Like many other technological devices, microscopes have a very long history. The earliest microscopes contained simple magnifying glass with low power (up to 10 times). They were used to observe small insects such as fleas.
The early versions of optical microscopes were developed in the late 15th century. Although the inventor is unknown, several claims have been made over the years. The use of microscopes to study organic tissue didn’t appear until 1644.
Today, we have microscopes that can provide a 50-picometer level of resolution with a magnification of up to 50 million times, which is enough to observe the ultrastructure of various inorganic and biological specimens.
Modern microscopes can be categorized in different ways. One way to group them is the way they interact with samples to create images. Based on the same factor, we have listed the 5 main types of microscopes and their uses.
1. Optical Microscopes
Optical microscopes are the most common microscopes that use light to pass through a sample to generate images. They can have a very simple design, although complex optical microscopes aim to increase resolution and sample contrast.
They can be further subdivided into two types: simple and compound microscopes. A simple microscope uses one lens (such as a magnifying glass) for magnification, while the compound microscopes use several lenses to enhance the sample’s magnification.
They are often equipped with a digital camera so the sample can be observed via a computer. This allows an in-depth analysis of the microscopic image.
Optical microscopes can provide a magnification of up to 1250 times with a theoretical resolution limit of 0.250 micrometers. However, the development of super-resolved fluorescence microscopy in the recent decade has brought optical microscopy into the nano-dimension.
A compound microscope
Variants of Optical Microscope
A) Stereo microscope: is designed for observing samples in 3D at low magnification.
B) Comparison microscope: is used to examine side-by-side specimens.
C) Polarizing microscope: is used in optical mineralogy and petrology to identify minerals and rocks in thin sections.
D) Two-photon microscope: allows imaging of living tissue up to 1 mm in depth.
E) Inverted microscope: studies sample from below; generally used for metallography and cell cultures in liquid.
F) Epifluorescence microscope: developed for analyzing samples that include fluorophores.
Basic optical microscopes are often found in classrooms and at home. The complex ones are extensively used in pharmaceutical research, microbiology, microelectronics, nanophysics, and mineralogy.
They are often used to examine tissue in order to study the manifestation of diseases. In clinical medicine, the examination of a biopsy or surgical specimen refers to histopathology.
2. Electron Microscopes
The electron microscope uses a beam of accelerated electrons to produce an image of the sample. Just like optical microscopes use glass lenses, electron microscopes use shaped magnetic fields to generate electron-optical lens systems.
Since the wavelength of an electron can be far shorter than that of photons, electron microscopes have a higher resolving power and magnification than conventional optical microscopes. They can reveal structures of picometer-sized objects.
The first electron microscope — which exceeded the resolution achieved with an optical microscope — was developed by a German physicist Ernst Ruska in 1933. Since then, numerous improvements have been made to further improve the microscope’s magnification and resolution.
Modern electron microscopes are capable of magnifying samples up to 2,000,000 times, however, they still rely upon Ruska’s prototype (developed in 1931) and his link between resolution and wavelength.
Electron microscopes do have some limitations: they are expensive to build, maintain, and must be placed in stable environments such as magnetic field canceling systems. Also, objects have to be to viewed in a vacuum.
A modern transmission electron microscope | Credit: David J Morgan from Cambridge, UK
Two Main Types of Electron Microscope
1. Transmission electron microscope: is used to observe thin specimens through which electrons can pass generating a projection image. It can capture fine details as tiny as a single column of atoms.
In this case, the specimen is usually a very thin section (<100 nanometers) and an image is generated from the interaction of the sample with electrons as the beam is passed through the specimen.
Modern hardware correctors can help these microscope achieve a high resolution of 50 picometers with magnifications above 50,000,000 times.
2. Scanning electron microscope: generates images of a specimen by scanning its surface with a focused beam of electrons. The electrons interact with atoms in the specimen and generate signals which contain data about the sample’s composition and surface topography.
Since this type of microscopy images only the surface (not interior) of specimens, it produces low image resolution compared to transmission electron microscopy. However, it can generate good quality 3D images of the sample surface.
Things you can observe with a scanning electron microscope include elements on the head of a pin, human inner ear hair cells, and the surface of a housefly’s eye.
Electron microscopes are widely used to examine the ultrastructure of various inorganic and biological specimens, such as metals, crystals, biopsy samples, large molecules, cells, and microorganisms.
Modern electron microscopes are equipped with special digital cameras and frame grabbers to record the structure of the sample and generate electron micrographs.
They are often used for industrial purposes (to assist in the manufacturing process) and in forensic science (to provide evidence for crime and law purposes).
3. Scanning Probe Microscope
Scanning probe microscopy was discovered in 1981 to image sample’s surface at the atomic level. It uses a physical probe to scan the sample and form highly magnified images.
Based on the purpose of the study, different methods are used in scanning probe microscopy.
For instance, the instrument may be set to ‘tapping mode’ in which cantilever oscillates so that the tip touches the sample surface intermittently. This is mostly used to examine samples that have soft surfaces.
In another method, the microscope may be set to ‘contact mode’ in which constant force is applied between the tip of the cantilever and the surface of the sample. This mode produces surface images quickly.
Unlike electron microscopy techniques, samples do not require to be placed in a specific vacuum environment. Instead, they can be imaged in air at room pressure and temperature or inside a liquid reaction vessel. However, they are often not useful for analyzing liquid-liquid or solid-solid interfaces.
A modern scanning probe microscope
Common Types of Scanning Probe Microscopes
A) Atomic force microscope: has a resolution on the order of fractions of a nanometer, thus it can image almost any type of surface, including glass, polymers and biological samples.
B) Near-field scanning optical microscopy: can achieve spatial resolution performance beyond the classical diffraction limit. It can be used for studying all conductive, non-conductive and transparent samples.
C) Scanning tunneling microscopes: can achieve 0.1 nm lateral resolution and 0.01 nm depth resolution. Samples can be imaged in extreme environments, at temperatures ranging from near absolute zero to more than 1000°C.
Moreover, the scanning tunneling microscope was the first microscope to make use of quantum concepts, which paved the way for the development of the quantum entanglement microscope and photoionization microscope.
Scanning probe microscopes are used in a wide range of disciplines of the natural sciences, including medicine, cell, and molecular biology, solid-state physics, polymer chemistry, and semiconductor science and technology.
In molecular biology, for instance, this microscopy technique is used to analyze the structure and mechanical characteristics of protein complexes and assemblies. In cellular biology, it is used to determine interactions between certain cells and distinguish normal cells and cancer cells based on the hardness of cells.
In solid-state physics, it is used to study the interaction between nearby atoms, and changes in the atomic arrangement through atomic manipulation.
4. Scanning Acoustic Microscopes
A scanning acoustic microscope measures variations in acoustic impedance using sound waves. It is mostly used for non-destructive evaluation, failure analysis and identifying defects in the materials’ subsurfaces including those found in integrated circuits.
This type of microscope was first developed in 1974 at the Microwave Laboratory of Stanford University. Since then, numerous improvements have been made to enhance its accuracy and resolution.
The microscope directly focuses sound from a transducer at a small point on the sample. Sound hitting the objects is either absorbed or scattered to different angles. These scattered pulses traveling in a specific direction give useful information about the sample.
The resolution of the sample image is either limited by the width of the sound beam (depends on the sound frequency) or the physical scanning resolution.
Unlike conventional optical microscopes that allow you to observe the surface of a specimen, acoustic microscopes focus on a particular point and obtain images from deeper layers. Also, they provide more accurate results and increased data while preserving the integrity of the sample.
Sonix HS 1000 scanning acoustic microscope
Many companies use this type of microscopy in analytical labs to determine the quality of their electronic components. Manufacturers also use it for quality control, vendor qualification, product reliability test, and research and development.
In biology, these microscopes provide useful data about the physical forces holding structures in certain shapes, such as elasticity of cells and tissues. This is extremely useful in examining cell motility process (an organism’s ability to move independently using metabolic energy).
5. X-Ray Microscope
X-ray microscopes generate magnified images of objects using electromagnetic radiation in the soft-ray. They are capable of delivering a 3D computed tomography image of relatively large samples at high resolution.
It uses a charge-coupled device detector to identify X-rays passing through the sample. Since X-rays easily penetrate matter, this type of microscopes can image the inside of specimens opaque for visible light.
Modern X-ray microscopes allow you to observe various samples, including those that have low absorption contrast and denser material such as ceramic composites. To achieve this, the microscope changes the X-ray wavelength which enhances contrast or penetration.
Its resolution lies between that of optical microscopy and electron microscopy. Unlike tradition electron microscopes, X-ray microscopes can image thick biological materials in their natural state.
ZEISS Xradia 510 Versa X-ray Microscope
X-ray microscopy has proved extremely beneficial in the field of medicine and materials science. It has been used to analyze structures of different tissues and biopsy samples.
In the field of materials science, X-ray microscopes can determine the structure of crystal down to the placement of individual atoms within its molecules. It also provides a non-destructive, non-invasive technique to find defects in three dimensions.