In uk.d-i-y, John wrote:
Because they are generally of the carriage lamp style and a normal
filament bulb goes up to the top of the glass and you can see the bulb
holder at the bottom. A CFL's ballast would fit but the effect would be
to move the lamp higher up - most of them do not appear to have very
much room above the top of the bulb. For the bulkheads, they are the
older squat type and when on you can see that the bulb is right against
the screw holding on the cover.

What can I say? I fitted CFLs into all my existing bulkheads (counting:
seven of them) and that was a few years ago. CFLs have shrunk quite a
lot since then. For instance the 11W CFL that I have in my hand is 99 mm
long overall. I can't tell you what the length of a normal incandescent
bulb is but I think it's probably about the same as that.

However I am interested in people saying that the newer CFLs may work in
PIR controlled lights - how can I tell whether the CFL and fitting are
compatible.

Dunno. Try it? You can bet that as soon as incandescents are in danger
of disappearing from the shelves, CFL compatibility will become apparent
on the box. As for existing fittings, just try it. I'm not aware of any
dangers. If there are any dangers, someone will be along soon to put me
right, I'm sure.

--
Mike Barnes