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I was looking for a sensor, then I caught this:

https://www.di-soric.com/pt/OGU-031-P3K-TSSL-44413.html

In the specs - emitted light section, it says the emitted light is "Red, Clocked", with a switching frequency of 4000 Hz. What I understand from this is the emitted light (and the output) is on and off 4000 times per second.

What kind of an advantage can it have against a continuous light? It definitely would have a simpler circuit (no clock source required).

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    \$\begingroup\$ An unmodulated always-on source is hard to distinguish from the background; a modulated one gives the receiver something specific to look for. IR TV remotes for example use a 38 KHz signal which comes and goes in a pattern to encode the command. \$\endgroup\$ – Chris Stratton Dec 19 '18 at 14:12
  • \$\begingroup\$ It also allows you to use a higher peak power for the same current budget and probably reduces the strain on the emitter (or at least doesn't exascerbate it). \$\endgroup\$ – Cristobol Polychronopolis Dec 19 '18 at 16:40
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If the emitted beam is mot modulated, it can be hard for the detector to distinguish between the emitter and stray background light. For instance, if the sun shines through a window onto the wall next to the emitter, then the detector may remain stuck on even if something breaks the beam.

Modulating the emitter allows the detector to filter out anything other than the emitted beam. A 4000 Hz bandpass filter would be one way to do this. A phase locked loop would be another. Either way, a steady light can be ignored and only a modulated one will be accepted.

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