WHY can't we have a real RED and YELLOW fluorescent tube, not one where they color the glass red?


Powell

One of the reason is the "natural" mercury emission in the visible range, mainly green and blue.
Even when it does not carry as much energy, it is (mainly the green - the eye is most sensitive for it and it is of too long waveform for any further conversion) very disturbing for the red lamp (it loose it's saturation), it need to be filtered out, so the red and/or yellow filter.

WHY can't we have a real RED and YELLOW fluorescent tube, not one where they color the glass red?


Powell

Panasonic lighting offers red in F40T10, and Philips in F54T5HO It's simply a rare earth fluorescent with just one of the three colors. 

I don't know about yellow. 

Red phosphors are comparatively inefficient to Orange ones.

Quite simply there are no phosphors which emit in the strong red region of the spectrum.  The only interesting phosphors we have in this region are MFG and YOX and both in fact emit more of a pinkish colour light - partly as Medved correctly states, because of mixing of their radiation with blue-green from the mercury discharge.  Some manufacturers today offer tubes called Red which use the Yttrium Oxide component of triphosphor tubes, but they are in fact pink when lighted.  A true red can only be made when combined with a dye material.  Magnesium fluoro-germanate has a similar effect but emits a rather broader spectrum than YOX.  MFG is mainly used today in special tubes recently introduced for collagen generation in the skin, used for anti-wrinkle treatment.  Both MFG and YOX could be combined with the red dye to produce a more saturated red colour light, but resulting lamp efficacy is rather low.  in practice the highest flux can be made for a red tube by using ordinary white-emitting calcium halophosphate materials, combined with the red dye to filter out blue and green.

Conversion from UV to strong deep red is efficacy-wise difficult from "nature":
- In red the eye is quite insensitive, so for given lumens you need quite a lot of radiated power.
- The energy of red photon is way lower (about 3..4x) then of the energy of the initial UV one, what mean even for 100% quantum efficiency (for each 1 absorbed UV photon the phosphor generate one radiated in red) you have only 25..30% energy efficiency, so for 1W generated by the phosphor in red you need to absorb about 3..4W of UV power.
- And now is the question about the quantum efficiency of those single, narrow band phosphors.

Here's a SPD of Panasonic Lighting's red fluorescent lamp.  It's on the shorter end of red spectrum. 

http://www.lighting-gallery.net/gallery/displayimage.php?album=1962&pos=1&pid=54112

@Medved: Our eyes are relatively inefficient at sensing purple, yet it is still easy to create with phosphors.

@Medved: Our eyes are relatively inefficient at sensing purple, yet it is still easy to create with phosphors.

Purple is mix of blue and red, so the sensitivity is according to these components.
Blue have quite high energy photons, what make them easy to generate with reasonable energy efficiency (for the same radiated power you need about half amount of blue compare to red photons)
And blue is way higher on the eye sensitivity then the red, so on the purple lamp the luminous flux come mainly from the blue component, the small amunt or red then give it the purple tint.

"purple" maybe a mix of blue and red, but "violet" which looks purple is shorter than blue, but longer than UV.

"purple" maybe a mix of blue and red, but "violet" which looks purple is shorter than blue, but longer than UV.

For that part yes, but i don't know about any lamp radiating there to be intended for (even decorative) illumination, output of lamps for other use is not rated in Lumens (it would not make any sense), but in Watts (the radiation power in desired wavelength range).
I had in mind purple decorative lamps, which are made as mix of red and blue.

Thanks for your replys -sorry for my naivety. I meant violet. Brain on the blink.

For that part yes, but i don't know about any lamp radiating there to be intended for (even decorative) illumination, output of lamps for other use is not rated in Lumens (it would not make any sense), but in Watts (the radiation power in desired wavelength range).
I had in mind purple decorative lamps, which are made as mix of red and blue.

What we see is reflected light from objects + fluorescence emitted by them. Many materials exhibit fluorescence in short blue to long UV, so to render them as close to possible as seen under the sun, it is necessary to include these parts of spectrum. 

Minerals　and gemstones are particularly sensitive the spectrum of light.  Neodymium glass is one good example, which looks pale light blue under triphosphor lamps, but lavender under incandescent and bluish raspberry color under continous spectrum light.

Special continuous spectrum high color rendition lamps like Chroma 50 are used for that and they emit in the violet region.
http://www.lighting-gallery.net/gallery/thumbnails.php?album=1962

@Luminaire: Then it is only part (one component) of the whole spectrum mix in the "white" lamp, but not narrow band color tube.
And if this UV content is to be rated, it would be again in Watts and not Lumens (of course, the visible part is rated in lumens)...