I know there are some pretty clever types on here, so I’m hoping someone can shed light (as it were) on something for me.

So, on your rear light, you’ve got a red filter over a white bulb.
The red filter takes out any wavelength of light that doesn’t correspond to red.

Considering the law of conservation of energy and all that…
What happens to the other wavelengths of light?

Best answer I can come up with is that red light passes through the filter and green and blue get caught up, and are turned into heat.
Not sure if that’s right though, or these things would get hot (aside from the heat from your bulb/led of course) wouldn’t they?
Or does light have such low energy that the heat it translates to is negligible?

turned to heat – but its only a few tenths of a watt

Yep. Heat, but a tiny amount. It’ll dissipate faster than it accumulates though.

For traditional bulbs, they produce a very large spread of wavelengths from blue down to infra-red, with most of it being infra-red ie heat. (This is why LEDs are more efficient, cos they don’t spew out all that heat, just the light you want.) The blue and green light you cancel out with a filter is only a tiny part of the spectrum, so you’re taking out a tiny slice of what, 1W of total energy for a small rear light.

EDIT actually.. some of it may be re-absorbed and re-emitted as something else – either heat or another wavelength, maybe even red. So the amount of red light you get from a source viewed through a red filter might actually be more than the red part of the original light.

DOUBLE EDIT: Original nonsense I wrote about LEDs deleted 🙂

As we’re talking about LED’s and science!

What was the breakthrough that allowed LED’s to be so much more powerfull? Years ago they were only just powerfull enough to light up a rear light, but now they can be used on cars!

Thanks guys.
I will be able to sleep tonight now!

molgrips, my next question was going to be whether a red led would be more efficient than a white led behind a red filter (or whether red leds were in fact just white leds with a red lens.)

I guess you’ve already answered that one too.

😀

LEDs work by putting fancy chemicals in the semiconductor that makes up a diode. Each fancy chemical emits a certain wavelength of light depending on how the electrons in the atoms are made up. So you use different chemicals to make different colours. Years ago, the stuff they used to use to make blue was really expensive, then I think they found something much cheaper. So now you can put red, green and blue in there to make white or any other colour. Sometimes it’s actually three different diodes in one so you can switch the colours at will and make all different colours.

I would guess that they have been working on different materials that can handle more current lately – more current equals more excited electrons and more light, basically. So if you can make it take enough current you’ll get bright light. You could make the diode bigger, but all those extra photons would not be able to get out of the semiconductor. Maybe the advance was to come up with transparent material that could be as large as needed to handle lots of current and still let the light out..? Not sure.

What was the breakthrough that allowed LED’s to be so much more powerfull?

All sorts of stuff – far too much to write down here, and I can’t be bothered GTFY – I’d start off looking for technical papers at http://www.cree.com

Years ago, the stuff they used to use to make blue was really expensive, then I think they found something much cheaper. So now you can put red, green and blue in there to make white or any other colour.

Though current white LEDs are actually blue LEDs with a phosphor coating (which does do clever stuff converting light from one frequency to another).

Yup, technically there’s no such thing as a white LED, they’re a very bright coloured LED driving a coating which absorbs the single wavelength and re-emits at a range of wavelengths.

For a back bike light, a red LED would be better than a white LED with a red filter – that’s just wasting energy.

Oh, I stand corrected. I knew they made red/green/orange ones with two junctions in the same package, I’d assumed that they did the same with the white ones.