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Prisms and Beamsplitters

Beamsplitters and prisms are not only found in a wide variety of common optical instruments, such as cameras, binoculars, microscopes, telescopes, periscopes, range finders, and surveying equipment, but also in many sophisticated scientific instruments including interferometers, spectrophotometers, and fluorimeters. Both of these important optical tools are critical for laser applications that require tight control of beam direction to precise tolerances with a minimum of light loss due to scatter or unwanted reflections. Illustrated in Figure 1 is a diagram of a typical binocular microscope observation tube configuration. In order to divert light collected by the objective into both eyepieces, it is first divided by a beamsplitter and then channeled through reflecting prisms into parallel cylindrical optical light pipes. Thus, the binocular observation tube utilizes both prism and beamsplitter technology to direct beams of light having equal intensity into the eyepieces.

Introduction - Prisms and beamsplitters are essential components that bend, split, reflect, and fold light through the pathways of both simple and sophisticated optical systems. Cut and ground to specific tolerances and exact angles, prisms are polished blocks of glass or other transparent materials that can be employed to deflect or deviate a light beam, rotate or invert an image, separate polarization states, or disperse light into its component wavelengths. Many prism designs can perform more than one function, which often includes changing the line of sight and simultaneously shortening the optical path, thus reducing the size of optical instruments.

Interactive Java Tutorials

Common Reflecting Prisms - The angular parameters displayed by various prism designs cover a wide gamut of geometries that dramatically extend the usefulness of prisms as strategic optical components. Reflecting prisms are often designed to be located in specific orientations where the entrance and exit faces are both parallel and perpendicular to the optical axis. This interactive tutorial explores image deviation, rotation, and displacement exhibited by common reflecting prisms.

Right-Angle Prisms - The right-angle prism possesses the simple geometry of a 45-degree right triangle, and is one of the most commonly used prisms for redirecting light and rotating images. This interactive tutorial explores light reflection and image rotation, inversion, and reversion by a right-angle prism as a function of the prism orientation with respect to incident light.

Refraction by an Equilateral Prism - Visible white light passing through an equilateral prism undergoes a phenomenon known as dispersion, which is manifested by wavelength-dependent refraction of the light waves. This interactive tutorial explores how the incident angle of white light entering the prism affects the degree of dispersion and the angles of light exiting the prism.

Transmission and Reflection by Beamsplitters - A beamsplitter is a common optical component that partially transmits and partially reflects an incident light beam, usually in unequal proportions. In addition to the task of dividing light, beamsplitters can be employed to recombine two separate light beams or images into a single path. This interactive tutorial explores transmission and reflection of a light beam by three common beamsplitter designs.

Dielectric Plate Beamsplitters - The simplest configuration for a beamsplitter is an uncoated flat glass plate (such as a microscope slide), which has an average surface reflectance of about 4 percent. When placed at a 45-degree angle, the plate will transmit most of the light, but reflect a small amount at a 90-degree angle to the incident beam. Plate beamsplitters are, as the name implies, optical crown glass plates having a partially silvered coating designed to produce a desired transmission-to-reflection ratio. These ratios usually vary between 50:50 and 20:80, depending upon the application.

Beam Steering by Wedge Prisms - Circular prisms having plane surfaces positioned at slight angles with respect to each other are termed optical wedges, and deflect light by refraction rather than reflection. Although wedges are prismatic in nature, they can be manipulated to act as beamsplitters or beam steerers. This interactive tutorial explores how two wedge prisms operate together to deflect an incident light beam.

Birefringent Polarizing Prisms - Polarizing prisms are utilized in a wide spectrum of applications ranging from optical microscopy and spectroscopy to complex laser systems. This interactive tutorial explores how various common birefringent polarizing prisms operate to split light waves into ordinary and extraordinary components.

Selected Literature References

Selected References - A number of high-quality review articles on prisms and beamsplitters have been published by leading investigators in the fields of optics and photonics. This section contains periodical and book location information about these articles, as well as providing a listing of the chapter titles for appropriate sections dealing with beamsplitters and prism systems.

Contributing Authors

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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