Electromagnetic Radiation – How is Light Emitted by an Incandescent Lamp?

Question

I am looking for better understanding of how light is produced in an incadescent lamp. More specifically: how is the kinetic energy of electrons converted to light?

• Are we dealing with interband transitions or with intraband relaxation involving photons? Is this Bremsstrahlung (electrons lose their energy as light when colliding with crystal impurities/defects)? Or is this a thetmal radiation resulting from Joule heating?
• How is the emission affected by presence of impurities and imperfections of the crystal lattice? Do phonons play a role?
• What properties make a material more suitable for use as a filament: should it be a metal? Should it have a crystalline structure? Will any metal produce light, if a high current is passed through it in vacuum?

Update
The term describing the processes in the incadescent lamp is thermal bremsstrahlung, see the posts on this subject here and here.

Answer 1

Supplementary answer to the OP's clarifying comment:

The main point that still needs clarification, in my opinion, is whether we are really dealing with thermal radiation here, i.e., whether the role of electric current is only to heat the material (since the current flow itself is not a thermal state

Good question, and...

Yes, we are really dealing with thermal radiation here!

The heating produced by the flow of conduction electrons in the bulk of the filament is not related to the thermal radiation coming from the few tens of atoms near the metal surface that are producing the photons that we see.

We know this because we can do some experiments:

1. When we turn on or off the current, the light produced ramps up or down with a radiative time scale of milliseconds, which is how much time it takes the filament to heat up or cool down from its previous equilibrium "OFF" or "ON" temperature.
2. We can try to measure the 100/120 Hz flicker of the bulb, and see that it's perhaps a percent. An incandescent bulb does not turn off 100 or 120 times per second. Its light stays relatively constant. We can make light trails with our eyes or cameras with pulsing light sources like some brands of LED lights (e.g. cheap battery operated portables) or florescent lights or some kinds of mercury or sodium vapor street lights, but we can't reproduce those effects with incandescent lights.

Now, this does not mean that electron collisions in metals can't make visible light, but the chances that a conduction electron can get 2 or 3 eV of kinetic energy before hitting another electron and that that also happens within tens of angstroms of the surface so that the light gets out is extremely small.

Basically the tungsten does two totally separate jobs at the same time:

• acts as a suitable temperature-dependent resistor such that it reaches thermal equilibrium and radiates 100 watts or whatever power it's supposed to
• acts as a thermal radiator, producing light when heated

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update: @Ruslan's comment links to two excellent videos!

• Incandescent light bulb turning on at 1000 fps - High-Speed Entertainment shows some low amplitude pulsations of the brightness of the light. I don't know how much they've slowed it down for display though.

• Incandescent light burning out at 1000 fps - High Speed Entertainment shows a filament burning in air and then breaking, and we can see the decay of the light emitted from each piece as it cools down.

Then it breaks, no current flows, and the light continues but starts to dim:

When it touches another part of the bulb, that part cools more quickly by conduction than by radiation, so it turns dark. But the bit at the right can't cool easily along the filament because it's thermal conductivity is low along the wire, so it's still glowing fairly brightly: