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Microscope Lenses Made Simple.

Operating and specifying microscopes and microscope components intimidates the uninitiated. However, reducing this problem to its basic elements--that is, the optical components--makes it more manageable. Each component has some rules that govern the best choice for your needs.

For eyepieces, the first parameter you may know is power. This is usually engraved on the eyepiece as a number--5x, 10x, 20x. Power is frequently misunderstood and almost always over stressed in the initial list of parameters.

Think of power in terms of diameters--1x power is the diameter of an object held 25.4 cm from the unaided eye. That distance is the near point, the closest distance at which a normal unaided eye can keep an object in focus. A power of 10x means the image in the magnifier or microscope is 10 times that of the same object held 25.4 cm from an unaided eye.

When in doubt, start with a 10x eyepiece because of its three-quarter inch eye relief--the distance between the observer's eyeball and the eyepiece. Because doubling the power halves the eye relief, 20x power is uncomfortable to use for prolonged viewing. It works well, however, for occasional use.

Field of view is another important consideration when choosing an eyepiece. It describes the diameter in mm of the portion of the object plane seen by the eyepiece, with a properly selected microscope objective and tube length.

If you are curious about your eyepiece, look into its bottom. If the first thing you see is the stop--the metal ring limiting the field of view--you are holding a Ramsden-type eyepiece with the stop on the bottom. Huyghens-type eyepieces have the stop between the lenses, where it's not readily accessible. They are usually lower power. Ramsden designs usually have a wider field angle.

The size of the reticle in practice is determined by the field of view. Most designers choose a number between 18 and 20mm, which accommodates most reticles. Reticles are usually fabricated from chrome on glass, which does not lend itself to edge illumination as well as older etch and fill types. However, the economy of fabrication has made chrome and glass reticles popular.

In many cases, stock reticles can be installed in either Ramsden-type or Huyghens-type eypieces. By far the most common reticule diameter for after market installation is 21 mm.

A stage micrometer is a very precise large reticle scale that is laid on the microscope stage in the plane of the object. It is used to calibrate the eyepiece reticle when very precise measurements are required. It can also be used to read dimensions directly when the eyepiece has no reticle.

The mechanical tube length is the distance from the eyepiece flange to the objective flange. The standard distance is 160 mm, though others exist. You can sometimes use a shorter tube--if it is available--to lengthen objective working distance, but this also reduces power. Some microscopes have variablelength eyepiece tubes to accommodate shorter distances. All tubes should accept the standard 23-mm eyepiece as a slip fit at one end and the Royal Microscopal Society (RMS) thread, a 0.800 inch 36 TPI Whitworth thread, at the objective end.

Objectives are characterized by power and numerical aperture, which are engraved on the lens. Objective power, which is stated as 1:1, 2:1, 5:1, etc., when multiplied by the eyepiece power equals the power of the microscope.

The objective working distance is fixed by the design and tube length and can be thought of as the mechanical clearance between the bottom of the objective and the sample or cover glass. The numerical aperture, defined as n.a.= n sin[Theta]. The term n is the index of the medium between the object and the microscope objective, usually air (n=1.0). And [Theta] is the half angle of the cone of light entering the objective.

Many different objectives are available, but, as with eyepieces, higher power reduces working distance, which frequently dictates microscope requirements and limits the power. This working distance/power limit at the objective may force the use of higher power in the eyepiece even though it is uncomfortable.

Ross is the president of Rolyn Optics Co., Covina, Calif.
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Author:Ross, John
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Sep 1, 1999
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