(d) How much does the precise wavelength matter? The study used 670nm but also mentions the range 650-1000nm. Lots of red-light gadgets are available on Amazon but how do you verify the wavelength? I was thinking of an optical spectrum analyzer[1] but at $28,000 they are shockingly expensive. Even with raw LEDs bought from a reliable electronics distributor like Digi-Key, it would be nice to be able to measure it somehow.
You can get a 1nm resolution "science grade" spectrometer for under £1k - I used to design them [1]. You can also buy used ocean optics kit for very reasonable prices on eBay (a few hundred for a USB2000 if you wait).
You might find this of interest - it seems to be a cottage-industry spectrometer selling for £67: "The i-Phos can see wavelengths from approximately 420 - 980nm and their relative (though not their absolute) intensities." http://chriswesley.org/spectrometer.htm
I suspect you do not need to use 670nm exactly, however I suspect you need to be above 650nm. Note that many red LEDs are at a lower wavelength (~630nm). So you need to search for deep red or by wavelength.
You can use a OSA, but yes they are expensive. Spectrometers are typically cheaper, but still expensive (you could go to a university optics group and ask them if they could measure it for you). That said if you buy from a reputable source you should get the right wavelength.
If you are trying this out be careful with the brightness.
Shouldn't it be quite cheap to determine the frequency using a prism, using the angle of refraction? I think you should be able to use known missing frequencies in sunlight to calibrate your set up. Back in a minute going for duckduckgo
So it looks like you need a bit more than a prism to see Fraunhofer lines, but there seem to be online description of how to do it with a with a prism and a CD, maybe you don't even need a prism, there seem to be 'make your own spectroscope' tutorials than mention building one with cereal box, and maybe a lens of some kind
(d) How much does the precise wavelength matter? The study used 670nm but also mentions the range 650-1000nm. Lots of red-light gadgets are available on Amazon but how do you verify the wavelength? I was thinking of an optical spectrum analyzer[1] but at $28,000 they are shockingly expensive. Even with raw LEDs bought from a reliable electronics distributor like Digi-Key, it would be nice to be able to measure it somehow.
[1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=52...