3 ms·
When you modulate the light, the unambiguous range is not determined by the wavelength of light -- it's determined by the wavelength of the modulation. This is
by beambot 11y ago
When you modulate the light, the unambiguous range is not determined by the wavelength of light -- it's determined by the wavelength of the modulation. This is a pretty fundamental concept in radar (see FMCW radars). It's sort of like looking for the 0-to-1 transitions of the light instead of looking at the phase of the reflected light itself.
If you were to directly compare unmodulated light (the light's phase), then you'd actually be creating an interferometer. The distance measurement would be ambiguous beyond 1 wavelength (a very small value!). For example, you might measure a difference of 0.3\lambda, but you don't know how many full wavelengths away you are on top of that. So you might be 10um or 5000010um away. This is where modulation helps.
- nomel 11y agoThis isn't that complicated, and would be crazy expensive if it was. The modulation is just a way to identify your light house and filter out others. Ir remotes use 40kHz or so, this uses mhz, probably with some sort of digital scrambling and filter, or boring channels. You would only need to have a few channels.
- beambot 11y agoI know how Lighthouse works (I wrote the article). :) Like I said, " Like many IR systems, the LEDs and lasers are actually modulated (Alan said, "on the order of MHz"). This is useful for a few reasons: (1) to distinguish the desired light signals from other IR interferers such as the sun; and (2) to permit multiple transmitters with different modulation frequencies." I was responding to the GP: "You would be able to get some distance information via the modulation on the signal." I was telling GP, (1) how Lighthouse cannot use time of flight, and (2) how time of flight works for laser rangefinders, where modulation is actually used for ToF measurements.
- nomel 11y agoLol, my mistake. I see I interpreted both of your comments incorrectly.
- darkmighty 11y agoAh yes you could use time of flight even in the MHz range, where the wavelength of light is 300 meters if you have an accurate enough receiver, but that should be a problem. I think the precision should be on the order of ~ (A/D relative precision) * (wavelength), so for mm accuracy you need an A/D converted with more then 10^(-9) precision (that is, a >30 bit D/A), which I don't think is cheap (but doable perhaps?). The other problem is this conflicts greatly with multipath and other kinds of interference.