In the main figure for HEGduino, it says that the low power of the sensor circuit protects tissue. While it is intuitive that an extremely high power EM field in close proximity would cause damage, I was wondering what backing there is for this statement specifically in relation to the device. Doing some searching on the subject, I found some relevant research, but it's a bit difficult for me to parse so I figured I'd ask and see if there was an easy answer.
Part of my concern comes from the fact I've been getting headaches after/during long (1-3hr) sessions using the device purely for recording, without attempting to control blood flow. Pain usually starts nearing the end of the first hour and continues for about 30-45mins after discontinuing use. It is possible that the discomfort comes from having the device strapped there and the hardness of the sensors/emitters, and I'll do a trial tomorrow to test that, but I find it unlikely.
Hey Ivan, First check none of the solder points are contacting your skin, it could be a 3V shock, which would build up heat slowly enough not to be noticeable for a period. Otherwise the wavelengths and power levels I'm using are safe and on par with other industry pulse oximeters (though mine isn't quite a pulse oximeter). The amount of power that actually penetrates your skin is very low anyway. The wireless stuff should not be a concern either, it's all toned down in power quite a bit and worst it can do is heat your skin on direct contact, though the ESP32 should not be on your skin if it's secured correctly. All things considered I've noticed this effect before after long term wearing without doing any training, and I wonder if it's a classical conditioning sort of reaction to seeing the flickering. You can see it plainly in the corner of your eyes if the sensor is close enough to them. Like maybe your brain is automatically stimulating blood flow in response to you sensing the flickering due to the association. This doesn't seem to happen to everyone mind you, but I would be curious what others' experiences are.
That's interesting then! Definitely something to narrow down the reason for. I'm using 950nm and 650nm LEDs. The ESP32 shouldn't be sending more than 40mA through them even though the resistors allow more than that. So there's your reference frame. Most pulse ox's have 20mA-100mA LEDs as far as I know, generally about 50mA. I've looked into red and IR-A (the segment of infrared we use) for damage concerns but found none for LEDs, esp at these amplitudes. No long term issues reported by the biofeedback people I know who've worked with this stuff for a couple decades. Only note I've gotten from someone I know was that ~840nm flickering at 40Hz *might* cause actual changes in brain state that are not apparent at other wavelengths, but we're at ~20Hz and ~950nm so a little off.
No visible solder points, so probably not that. I doubt the classical conditioning theory too since I've done no biofeedback training and have been using the device for solely recording, but something in that vein is definitely possible. In terms of photochemical effects, some of the research I've read on photobiomodulation seems sound and looks to come from reliable journals, though I have a feeling most of the commercial "applications" of it are definitely quack science.
It's interesting. It's probably a case of sensitivity differing between people, if it is something comparable to a photochemical effect, but I'll definitely look into it more. I'm curious if other similar applications of near-infrared and visible light in the brain have run into this.
For reference
We are in the Optical range with our LEDs. So there are potential photochemical and thermal effects but I've yet to find convincing literature on that (google photobiomodulation, though I've yet to figure out if all of it's quack science or not, so at your own risk). It's not really until you hit Ultraviolet that you get into trouble, however.