times faster and antennae a million times smaller.

There are fundamental reasons that get a bit more complicated. in simple terms with real metals, you could never make the electrons move around fast enough to get a visible light emitted. See plasma Frequency

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It's easy to get an antenna to emit visible light. Reduce the cross section of the antenna rod and you will see how it starts to glow. But the light intensity does not follow the frequency of the generator, because the energy losses are very large and also the heat dissipation is very large.

In an ideal antenna the electrons accelerations lead to synchron photon emission from this electrons. The modulation of this emission is what your receiver filters out. The point, is that the emitted photons are not of the wavelength of the radio frequency. That is the reason why you need only a tiny receiver antenna rod. You filter out photons in the frequency range of the generators frequency.

The same has be done for visible light too. Remember the experiments with LED light an the ceiling and a receiver on a computer for information transfer. This works as a scanner too. Modulated visible light is send out and the receivers electronic filters out the noise from daily light.

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    $\begingroup$ Yes that's true, with enough power and a poorly matched impedance you could make the antennae emit visible light - fairly briefly $\endgroup$
    – Martin Beckett
    Commented Jun 4, 2015 at 17:15

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