The objective lens forms the primary image of the microstructure and is the most important component of the light microscope. It is the lens closest to the object of interest. It collects as much light as possible from the specimen and combines this light to produce the image.
The condenser is an adjustable lens placed in front of the light source to focus the light at the desired point in the optical path. A field diaphragm is placed in front of this lens to minimize internal glare and reflections within the microscope. The field diaphragm is stopped down to the edge of the field of view, so the size of the illuminated area is limited to the observed field to minimize stray light.
The vertical illuminator usually provides only one or two types of illumination, such as bright-field and dark-field illumination, or bright-field and polarized light illumination. However, universal vertical illuminators are available that provide all types of illumination with one vertical illuminator and one set of objectives. Modern light microscopes are based on the principle of infinity corrected optics. This means that the reflected light forms a parallel beam between the objective and the tube lens. This principle allows placement of reflectors, prisms, and other components in the vertical illuminator without altering the magnification or the formation of the secondary image in the eyepieces.
The tube length is the length of the body tube from the eye line of the eyepiece to the objective thread. Most objectives are designed for use with a certain tube length, generally 160–250 mm (6–10 in.) and generally cannot be interchanged without changing the total magnification of the optical system.
The most commonly used objective is the achromat, which is corrected for spherical aberration for one color (usually yellow–green) and for longitudinal chromatic aberration for two colors (usually red and green). Therefore, achromats are not suitable for color photomicroscopy, except at low magnifications.
The parfocal distance of the objective is an important limiting factor for working distance. Working distance decreases as magnification of the objective increases. With parfocal lens systems, each objective on the nosepiece turret will be nearly in focus when the turret is rotated, preventing the objective front lens from striking the specimen when lenses are switched.
Certain objectives are designed for use with oil between the specimen and the front lens of the objective. Oil-immersion lenses are rarely used, because the specimen and lens must be cleaned after use. However, they do provide higher resolutions than can be achieved when air is between the lens and specimen. Use of oil also increases contrast and reducing glare of the image, which is valuable when examining low-reflectivity specimens, such as polymers or ceramics.
The eyepiece, or ocular, magnifies the primary image produced by the objective; the eye can then use the full resolution capability of the objective. The microscope produces a virtual image of the specimen at the point of most distinct vision, generally 250 mm (10 in.) from the eye. The eyepiece magnifies this image, permitting achievement of useful magnifications. The standard eyepiece has a 24 mm diameter field of view.
A ×10 magnification eyepiece is usually used; to obtain standard magnifications, some systems require other magnifications, such as ×6.3. The overall magnification is found by multiplying the objective magnification by the eyepiece magnification. The upper limit of useful magnification in a light microscope is approximately ×1500, and the fundamental limitations of light optic systems limit resolution to features of ~0.2 μm or larger.
Eye clearance is the distance between the eye lens of the ocular and the eye. For most eyepieces, the eye clearance is 10 mm or less—inadequate if the microscopist wears glasses. Simple vision problems, such as nearsightedness, can be accommodated using the fine focus adjustment.
A second adjustable-iris diaphragm, the aperture diaphragm, is placed in the light path before the vertical illuminator. Opening or closing this diaphragm alters the amount and angle of light entering the objective lens. Opening this aperture increases image resolution, but reduces contrast; closing the aperture increases contrast, but diminishes image resolution.
Light filters are used to modify the light for ease of observation, for improved photomicroscopy, or to alter contrast. Neutral-density filters are used to reduce the light intensity uniformly across the visible spectrum. Most light microscopes offer at least two such filters.
Selective filters are used to balance the color temperature of the light source to that of the film. This is often necessary for faithful reproduction of color images, depending on the light source used and the film type. Most objectives require such filtering for best results, as these objectives are not fully corrected for chromatic aberrations.
An example of a microstructure revealed through optical microscopy of a common titanium base alloy is shown in Fig. 4.