the optical (light) microscope Download

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THE OPTICAL (LIGHT) MICROSCOPE
 It is the most important tool to study of microstructure,
despite the fact that more sophisticated electron
metallographic instruments, such as scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM), have been evolved.
 Electron microscopy should be used in conjunction with
optical microscopy, rather than as a substitute.
 All examinations of microstructure should begin with use of
the optical microscope, starting at low magnification, such
as 100×, followed by progressively higher magnifications to
assess the basic characteristics of the microstructure
efficiently.
 Most microstructures can be observed with the optical
microscope and identified based on their characteristics.
 Identification of questionable or unknown constituents may be
aided by observation of their hardness relative to the matrix, by
their natural colour, by their response to polarized light, and by
their response to selective etchants.
 These observations are compared to known details about the
physical metallurgy of the material being examined. If doubt still
remains or if the structure is too fine to observe, more sophisticated
techniques must be implemented.
 Polished or etched metallographic specimens are examined by
optical microscope.
 Some constituents such as inclusions, nitrides, certain carbides, and
intermetallic phases can be readily observed without etching.
 Specimens that respond to polarized light, such as materials with
non-cubic crystal structures, are also examined without etching.
 Other phases may be more easily examined if some relief is
introduced during final polishing. The specimen must be adequately
prepared free from artifacts for correct observation
 Etching must be performed carefully to observe the
microstructure.
 A general-purpose etchant is normally used first to reveal the
grain structure and the phases present, followed by selective
etchants that attack or colour specific phases of interest.
 Selective etchants are widely used for quantitative
metallography, particularly if performed using an automated
device.
Microscope Components
 Reflected light is used for the study of metals.
 Transmittedlight microscopes are used to study minerals and
polymers, which can also be examined using reflected light.
 Optical microscopes are also classified as "upright" or "inverted";
these terms refer to the orientation of the plane of polish of the
specimen during observation.
 Figure 1 illustrates the light path in the two designs.
 The simplest optical microscope is the bench type (usually
upright).
 Photographic capabilities can be added to some units depending
on the rigidity of the stand. Figure 2 illustrates basic bench
microscopes, and Figure 3 shows research-quality bench
microscopes suitable for photographic work.
Fig. 1 Light paths in (a) an upright incident-light microscope and (b)
an inverted incident-light microscope. (E. Leitz, Inc.; C. Zeiss, Inc.)
Fig. 2 (a) Upright bench microscope. (b) Inverted bench microscope
Fig. 3 Research-quality optical microscopes. (a) Upright. (b) Inverted.
(E. Leitz, Inc.; Unitron Instruments, Inc.)
Fig. 4 Moderately priced
inverted metallograph. The
small box to the right is an
automatic exposure control.
(Nikon, Inc.)
Fig. 5 Research-quality metallograph
with a projection screen for group
viewing. (E. Leitz, Inc.)
Illumination System.
A variety of light sources for optical microscopy are available.
 The low-voltage tungsten filament lamp used primarily with bench
microscopes has adequate intensity for observation, but not for
photography. Altering the current to the bulb controls light intensity.
 Carbon-arc illumination systems, once common on metallographs,
have been replaced by arc or filament light sources.
 The xenon-arc light source is prevalent because of its high intensity
and daylight colour characteristics. Light intensity, however, can be
adjusted only by the use of neutral-density filters.
 Tungsten-halogen filament lamps are also widely used for their high
intensity and high colour temperature. Light intensity can be
controlled by varying the current or by use of neutral-density filters.
 Other light sources, such as the zirconium-arc, sodium-arc, quartziodine, or mercury-vapor lamps, are less common.
Condenser.
 An adjustable lens free of spherical aberration and coma is 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.
 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 of light and the
angle of the cone of light entering the objective lens.
 The optimum setting for this aperture varies with each objective lens and
is a compromise among image contrast, sharpness, and depth of field.
 Opening this aperture increases image sharpness, but reduces contrast;
closing the aperture increases contrast, but impairs image sharpness.
 The aperture diaphragm should not be used for reducing light intensity. It
should be adjusted only for contrast and sharpness.
Light filters
They are used to modify the light for ease of observation, for
improved photomicroscopy, or to alter contrast.
 A green or yellow-green filter is widely used in black-and-white
photography to reduce the effect of lens defects on image quality.
Most objectives, particularly the lower cost achromats, require
such filtering for best results.
 Polarizing filters are used to produce plane-polarized light (one
filter) or crossed-polarized light (two filters rotated to produce
extinction) for examination of noncubic (crystallographic)
materials. Materials that are optically anisotropic, such as
beryllium, zirconium, α-titanium, and uranium, can be examined
in the crossed-polarized condition without etching. A sensitivetint plate may also be used with crossed-polarized light to
enhance coloration.
The objective lens
It is used to form the primary image of the microstructure and is the
most important component of the optical microscope. The objective
lens collects as much light as possible from the specimen and
combines this light to produce the image. The numerical aperture
(NA) of the objective, a measure of the light-collecting ability of the
lens, is defined as:
NA = n sin α
(Eq 1)
where n is the minimum refraction index of the material (air or oil)
between the specimen and the lens, and α is the half angle of the
most oblique light rays that enter the front lens of the objective.
Light-collecting ability increases with α. The setting of the aperture
diaphragm will alter the NA of the condenser and therefore the NA of
the system.
The most commonly used objective is the achromat, which is corrected
spherically 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.
Use of a yellow-green filter and orthochromatic film yields optimum
results. However, achromats do provide a relatively long working
distance, that is, the distance from the front lens of the objective to the
specimen surface.
Working distance decreases as magnification of the objective
increases. Most manufacturers make long-workingdistance objectives
for special applications, for example, in hot-stage microscopy.
Achromats are strain free, which is important for polarized light
examinations. Because they contain fewer lenses than other more
highly corrected lenses, internal reflection losses are minimized.
 Semiapochromatic or fluorite objectives provide a higher degree of
correction of spherical and chromatic aberration. Therefore, they
produce higher quality images than achromats.
 The apochromatic objectives have the highest degree of correction,
produce the best results, and are more expensive.
 Plano objectives have extensive correction for flatness of field,
which reduces eyestrain, and are often found on modern
microscopes.
 Figure 6 illustrates three plano-type objectives. Each is coded as to
the type of objective, its magnification, and numerical aperture.
 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.
Fig. 6 Plano-type objective lenses and cross sections through each. The lens shown
in (c) is a 14-element oilimmersion objective, with a numerical aperture (NA) of 1.32.
Because the lens and specimen must be cleaned between each use, oil immersion is
rarely used; it does provide higher resolution and a crisper image, which is valuable
for examining low-reflectivity specimens. (E. Leitz, Inc.)
The eyepiece (ocular),
 It 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-diam field of view; wide-field
eyepieces for plano objectives have a 30-mm-diam field of view (Fig.
7), which increases the usable area of the primary image.
 The simplest eyepiece is the Huygenian, which is satisfactory for use
with low- and medium-power achromat objectives.
 Compensating eyepieces are used with high NA achromat and the
more highly corrected objectives. Because some lens corrections are
performed using these eyepieces, the eyepiece must be matched
with the type of objective used.
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. Vision problems
such as astigmatism cannot be corrected by the microscope, and
glasses must be worn.
High-eyepoint eyepieces are available to provide the eye clearance
of approximately 20 mm necessary for glasses (Fig. 8).
Fig. 7 Cross sections of
typical eyepieces. (a)
Standard (24-mm) field of
view. (b) Wide (30-mm)
field of view. The widefield eyepiece increases
the usable area of the
primary image. (E. Leitz,
Inc.)
Fig. 8 Comparison between the position of the eye with (a) a
standard eyepiece and (b) a high-point eyepiece. Eye clearance with a
standard eyepiece is approximately 10 mm (0.4 in.); a high-point
eyepiece allows clearances of approximately 20 mm (0.8 in.) (E. Leitz,
Inc.)
Eyepieces are commonly equipped with various reticles or
graticules for locating, measuring, counting, or comparing
microstructures. The eyepiece enlarges the reticle or graticule
image and the primary image.