LED, Incandescent, and Fluorescent

[Donut.Hole]

What's the difference between the three and what are the advantages of each?

 

Thanks!

 

(I just know that this is some dumb question that slipped my mind)

[Klaynos]

Well they're 3 completely different technologies, the main advantage people talk about is energy efficiency....

[swansont]

Incandescent depends on heating, and emission of a spectrum approximating a blackbody, so a fair fraction of the energy is outside the visible range, giving rise to the "all heat but no light" misnomer.

 

Fluorescent relies on a discharge to excite/ionize a material, and the photons emitted (usually in the UV) to excite a material that (wait for it) fluoresces, i.e. emits lower-energy photons. The "wasted" energy is often smaller, because the difference in energy from the UV down to the visible can be made relatively small.

 

LEDs excite electrons across a bandgap in a semiconductor, and they emit photons when the drop back down. Also can be made to be very efficient, if the excitation closely matches the photon energy, so much of the input energy is delivered as photons.

[miknsk]

But it seems that the color reproduction of luminous tubes is much better than that of "white LEDs".

The "wasted" energy is often smaller, because the difference in energy from the UV down to the visible can be made relatively small.

Really? And how can the difference in energy be made smaller? If we assume the quantum yield to be equal to 1 (1 UV photon with energy 4.8 eV produces 1 "visible" photon with energy ~2.4 eV), the energy losses should be about 50%, in any case! (as I know, it's quite difficult to make the quantum yield of luminophor large than 1).

[swansont]

But it seems that the color reproduction of luminous tubes is much better than that of "white LEDs".

 

Really? And how can the difference in energy be made smaller? If we assume the quantum yield to be equal to 1 (1 UV photon with energy 4.8 eV produces 1 "visible" photon with energy ~2.4 eV), the energy losses should be about 50%, in any case! (as I know, it's quite difficult to make the quantum yield of luminophor large than 1).

 

The top of the visible is a tad smaller, though — 400 nm is about 3.1 eV, and 700 nm is about 1.75 eV, so the efficiency from that standpoint can be above 50%. Normal fluorescent bulbs have an overall luminous efficiency of around 10%, but incandescent bulbs are somewhere around 2%. Incandescents waste a lot of energy in the IR. (Keep in mind in evaluating those numbers that the ideal white light source has an efficiency of 35.5%, because luminosity is defined in terms of light at 555 nm, where the eye is most sensitive.) So the "relatively small" here has to be taken in context — even though 10% doesn't seem very efficient, it's still five times better.

 

http://en.wikipedia.org/wiki/Luminous_efficacy

 

One advantage that LEDs would seem to have over fluorescent bulbs is the ability to choose the bandgap rather than rely on a molecular or atomic transition, usually mercury (at 254 nm IIRC)

[miknsk]

Sodium lamps of low-pressure also should be noted, because theirs efficiency is two times larger than that of fluorescent bulbs...

[NeonBlack]

LEDs virtually never burn out.

They also use low voltage DC, which may be good or bad.

Fluorescents must be tube shaped

Incandescents use a lot of power for the amount of light they put out (although it probably doesn't matter in the winter).

[Mr Skeptic]

LEDs are solid-state, extremely efficient, and very long-lasting. However, they are expensive and small, use a smaller portion of the spectrum (read: ugly), and require low voltage DC. This makes them ideal for electronics or battery power, incidentally.

 

Fluorescents are relatively efficient. They use high voltage electricity passed through a tube containing low-pressure mercury (though other material may be used), exciting the gas to release UV light. The UV is used to excite a fluorescent powder to emit visible light.

 

Incandescents are dirt cheap to produce, but very inefficient. They function by heating a filament till it glows, however much of the glow is in the infrared and the bright filament is harsh to look at. They can run at pretty much any voltage, AC or DC. They usually burn out the most frequently.

 

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Also, compact fluorescents are now at about the level where they could replace most incandescents, and are more efficient and a longer lifetime. They fit in the screw sockets of incandescents, and now some are actually smaller than incandescents.

[SH3RL0CK]

One thing that hasn't been noted here yet is cost.

 

Incandescent bulbs are the cheapest to buy, followed by fluorescent, and then the most expensive is LED lighting (I haven't even seen these for sale in Home Depot yet).

 

Fluorescents have gotten cheaper (especially the compact versions that screw into the normal "incandescent" light bulb fittings), but are still more expensive initially than incandescent. However, they make up the cost difference due to the electricity saved and because they typically last so much longer.

 

I don't think it would be fair to compare LEDs since they aren't in common use yet and as I understand it, there are factors that can affect their longevity.