You would be able to get some distance information via the modulation on the signal. If your photodiode response was able to pick up phase changes, you can estimate distance. The light arriving at closer diodes will have a different phase than the further diodes. The article mentiones "MHz" intensity modulation on the does which gives you several meter wavelength.
Time of flight as you describe could work at the transmitter, but could not work at the Lighthouse receiving photodiode. To measure time of flight, you need to know (very precisely) when the light was initially transmitted. The synchronization flash is insufficient for this, as an error of just 1ns results in a 1ft error -- modulation or not. Even with modulation, the receiving photodiode doesn't have another signal with which to compare for ToF measurements.
Normal laser rangefinders are transmitting the light (modulated), and looking for the reflected return (modulated) at the same location as the transmitter. They use a PLL to determine the phase difference between the TX and RX (modulated) signals -- since the transmitter has both versions readily at hand. The phase difference corresponds to a time difference => distance. Note that the "reflector" (where the photodiode sits in Lighthouse) is not part of the equation.
In other words, laser rangefinder ToF measurements require coherent demodulation at the location of transmission.
I wasn't referring to ToF measurement, I was referring to two diodes measuring the difference in phase of a subcarrier intensity modulated signal. Essentially the subcarrier is a intensity modulation of the light. At XX MHz the optical output power will go from 100% to 0% and then back. If two diodes receive the light at the same distance then difference in phase will be zero (i.e. correlating the two signals will have a 0 lag). If the two diodes are separated by a distance, then one diode will receive a greater optical signal and one will receive a smaller optical signal. Correlating the two received signals (or measuring the phase difference) will directly correspond to the difference in distance between the two diodes - this is because the light travels at a fixed speed and the intensity of the signal directly corresponds to the physical distance. Obviously there isn't a unique solution since the diodes could be spaced at multiples of the wavelength, but for a fixed area this isn't a problem.
This only requires knowledge of the subcarrier modulation frequency, as any diode can be picked as the reference and the others matched to it. This isn't an ideal way of doing distance measurement because noise will greatly effect the estimate, but it's certainly do-able.
I know how Lighthouse works -- I wrote the article. :) 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.
> MHz intensity modulation gives you several meter wavelength
What do you mean by this? The light itself is not at MHz frequency, just the modulation of its intensity. It is infrared, which is ~700nm => 100s of THz.
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.
This 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.
I 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.
Ah 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.
I am referring to subcarrier intensity modulation. If you were to turn the optical power from 100% to 0% and then back to 100% in a sinusoid fashion, you would be modulating the intensity of the output. The diode observes the extremely high optical frequency as DC signal, onto which you are modulating another signal at a lower frequency. e.g. I(t) = cos(f_opticalt)cos(f_modulation*t), where I(t) is intensity vs time, f_optical is extremely high (THz) and f_modulation is the lower modulation frequency (MHz)
