Welcome to Electron Microscope .COM website.  We are glad you have chosen to visit us.  The purpose of this website is to provide an internet location where news articles, references, website links, informative articles, and advertisers can all meet to discuss the exciting field of electron microscopy.  We also provide an introduction to the fundamentals of electron microscopy and the use of the electron microscope.  In short, an electron microscope is an instrument that produces images from a specimen by means of a beam of electrons.  This is in contrast to a compound light microscope that uses light waves in the visible spectrum to produce a microscopic image.

There are two main types of electron microscopes.  One is the transmission electron microscope, also know as T.E.M. and the next is the scanning electron microscope, S.E.M..  There are other types as well but these are the main types of electron microscopes.  Both the T.E.M. and the S.E.M. utilize a beam of free electrons focused on the specimen that are discharged from an electron gun.  The most common type of electron microscope is the transmission electron microscope.  It is often used in medical diagnostic and research laboratories.

It is a great tool for the fields of biology and histology and electron photomicrographs from it are often used in textbooks relating to those fields.  Just as the name implies, the transmission electron microscope “transmits” a beam of electrons through the specimen.  This is similar to a compound light microscope that uses transmitted illumination to pass the focused light throught the specimen on a slide.  And as the name implies, the scanning electron microscope scans the surface of a specimen with a focused beam of electrons, reflecting them off the specimen.
This provides the microscopist a high level of surface detail. This can be compared to the low power stereoscopic microscope that uses light from above (incident light) to reflect the light off of the specimen.  The stereo microscope is also used for 3D spacial viewing of microscopic specimens.  The same goes for the scanning electron microscope, as it provides three dimensional photomicrographs.  The electron microscope allows us a detailed view of specimens far beyond the viewing capacity of the compound light microscope and the stereo microscope.  But, all electron microscope photos are obtained in black and white.  They sometimes will be colored by an illustrator for a textbook.  The electron microscope is not capable of viewing or imaging in color.

Advantages of electron microscopy are numerous.  The most obvious benefits of using an electron microscope are the higher level of magnification and the extremely fine microscopic imaging resolution.  The resolution of an instrument is dependent on the wavelength of the illumination radiation.  The smaller the wavelength of radiation, the more detailed and greater will be the resolution.  It is an inversely proportional relation.  White light, as used in the common compound light microscope, is between 400nm and 800nm.  Theoretical calculations show us that the light microscope has a maximum resolution of about 0.2 micrometers.

The electron microscope can provide a wavelength of electron beams much smaller.  The accelerating voltage of the electron microscope determines the wavelength but is going to be around 0.5 nanometers to 0.09 nanometers.  The theoretical calculations show us that the electron microscope has a maximum resolution of 0.2 nanometers to 0.07 nanometers.  This means that the electron microscope is capable of around 1000 times greater resolution than the conventional light microscope.  This resolution is directly dependent on the level of detail that can be viewed from the specimen.  Even when used at the same magnification as a light microscope, the electron microscope is capable of a much greater image resolution providing stunning images with unparalleled detail.

The parts of an electron microscope can be compared to the parts of a light microscope.  The imaging media and construction are different, but the purpose is essentially similar.  The transmission electron microscope passes the electron beam through the specimen and obtains a resultant image.  The beam of electrons passed through the specimen contain some electrons that change direction and velocity.  This is similar to the brightfield transmitted compound light microscope passing light through the specimen.  In a light microscope, the illumination source is typically a high intensity halogen bulb.
In an electron microscope, a heated tungsten filament is used to generate the beam of electrons.  The electrons are accelerated using the high voltage differential between the cathode and the anode of the electron gun.  The accelerating voltage is adjusted to change the velocity and wavelength of the electron beam.  The typical voltage in an electron microscope is about 100 kV but can go up to 1000kV for special applications.
The electron microscopist adjusts the voltage based on what type of specimen is being examined and the desired magnification.
One unique difference between light microscopy and a electron microscopy is that the electron microscope needs a vacuum to operate in.  This is because the electron beam will collide with molecules in the air, such as nitrogen and oxygen, if the air has not been evacuated.  An advanced high vacuum pumping system is employed in the electron microscope.  A common vacuum level is 1.33 x 10 (exp. -4) Pascals.  It is estimated that the electrons can travel for about 2.5 meters before they would collide with a gas molecule in the vacuumed air.  The electron microscope contains a number of air locks to facilitate the changing of specimens.  The specimen is within the vacuum, but the air locks allow some of the microscope to be under vacuum while specimen changes are made.

In a light microscope, the light is manipulated to form the desired image of the specimen.  Similarly, in an electron microscope, the electron beam is manipulated but it is done with electro-magnetic lens, not with the glass lens as found in the light microscope.  Glass lens would interfere with the electrom beam.  The electro-magnetic lens can be thought of as magnets that shape and control the focused beam of electrons.  The electron microscope has a series of electro-magnetic lens, each being independantly able vary the results by changing the electrical current that passes through each of them.
In a compound light microscope, glass objective lens are changed to give different results, but in an electron microscope, the electical current associated with each electo-magnetic lens is independently varied to give the final toughups to the electron beam.  The transmission electron microscope usually has two electro-magnetic condenser lens before the electron beam reaches the specimen.  After the specimen, it has an electro-magnetic objective lens, intermediate lens, and a projection lens.

The final projection lens projects the electron beam on a fluorescent green or photographic plate for the microscopic image to be viewed.  The first objective lens enlarges the electron beam image from the specimen.  The intermediate lens enlarges it more, and so does the final projection lens.  Each of these lens acts as an image magnifier.  The electron microscope is capable of magnifications as low as 10x and as high as 1,000,000x, all while maintaining an incredibly level of detail and resolution.  Most electron microscope magnifications are done up to 150,000x.

As in a compound light microscope, the objective lens of an electron microscope is primarily responsible for determining the resolution. The following intermediate lens and projection lens simply “compound” the magnification, increasing it as is done in the compound light microscope using an objective and an eyepiece to produce a final compounded magnification.  Just as the objective lens in a light microscope is subject to lens aberrations, so is the objective lens in an electron microscope.  It is subject to electronic optical aberrations.

The electron microscope’s depth of field in the objective lens is comparatively much greater than in a compound light microscope, considering the magnification.  It can be about 200 nanometers (0.2 micrometers).  This is in the range of the thickness of the specimen, as the specimen is much thinner for a transmission electron microscope than for a light microscope.  In light microscopy, using a high power compound objective lens, the microscopist must use the fine focus frequently to see detail within the thin biological specimen.

The transmission electron is capable of viewing the whole image within its depth of field range, meaning it is all in focus.  Specimen preparation for a transmission electron microscope is challenging.  A typical biological compound light microscope in a medical research laboratory may take a microtome and cut a specimen about two to six micrometers thick.  The thickness of a specimen in a transmission electron microscope is in the range of 50 to 300 nanometers thick.  If the specimen is too thick, the electrons will not be able to be transmitted through it.

The projector lens of the electron microscope activates a fluorescent screen or produces a visible image on a photographic plate.  The electron beam is invisible to the human eye, and so is the direct image formed by it.
The scanning electron microscope cam available much later than the transmission electron microscope.  The SEM first sold in year 1965.  It also has a large depth of field.  It can be compared to a low power stereoscopic microscope.  A stereo microscope has a high depth of field and produces 3D images.  The scanning electron microscope can provide high resolution images of great detail, in a 3D type view, and all with an incredible depth of field.  The scanning electron microscope’s depth of field is approximately 500 times greater than for a light microscope at the same magnification.
The scanning electron microscope has an incident electron beam that scans the specimen’s surface.  The SEM utilizes a fluorescent screen or photographic film to view the specimen.  The magnification can be from 10x to 100,000x.  It has such high resolution it is able to distinguish features in specimens that are only 5 to 10 nanometers in diameter.  The scanning electron microscope often works in the magnification range of 1000x to 5000x, that is just bordering the limits of the light microscope and going beyond.  Anything more powerful than about 1250x in a light microscope is going to give you empty resolution.

One benefit of the scanning electron microscope compared to the transmission electron microscope is the former is able to view specimens that are not so thin.  It can exam most anything, such as crystals, insects, biological tissue samples, blood cells, and more.  But a drawback of the transmission electron microscope is that most of the specimens need a type of coating to prepare the specimen for imaging.  This can be a thin layer of vacuum-evaporated metal such as gold, palladium alloys, chromium, aluminum, and more.  These special preparation techniques add to the skill level required by the electron microscopist.

The advent of the electron microscope in the past century has advanced mankind to a new level of understanding in the micro world.  Just as the first compound microscopes developed an unparalleled excitement about the new world just beyond our eyesight, the age of electron microscopy has brought about another incredible scientific tool.  We hope you enjoy the articles and infomation about electron microscopes in this website.  Please visit us often.