Continuously and apparantly continuously operating xenon lamps

updated 3/8/2000.

So you want a xenon lamp that operates continuously or flashes fast enough to appear to operate continuously? Well, so do I. But if you want a small light source more efficient than a halogen lamp, things get difficult and usually expensive. For high end bicycle lights and the like, specialty metal halide lamps are used.

Brief Discussion of Short Arc Xenon Lamps
Brief Discussion of Long Arc Xenon Lamps
Continuous Operation of a Flashtube
Rapidly Flashing a Flashtube
Efficiency of Xenon Flashlamps

Brief Discussion of Short Arc Xenon Lamps

There are continuously operating xenon lamps, mostly of the short arc type. All of these are expensive and are largely unavailable through most of the usual hobbyist electronic suppliers and photo equipment shops. Short arc lamps have special hazards and troubles. I have more info on short arc lamps in my Short Arc Lamp Page.

Note that xenon short arc lamps have an overall luminous efficacy of:

Approx. 13 lumens/watt in a 75 watt size
Approx. 13-20 lumens/watt in 150 watt sizes
Approx. 35-40 lumens/watt in 1-1.6 kilowatt sizes
Approx. 45-51 lumens/watt in 10-20 kilowatt sizes

Compare to 35-65 for ordinary mercury vapor lamps and 60-110 for metal halide lamps.

Please note that automotive high intensity discharge lamps are not so much xenon lamps as they are specialized metal halide lamps, and they are expensive and the ballasts for them are expensive and these lamps have more severe starting voltage requirenments than more ordinary metal halide lamps. I have more info on these in my Automotive HID Lamp Page.

Brief Discussion of Long Arc Xenon Lamps

These do exist, but they are not common. They are substantially less efficient than ordinary high pressure mercury vapor lamps and have efficiency roughly in the ballpark of that of halogen lamps. These are mainly used where a spectrum very much like that of sunlight is needed - such as in test laboratories to simulate heavy exposure to sunlight. The spectrum of a xenon arc lamp is like that of daylight in much of the ultraviolet, through the visible spectrum, and in much of the infrared. However, as in many other xenon lamps and even more so in long arc xenon lamps, a cluster of strong spectral lines in the infrared around 820-1000 nm detracts from the efficiency of producing other wavelengths.

Continuous Operation of a Flashtube

Flashtubes do not work well if you have a continuously operating arc in them.

One reason is the thermal conduction loss - which is sometimes referred to as the "watt per centimeter" loss. This loss is usually around 10 watts per centimeter of arc length in any xenon arc of length greatly more than the tubing diameter.
(In short arcs, the loss is more than 10 watts per centimeter.)
So, you have to exceed what approximates a 10 watt per centimeter threshold to get much radiation. If you dump 20 watts per centimeter of arc length into the arc, about half of this will become heat and about half will be radiated. If you only put in 10 watts or less per centimeter, nearly all of this will become heat.

Please note that most quartz flashtubes 6 to 12 millimeters in diameter will overheat with 10 watts per centimeter of arc length, but often survive this briefly with little damage. Glass flashtubes are usually quickly ruined by this much heat.

Another problem is electrode losses. Each electrode has a roughly 10 volt voltage drop in the process of getting electrons from metal to gas and vice versa. The cathode drop or "fall" is even higher if the current is lower than usual, especially if the cathode is cold with low current.

Multiply at least 20 volts by the current flowing through the tube to get the electrode losses in watts. Subtract this from the total wattage dumped into the tube, and subtract 10 watts per centimeter of arc length from the remaining wattage to get (approximately) the radiated wattage.

At any current remotely survivable on a continuous basis, the voltage across a flashtube (minus electrode drops totalling at least 20 volts) will be low. You need lots of current to get that 10 watts per centimeter of arc length just to get much of anything. Chances are, this much current will get the electrodes blazing hot and might melt them. The tubing around the electrodes can easily overheat even if the tubing elsewhere does not. You could easily evaporate electrode material and discolor nearby parts of the tubing.

Another consideration with xenon at currents of only several amps or less is the spectrum. Much of the radiated output is in a cluster of spectral lines around 820 to 1000 nanometers in the near infrared.

In a short arc lamp with a pressure of several atmospheres or more at room temperature and dozens to maybe over a hundred atmospheres hot, it takes hundreds of watts per centimeter to make the xenon effeciently radiate a daylight-like continuous spectrum. In a flashtube where the pressure is less, even more wattage per centimeter is necessary (amounting to several kilowatts to maybe hundreds of kilowatts) or else much of the output will be those infrared spectral lines. There is no way a flashtube will withstand many kilowatts continuously.

There is a continuously operating "long arc" xenon lamp made by Advanced Radiation Corporation. It is probably a little more efficient than halogen lamps in producing visible light, with the light having a color temperature in the low or mid 5,000's Kelvin. It is not likely worth the likely couple hundred-plus dollars cost except in applications requiring the exact spectrum including ultraviolet and infrared (which has those strong lines in addition to the 5,000's Kelvin continuous spectrum). For more ordinary lighting purposes, there is certainly a metal halide lamp that is more efficient and more cost-effective.

Rapidly Flashing a Flashtube

Often enough, I get inquiries about making a xenon flashtube flash fast enough to appear to glow continuously. I have tried this extensively and failed to make a flashtube shine brightly like this.

I found some reasons why this does not work well with most flashtubes!

One thing to consider is that there is an "economy of scale" in flash energy. The lower the flash energy, the less efficiently the xenon glows. This almost but not quite simulates a "threshold" energy, in which a portion of the flash energy is used to put the xenon into an efficiently radiating state.
Many flashtubes have a xenon pressure near 80 Torr, which is favorable for efficiency at lower flash energy. Pressure less than 80 Torr impairs efficiency with little improvement in energy requirements. Pressures above 80 Torr favor a slight to moderate improvement in efficiency, but with higher flash energy requirements.
To get half of a given flashtube's ultimate efficiency, you need approx. 2 joules per cubic centimeter of the portion of the flashtube volume that is between the electrodes. This is at low xenon pressure near or below 80 Torr. With higher xenon pressure, you need even more energy.
To get 80 percent of a given flashtube's ultimate efficiency, you need about 7 joules per cubic centimeter - more if the room-temperature xenon pressure is above 80 Torr.

Energy requirements are somewhat less if you use unusually high voltages. Please note that use of unusually high voltage at low energy will not reach the ultimate efficiency; merely give you a higher fraction if the energy is too low to approach the ultimate efficiency. If you use high voltage and low energy, there are some things to be aware of:

1. You should have particularly conductive capacitors. These would be axial lead foil types, maybe motor run capacitors or especially paralell banks of low value motor run capacitors, or other especially low impedance capacitors. "Glassmike" ones are good for this. Axial lead electrolytic types with one lead connected to the can are generally better than "radial" or "PC-mount" or "snap-cap" types, but not as good as motor run capacitors.

2. The spectrum will be rich in xenon ion lines. I show that spectrum among others in my Spectra Page. This spectrum is low on mid-red and deep red, which will make color photos look blue-green. Color film is more sensitive to deep red and less sensitive to orangish red (in balance with overall visible light sensitivity) than human eyes are. Many sources of artificial light look low on red to anything other than human eyes since their red output is optimized for human eye red response. This is accidental with xenon in the ion spectrum mode, but deliberate with fluorescent lamps and phosphored mercury lamps and most metal halide lamps.

3. With low energy, the xenon arc does not uniformly fill the flashtube. The arc often does not fill the tube in the same pattern from one flash to the next, and a flickering effect can result.

Now, even with the tricks and disadvantages, the flash energy is probably too high to be safely repeated frequently enough to make the flashtube appear to brilliantly glow continuously. This is especially true of flashtubes made of glass and not quartz. Smaller tubing diameter is favorable by reducing the ratio of volume to surface area, but narrower tubes often have a higher xenon pressure.

Forced air and liquid cooling do surprisingly little to increase the power handling capability of a glass flashtube. You will usually only double the power handling of a glass flashtube this way. Glass flashtubes conduct heat poorly and a large temperature difference between the inner and outer surfaces of a glass flashtube is bad.

Quartz flashtubes have much higher power handling, and are improved more by forced air and liquid cooling. Some are not designed to take advantage of forced air or liquid cooling well; others are. A flashtube that efficiently flashes rapidly enough to appear to glow continuously is almost certainly made of quartz and is usually cooled by forced air or flowing liquid.

One more thing: Many people underestimate the flash rate needed to appear to glow continuously. It is widely known that movies have 24 frames per second, but it is not so well known that the duty cycle is something like 80 percent. With the really low duty cycle of a xenon strobe (1 millisecond or less of on time), you need 45 to 60 flashes per second to eliminate flicker.

There is still another difficulty: At low flash energy which compromises efficiency of producing light, the efficiency of producing heat is increased. A flashtube may fail to withstand its rated average power when flashed rapidly with low flash energy.

I mention some characteristics of some popular and widely available smaller, inexpensive flashtubes in my Popular Small Flashtube Actual Ratings and Characteristics Page. Energy requirement for a given level of efficiency is in comparison to ultimate efficiency with everything done at a usual to lowish-usual voltage; you can almost certainly get some improvement using higher voltages. What I mention in that file mostly indicates that a flash energy low enough to get only half of ultimate efficency is high enough to be withstandable only for 3-5 flashes per second. You can improve on this by using higher voltages, but only to some extent. You won't safely flash a cheap glass flashtube more than 6-10 times a second with over half the ultimate efficiency, maybe double that to 12-20 flashes per second with forced air cooling.

Perkin Elmer makes heavy duty quartz linear flashlamps, some of which are suitable for liquid cooling. Here is the link:

Perkin Elmer Optoelectronics Linear Flashlamps, etc.

Efficiency of Xenon Flashlamps

As for what the ultimate efficiency of a xenon flashtube is? A xenon flashtube's efficiency is usually maximized at a higher arc temperature than the theoretical optimum of approx. 6600 degrees Kelvin. For a blackbody, 6600 Kelvin minimizes the combined percentage of IR and UV and maximizes the percentage of radiation in the visible, despite a peak wavelength near 450 nM in the deep blue. Xenon favors even higher temperatures since lower temperature favors xenon's favorite spectral lines in the infrared. Note that flashtube manufacturers prefer lower temperature, since "daylight" color film is made to consider an "average daylight" of 5500 Kelvin to be pure white. It is noted that efficiency of producing 5500 Kelvin light from xenon is maximized by using a higher temperature and correcting the color with a yellowish filter.
6600 Kelvin theoretically has a luminous efficacy around 95 lumens per watt. With thermal conduction losses, electrode losses, infrared spectral line losses, ion/electron recombination losses, other losses, and some energy used just to get the xenon to the operating temperature, and temperature varying during a flash (and deviating from optimum in the process) I expect around 60 lumens per watt when everything is going right. A major flashtube manufacturer's catalogs (from two different divisions even!) give an even more pessimistic figure of 40 lumens per watt! Half of this figure is obtainable with halogen lamps!

Written by Don Klipstein.

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