I found graphs in a 1978 GE miniature lamp data book, giving life,
candlepower, and current as a function of lamp voltage. It has a bunch
of qualifications:

"Calculations of characteristics shown in Chart I are approximate only
between 95% and 110% of rated voltage for lamp types with 5,000 hours
life or less. Certain lamp types will vary widely from calculated
values. This chart will not apply to lamps with lives in excess of 5,000
hours. This chart does not apply to halogen cycle lamps."

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

In the other direction, at 140% of rated voltage, you get about 120% of
rated current, about 325% normal brightness, and about 0.016 times
normal life.

"Double life" bulbs are popular. You can make any bulb into a "double
life" bulb (according to the graphs) by running it at 95% of normal
voltage. Current will drop 3 or 4 percent, and you'll get about 85% of
normal light output.

I'm sure more information is available on the web for anyone who's
interested.

Roy Lewallen, W7EL

Thanks for the confirmation of 12th power.

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

The 12th power approximation does originate at GE Lighting,
AFAIK, but is only valid for voltages rather close to the rated
operating voltage, and for typical high voltage incandescent
lamps. Long-life and halogen bulbs WON'T behave the same.

The urban legend about halogen lamps is that reducing the voltage even slightly
causes the filament to burn out prematurely. The reasoning is that the slight
drop in temperature reduces the halogen self-healing effect much more than it
reduces the evaporation of the filament. I believe this is correct.

Thanks for the references.

Now... Does anyone know anything about helium reducing the life of incandescent
lamps? grin


http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.

The 12th power approximation does originate at GE Lighting,
AFAIK, but is only valid for voltages rather close to the rated
operating voltage, and for typical high voltage incandescent
lamps. Long-life and halogen bulbs WON'T behave the same.

The urban legend about halogen lamps is that reducing the voltage even slightly
causes the filament to burn out prematurely. The reasoning is that the slight
drop in temperature reduces the halogen self-healing effect much more than it
reduces the evaporation of the filament. I believe this is correct.

Thanks for the references.

Now... Does anyone know anything about helium reducing the life of incandescent
lamps? grin


http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.

On Sun, 31 Aug 2003 10:38:58 GMT, Spehro Pefhany
wrote:

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm


The problem with Halogens is LOW voltage reduces the life of the bulb
as well. The Halogen Cycle requires a minimum temperature in order to
re-deposit the tungsten on the filament.Low voltage boils the tungsten
off and deposits it on the glass envelope. Eventually the glass gets
dark and the filament gets too thin and burns out. Specified voltage
maintains proper temperature for thr reddepositing of tungsten on the
filament.
This operating heat is why quartz glass envelopes are generally used
for halogen bulbs.
This is copied from elsewhere on the web:

What is the difference between the internal conditions and mass
transports happening inside the noble gas fill, and the halogen
cycle incandescent bulbs?

Noble gases do not react with the tungsten vapor, leading to a layer
of semi-opaque condensed tungsten on the inner surface of the
bulb. Since the bulb is cooler than the boiling point of tungsten,
tungsten is gradually transfered from the filament to the bulb until
the filament burns out.

Halogens react with tungsten vapor, resulting in a layer of tungsten
halide on the inner surface of the bulb --- but since tungsten
halide is transparent, less light is absorbed. Also, tungsten halide
has a low enough boiling point that it can re-evaporate when the
bulb is hot enough. Finally, if tungsten halide molecules get close
enough to the fillament, they can disassociate back into tungsten
and halogen atoms, and the tungsten can be re-deposited onto the
filament, extending its lifetime, while the halogen goes back into
the fill gas.

There is an important elegance to the tungsten-halogen cycle. Because
the filament is a series resistance with a positive temperature
coefficient of resistance, any part that thins by sublimation runs
hotter than the rest. In a conventional bulb the heavy gas fill acts
to reduce sublimation and thermally insulate the filament so a given
power level gives more visible light. However, a localized thinning
gives positive feedback and failure.

In a tungsten-halogen bulb, sublimated tungsten reacts to give
volatile tungsten halides that thermally decompose and redeposit metal
at the hottest spots. The bulb does not darken from transported
tungsten. The hottest spots get rebuilt. Negative feedback allows a
filament to be run very hot indeed.

There is a price to be paid. The envelope must be made of fused
silica to take the high temperature. A mere trace of sodium (a
fingerprint) catalyzes crystallization of fused slica to cristobalite
at temp. When the envelope cools or heats it cracks from differential
coefficients of thermal expansion. When tungsten halogen bulbs fail
in use the results are often quite... attention-getting. Internal
pressure at operating temperature can be several atmospheres.
Tungsten halogen bulbs also have a pretty good UV component compared
to ordinary incandescents unless the envelope is doped with cerium or
such.

On Sun, 31 Aug 2003 10:38:58 GMT, Spehro Pefhany
wrote:

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm


The problem with Halogens is LOW voltage reduces the life of the bulb
as well. The Halogen Cycle requires a minimum temperature in order to
re-deposit the tungsten on the filament.Low voltage boils the tungsten
off and deposits it on the glass envelope. Eventually the glass gets
dark and the filament gets too thin and burns out. Specified voltage
maintains proper temperature for thr reddepositing of tungsten on the
filament.
This operating heat is why quartz glass envelopes are generally used
for halogen bulbs.
This is copied from elsewhere on the web:

What is the difference between the internal conditions and mass
transports happening inside the noble gas fill, and the halogen
cycle incandescent bulbs?

Noble gases do not react with the tungsten vapor, leading to a layer
of semi-opaque condensed tungsten on the inner surface of the
bulb. Since the bulb is cooler than the boiling point of tungsten,
tungsten is gradually transfered from the filament to the bulb until
the filament burns out.

Halogens react with tungsten vapor, resulting in a layer of tungsten
halide on the inner surface of the bulb --- but since tungsten
halide is transparent, less light is absorbed. Also, tungsten halide
has a low enough boiling point that it can re-evaporate when the
bulb is hot enough. Finally, if tungsten halide molecules get close
enough to the fillament, they can disassociate back into tungsten
and halogen atoms, and the tungsten can be re-deposited onto the
filament, extending its lifetime, while the halogen goes back into
the fill gas.

There is an important elegance to the tungsten-halogen cycle. Because
the filament is a series resistance with a positive temperature
coefficient of resistance, any part that thins by sublimation runs
hotter than the rest. In a conventional bulb the heavy gas fill acts
to reduce sublimation and thermally insulate the filament so a given
power level gives more visible light. However, a localized thinning
gives positive feedback and failure.

In a tungsten-halogen bulb, sublimated tungsten reacts to give
volatile tungsten halides that thermally decompose and redeposit metal
at the hottest spots. The bulb does not darken from transported
tungsten. The hottest spots get rebuilt. Negative feedback allows a
filament to be run very hot indeed.

There is a price to be paid. The envelope must be made of fused
silica to take the high temperature. A mere trace of sodium (a
fingerprint) catalyzes crystallization of fused slica to cristobalite
at temp. When the envelope cools or heats it cracks from differential
coefficients of thermal expansion. When tungsten halogen bulbs fail
in use the results are often quite... attention-getting. Internal
pressure at operating temperature can be several atmospheres.
Tungsten halogen bulbs also have a pretty good UV component compared
to ordinary incandescents unless the envelope is doped with cerium or
such.

On Thu, 28 Aug 2003 12:44:54 -0700, the renowned "William Sommerwerck"
wrote:

Thanks for the confirmation of 12th power.

The 12th power approximation does originate at GE Lighting, AFAIK, but
is only valid for voltages rather close to the rated operating
voltage, and for typical high voltage incandescent lamps. Long-life
and halogen bulbs WON'T behave the same.

http://www.eaoswitch.com/about/lamps.htm

Here's a rule of thumb for low-voltage halogens:

http://www.ndlight.com.au/low_voltage_lighting.htm

They claim a 5% voltage increase will reduce life by 50%, which is
more like the 13.5th power.

The one time I checked the 12th power approximation against actual
testing of low wattage high voltage (mains) lamps it was off by more
than an order of magnitude, so take the whole thing with a grain of
salt, IMHO, unless your lamp type matches the type used for testing.
I'm sure a real lamp specialist could go on for hours about this sort
of thing.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Thanks for the confirmation of 12th power.

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

"Roy Lewallen" wrote in message
...
I found graphs in a 1978 GE miniature lamp data book, giving life,
candlepower, and current as a function of lamp voltage. It has a bunch
of qualifications:

"Calculations of characteristics shown in Chart I are approximate only
between 95% and 110% of rated voltage for lamp types with 5,000 hours
life or less. Certain lamp types will vary widely from calculated
values. This chart will not apply to lamps with lives in excess of 5,000
hours. This chart does not apply to halogen cycle lamps."

It extends from 60% to 140% of rated voltage. The text accompanying the
graph says that, "as approximations", the light output varies as the 3.6
power of the voltage and the life varies inversely as the 12th power of
the voltage. It also says of the graphs that "Indicated values (except
for long life lamps) are reasonably valid, between 95% and 110% rated
volts. Beyond that, indicated characteristics may not be realized
because of the increasing influence of factors which cannot be
incorporated into the chart." I assume that long life bulbs are excluded
because they're already running a a considerably lower than "normal"
voltage.

At 60% of rated voltage, the graphs show that the current had decreased
to about 75% of rated current, while the candlepower has dropped to
something like 17% of rated output. (This means the efficiency is around
38% of normal.) Life is around 650 times the rated life.

In the other direction, at 140% of rated voltage, you get about 120% of
rated current, about 325% normal brightness, and about 0.016 times
normal life.

"Double life" bulbs are popular. You can make any bulb into a "double
life" bulb (according to the graphs) by running it at 95% of normal
voltage. Current will drop 3 or 4 percent, and you'll get about 85% of
normal light output.

I'm sure more information is available on the web for anyone who's
interested.

Roy Lewallen, W7EL


Running a lamp undervoltage sure does work. I put 220 volt bulbs in a 110v
porch light. Bright
enough to see ok and they have lasted for 8 years now being on nearly every
night!
I don't care if I get as lumens per watt as the house next door. It works!
Ghost