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Why LEDs are more efficient than CFL

Discussion in 'Lighting' started by Benis Apucnoids, Jun 29, 2011.

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  1. 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?

  2. TKM

    TKM Guest

    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

    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
  3. 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
  4. 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.
  5. 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.
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