CW fNIRS - Continuous or sequential light activation?


Continuous-wave (CW) fNIRS systems often employ two different wavelengths, e.g. 730 and 850 nm to target respectively deoxy and oxyhemoglobin.

My doubts:

In CW-fNIRS, if each emitter uses two different wavelengths, it means that both wavelengths are active at the same time, or they are activated sequentially for short periods?

If the first case is true (two wavelengths activated at the same time continuously) , I wonder then how would be possible to discriminate the amount of light absorbed by oxy and deoxy concerning a specific wavelength (light is composed of both)?

If the second case is true (two wavelengths but activated one at a time sequentially), I wonder if:

  • the interval time between the activation of one wavelength and the other impact the measurement or can be ignored.

  • Do all emitters, placed at different scalp locations, turn on (and off) a specific wavelength (e.g. 850 nm) at the same time? In other words, are emitters perfectly synchronized?

Thanks in advance,


So this is dependent on the device you are using. Up to my knowledge, the two main technologies uses either time-multiplexing or frequency multiplexing[1].

When using frequency multiplexing, all the source are activated at the same time and the amplitude is modulated at a specific frequency so that the detector can link the signal it receive with the source it emitted.
With time-multiplexing, the sources are activated one after the other. In some cases, if two sources are sufficiently separated, you can activate multiple source at the same time as they won’t interfere with each other. In that case, it’s true that the time of acquisition is not exactly the same for all channels, but I never saw anyone doing any correction for nirs (as people do for fMRI).

[1] Frequency Division Multiplexed (FDM) NIRS for Wearable Applications


1 Like

Thank you @edelaire for your answer and reference (unfortunately I cannot access it).
The only doubt I have based on what you said is that the ones you mentioned look to refer to frequency-domain (FD) and time-domain (TD) systems while my questions were related to the Continuous-wave (CW) fNIRS systems (the most popular).

I don’t know if I’ve misunderstood something, however would be glad to know your point of view, maybe also someone else may integrate :wink:
thank you