What are the major types of optical microscopes?
November 23, 2008
The most common types of microscopes are classified according to the type of view they provide. A high-power or compound microscope gives a highly magnified view of a tiny cell, mineral grain, or pixel on a chip. With its straight barrel leading from specimen to eyepiece, this is the kind of instrument most people visualize when thinking of the word microscope. A low-power or dissecting microscope is used for broader views of a whole insect or circuit. Low-power microscopes are most often built as two separate optical tubes providing a 3D image: in this form they are called stereo microscopes.
A compound microscope gets its name from its use of several lenses in series to provide its high magnifications. Usually there are two lenses used to produce an image: the objective and the eyepiece. Each lens has a magnification rating and the overall power of the microscope is calculated by multiplying the objective’s power by the eyepiece’s power. Compound scopes often have turrets mounting multiple (2-5) objectives just above where the object to be viewed is positioned, so magnifications can be quickly switched by rotating the turret to a lower- or higher-powered objective. The lowest-power objectives have shorter barrels. The magnifications can range from 40x to over 2000x. The final image the eye sees is two dimensional (2D) and usually reversed and upside down.
Because they are widely used in biology or medicine, compound microscopes are often called biological or research microscopes. Common features are an adjustable stage (viewing platform), illuminator (a light source usually transmitting or shining light up through the specimen), and coarse and fine focusing knobs.
Stereo microscopes are optimized for lower-power, 3D views where the specimen or object can be observed and manipulated at the same time. The principal idea is to see much of a specimen like a flower, coin, or stamp at once. The distinctive paired optics and associated prisms provide an upright and normal image and a magnification range from 10x to 80x. Common features include coarse and fine focus knobs, adjustable stage, diopter (individually adjustable) eyepiece, and illuminators for both transmitted and reflected light.
Note: Binocular microscopes are different than stereo microscopes. Stereo microscopes have two complete optical systems or barrels, each consisting of objective, prism, and eyepiece. Since the objectives are looking at the specimen from two different angles, true 3D views are produced. Binocular microscopes have just one barrel and a prism assembly mounted on top that splits the light, directing it to two eyepieces, so there is no true 3D vision; just the comfort of using two eyes at once.
Low-power microscopes are sometimes seen in single-barrel designs as well. These dissecting or inspection microscopes look similar to and function like compound microscopes, providing 2D views. Since they have only one barrel, they are less expensive than stereo microscopes.