There currently exist a wide variety of compact flourescent lights consisting of a spiral tube with phosphor on its inner surface, inside of a decorative (or sometimes functional) outer shell.

Typically this shell will be (partly) clear, such as outdoor post lamps, or the front surface of a reflector, or white, such as in globes, to diffuse and soften the light produced.

Why not move the phosphor from the inner surface of the spiral to the inner surface of the shell?

This would hide the spiral tube, which many people consider ugly, and thus improve their attractiveness to people who might not otherwise buy them.

Additionally, since phosphors absorb a small amount of the visible light that the mercury arc produces, I would excpect that not having one white tube cross in front of another would increase the total efficiacy of the lamp.

As an added benefit, in some small sizes of lamp, there might be less phosphor used total, coating the shell instead of coating the tube.

In the case of a floodlight, which has a cfl inside a reflector, only the front surface would get the phosphor coating: the UV produced by the mercury arc would bounce off of the reflector, then hit the phosphor on the front.
-- goldbb, Jan 20 2009

For floodlight use, placing the phosphor on the front would defeat the purpose, with the front surface becoming a single large diffuse source of light instead of a semi-focused beam.

I think there may be other reasons to place the phosphor on the tube, but in concept I like this idea. [+]
-- Freefall, Jan 21 2009


The problem, surely, is that glass aborbs UV to a considerable extent. So, if the UV has to go through the glass of the inner spiral before hitting the phosphor, you'll lose quite a lot. I think.
-- MaxwellBuchanan, Jan 21 2009


Switching from spiral to sphere would decrease surface area / volume and so decrease light output.

One might make a fluorescent light that is a sphere with multiple deep invaginations - sort of a reverse sea urchin. The crossing across problem becomes more of an issue and also I am sure it is easier to coil a glass tube into a spiral than make a reverse sea urchin.
-- bungston, Jan 21 2009


MaxwellBuchanan: Different types of glass absorb different wavelengths of light. Some types of glass absorb UV, while some don't absorb any. I would expect that if a CFL were made with no phosphor on the inner tube, just on the outer shell, the inner tube would be made of a type of glass which absorbs little or no UV.

Bugston, are you sure that amount of surface area has a signifcant effect on light output?

I thought that it was based almost soley on the amount of UV emitted by the mercury, which is basically dependent on the length of the spiral tube. This is the reason why a 4 foot 32 watt flourescent bulb emits nearly the same amount of light as a 4 foot 34 watt bulb, or a 4 foot 40 watt bulb.
-- goldbb, Jan 21 2009


[goldbb] true, there are more and less UV-absorbtive glasses, but I think that fully UV-transparent glass is probably either costlier or (like pure silica, which I think is UV-transparent) much harder to work than other glasses.
-- MaxwellBuchanan, Jan 22 2009


//invaginations// - excellent, [bungston]. Had I been drinking tea while reading that, I would have snorted it all over the keyboard.
-- hippo, Jan 22 2009



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