Lamps


Introduction

Lamps convert electrical energy into radiation. Different designs and materials are needed to produce light in different parts of the electromagnetic spectrum. The following sections describe several different types of lamps that are useful in spectroscopy.

Blackbody Sources

A hot material, such as an electrically-heated filament in a light bulb, emits a continuum spectrum of light. The spectrum is approximated by Planck's radiation law for blackbody radiators:
B = {2hnu3/c2}{1/exp(hnu/kT)-1}   
The most common incandescent lamps and their wavelength ranges are:
tungsten filament lamps : 350 nm - 2.5 um
glowbar : 1 - 40 um
Nernst glower : 400 nm - 20 um

Tungsten lamps are used in visible and near-infrared (NIR) absorption spectroscopy and the glowbar and Nernst glower are used for infrared spectroscopy.


Discharge Lamps

Discharge lamps, such as neon signs, pass an electric current through a rare gas or metal vapor to produce light. The electrons collide with gas atoms, exciting them to higher energy levels which then decay to lower levels by emitting light. Low-pressure lamps have sharp line emission characteristic of the atoms in the lamp, and high-pressure lamps have broadened lines superimposed on a continuum.

Common discharge lamps and their wavelength ranges are:
hydrogen or deuterium : 160 - 360 nm
mercury : 253.7 nm, and weaker lines in the near-uv and visible
Ne, Ar, Kr, Xe discharge lamps : many sharp lines throughout the near-uv to near-IR
xenon arc : 300 - 1300 nm

Deuterium lamps are the uv source in uv-vis absorption spectrophotometers. The sharp lines of the mercury and rare gas discharge lamps are useful for wavelength calibration of optical instrumentation. Mercury and xenon arc lamps are used to excite fluorescence.


Hollow-cathode Lamps

Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element of interest. The electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light.

Schematic of a hollow-cathode lamp

Hollow-cathode lamps have become the most common light source for atomic absorption (AA) spectroscopy. They are also sometimes used as an excitation source for atomic-fluorescence spectroscopy (AFS).


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Copyright © 1996 by Brian M. Tissue

updated 2/21/96