[Footnotes at the bottom, to avoid clutter]
All modern lighting technologies will gradually
decrease in brightness over the course of their
lifespans... the longest lived lights, at the end of their
rated lifespan, are 30% less bright than they were when
will tell us that this depreciation
isn't a problem for a variety of reasons, but in reality,
it is indeed a problem, for a variety of other reasons.
This idea is to design a lamp which has an electronic
light sensor (located so that it can "see" some of the
light produced by the lighting element) in it's circuit,
and a microcontroller to regulate the amount of power
sent to the lighting element, such that the lamp's
brightness remains constant until such time that the
lighting element cannot efficiently produce the targeted
number of lumens, after which it would shut down.
Obviously, this means that the lamp's brightness at the
beginning of it's life is rather less than what the brand
new lighting element could potentially produce, but the
constancy of brightness, and the extend lifespan, make
up for it.
 This applies to all of: Incandescent lamps (including
tungsten and halogens), fluorescent and other arc lamps
(including linear fluorescents, circlines, CFLs, and
induction lamps), LED lamps, Electroluminescent wire,
EL tape, and Electron Stimulated Luminescent lamps.
There are probably others which I'm not aware of, but I
think I've got most of the bases covered.
 If a lamp is so old that it's maximum brightness is 70%
of what it was when it was new, then it has, /by
definition/ reached the end of it's lifespan, even if it is
still functioning. Many long-lived lamps continue to
function, and continue to grow ever dimmer, even
beyond their proper lifespan. I call such bulbs, which
are overdue for replacement, zombie bulbs :).
[3, 4] Supposedly, because the loss of brightness is
gradual, a human won't notice that an old lighting
element produces less light than when it was new...
This is true, only if the lamp is in isolation. If I have a
light fixture with several lamps, or a ceiling with
several recessed fixtures, and I replace one, I will
probably notice that the other, similarly old lamps, are
dimmer than the new one... potentially leading me to
replace the old lamps, even if they had a few good
years left in them.
Futhermore, if a person /doesn't/ realize that a lamp
has dimmed over it's life, then when it burns out and he
has to replace it, he will believe (falsely) that it had
been just as bright when it had been new... and since it
was quite dim when it burnt out, it must have been
quite dim when it was new! This would lead that
person to replace the burnt out lamp with a higher
wattage one, resulting in too much light, and too much
energy use. This is especially true of zombie bulbs.
Lastly, lighting manufacturers tell us that if a lamp gave
ample light when it was new, it will be just sufficient
when it reaches the end of it's life. The problem is that
many people purchase lights which produce just barely
enough light when new, and which become insufficient,
before the end of their lifespans. In such a case, the
bulb's practical lifespan is less than it's rated lifespan,
because as soon as the consumer tosses it in the
garbage, it's effectively dead.
PS: I happen to work in retail, and one of the things the
store I work in sells is light bulbs. Many people refuse to
by CFLs because, supposedly, "They're too dim." This
reason isn't the least bit logical, since one can always
choose to purchase the higher wattage CFL. The only
reason I can think that any person would make such a
complaint is that they don't realize that CFLs dim with
PPS: It might make sense for some exceptionally long-
lived lamps to have a manual switch which selects from
two different luminosities. When the lighting element
is too old to produce the highest luminosity, the user
removes it from the fixture, sets the switch to a lower
setting, then installs it in a different light fixture, where
the lower light level is correct for that fixture. Does
anyone know a non-evil sounding term for an undead