DC Arc Lamps
Other than lasers, short arc lamps are the brightest manufactured sources of DC radiation. A large portion of their output is in the UV-VIS, which makes them useful for UV spectroscopy and UV photochemistry applications. Our DC xenon short arc lamps have a correlated color temperature close to that of the sun, ~5800 K. Our mercury arc lamps emit intense ultraviolet and visible radiation with very strong UV lines. The arc region between the anode and the cathode is so small that for many purposes, these lamps are effectively point sources. These small, bright sources can produce high intensity collimated beams using our condensing lens assemblies or be reimaged onto fiber optic bundles. We offer 50 to 1600 W mercury, xenon and mercury (xenon) arc lamps. For most models, we carry ozone free versions, a safer alternative if you don’t need output below 260 nm.
Arc Lamp Construction
Fig. 1 shows the construction of typical arc lamps. The anode and the cathode are sealed in a clear quartz envelope. Quartz is used for mechanical and thermal durability. The type of quartz used depends on the desired ultraviolet output. Some of our lamps use high quality UV grade quartz to transmit output to below 200 nm. Others use doped quartz, which absorbs short wavelengths to minimize ozone generation; we call these our ozone free lamps. The quartz bulb is carefully shaped to withstand the thermal gradients and shock inherent in running these lamps. The bulb has a small sealed-off tip.
Anode and Cathode
The anode and cathode are made of tungsten. The tungsten used in the cathode is doped with materials such as thorium dioxide to enhance electron emission. The cathode is small and pointed to ensure that the tip reaches a high temperature for efficient electron emission. The anode is more massive to withstand the electron bombardment and efficiently dissipate the heat produced. Mercury arc lamps have coils on one or both electrodes to help in arc formation. The anode-cathode gap can be from 0.25 mm to several mm, depending on the lamp power rating.
Arc lamps are filled with either rare gas at several atmospheres pressure, or a little rare gas and an exact amount of mercury. When the lamps run, the internal pressure increases to 12 to 75 bar, depending on the lamp type. The high pressures demand special care in the handling and operation of these lamps.
The two metal terminals at the ends of the lamp are for the electrical connections and mechanical support of the lamp. The terminals are connected to the electrodes by molybdenum foil strips inside the glass stem, or, for the higher current lamps, by tungsten rods.
Which Lamp Do I Choose?
Your application determines which arc lamp is best. Ask yourself two questions:
Types of Short Arc Lamps
Xenon Short Arc Lamps*
Xenon lamps are filled with purified xenon at 5-20 bar. The pressure triples during operation. Xenon lamps operate with the anode at the top. The small intense arcs radiate like 5500-6000 K full radiators with some xenon lines superimposed. The xenon lines dominate between 750 and 1000 nm, but the spectrum is almost featureless through the ultraviolet and visible. Xenon lamps are popular for demanding absorbance and fluorescence applications involving source spectral scanning, and as high intensity broadband sources. Because of its sun-like spectrum, the xenon lamp is used for solar simulation.
Mercury Arc Lamps
Mercury arc lamps contain an exactly measured amount of mercury and either argon gas or xenon, which acts as a starter gas, as the mercury vaporizes. When the lamps are cold, the gas pressure inside is lower than atmospheric and you can see small globules of mercury inside the lamp. When the lamps run, this mercury vaporizes and reaches pressures of up to 75 bar. So, these lamps pass from being a rare gas lamp to a mercury lamp. During the five to ten minute transition phase, the lamps run at a higher than normal current and the anode must be at the bottom to ensure proper vaporization of the mercury. Some mercury lamps have a reflective coating on part of the bulb. This speeds the transition phase and improves the thermal distribution. Because the bulb temperature influences the mercury pressure significantly, these lamps are sensitive to airflow over the bulb.
Mercury (Xenon) Lamps
Mercury(xenon) lamps have similar output spectra to mercury lamps, with some xenon lines in the infrared, and enhancement of the low level continuum in the visible. These lamps use xenon as the starter gas at above atmospheric pressure and operate with the anode at the top.
High Stability Lamp
We offer a 75 W Xenon High Stability Lamp, also commonly called a “Super-Quiet” Lamp. Most arc lamps suffer from “arc wander,” the gradual movement of the arc point over time, caused by a lack of electrons emitted from the cathode. This results in the need to re-adjust the lamp, over its lifetime. This lamp addresses the problem of the arc wander by using a high-performance cathode. The radiant intensity won’t drop off over the lamp’s lifetime.
Our 200 W EmArc™ Lamp is an enhanced metal arc lamp that combines the advantages of a xenon, a mercury, and a halide lamp, into a single source. It is similar to a metal halide lamp in luminous efficacy, but has a much longer life. The spectral output is similar to that of a mercury short arc lamp. The very small arc gap (1.2 mm) makes it useful as the source for a fiber or monochromator illuminator.
Powering an Arc Lamp
Arc lamps, particularly xenon and mercury(xenon) lamps, require high voltage sparks to ignite. They then need carefully regulated current for continued operation. An ignitor provides the high voltage sparks while a power supply provides the regulated current. Our Research Housings have a built-in ignitor for ease of use and compliance with CE regulations.
Arc Brightness Map
Figs. 2 and 3 show typical contours of equal luminance for mercury and xenon lamps. Both lamps have "hot spots" that may be imaged onto pinholes, fibers or other small targets for maximum illumination.
Distribution of Luminous Intensity
Figs. 4 and 5 show typical luminous intensity distributions for mercury and xenon arc lamps. The pattern shows good rotational symmetry. Note the shadowing effect of the electrodes. Mercury(xenon) lamps have similar distributions to xenon. These patterns can be important in the design of light collecting mirror systems such as in our PhotoMax™ Lamp Housing and ourExtremely Large Telescope Metrology .
Fig. 2 Typical contour map of mercury arc lamp.Fig. 3 Typical contour map of low power xenon arc lamp.Fig. 4 Typical luminous intensity distribution of 6286350 W Mercury Lamp.Fig. 5 Typical luminous intensity distribution of 6253150 W Xenon Lamp.
Our arc lamps have electrodes that are designed to minimize spatial variations in the arcs. Convection currents inside the lamp, and arc migration on the electrodes are two sources of variation. We evaluate the performance of complete systems, not just the lamps, by imaging the arc on a narrow pinhole or slit and measuring the light that passes through. Changes indicate arc fluctuations. The combination of Oriel Lamp Housings with baffled air flow, and Oriel Power Supplies results in minimal signal change. Our Light Intensity Controller greatly reduces the impact of any changes.
Tungsten from the electrodes evaporates slowly with use, and deposits on the inside of the lamp envelope. This reduces the radiated output. Fig. 7 shows the spectral output of a new lamp compared to that of a lamp run for 1200 hours. The ultraviolet light is reduced to a little more than visible. Electrode “burn back”, opening the anode-cathode gap, is a secondary result of the tungsten erosion. We quote typical lamp life below; this is the average period for the visible output to fall to 75% of the initial value. It is based on running the lamp for 30 minutes between ignitions. There can be significant tungsten removal from the cathode during ignitions so frequent start can reduce lamp life. Adjusting the power input during the operating period can compensate for fall in output. Our 68950 Digital Exposure Control Systems, control power input (on most Oriel Power Supplies) automatically. The 68950 works with our small area sources.
Arc lamps operate at very high pressures and temperatures, and emit ultraviolet radiation. They must be operated in a fully enclosed housing. All arc lamps must be handled properly to prevent contamination of the bulb and resultant thermal stress. Xenon and mercury (xenon) lamps have supra atmospheric pressure even when cold. Handle these with appropriate safety equipment. Heavy gloves and impact resistant goggles are recommended.
These lamps are widely used for their production of ultraviolet radiation. Protect your eyes and skin with Ultraviolet Safety Equipment if they might be exposed to ultraviolet.
Ultraviolet radiation below 242 nm produces toxic ozone. Consider an ozone free lamp if you don’t need the very short wave ultraviolet below 260 nms.
1. It is good practice to replace any arc lamp after the rated life to avoid rapid deterioration in output and potential dramatic lamp failure.
For disposal of mercury-containing lamps, refer to www.lamprecycle.org.