Choosing the Right Microscope

Carl Zeiss Microscopy optical microscope

There are two main types of microscopes: optical microscopes and electron microscopes.

The main difference between these two types of microscopes lies in the way the sample to be observed is prepared and passed through. This is what determines the quality of the image (magnification, color, black and white).

With an optical microscope, the preparation is placed on a glass slide and irradiated with light rays:

• The resolution is in the range of 200 nanometers.
• For example, it is possible to observe an entire cell.
• However, the magnification is rather limited, meaning that details smaller than 200 nanometers cannot be observed.

With an electron microscope, an electron beam is passed through the prepared sample:

• The magnification is higher.
• However, the image is in black and white.
• Colors can be added digitally on the computer afterwards.

Jonel electron microscope

There are two types of electron microscopes: scanning and transmission.

• Scanning Electron Microscope (SEM):
  • This type of microscope emits electrons that sweep the surface of the prepared sample.
  • The resolution is very high, in the range of 0.4 to 20 nanometers, which makes it possible to differentiate between two points less than a nanometer apart.
  • The relief image enables the structure and the form of the sample to be studied.
  • This type of microscope is mainly used by biological research institutes to obtain the shape of cells or organs.
  • They can cost between 150,000 and a million euros.

• Transmission Electron Microscope (TEM):
  • This type of microscope emits electrons that go through the prepared sample.
  • This means the resulting image provides even the most minute details of the sample.
  • This type of microscope is used in cell biology because it is the only way to obtain accurate images of the inside of a cell.

There are several types of optical microscopes to choose from, depending on the sample you want to observe. First you have to choose between upright and inverted microscopes.

Upright microscopes:

• The light source is placed below the sample and the sample is observed from above.
• The upright microscope is used for observation and magnification of specimens on slides.

Inverted microscopes:

• The light source is placed above the sample and the objectives are below.
• The inverted microscope is used in the observation of cells in in vitro culture, and makes it possible to inspect thick objects or objects located on the bottom of petri dishes.

Microscopes are also available with different types of eyepieces.

Optika Srl monocular microscope

Monocular microscopes:

• The sample is observed with only one eye.
• They are inexpensive.
• They are uncomfortable and not suitable for prolonged observations.

ATM Binocular microscope

Binocular microscopes:

• With these microscopes, you observe the sample with both eyes, which reduces eye and muscle fatigue.
• They are suitable for prolonged use.
• They are also used for stereoscopic observation in order to perceive the depth of the sample.

EchoLAB trinocular microscope

Trinocular microscopes:

• Both eyes are used to observe.
• A camera, acting as a sort of “third eye”, is attached to the upper part of the microscope to film and record observations.
• These are mainly intended for experienced users.
• They are very expensive.

Keyence digital microscope

Digital microscopes:

• They have an LCD screen which allows several people to make observations at once.
• They are very comfortable to use as observation is carried out on the screen.
• The magnification is high, which allows for the observation of sample details.
• The image provided by this type of microscope is very accurate compared to other microscopes.
• It is also possible to film or take pictures of the observation.
• These microscopes are often used in laboratories. They are very expensive.

Huygens eyepieces:

• They have a single eyepiece.
• They are easy to use and inexpensive.
• They are suitable for beginners.
• They have a reduced field of view so they are not very comfortable.

Wide-field eyepieces:

• They have a wider field of view and are more comfortable.
• This type of eyepiece is higher quality, which explains its higher price.

Eyepiece micrometers:

• With these eyepieces you can observe and compare the dimensions of samples.
• They can also be used with other devices such as an object micrometer.
• They are primarily intended for experienced users who want to carry out in-depth observation of a sample.

The head of a microscope is the top part that the eyepieces are attached to. Different types of heads exist: straight heads and inclined heads.

Straight heads:

• The stage is usually fixed on these microscopes and the head is moved in order to focus.
• They are used with monocular microscopes with a single eyepiece.
• Straight-headed microscopes are suitable for beginners and occasional use because they are not very comfortable.

Inclined head:

• These microscopes are equipped with a mobile stage, the user does not need to move during focusing, the microscope head remains fixed.
• Microscopes with an inclined head are more expensive than straight-headed microscopes, but they are also more comfortable.
• They are intended for regular use and can be a used for a long period of time. They are more ergonomic and therefore more suitable for professional use.

Choosing the right microscope objective is critical in order to obtain an accurate image of a specimen (i.e. an image that provides magnification of the object but does not distort it.)

The objective you choose depends first of all on the desired magnification. There are three types of magnifications:

• low magnifications (from x1 to x10)
• medium magnifications (from x10 to x40)
• high magnifications (above x40)

The objective must also render an accurate image of the observed sample and correct defects in optical systems, also known as optical aberrations.

There are two types of optical aberrations: chromatic aberrations and monochromatic aberrations.

Chromatic aberrations:

• Chromatic aberration is a color defect, the result of the decomposition of white light into different colors.
• This decomposition results in different focuses depending on the wavelength.
• This means the image is blurry.
• Achromatic lenses compensate for this aberration and deliver a sharp image.

Monochromatic aberrations:

• Monochromatic aberration depends on geometric parameters (angle of field, pupil position) and impacts the microscope’s ability to distinguish details.
• This means the image is curved.
• Plan achromatic objectives are designed to correct image planeness while also compensating for chromatic aberration.

Assembly and quality inspection:

• Assembly and quality control microscopes are used to inspect materials and finished products.
• These microscopes are often equipped with a screen so that the items being viewed can easily be observed by several people.

Electronic inspection:

• With magnification of up to 400x, stereoscopic microscopes allow you to observe and find possible manufacturing defects in objects that fit in your hand such as printed circuit boards or workpieces.

Measurement:

• Measuring microscopes are used to make non-contact measurements on a sample. By using a telecentric objective, this type of microscope gives a perfectly flat image, ideal for dimensional measurements. Its precision stage means it can be used to make micromovements of the sample.
• This type of microscope provides accurate and reliable measurements on the X, Y and Z axes.
• These microscopes are particularly suited for applications when precision is very important.

Metallurgical:

• Metallurgical microscopes are designed to inspect bright, flat and/or reflective materials.
• They offer a higher magnification than stereoscopic microscopes.

Geology:

• Polarizing microscopes, also known as polarizing light microscopes, can reveal the chemical composition of many organic and inorganic materials.
• They are often used to study minerals in thin rock sections.
• Other materials observable with such microscopes include cement, ceramics, polymers, crystalline molecules, starch and wood.

In laboratories and hospitals:

• Biological microscopes are mainly used in laboratories to examine biological samples for research or diagnostic purposes.
• The source of light on a biological microscope always comes from underneath the stage because the samples being observed must allow the light to pass through so that, for example, blood cells or bacteria can be observed.