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.
Originally Posted by David Brown
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.
Incandescents are fundamentally different in that they're a chunk of hot metal, i.e. a blackbody radiator which produces continuous spectrum all the way up to UV. One applies filtering to make it safe(r) but there is no such thing as perfect filtering cutoffs, etc, i.e. we can achieve only a certain dynamic range between the blocked and passed wavelengths. Attenuating the shorter wavelengths enough for paper to be safe to half an hour means attenuating the longer wavelengths down to annoyingly-dim levels.
LEDs though, if monochromatic, you can make them as blindingly bright as you want and the paper cannot pick it up. The photon energy is too small to chemically activate the paper, regardless of how many of the photons are present.
Or to re-state it: there is a bandgap in the LED that produces a photon, and a bandgap in the paper that can absorb the photon. If the energy available to create the photon is less than the energy required to activate an electron in the paper, the electron can never be activated. The number of photons (brightness) is completely irrelevant.
Last edited by polyglot; 06-14-2013 at 01:37 AM. Click to view previous post history.