I received both the 670nm single mode (C01) and the 660nm 3-mode (C01R) versions the other day.
By eye, I can easily perceive the difference in wavelength between these and a 625nm light I own. However, in all honesty, I cannot perceive the difference in shade between 660 and 670nm. The trouble is they have different brightness and different beam patterns, which probably makes it difficult.
The 670nm has a distinct central hot spot, whereas the 660nm is quite floody. The floody beam is better for its intended use, which is close work; not bothered about throw for a light like this.
The 3 modes on the 660nm are very well separated, very dim on low and surprisingly bright on high. I would’ve preferred the middle setting to be a bit lower.
I am happy with these for the price, so much so that I have ordered two more so I have spares. My ideal would be AA sized, 670nm, with floody beam, 3 modes with the middle setting a bit lower than on the C01R.
