Quote Originally Posted by polyglot View Post
The thing about LEDs is that they are bandgap devices, they (usually) emit light at exactly one wavelength only. No other wavelengths are possible, because the wavelength is defined by the photon energy, which is defined by the difference in chemical energy levels in the semiconductor.

Some manufacturers will deliberately mix multiple dopants to broaden the spectrum so this is clearly not true of all LEDs, but the basic simplest cheapest red LED you can make is a single-bandgap device. Obviously one must still test to make sure they don't have a deliberately spread-spectrum device or that the paper they're using doesn't somehow have a tiny bit of sensitivity out to the longer wavelengths.
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That's not my experience. A simple test: view the light reflected from the base of a CD at the angle that gives the "rainbow" effect and see if there are other colours present. I now have safe red LEDs but only by choosing some extreme deep red units. Most red LEDs that I tested showed orange..yellow..even cyan.

If you have some that are a single wavelength could you let us know the spec because Id' be interested in trying them?