LEDs are more efficient because they focus all their light in the direction
it is to be used?
The tubular shape of CFL mean lots of lumens going in the wrong direction
towards inefficient reflectors.
Would CFL efficiency improve if better reflectors were used?
What do you think?
If you start with a straight tube and fold it in half to make a
retrofit CFL, the first problem you have is that one limb of the
tube shields the other limb of the tube creating a shadow. This
is the first level of efficiency drop when comparing a CFL with
a linear fluorescent tube. Most retrofit CFLs are folded more
than just once, and significant light is lost in the shadows
created by the tube limbs. You can see this if you hang a bare
folded tube CFL on a pendant cord, and twist the cord so the
lamp turns - you will see that the light output varies by direction
due to some limbs shielding other limbs by differing amounts
depending on direction.
This effect can be minimised by spacing the tube folds as far
apart as possible so the shadows are smaller, and less of the
emitted light is reabsorbed into some other part of the tube.
However, this makes a lamp which is larger and less likely to
actually fit as a retrofit. It also means the light source is
not in the same position it was in the original GLS/A-line
filament lamp, so any directing of light by reflectors etc may
be misaligned with the retrofit CFL, further reducing the
efficiency of the luminare. Conversely, the more compact tube
the arrangement is, the more likely it is to have the light
source nearer the designed position, even though the CFL itself
is less efficient. The large light source of a retrofit CFL
does not make for a good directional light source in any case,
as any reflectors tend to need to be large relative to the
light source size.
The most efficient retrofit CFL designs tend to be the well-
spaced spiral ones without any outer bulb, which also tend to
be physically largest making them unsuitable in some cases.
One thing I've often thought about is why not make CFLs out
of aperture tubes, with the aperture facing outwards,
minimising the losses in other directions where light output
from the tube is most likely to be lost. The light output
from the phosphor side facing the tube centerline is
significantly higher than it is from the phosphor side
facing out of the tube, and the phosphor itself is quite
opaque.