Why LEDs are more efficient than CFL

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?


BA

 

I work in residential lighting and a common "task" in residential lighting
is illuminating a whole room. That's usually done with general lighting on
the ceiling or several table and floor lamps emitting light in all
directions.

As you say, LEDs emit directional light and so have to be forced (via
optics) to do such general lighting. Good examples are the LED "A-line"
bulbs now available for replacing standard GLS lamps. Their optics are
complicated, fascinating and innovative. No wonder the U.S. DOE has offered
millions of dollars in prizes (via the DOE "L Prize" Competition) for LED
screw-in products that can efficiently emit light in all directions.

So, your first sentence is a conversation starter; but it's also misleading
as virtually any light source can be the most efficient choice if it's the
best match for a particular lighting application.

Terry McGowan

 

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.

 

I see two problems:

1: Glass tubing that is transparent to the main phosphor-exiting
wavelengths of low pressure mercury vapor (253.7 and 184.9 nm) is
special stuff, costing more than ordinary glass.

2: Low pressure mercury vapor is not all that transparent to 253.7 and
184.9 nm. At these wavelengths, low pressure mercury vapor looks
like fog - and not a white fog, but a grayish one.

Mercury vapor atoms very easily absorb photons of these wavelengths,
thern re-emits them. (This phenomenon is known as "resonance", and
is typical of atomic vapors at wavelengths whose atomic transitions
have their lover levels being the "ground state".)

In fact, photons of these wavelengths inside a fluorescent lamp
are usually absorbed and re-emitted many times before they make it
out to the inner surface of the tubing. (Check out "imprisonment"
in relevant context.)

Sometimes, the absorbed quantum of energy is not re-emitted but
gets lost due to quenching triggered by a collision (gets converted
to atom/molecule kinetic energy - heat).

But in a spiral lamp made of transparent tubing, some of the
shortwave UV photons escaping the tubing go back into the tubing -
to run into more opportunities for their quanta of energy to be
converted to heat. My impression is that visible light losses in
phosphor coatings have been tweaked to a less serious loss.

 

Wouldn't it be even easier - you mount the tool on the end of
a length of straight springy wire and pulling or pushing round
the spiral will force it against the outside of the spiral?
Probably need a teflon or similar sleeve on the wire to minimise
scratching of the coating elsewhere, although if pushed, it
should be rubbing against the part you've wiped off anyway.

The other option would be to do it to the tube before it's
wound into a spiral. I'm not sure if the bending or coating
is normally done first nowadays. I know both methods have been
used, but heating the phosphor enough to bend the tube does
permanently reduce the phosphor's efficiency.