Question about source and detector in the same optode module
In the optode module, you put the source and detector in the same module. For recording fNIRS data, we should organize the source-detector channel on the head at a distance of around 2-3 cm which means we will use the source from one module and the detector from the other module to build a channel. It seems it is a waste to have a source and a detector in the same module and it would sacrifice the temporal resolution if we are going to light up every source sequentially. Is there any specific purpose for putting them in the same module?
The purposes of having a detector and a source on the same module are:
- So an optode can be configured as a short separation channel, which is very useful to estimate the contribution of systemic physiology to the fNIRS signal
- To increase the versatility and modularity of the optodes.
- To improve coupling of the optode to the scalp through the hair
Do note that for ninjaNIRS 2020, each optode is either set as a source (source high, detector "off"), as a detector (source off, detector "on), or as a short separation channel detector (source low, detector "on"). Its firmware does not provide support for temporal multiplexing. Channel discrimination is achieved purely through spatial and frequency multiplexing. On the other hand, the sampling rate is 25 Hz, so it would be possible to perform temporal multiplexing through software while still maintaining a higher sampling rate than many commercial systems.
That said, ninjaNIRS 2021+ has single optodes (each is either a source or a detector) because of other design constraints. We plan to implement temporal multiplexing firmware support for 2022+ as we want to achieve full head high density coverage.