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).

Or make your own? It'll probably be good enough https://publiclab.org/w/spectrometry

ThorLabs also sell cheaper spectrometers by the way. You don't need a spectrum analyser with seven significant figures! https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=34...

You can also get light testing devices used for film/industrial illumination https://www.sekonic.com/color-meter/c700 (about 1.5-2k)

Or even single chip spectrometers with broad 20nm resolution from Hamamatsu (about £400 on group gets). https://groupgets.com/manufacturers/hamamatsu-photonics/prod...

[1] https://is-instruments.com/products/miniature-spectrometer/

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.

Instead of trying to emit 670nm, why not just use a wide spectrum source and a 670nm bandpass filter?

Like the sun and your closed eyelids. Light transmission goes up with wavelength https://www.spiedigitallibrary.org/journals/journal-of-biome... (see figure 5.) To speculate, perhaps we have evolved to benefit from closing our eyes in the sun.

That's a great connection. If it's that simple (and explained by human evolution), that would be awesome.

I got one of these $25 kits in a physics class that gives you the eyeball version of the fancy digital spectrometer: https://shop.sciencefirst.com/starlab/kits/5800-cardboard-sp...

We like these (or rather, the whole line):

https://www.avantes.com/products/spectrometers/compactline/i...

You can get them used for below 1000$, eg this listing:

https://www.ebay.com/itm/Avantes-Compactline-Avaspec-Mini-/3...

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

The D line is probably more easily seen in a colored flame made by adding salt water to an alcohol-burner flame:

https://en.wikipedia.org/wiki/Emission_spectrum#/media/File:...

that sounds like a project to do with my kids, thanks : )

Most LED have their wavelength in their datasheet.

Just select one that has the one you are looking for, it's going to be cheaper and easier than getting an expensive or inaccurate spectrometer.

For instance, this classic one is at 660nm:

https://www.mouser.com/ProductDetail/Lumex/SSL-LX5093SRD-D