Microscopes
The microscope:
high precision optical instrument that uses a lens or a combination of lenses to
produce highly magnified images of small specimens or objects especially when
they are too small to be seen by the naked (unaided) eye. A light source is used
(either by mirrors or lamps) to make it easier to see the subject matter.
Microscopy:
Microscopy is the use of a microscope or investigation by a microscope.
Specific categories are microscopes used for or by:
 Hobbyists – gems, coins, stamps, collectibles, learning and discovery, etc.
 Education – chemistry, biology, botany, zoology
 Medical – microbiology, hematology, pathology, entomology, dermatology,
dental usage, veterinary use,
 everyday analysis to advanced research. From medical schools to labs to
hospitals
 Industry – inspection of electronic assembly components and many different
materials such as metals, textiles,
 plastics, etc. Used in agriculture, wineries, breweries, and for fine engravings
and mining inspection. Used by
 jewelers and geologists
 Science – for the study of archeology, oceanography, geology, metallurgy,
and numerous other fields
 Government – many areas for public health and safety such as water quality,
pharmaceuticals, forensics,
 asbestos, lab work, military applications, etc.
Types of Microscopes
The majority of microscopes are called light (bright field) microscopes since
they rely on light to observe the magnified image of a specimen or object. Within
this category are two main categories
(1) compound (high power microscopes)
(2) stereo or dissecting (low power microscopes).
Compound Microscope
This is the most common type of microscope. It can also be referred to as a
biological or research microscope. The compound microscope is what many refer to
as a high power microscope. The magnification (power) can have a range from about
40x to 1000x and some can go up to 1500x or 2000x. Compound refers to the fact
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that in order to enlarge an image, a single light path passes through a series of lenses
in a line where each lens magnifies the image over the previous one. In other words,
one light path with multiple lenses equals a compound microscope.
In the standard form the lenses consist of an objective lens (closest to the object
or specimen) and an eyepiece lens (closest to the observers’ eye) and a means of
adjusting the focus and position of the specimen or object. In addition, a compound
microscope uses light (reflected from a mirror, from indirect sunlight, from desk
lamps or other interior light sources, or from built-in lamps) to illuminate the
specimen or object so that you can see it with your eye.
The objective lens usually consists of three or four lenses (sometimes even five)
on a rotating nosepiece (turret) so that the power can be changed. The image
produced at the eye is two dimensional (2-D) and usually reversed and upside down.
The most used light method is trans-illumination (light projected from below to pass
through the specimen). At 400x much detail can be seen at the cellular level of
biological specimens. Learning about cells and microorganisms is
both educational and important for medical and science applications.
Other Types of Microscopes
These are usually advanced and expensive type microscopes made for specific
usages mainly in advanced medical and research. There are many, many types but
some of the more popular types are listed below.
1. Phase Contrast
2. Polarizing
3. Fluorescence
4. Metallurgical
5. Electron Beam
6. Digital
7. Handheld Digital Microscopes
Parts of a Compound Microscope
A- Structural components
The three basic, structural components of a compound microscope are the head,
base and arm.
 Head houses the optical parts in the upper part of the microscope
 Base of the microscope supports the microscope and houses the illuminator
 Arm connects to the base and supports the microscope head. It is also used to
carry the microscope.
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B- Optical components
There are two optical systems in a compound microscope: Eyepiece Lenses and
Objective Lenses:
1. Eyepiece or Ocular is what you look through at the top of the microscope.
Typically, standard eyepieces have a magnifying power of 10x. Optional
eyepieces of varying powers are available, typically from 5x-30x.
2. Eyepiece Tube holds the eyepieces in place above the objective lens.
3. Objective Lenses are the primary optical lenses on a microscope. They range
from 4x-100x and typically, include, three, four or five on lens on most
microscopes. Objectives can be forward or rear-facing.
4. Nosepiece houses the objectives. The objectives are exposed and are mounted
on a rotating turret so that different objectives can be conveniently selected.
Standard objectives include 4x, 10x, 40x and 100x although different power
objectives are available.
5. Coarse and Fine Focus knobs are used to focus the microscope.
Increasingly, they are coaxial knobs - that is to say they are built on the same
axis with the fine focus knob on the outside. Coaxial focus knobs are more
convenient since the viewer does not have to grope for a different knob.
6. Stage is where the specimen to be viewed is placed. A mechanical stage is
used when working at higher magnifications where delicate movements of the
specimen slide are required.
7. Stage Clips are used when there is no mechanical stage. The viewer is
required to move the slide manually to view different sections of the
specimen.
8. Aperture is the hole in the stage through which the base (transmitted) light
reaches the stage.
9. Illuminator is the light source for a microscope, typically located in the base
of the microscope. Most light microscopes use low voltage, halogen bulbs
with continuous variable lighting control located within the base.
10.Condenser is used to collect and focus the light from the illuminator on to the
specimen. It is located under the stage often in conjunction with an iris
diaphragm.
11.Iris Diaphragm controls the amount of light reaching the specimen. It is
located above the condenser and below the stage. Most high quality
microscopes include an Abbe condenser with an iris diaphragm. Combined,
they control both the focus and quantity of light applied to the specimen.
12.Condenser Focus Knob moves the condenser up or down to control the
lighting focus on the specimen.
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Electron Microscope
An electron microscope is a microscope that uses a beam of accelerated
electrons as a source of illumination. Because the wavelength of an electron can be
up to 100,000 times shorter than that of visible light photons, the electron
microscope has a higher resolving power than a light microscope and can reveal the
structure of smaller objects.
A transmission electron microscope can achieve better than 50 pm resolution
and magnifications of up to about 10,000,000x whereas most light microscopes are
limited by diffraction to about 200 nm resolution and useful magnifications below
2000x.
Electron microscopes are used to investigate the ultrastructure of a wide range
of biological and inorganic specimens including microorganisms, cells, large
molecules, biopsy samples, metals, and crystals. Industrially, the electron
microscope is often used for quality control and failure analysis. Modern electron
microscopes produce electron micrographs using specialized digital cameras and
frame grabbers to capture the image.
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Types of Electron Microscope
1. Scanning Electron Microscope (SEM)
In a scanning electron microscope or SEM, a beam of electrons scans the surface
of a sample. The electrons interact with the material in a way that triggers the
emission of secondary electrons. These secondary electrons are captured by a
detector, which forms an image of the surface of the sample. The direction of the
emission of the secondary electrons depends on the orientation of the features of
the surface. There, the image formed will reflect the characteristic feature of the
region of the surface that was exposed to the electron beam.
2. Transmission Electron Microscope (TEM)
In a transmission electron microscope or TEM, a beam of electrons hits a very
thin sample (usually no more than 100 nm thick). The electrons are transmitted
through the sample. After the sample, the electrons hit a fluorescence screen that
forms an image with the electrons that were transmitted. You can better
understand this process by imagining how a movie projector works. In a
projector, you have a film that has the negative image that will be projected. The
projector shines white light on the negative and the light transmitted forms the
image contained in the negative.
3. Scanning Transmission Electron Microscope (STEM)
A scanning transmission electron microscope or STEM combines the capabilities
of both an SEM and a TEM. The electron beam is transmitted across the sample
to create an image (TEM) while it also scans a small region on the sample (SEM).
The ability to scan the electron beams allows the user to analyze the sample with
various techniques such as Electron Energy Loss Spectroscopy (EELS) and
Energy Dispersive X-ray (EDX) Spectroscopy which are useful tools to
understand the nature of the materials in the sample.
Terms Associated with Microscopes
Magnification (power):
The magnification of a microscope is determined by multiplying the power of
the eyepiece by the power of the objective lens being used. As an example, a
40x objective lens times 10x eyepiece = 400x.
Lower powers allow for brighter, sharper images combined with a wide field
of view. Higher powers, often for examining slide specimens, present larger
but dimmer images with a narrower field of view. When observing, always
start with the lowest power on your microscope (like 4x) and progress to
higher powers.
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Field of View:
Is the diameter of the circle of light that you see when looking into a
microscope and it is measured in millimeters (mm). The lowest powers have
the widest field of view. As you increase power, the field of view gets smaller.
Depth of Field:
How much depth of field is a function of the objective lenses and means the
farthest and nearest points in the field of view are in simultaneous sharp focus.
Low magnification objectives have more depth of field than high
magnification objectives.
Depth of Focus:
Means the farthest and nearest points in the film plane (photomicrography) or
the CCD plane (video microphotography) which are simultaneously in focus.
It is just the reverse of the depth of field, where here greater depth of focus
occurs with high magnification objectives.
Flatness of Field:
A quality describing the appearance of the field of view as being flat from
edge-to-edge.
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