Image Formation

Section Overview:

In the optical microscope, image formation occurs at the intermediate image plane through interference between direct light that has passed through the specimen unaltered and light diffracted by minute features present in the specimen. The image produced by an objective lens is conjugate with the specimen, meaning that each image point is geometrically related to a corresponding point in the specimen. It follows that each point in the specimen is therefore represented by a corresponding point in the image.

Image resolution and contrast in the microscope can only be fully understood by considering light as a train of waves. Light emitted by a particular point on a specimen is not actually focused to an infinitely small point in the conjugate image plane, but instead light waves converge and interfere near the focal plane to produce a three-dimensional diffraction pattern. The ensemble of individual diffraction patterns spatially oriented in two dimensions, often termed Airy patterns, is what constitutes the image observed when viewing specimens through the eyepieces of a microscope. These and related concepts are discussed more fully in the sections listed below.

Review Articles

Overview of Image Formation

When direct or undeviated light from a specimen is projected by the objective, it is spread evenly across the entire image plane at the diaphragm of the eyepiece. The light diffracted by the specimen is brought to focus at various localized sites on the same image plane, and there the diffracted light causes destructive interference. A consequence is the reduction in light intensity resulting in more or less dark areas. These patterns of light and dark are what we recognize as an image of the specimen. Since our eyes are sensitive to variations in brightness, the image then becomes a more or less faithful reconstitution of the original specimen.

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Selected Literature References

Selected Literature References

An understanding of the distribution of light intensity throughout images observed in the optical microscope involves the laws of physical optics. Of primary consideration is the diffraction pattern exhibited by the specimen, which is composed of an array of elementary constituents known as the Airy disk. These and related concepts are reviewed in the reference materials listed in this section.

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

Brian O. Flynn, Kirill I. Tchourioukanov, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.