Microscope Optical Components
Conjugate Field Planes
The geometrical relationship between image planes in the optical microscope configured for infinity correction with a tube lens is explored in this tutorial. In all of the imaging steps, with the exception of Image Plane (3'), the image is real and inverted.
The tutorial initializes with the Object Height slider set to the largest available value. The microscope optical tube length, which is the distance between the tube lens rear principal plane and the intermediate image plane, is indicated in the tutorial diagram. Light focused by the objective produces an image at I(3) (the intermediate image) that is further magnified by the eyepiece to produce an image on the retina at I(4). When the microscope eyepiece is used for direct viewing rather than for projection, the image at I(3') is not real but virtual and is not inverted relative to the intermediate image. The human eye will not perceive the image on the retina as inverted, even though the image is inverted in relation to the intermediate image and the virtual image.
A majority of modern research microscopes are equipped with infinity-corrected objectives that no longer project the intermediate image directly into the intermediate image plane. Light emerging from these objectives is instead focused to infinity, and a second lens, known as a tube lens, forms the image at its focal plane. Wavetrains of light leaving the infinity-focused objective are collimated, allowing beamsplitters, polarizers, Wollaston prisms, etc., to be introduced into the space between the objective and tube lens.
In a microscope with infinity-corrected optics, magnification of the intermediate image is determined by the ratio of the focal lengths of the tube lens and objective lens. Because the focal length of the tube lens varies between 160 and 250 millimeters (depending upon the manufacturer and model), the focal length of the objective can no longer be assumed to be 160 millimeters divided by its magnification. Thus, an objective having a focal length of 8 millimeters in an infinity-correct microscope with a tube lens focal length of 200 millimeters would have a lateral magnification of 25x (200/8).
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
Matthew J. Parry-Hill, John C. Long, 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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