Xenon Incandescent Lamps
Xenon flashlight bulbs, and the like!
Background on Gas Filled Incandescent Lamps/Bulbs:
Why Some Bulbs Still Have a Vacuum
Now for Premium Fill Gases!
Another Reason Why Premium Flashlight Bulbs Are Brighter!
Why premium gases mostly only in small bulbs?
Halogen lamps with krypton or xenon
Incandescent lamps (AKA regular lightbulbs) originally had a vacuum since
air oxidizes the white-hot filament.
As it turns out, an inert gas slows down evaporation of the filament. The
atoms of the gas bounce evaporated atoms of filament material (usually
tungsten) back onto the filament. This permits a higher filament temperature
for a given life expectancy.
Hotter is better, since maximizing the percentage of thermal radiation
that is in the form of visible light requires (idealy in an oversimplified
case) a temperature of about 6600 degrees K or about 6300 degrees C or
about 11,000 degrees F! This is slightly hotter than the surface of the
sun and much hotter than any light bulb filament material can withstand
even for a second.
The usual fill gas is a mixture of argon and nitrogen. Both of these are
cheap, argon is slightly less conductive of heat than nitrogen, but
nitrogen is necessary in many light bulbs to impair formation of
destructive electric arcs that form easily in pure argon.
A gas fill has advantages in permitting a higher filament temperature for
a given life expectancy. But the gas also conducts heat - this permits
energy to exit the filament by means other than radiation so this is a
loss. If the heat conduction of the gas reduces efficiency more than the
higher allowed filament temperature increases efficiency, then the light
bulb design is better with a vacuum than with a gas.
The break-even point between vacuum and the usual argon-nitrogen fuill is
typically around the point where the light bulb's wattage is about 8 watts
per centimeter of visibly apparent filament length. Heat conduction is
surprisingly independent of filament diameter - a wider filament has a
thicker "boundary layer" of immediately surrounding gas which has a
"thermal insulation" nearly proportionately higher with the increased
area. At least usually! Lower current bulbs (most types under .2 amp and
some types .2 to .35 amp) usually have a vacuum and higher current ones
usually have the argon-nitrogen mixture.
Extremely short filaments where the ends are a significant portion of the
exposed surface have greater heat conduction losses. So some low voltage
bulbs (3 volts or less) of design current as high as maybe half an
amp usually get a vacuum. This includes some flashlight bulbs.
As it turns out, argon and the argon-nitrogen mixture are not the best.
Krypton conducts heat less than argon does, and xenon conducts heat less than
even krypton does. So these are better than argon for more efficiency.
In fact, in cases where argon is only slightly worse than a vacuum,
krypton and xenon are better than a vacuum.
In addition, the larger atoms of krypton and xenon are better for bouncing
evaporated tungsten atoms back onto the filament than the smaller atoms of
argon and nitrogen are.
Flashlight bulbs with premium fill gases are usually designed to draw .7
to .9 amp of current, as opposed to the roughly .5 amp drawn by the
standard ones they replace. Due to economies of scale, increasing the
design wattage increases the efficiency.
One economy of scale is that in higher current lamps, the percentage of
input power lost to heat conduction decreases. Another is that a thicker
filament lasts longer at a given temperature, or can be operated at a
slightly higher temperature for the same life expectancy.
So for many reasons, premium flashlight bulbs can be about twice as bright
as standard ones, sometimes even brighter.
One reason is that the performance differences between the different gases
are greater where conducted heat is a higher percentage of the power going
into the lamp. This means the differences are greatest in the lowest
current/wattage lamps that have a gas fill and least in high wattage lamps.
Another reason is that the higher cost of premium gases is more
significant where the bulb size is larger. The cost difference between
xenon and argon can be a couple hundred times greater in a standard
household light bulb than in a flashlight bulb!
As it turns out, a halogen "capsule" or bulb has a gas fill that is
normally over 99 percent inert gas and less than 1 percent halogen vapor
(traditionally iodine vapor). The inert gas can be argon, krypton, or
xenon. If the manufacturer wants to go cheap, argon is used. For a little
better performance with an only slightly higher cost, krypton is used and
this is common. For slightly better performance than krypton, xenon is used.
When xenon is used, the manufacturer usually says so since xenon is an
effective buzzword. Strangely enough, Osram's HPR series halogen
flashlight bulbs (or at least some of them) have xenon and they don't
bother to make a big deal of this.
Some "xenon" automotive headlight bulbs are halogens with a xenon gas fill
as opposed to krypton or argon. Some of these merely have a blue tint
added to the glass or in a coating on the glass and have nothing to do
with xenon except for producing light with a more xenon-like color!
The true xenon car headlight bulbs are very expensive and require
expensive ballasts and do not have filaments but contain an electric arc.
Furthermore, these are not so much xenon lamps as they are metal halide
lamps. They do contain xenon which is useful for producing some useful
white light output while you are waiting for the metal halide ingredients
and the mercury in these bulbs to vaporize as the bulb warms up.
More info on these is here.
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Written by Don Klipstein.
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