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I assume you had a cut-n-paste failure with that link. I think you meant https://www.frontiersin.org/journals/neurology/articles/10.3..., which I assume you picked because the title. It is an interesting article, should you ever decide to read it beyond cherry picking the caption from 'Figure 2'. But TL;DR: It doesn't say what you apparently think it says. The author is questioning the therapeutic value of low power infrared light therapy, and demonstrating what is required for medical effect. He is not questioning whether or not infrared light can penetrate skin and bone, because it does. For example: We have demonstrated that our multi-watt NIR data delivers an estimated 1.65–3.7 J/cm2 to a depth of 30 mm. As shown above, this is within the biologically meaningful fluence range (1, 2, 4, 6, 47) and is more than 100-fold greater than the fluence delivered by an LED system or by a low-power infrared light system according to the findings of the authors cited above (7, 18, 21, 37, 38, 48). and Patients receiving 10–20 treatments of multi-watt infrared light, each lasting approximately 20–30 min, have experienced significant, and often, dramatic improvements (47, 48). The fluence of combined 810 and 980 nm light delivered during each of these treatments was, on average, 81 J/cm2/treatment. Correcting for forehead skin, skull, and 1 cm of brain tissue, this delivered a fluence of fluence of 0.41 J/cm2 to the neurons 1 cm below the cortical surface. The authors paper listed as citation #4 (https://www.tandfonline.com/doi/full/10.2147/NDT.S78182#medi...) has more detail on his methods, and the abstract sums it up pretty well: NIR in the power range of 10–15 W at 810 and 980 nm can provide fluence within the range shown to be biologically beneficial at 3 cm depth. You can't ELI5 more than that. Understanding does take work, and given your posting history of mostly low effort negative snark, I probably spent more time than I should have. But it was an interesting diversion into something I otherwise wouldn't have known. |
Instead of light getting through bone you said
From your own link:
https://www.frontiersin.org/journals/neurology/articles/10.3...
Now, the skin of the forehead overlying a portion of the frontal lobes is approximately 2 mm thick. It is possible that tiny amounts of infrared light from lower powered emitters could penetrate the forehead skin; however, only 9–11% of the light from a 10 W emitter penetrated that thickness of skin. Nevertheless, the remainder of the scalp, over which hoods, helmets, and posteriorly placed LED pads are emitting low-power light, is an average of 5.1–5.8 mm thick.
Simply put, it does not matter how long an LED is shone on a human head if the light energy from that LED cannot penetrate through human skin further than 3 mm. The energy of low-power devices simply will not penetrate the thickness of the scalp overlying much of the skull.
Some have suggested that NIR energy from low-power devices penetrates deeper if longer exposure times are used. This reflects a fundamental misunderstanding of the roles that scatter, absorption, and refraction play in degrading NIR energy as it passes through tissue.
The energy delivered to the skin surface is different from the energy that penetrates to the depth of the target tissue – often several cm below the surface.
Longer exposure times will simply pump more energy into the epidermis and dermis of the skin/scalp.
Longer exposure times do not yield deeper penetration. These limitations on penetration only take into consideration the skin and scalp; however, the skull is a formidable barrier to light penetration, as well.
This is all moot however, as NIR is not light, because people can't see it (and possibly no animals can). If you drop the frequency enough something will get through (a tiny percent) and if you keep calling it light then you can make the false claim that light passes through bone, when we know that isn't true because we can see bones and they aren't transparent. We could say radio waves pass though people too and therefore light goes right though people.