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Microscope Light Sources

Microscope Light Sources

The overall 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. These, in turn, are affected by the shape and position of lenses and mirrors within the 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.

Review Articles

Fundamentals of Illumination Sources for Optical Microscopy - This discussion addresses brightness, stability, coherence, wavelength distribution, and uniformity in the most common light sources currently employed for investigations in transmitted and fluorescence microscopy.

Tungsten-Halogen Lamps - Incandescent tungsten-halogen lamps have been successfully employed as a highly reliable light source in optical microscopy for many decades and continue to be the one of the illumination mechanisms of choice for a variety of imaging modalities.

Mercury Arc Lamps - The mercury arc lamp remains a workhorse in fluorescence microscopy and is still considered one of the best illumination sources, especially for those fluorophores whose excitation maxima coincide with the spectral lines emitted by the hot mercury plasma.

Xenon Arc Lamps - The xenon arc lamp, which features a largely continuous and uniform spectrum across the entire visible spectral region, is suited to stringent applications requiring the simultaneous excitation of multiple fluorophores over a wide wavelength range in analytical fluorescence microscopy.

Metal Halide Lamps - Metal halide illumination sources are rapidly emerging as a serious challenger to the application of mercury and xenon arc lamps for investigations in fluorescence microscopy. These light sources feature a high-performance arc discharge lamp housed in an elliptical reflector that focuses the output into a liquid light guide for delivery to the microscope.

Light-Emitting Diodes (LEDs) - Among the most promising of emerging technologies for illumination in optical microscopy is the light-emitting diode (LED). High-power diodes generate sufficient intensity to provide a useful illumination source for a wide spectrum of applications in fluorescence microscopy.

Light Source Power Levels - Choosing the appropriate light source for investigations in optical microscopy is highly dependent upon the illumination strategy (transmitted or episcopic), specimen parameters, microscope configuration, and the detector sensitivity.

Interactive Tutorials

Arc Lamp Instability - Illumination sources based on plasma discharge (arc lamps) require a considerable period after ignition to reach thermal equilibrium, a factor that can affect temporal, spatial, and spectral stability. This tutorial examines several of the origins of arc lamp instability, including wander, flare, and flutter.

Halogen Regenerative Cycle - In the halogen regenerative cycle, which operates in tungsten halogen incandescent lamps, vaporized tungsten reacts with hydrogen bromide to form gaseous halides that are subsequently re-deposited onto cooler areas of the filament rather than being slowly accumulated on the inner walls of the envelope. This interactive tutorial demonstrates how halogens combine with tungsten and oxygen to complete the halogen regenerative cycle in incandescent tungsten halogen lamps.

Coherence of Light - One of the important parameters of illumination sources is their coherence, which is somewhat related to brightness due to the fact that extremely bright light sources are more likely to be highly coherent. This tutorial examines how incoherent light emitted by an arc lamp can be passed through a slit and filter to increase coherence and narrow the wavelength band.

Elliptical Reflectors - Advanced light sources suitable for use in high-performance fluorescence microscopy couple metal halide arc lamps with elliptical collection mirrors and high-speed filter wheels for rapidly shifting the output wavelength. These sources also provide fiber optics or liquid light guides for coupling the output to the microscope optical train. This interactive tutorial explores how careful positioning of the arc with respect to elliptical reflector focal points is critical to the formation of a focused beam at the input of a liquid light guide.

Mercury Lamphouses - High pressure mercury plasma arc-discharge lamps are highly reliable, produce very high flux densities, and have historically been widely used in fluorescence microscopy. This interactive tutorial examines advanced mercury arc lamphouses that are capable of automatic bulb alignment and intensity control.

Light-Emitting Diode Operation - Among the most promising of emerging technologies for illumination in optical microscopy is the light-emitting diode (LED). These versatile semiconductor devices possess all of the desirable features that incandescent (tungsten-halogen) and arc lamps lack, and are now efficient enough to be powered by low-voltage batteries or relatively inexpensive switchable power supplies. This interactive tutorial explores how two dissimilar doped semiconductors can produce light when a voltage is applied to the junction region between the materials.

LED Illumination for Microscopy - Among the most promising of emerging technologies for illumination in optical microscopy is the light-emitting diode (LED). These versatile semiconductor devices possess all of the desirable features that incandescent (tungsten halogen) and arc lamps lack, and are now efficient enough to be powered by low-voltage batteries or relatively inexpensive switchable power supplies. The interactive tutorial featured in this section explores the ZEISS Colibri LED illumination system for widefield fluorescence microscopy.