UPDATE - 3/23/2009: LEDs are now available with power handling and efficiency suitable for use in superior flashlights and bicycle headlights. The need for miniaturized HID lamps is decreasing.
As for flashing a xenon strobe rapidly with low energy flashes - there is
a bit of a problem. Performance of xenon flashtubes largely gets better
with higher flash energy and worse with lower flash energy. If the energy
is high enough to give satisfactory improvement of energy efficiency over
halogen lamps, any economical flashtube including all made of glass as
opposed to quartz will not survive such flashing repeated rapidly enough
to appear to glow continuously.
NOTE - It takes 50-60 flashes per second to avoid flicker! Movies avoid
flicker at 24 frames/second by having the "on" fraction of each cycle
being about 80 percent of the cycle as opposed to the fraction of 1 percent
that a fast xenon strobe would have. (NOTE - someone tells me that
movie projectors "blink" twice per frame for 48 "blinks" per second, and
this may well be true for just some projectors.) You need the off time to
be under 20 or preferably under about 16 milliseconds to make the light
appear continuous.
Further discussion of this is in a separate file on making xenon glow continuously.
Now back to miniature HID lamps:
One obstacle is the thermal conduction loss from the arc. This is
surprisingly proportional to the overall length of the arc and surprisingly
independent of the width of the arc or the gas/vapor pressure or the power.
For the really technical explanations why, look in the book "The High Pressure
Mercury Vapor Discharge" by W. Elenbaas published by the North Holland Publishing
Co. in Amsterdam. The principles are relevant to gases/vapors other than
mercury.
One thing that happens is that if the arc diameter is reduced, the temperature
gradient around the arc steepens and this makes the loss largely independent
of the arc diameter.
This loss is largely independent of gas/vapor pressure since increasing the
concentration of gas molecules also reduces their mobility by having more in
each other's way.
Increasing power delivered to the arc increases the arc temperature by a
surprisingly small amount. In the most oversimplified case radiated power
is proportional to temperature to the fourth power. In addition, the emissivity
of the gas/vapor increases (usually drastically) with temperature, making the
radiated power proportional to arc temperature raised to a power much higher
than 4. So the arc temperature is substantially more constant than proportioanl
to the fourth root of radiated power!
For an arc in mercury vapor or xenon with the arc length much greater than the diameter of the arc, this loss is roughly 10 watts per centimeter of arc length. For short arcs, thermal conduction from the ends of the arc is also significant which makes this loss substantially more than 10 watts per centimeter of arc length for short arcs.
Now, one would wonder why then it would not be simple to have an arc only
a fraction of a millimeter long so that the thermal conduction loss would
be a watt or less?
The reason is that the voltage drop across the extremely short arc would be
low compared to the voltage drop in the processes of electrons entering and
exiting the arc. Ordinarily, the "cathode fall", or the process of getting
electrons from metal into the arc is about 10 volts. In the very intense
heat of the cathode region of a short arc lamp, this can be a little less but
is certainly over 6 and probably at least 8 volts. This figure normally
varies slightly inversely with power and intensity of heating of the
electrodes!
If the voltage drop across the short arc is low, it will be mostly electrode
drop. If the voltage across the arc is only about 15 volts (common in lower
wattage short arc lamps like 150 watts or less), a majority of this voltage
drop is in electrode losses. Multiply the current (amps) by something like 9
volts to get the wattage dissipated into the electrode drops and multiply the
current by something like 6 volts to get the wattage dumped into the body of
the arc. And subtract from that maybe 2-3 watts per millimeter of arc length
for the thermal conduction loss!
It is easy to increase the voltage drop of a short arc by increasing the
gas/vapor pressure. But there are limits! Hot quartz bulbs can reliably at
most contain a pressure of a couple hundred atmospheres! Even at such a high
pressure, the voltage across the main body of an arc one millimeter long will
only be several to maybe a couple dozen volts. Figure on the low side since
the intense power level in such arcs makes them a bit hotter than usual and
makes them more conductive than usual.
So now we have the situation for a 1 millimeter arc that about half or more
of the lamp wattage is wasted in electrode losses and about 2-3 watts of
what's left goes to the thermal conduction loss.
One other problem - remember that thermal conduction from the arc is proportional to the length of the arc. But if you miniaturize any given existing lamp design, the area of the bulb varies with the square of the length! Miniaturization increases conducted heat per unit bulb area! Miniaturize the bulb and you may need forced air cooling. How energy efficient is a 3 watt mercury vapor lamp that needs a 2 watt fan to keep it adequately cool?
Still another problem: If you scale the size of a lamp, life expectancy is roughly proportional to the linear dimensions of the lamp. So if you scale some existing lamp design in half and keep the temperature of all the various parts the same (despite doubling conducted heat per unit area), you cut the life expectancy in half. This is because (in part) the thickness of whatever regions of the electrodes is halved. And with half heating (remember the thermal conduction loss varies linerly with length) on 1/4 bulb area, expect double condensation of electrode vapors per unit area of the bulb. This is a bit oversimplified but you will find it to be true!
And there is another problem of short arcs: The electrodes are close to
all parts of the bulb and evaporated electrode material condenses onto all
parts of the bulb. Because of this, short arc lamps mostly have life
expectancy around 500 hours or so, and that is in the available wattages -
mostly 100 watts to kilowatts!
HID lamps of long life expectancy have longer arc tubes with most of the
tubing safely far from the electrodes by having the tubing length great
compared to the tubing diameter. Since you need some significant tubing
diameter to not overheat from 10 watts per centimeter of conducted heat, you
need an arc tube length of a few centimeters for good efficiency and long
life. Now, with a few centimeters of arc length and 10 watts per centimeter
of losses, you need lots of watts for good efficiency. This does not work
for good efficiency at low wattage!
One possible solution for cleaning the arc tube or bulb is adding iodine or
another halogen to take advantage of the halogen cycle. You hope for the
halogen to attack any condensed electrode material vapors on the inner
surface of the bulb or arc tube. But remember that there will be parts of
the electrode structure at the same temperature as the inner surface of the
bulb and these parts of the electrode structure will also be attacked by the
halogen. Also note that traces of halogen vapor impair ionization of the
gas in the bulb, necessitating higher starting voltages.
But the halogen cycle has been utilized with limited success in lower
wattage metal halide lamps. The automotive headlight HID lamps are 35 watt
metal halide lamps that supposedly have an average life expectancy of
2700 hours. How practical is it to you to spend about $100 (or more) on a
lightbulb that lasts 2700 hours just to get the light of a 100 to 150 watt
halogen lamp for 35 watts (42 watts and maybe more including ballast
losses)?
The Philips 50 watt standard metal halide lamp lasts longer than that and
is available at Home Depot for maybe around $35. But 50 watts is not some
really low wattage good for bicycle lights and flashlights.
Believe me, there is a lot of money to be made from low wattage HID lamps suitable for bike lights and flashlights. So how low in wattage have practical HID lamps been made?
As for standard type general purpose metal halide lamps - there is the 50 watt Philips one available at Home Depot for maybe around $35. General
Electric and Philips make a 39 watt general purpose metal halide lamp sometimes
referred to as a "35 watt" lamp, but to get these you probably have to go
to an electric/lighting supply shop and order them, probably by the case.
Automotive headlight HID lamps are specialized 35 watt metal halide lamps.
There is xenon in them at high pressure to enable some useful light output
while they are warming up. They require multikilovolt starting pulses and
cost around $100 or more. More info on these is Here.
High pressure sodium lamps are available in a 35 watt version. You can get
them at many home centers for around $25 or in some hardware stores for a
little more.
The lowest wattage standard mercury lamp is the 40 watt one, or more common
the 40/50 watt one. This thing, after ballast losses, is only a little more
efficient than the more efficient halogen lamps.
Now for available lower wattage HID lamps, but these are expensive specialty lamps probably costing over $150:
Welch Allyn makes, among their other specialty lamps, the "Solarc" or "Hi Lux" miniature metal halide arc lamps. Their 21 watt one is used in the now-discontinued Cat Eye "Stadium Light", which is a high-end bicycle headlight costing $400-500 or so. Cat Eye is currently using LEDs in all of their better bike headlights.
UPDATE 8/27/2000 - Welch Allyn at that time had a 10 watt lamp in production. They have at best a minimum order requirement apparently designed to discourage casual users. I have heard of them developing working laboratory prototypes as small as 2 watts, but they have no plans to put any under 10 watts into production due to cost and general unworkability.
UPDATE 10/1/2000 - the 10W lamp has an initial luminous efficacy of 45 lumens/watt and a life expectancy of 1,000 hours according to the PDF datasheet available from their 10 Watt Lamp Datasheet. (link not working as of 12/19/2015)
Advanced Radiation Corporation makes a 20 watt xenon short arc lamp. Expect efficiency less than the 13-14 lumens per watt typical of 75 watt xenon short arc lamps. This lamp is only practical where a hundred or two lumens must be emitted by a light source much more compact than a 10 or 20 watt halogen lamp filament.
As for why specialty HID lamps cost so much? One reason is that they are made
of fused quartz. Quartz is as hard as a rock at the melting point of iron
(1535 degrees C). Steel tools melt before quartz gets workable. And you need
torches using oxygen (and/or acetylene update 5/15/2020) to get hot enough - a
regular Bunsen burner or even a "MAPP" gas blowtorch from a hardware store is
not hot enough.
And if you have the heat and the tools, there is still another problem -
quartz has a narrow plastic range of temperature. The temperature needed
to get quartz barely as soft as taffy or frozen chewing gum is only a few
dozen degrees short of making quartz liquid enough to pour. Quartz is trickier
to work than glass and glassblowers that can work quartz will not be
working for minimum wage! Specialty quartz lamps are hand-made by
adequately compensated skilled craftsmen and will cost many times the
cost of a mass-produced HID lamp.
Production machinery that can mass-produce quartz lamps is so expensive that
only lamps that will sell in huge quantities (hundreds of thousands) can
be made economically by such machinery. Otherwise it would cost even more to
get the machines made and set up than it would be to hire those highly
skilled glassblowers. If you have a specialty HID lamp design that would
use the same bulb and the same leads as a halogen lamp that is in production,
then maybe you could make a deal with an existing lamp manufacturer. (Expect
a minimum production run of thousands - perhaps many thousands - of lamps at
a price at least a few times that of a halogen lamp in order to make it
worthwhile to the manufacturer.)
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