Sources of Visible Light
Visible light comprises only a tiny fraction of the entire electromagnetic radiation spectrum, yet it contains the only region of frequencies to which the rods and cones of the human eye will respond. The wavelengths that humans are typically able to visualize lie in a very narrow range between approximately 400 and 700 nanometers. Humans can observe and respond to stimuli created by visible light because the eyes contain specialized nerve endings that are sensitive to this range of frequencies. The remainder of the electromagnetic spectrum is invisible to humans.
Introduction to Visible Light Sources - A wide variety of sources are responsible for emission of electromagnetic radiation, and are categorized according to the spectrum of wavelengths generated by the source. Relatively long radio waves are produced by electrical current flowing through huge broadcast antennas, while much shorter visible light waves are produced by the energy state fluctuations of negatively charged electrons within atoms. The shortest form of electromagnetic radiation, gamma waves, results from decay of nuclear components at the center of the atom. The visible light that humans are able to see (the spectrum is illustrated in Figure 1) is usually a mixture of wavelengths whose varying composition is a function of the light source.
Introduction to Lasers - Ordinary natural and artificial light is released by energy changes on the atomic and molecular level that occur without any outside intervention. A second type of light exists, however, and occurs when an atom or molecule retains its excess energy until stimulated to emit the energy in the form of light. Lasers are designed to produce and amplify this stimulated form of light into intense and focused beams. The word laser was coined as an acronym for Light Amplification by the Stimulated Emission of Radiation. The special nature of laser light has made laser technology a vital tool in nearly every aspect of everyday life including communications, entertainment, manufacturing, and medicine.
Light Emitting Diode Fundamentals - The past few decades have brought a continuing and rapidly evolving sequence of technological revolutions, particularly in the digital arena, which has dramatically changed many aspects of our daily lives. The developing race among manufacturers of light emitting diodes (LEDs) promises to produce, literally, the most visible and far-reaching transition to date. Recent advances in the design and manufacture of these miniature semiconductor devices may result in the obsolescence of the common light bulb, perhaps the most ubiquitous device utilized by modern society.
A Brief Introduction to Microscope Light Sources - Modern microscopes usually have an integral light source that can be controlled to a relatively high degree. The most common source for today's microscopes is an incandescent tungsten-halogen bulb positioned in a reflective housing that projects light through the collector lens and into the substage condenser. Other sources include arc-discharge lamps, light emitting diodes (LEDs), and lasers.
Light Sources for Optical Microscopy - The performance of the various illumination sources available for optical microscopy depends on the emission characteristics and geometry of the source, as well as the focal length, magnification and numerical aperture of the collector lens system. In gauging the suitability of a particular light source, the important parameters are structure (the spatial distribution of light, source geometry, coherence, and alignment), the wavelength distribution, spatial and temporal stability, brightness, and to what degree these various parameters can be controlled.
Laser Systems for Optical Microscopy - The lasers commonly employed in optical microscopy are high-intensity monochromatic light sources, which are useful as tools for a variety of techniques including optical trapping, lifetime imaging studies, photobleaching recovery, and total internal reflection fluorescence. In addition, lasers are also the most common light source for scanning confocal fluorescence microscopy, and have been utilized, although less frequently, in conventional widefield fluorescence investigations.
Fluorescence Microscopy Light Sources - In order to generate enough excitation light intensity to furnish secondary fluorescence emission capable of detection, powerful light sources are needed. These are usually either mercury or xenon arc (burner) lamps, which produce high-intensity illumination powerful enough to image faintly visible fluorescence specimens.
Focusing and Alignment of Arc Lamps - Mercury and xenon arc lamps are now widely utilized as illumination sources for a large number of investigations in widefield fluorescence microscopy. Visitors can gain practice aligning and focusing the arc lamp in a Mercury or Xenon Burner with this Nikon MicroscopyU interactive tutorial, which simulates how the lamp is adjusted in a fluorescence microscope.
Interactive Java Tutorials
Lightning: A Natural Capacitor - Lightning is one of the naturally occurring mechanisms that provided early mankind with the ability to understand and harness fire. This meteorological phenomenon occurs when water-filled clouds and the ground act in unison to mimic a huge natural capacitor. View the build-up of static electrical charges between storm clouds and the wet ground during a thunderstorm with this tutorial, which simulates capacitor-like lightning discharges.
Color Temperature - Investigate the apparent "color" of a virtual radiator (in this case, a black metal pot) as it is slowly heated through a wide temperature range by external energy. The concept of color temperature is based on the relationship between the temperature and radiation emitted by a theoretical standardized material, termed a black body radiator, cooled down to a state in which all molecular motion has ceased. Hypothetically, at cessation of all molecular motion, the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.
Incandescent Lamp Filaments - Nearly every source of light depends, at the fundamental level, on the release of energy from atoms that have been excited in some manner. Standard incandescent lamps, derived directly from the early models of the 1800s, now commonly utilize a tungsten filament in an inert gas atmosphere, and produce light through the resistive effect that occurs when the filament temperature increases as electrical current is passed through. This interactive tutorial demonstrates the sub-atomic activity within a conducting incandescent lamp filament that results in resistance to current flow, and ultimately leads to the emission of infrared and visible light photons (with a corresponding rise in color temperature).
Compact Disk Lasers - A pre-recorded compact disk is read by tracking a finely focused laser across the spiral pattern of lands and pits stamped into the disk by a master diskette. This tutorial explores how the laser beam is focused onto the surface of a spinning compact disk, and how variations between the height of pits and lands determine whether the light is scattered by the disk surface or reflected back into a detector.
Light Emitting Diodes - Light emitting diodes (LEDs) are a general source of continuous light with a high luminescence efficiency, and are based on the general properties of a simple twin-element semiconductor diode encased in a clear epoxy dome that acts as a lens. This interactive tutorial explores how two dissimilar doped semiconductors can produce light when a voltage is applied to the junction region between the materials.
Argon-Ion Lasers - As a distinguished member of the common and well-explored family of ion lasers, the argon-ion laser operates in the visible and ultraviolet spectral regions by utilizing an ionized species of the noble gas argon. Argon-ion lasers function in continuous wave mode when plasma electrons within the gaseous discharge collide with the excited laser species to produce light.
Diode Lasers - Semiconductor diode lasers having sufficient power output to be useful in optical microscopy are now available from a host of manufacturers. In general these devices operate in the infrared region, but newer diode lasers operating at specific visible wavelengths are now available. Diode lasers coupled to internal optical systems that improve beam shape have sufficient power and stability to rival helium-neon lasers in many applications. This interactive tutorial explores the properties of typical diode lasers and how specialized anamorphic prisms can be utilized for beam expansion.
Ti:Sapphire Mode-Locked Lasers - The self mode-locked Ti:sapphire pulsed laser is currently one of the preferred laser excitation sources in a majority of multiphoton fluorescence microscopy investigations. This tutorial explores the operation of Ti:sapphire lasers over a broad range of near-infrared wavelengths with variable pulse widths and an adjustable applet speed control.
Nd:YLF Mode-Locked Pulsed Lasers - An increasing number of applications, including new illumination techniques in fluorescence optical microscopy, require a reliable high average-power laser source that enables efficient frequency conversion to ultra violet and visible wavelengths. Several variants of the diode-pumped solid state laser have been developed, and of these, the Nd:YLF (neodymium: yttrium lithium fluoride) laser produces the highest pulse energy and average power in the repetition rate ranging from a single pulse up to approximately 6 kHz. This tutorial explores the operation of a Nd:YLF multi-pass slab laser side-pumped by two collimated diode-laser bars.
Selected Literature References
Reference Listing - The reference materials listed in this section, gathered from our vast optical microscopy library, are an excellent source of additional information on microscope illumination. Included are references to review articles and original research reports that discuss how microscopes are configured to take advantage of various illumination scenarios and the critical illumination required for high-quality photomicrography. In addition, basic articles describing the properties of a variety of natural and artificial light sources are included.
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
Matthew J. Parry-Hill, Thomas J. Fellers, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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