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Physics/Energy balances of light sources


With the development of led light sources, you can hear a lot about the improved efficiency. Searching for easy ways to explain this to a non-physicist public, I had the idea of showing energy balances of the different light sources. By using this, I thought I would be able to show quickly that the big difference between incandescent and discharge and led sources is amongst others the fact that it emits a lot of IR radiation. My energy balance for incandescent would be: 5% visible, 10% heat (conductive and convective) and 85% IR. I read some things on energy balances for other types of light sources, and found:
- mercury vapor fluorescent sources: 35% IR, 35% heat, 30% visible
- led: 35% visible, 65% heat

But why would a mercury vapor lamp emit that much IR? I thought it would be linked to the temperature at the elektrodes and the losses in the discharge column, but is that the reason? Why is it that a led doesn't emit any IR?
I gave this a lot of thoughts but up until now I don't seem to find the answer...

Thank you for your help!

Your estimate of mercury vapor efficiency might be optimistic for visible light, but these seem pretty accurate.  

Mercury vapor has hot parts, obviously, it works by electrical conduction through the aforementioned vapor.  The lights are not that cool.  Also, it emits light via transitions between many excited states and the ground state.  Some of these are in the UV, which excites the phosphors (with some loss of energy in that process), some are visible, and some transitions are low enough in energy difference between the excited states to be down in the IR.  So there are several places it emits infrared radiation, from the ballast to the bulb to the actual transitions in the IR range.

An LED light, on the other hand, is engineered to operate with electrical conduction causing a certain amount of heating in the semiconductor and a quantum process which has only the transitions engineered into it.  Hence the pure colors they usually emit.  White LEDs are made in several ways, combining transitions between energy levels to make up the different wavelengths of light.  


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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