Arc lamp (xenon, mercury)
A lamp in which light comes from an electric arc through a gas or vapor between two electrodes. Xenon short-arc lamps give a nearly flat continuum from the ultraviolet to the near infrared, close to a 6000 K blackbody; mercury lamps emit strong lines at 254, 365, 405, 436 and 546 nm. Typical electrical powers range from tens of watts to several kilowatts.
An arc lamp produces light from a sustained electrical discharge between two tungsten electrodes in a sealed quartz envelope filled with xenon, mercury vapor or a mixture. The discharge forms a small, very hot plasma, and its spectrum depends on the fill and its pressure. A high-pressure xenon lamp gives a broad continuum from about 200 nm to beyond 2 µm, approximating a blackbody near 6000 K in the visible, with a cluster of xenon lines between 800 and 1000 nm on top. A mercury lamp concentrates its output in discrete lines, notably 253.7, 365.0, 404.7, 435.8 and 546.1 nm. Laboratory short-arc lamps run at tens of watts to a few kilowatts, with arc gaps from about 0.5 mm in the smallest lamps to several millimeters in kilowatt lamps.
Types and spectra
Short-arc lamps (xenon, mercury, mercury-xenon) run at high internal pressure, tens of atmospheres when hot, with electrodes a fraction of a millimeter to a few millimeters apart. The small, bright arc has high radiance, which makes these lamps the usual broadband source for imaging onto a slit or a fiber. Pressure broadening widens the mercury lines to several nanometers, and they sit on a weaker continuum.
Low-pressure lamps, such as mercury-argon pen lamps and other discharge lamps of neon, argon or krypton, emit narrow atomic lines with little background. Their line positions are known accurately, so they are used to calibrate the wavelength axis of a spectrometer or monochromator. The 253.7 nm mercury line also drives germicidal lamps and appears in the UV-C band of the ultraviolet.
Flashlamps and long-arc lamps, in linear tubes filled with xenon or krypton, are pulsed or run continuously to pump solid-state lasers by optical pumping; the Nd:YAG laser entry describes how diode pumping replaced them in most designs.
The photon energies of the main mercury lines follow from = 1239.84 eV·nm divided by the wavelength:
| Line | Energy |
|---|---|
| 253.7 nm | 4.89 eV |
| 365.0 nm (i-line) | 3.40 eV |
| 404.7 nm (h-line) | 3.06 eV |
| 435.8 nm (g-line) | 2.84 eV |
| 546.1 nm | 2.27 eV |
The g, h and i letters (and e for 546.1 nm) are the standard spectral-line designations used in optical glass data; photolithography adopted them for the g-line and i-line steppers that preceded excimer-laser exposure at 248 and 193 nm, and i-line steppers are still used for less critical layers.
Where arc lamps are used
Xenon lamps are the standard excitation source in scanning fluorescence spectrometers, where a monochromator selects a narrow band from the continuum, and the light source in many solar simulators because their visible spectrum resembles sunlight. Mercury and metal-halide lamps were for decades the default illumination in fluorescence microscopy, with filter cubes picking out the 365, 405, 436 and 546 nm lines; high-power LEDs have largely displaced them there because LEDs switch in microseconds, do not drift as the arc ages and contain no mercury. Arc lamps remain in use where a continuous spectrum deep into the ultraviolet is needed, and for UV curing.
Coupling and étendue
An arc cannot be focused to a smaller spot than its own image at a given numerical aperture, so the useful power delivered to a slit or fiber is limited by étendue. As a worked example, an arc 0.3 mm × 1.0 mm collected by a condenser of numerical aperture 0.45 has an étendue of about ≈ 0.19 mm²·sr. A 200 µm core fiber with numerical aperture 0.22 accepts ≈ 0.0048 mm²·sr, about 40 times less, so at most about 2.5% of the collected light can enter the fiber, whatever the coupling optics. The delivered power is therefore set by the arc's radiance; a larger arc of equal radiance adds only light that the fiber cannot accept.
Practical points
- Ignition and warm-up. A high-voltage pulse of tens of kilovolts starts the arc. Output and spectrum settle over several minutes, and the ignition pulse can disturb nearby electronics.
- Stability. The arc moves on the electrode tips (arc wander), producing intensity fluctuations of order a percent and a slow drift as the electrodes erode. Ratiometric measurement with a reference detector, or optical feedback, corrects for both.
- Aging. Electrode material deposits on the envelope, darkening it and shifting the spectrum; usable life is typically hundreds to a few thousand hours.
- Ozone. Lamps transmitting below about 240 nm generate ozone in air; ozone-free envelopes absorb that range at the cost of deep-UV output.
- Pressure and heat. Short-arc lamps operate under high internal pressure and must be handled cold in their protective housing.
Common questions
What is the difference between a xenon and a mercury arc lamp?
Xenon gives a smooth continuum suited to spectroscopy at any wavelength; mercury puts most of its output into a few strong lines, which is efficient when the application needs one of those wavelengths, such as 365 nm curing or 546 nm illumination, and inconvenient otherwise.
Why are arc lamps used in spectrometers?
They combine a broad, continuous spectrum with a small, bright source that can be imaged onto an entrance slit. Incandescent halogen lamps are smoother and more stable but have little ultraviolet output; deuterium lamps cover the ultraviolet but are weak in the visible.
References: E. Hecht, Optics, 5th ed. (Pearson, 2017); B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019); J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed. (Springer, 2006).