It is assumed the Time Of Flight (TOF) approach uses short pulses of light and short sampling windows. While the Phase approach modulates the light such that the phase of the received light is compared to the transmitted light. These approaches appear very different. Yet the documentation of these light based distance measurement devices rarely, if at all, discriminate between the two.

So are there two widely used approaches? Or is one rarely used such that everyone assumed all devices are of only one type.

  • \$\begingroup\$ "Yet light based distance measurement devices rarely, if at all, discriminate between the two." I think you mean their documentation rarely discriminates. \$\endgroup\$
    – Transistor
    Jul 17, 2016 at 20:21
  • \$\begingroup\$ Yes, you are correct. I will edit the question. \$\endgroup\$
    – st2000
    Jul 17, 2016 at 21:20

1 Answer 1


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Ref: Optical methods for distance and displacement measurements by Garry Berkovic and Ehud Shafir.

The main features that are evident from the graph are that the resolution of most techniques decreases as working distance increases; the exception is laser interferometry, which offers a wide distance dynamic range for a reasonably constant accuracy. This technique, as mentioned in the text, is more complex and costly than the other simpler techniques.

In section 2.3 we read (emphasis mine):

At distances shorter than tens of meters accurate time-of-flight measurements need to take into account the temporal pulse shape in order to correctly measure the time delay between the peaks of the input and returned pulses. Eventually the input and returned pulses will overlap in time, and photon-counting techniques [32] or very fast detection with autocorrelation algorithms [33] should be used to evaluate the time delay.

An alternative approach to the pulse illumination is to use amplitude modulated continuous light [34,35] (see Fig. 10(b)). In this case a phase shift in the modulation signal is measured between the launched and the returned light, and the time-of-flight is determined by dividing the phase shift by the modulation frequency. The true phase shift is the measured residual phase shift plus an integral number of full cycles (2𝜋 phase shifts). This ambiguity can be eliminated and the true phase shift found by measuring at an additional (nonharmonic) modulation frequency. This approach is most practical for measuring distances in the intermediate region from a few meters up to 50 m (times of flight a few times larger than typical short pulses), but more difficult for distances shorter than 1 m, as the required modulation rate approaches the gigahertz range.

enter image description here

The article is worth a read. It contains a lot more detail than I could cover in an answer.

  • \$\begingroup\$ @st2000: I recommend that you un-accept my answer for a day or two. You might then attract an answer from someone who actually understands the subject. ;^) \$\endgroup\$
    – Transistor
    Jul 17, 2016 at 20:50
  • \$\begingroup\$ I'll do that. As I did ask specifically about how to tell one device from another (which, I'm betting, will be difficult with out contacting the manufacturer). However, you did provide a very interesting resource! Thanks! \$\endgroup\$
    – st2000
    Jul 17, 2016 at 21:25
  • \$\begingroup\$ en.wikipedia.org/wiki/LIGO uses laser inferometry for gravity waves, with "an equivalent of approximately 280 trips down the 4 km length to the far mirrors and back again," arguably putting its working distance to over 1000 km. \$\endgroup\$ Jul 17, 2016 at 22:28

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