It made me think - why wait for white LEDs? Very small fluorescent tubes
    are available now, are more efficient than incandescent lamps, and have
    a long life.

They're high voltage devices, and not at all directional, though, leaving
them out of some applications where LEDs would be especially suitable.
There's a web page somewhere that opines that the brightest white LED
currently available is only as part of one of those "keychain" lights (sold
by gateway and perhaps others.)


    The smallest tube available seems to be 150mm (6 inches) long and rated
    at 4 watts. There is one in the Maplin catalogue; since it only costs
    UKP 5 (8$), I bought one.

Those are commercial fluorescents aimed at lighting.  There are smaller
"cold cathod fluorescent lights" aimed at backlighting in laptops and
such.  A typical tube is about 1/8 inch diameter, and can range from 1
inch to about 8 inches long (the longer ones look very fragile.)  You
can sometimes find similar lamps intended for solar-powered lawn lights
and such - it's not cheap anymore, though...


    Size is 160 x 40 x 38mm. It runs on 4 AA cells, and takes 0.4 amps, so
    the wattage is 6 x 0.4 = 2.4 watts. The 4 watt tube is therefore
    under-run. These tubes need a high voltage, so the 6 volts DC is
    "chopped" into AC by a transistor, and then stepped-up with a small
    transformer. Inside, there is about 60 pence (1$) worth of parts. It is
    very cheaply done, with no attempt at regulation (there could be, with
    better electronics); the tube starts glowing with a 3-volt supply, and
    gets brighter as the voltage increases; I stopped testing at 7.5 volts.

Thanks to laptops, there's a lot of interest in high efficiency power
supplies for CCFL tubes, with dimming capabilities and so on.  Check out
Linear Technology's web page - they think they have assorted patents on some
of the technology.

The tiny AMLCD screens that hit the surplus market over here a couple months
back came with equally tiny CCFL backlights (about 1x1x.5 inch) - designed
to run off 5V if provided with a periodic 15 uS pulse or somesuch if I read
the skimpy datasheet right (ie they contained a power mosfet and a big
inductor.)  My immediate reaction was to drive them with a PIC (actually a
basic stamp), since that's about the most convenient "pulse generator" I've
got...  (there!  Now it's PIC related!)

BillW