D
Don Klipstein
thanks for a great response... information that I'm familiar with, though
probably not backed by the knowledge you have. To continue the conversation
back to the original thought - can you make a similar statement concerning
the luminous efficacy of fluorescents when supplied 50% reduction in
wattage? I often give training seminars that are product related at
electrical distributors, and when this subject hits the floor, I'm somewhat
at a loss to give useful information.
50% power reduction for fluorescents:
Overall luminous efficacy should not change much, and could change in
either direction.
I remember from a few classic books (maybe Waymouth or Elenbaas or
both), and from a fluorescent lamp catalog from the late 1970's that
fluorescent lamps in general, and especially T12 ones, operate slightly
more efficiently when slightly to moderately dimmed - if the shape of the
current waveform stays the same.
There may be some exceptions.
Then comes the question of what happens when the temperature of any
given lamp changes as a result of change in input power, and the mercury
vapor concentration in it changes. The temperature could become closer to
or farther from that at which luminous efficacy is maximized.
Ideally, the lamp's temperature is optimal when the lamp gets full
power, so when power input is reduced enough to make much change in the
lamp temperature and the lamp cools, I would think that its efficiency
should normally decrease slightly. With only slight dimming, the
temperature could easily remain close enough to optimum to have less
effect on efficiency in producing light than the change in current density
within the lamp - so overall luminous efficacy could very slightly
increase.
However, for minor to moderate dimming, I would think that this usually
does not change luminous efficacy much in either direction.
Dimming ballasts may provide extra filament/cathode heating so that the
cathodes do not operate at too low a temperature during dimming. That
would indicate some decrease in overall luminous efficacy from dimming,
since dimming requires an increase in percentage of input power being used
to keep the cathodes hot.
Overall, I would say overall luminous efficacy could make a minor change
in either direction if a fluorescent lamp is operated at half power and
working OK when dimmed to such an extent. And I would not expect the
change to be the same at every installation.
- Don Klipstein ([email protected])