How to select correct wavelength when make red therapy light

Light Window: light enters the human body 600-1100nm

Let's first establish a physical fact: There is only one "window" through which light enters the human body. Not all wavelengths can penetrate into the flesh. 

There are two "absorption walls" in human tissues:

Those shorter than 600 nm :

are strongly absorbed by hemoglobin and melanin, and blue-green light is basically consumed within 1 mm of the epidermis;

Longer than 1100 nm:

strongly absorbed by water, most of the light energy becomes heat, not photochemical reaction.

The wavelength range of 600-1100 nm in the middle is called the "optical window" (also known as the therapeutic window),where hemoglobin and water are absorbed less,
allowing light to travel deeper.

The 630-1064 nm wavelength used for red light therapy is all within this window. So choosing a wavelength is essentially picking a position within this window.
if you pick the wrong one, either you won't be able to hit the target layer, or everything will turn into heat.

3 Layers: how depth can travel on human body

Key conclusion: 810/850 nm is the sweet zone - hemoglobin and water absorption are the smallest, and penetration is the deepest, which is why almost
all mainstream panels on the market have 850 nm.On the other hand, 940-1064 nm, because water absorption increases with wavelength,
penetration depth actually shrinks back compared to 850 nm, and more people take the "thermal effect" route. When making skin contact devices,
more careful temperature control is necessary. 

Bipolar dose-response: It's not that the higher dose is better

Why 'multi wavelength ≠ more efficient': energy is shared, dose has a window

Firstly, when the total power remains constant, the more wavelengths there are, the less energy each wavelength receives. If you mark 100 W and divide it into 3 wavelengths,
it will be around 33 W each; Dividing two wavelengths results in 50 W each. Adding a third wavelength does not mean 'more intense', often 'all three become weaker'

Secondly, photobiomodulation (PBM) has a "biphasic dose-response" (Arndt Schulz's law): too low a dose results in no signal, while too high a dose actually inhibits.
In public literature, the effective dose window is roughly in the range of 4-30 J/cm ² (any higher will go downhill, and over 60 may even have a counterproductive effect).

So the core of selection is not "stack wavelength", but to make the target layer fall within the dose window - this relies on "selecting the right wavelength and providing
enough irradiance and time", not the number of wavelengths.

How to combine: single/double/tree wavelengths, with different shapes