Microscopy

Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.

Optical microscopy and electron microscopy involve the diffraction, reflection, or refraction of electromagnetic radiation/electron beams interacting with the specimen, and the collection of the scattered radiation or another signal in order to create an image. This process may be carried out by wide-field irradiation of the sample (for example standard light microscopy and transmission electron microscopy) or by scanning a fine beam over the sample (for example confocal laser scanning microscopy and scanning electron microscopy). Scanning probe microscopy involves the interaction of a scanning probe with the surface of the object of interest. The development of microscopy revolutionized biology, gave rise to the field of histology and so remains an essential technique in the life and physical sciences. X-ray microscopy is three-dimensional and non-destructive, allowing for repeated imaging of the same sample for in situ or 4D studies, and providing the ability to "see inside" the sample being studied before sacrificing it to higher resolution techniques. A 3D X-ray microscope uses the technique of computed tomography ( microCT), rotating the sample 360 degrees and reconstructing the images. CT is typically carried out with a flat panel display. A 3D X-ray microscope employs a range of objectives, e.g., from 4X to 40X, and can also include a flat panel.

History

The field of microscopy ( optical microscopy) dates back to at least the 17th-century. Earlier microscopes, single lens magnifying glasses with limited magnification, date at least as far back as the wide spread use of lenses in eyeglasses in the 13th century but more advanced compound microscopes first appeared in Europe around 1620 The earliest practitioners of microscopy include Galileo Galilei, who found in 1610 that he could close focus his telescope to view small objects close up and Cornelis Drebbel, who may have invented the compound microscope around 1620Raymond J. Seeger, Men of Physics: Galileo Galilei, His Life and His Works, Elsevier - 2016, page 24J. William Rosenthal, Spectacles and Other Vision Aids: A History and Guide to Collecting, Norman Publishing, 1996, page 391 Antonie van Leeuwenhoek developed a very high magnification simple microscope in the 1670s and is often considered to be the first acknowledged microscopist and microbiologist.

Optical microscopy

Optical or light microscopy involves passing visible light transmitted through or reflected from the sample through a single lens or multiple lenses to allow a magnified view of the sample. The resulting image can be detected directly by the eye, imaged on a photographic plate, or captured digitally. The single lens with its attachments, or the system of lenses and imaging equipment, along with the appropriate lighting equipment, sample stage, and support, makes up the basic light microscope. The most recent development is the digital microscope, which uses a CCD camera to focus on the exhibit of interest. The image is shown on a computer screen, so eye-pieces are unnecessary.

Limitations

Limitations of standard optical microscopy ( bright field microscopy) lie in three areas;
* The technique can only image dark or strongly refracting objects effectively.
* There is a diffraction-limited resolution depending on incident wavelength; in visible range, the resolution of optical microscopy is limited to approximately 0.2   micrometres (''see: microscope'') and the practical magnification limit to ~1500x.
* Out-of-focus light from points outside the focal plane reduces image clarity.

Live cells in particular generally lack sufficient contrast to be studied successfully, since the internal structures of the cell are colorless and transparent. The most common way to increase contrast is to stain the structures with selective dyes, but this often involves killing and fixing the sample. Staining may also introduce artifacts, which are apparent structural details that are caused by the processing of the specimen and are thus not features of the specimen. In general, these techniques make use of differences in the refractive index of cell structures. Bright-field microscopy is comparable to looking through a glass window: one sees not the glass but merely the dirt on the glass. There is a difference, as glass is a denser material, and this creates a difference in phase of the light passing through. The human eye is not sensitive to this difference in phase, but clever optical solutions have been devised to change this difference in phase into a difference in amplitude (light intensity).

Techniques

To improve specimen contrast or highlight structures in a sample, special techniques must be used. A huge selection of microscopy techniques are available to increase contrast or label a sample.

tissue paper. 1.559 μm/pixel." align="center">
Image:Paper_Micrograph_Bright.png, Bright field illumination, sample contrast comes from absorbance of light in the sample
Image:Paper_Micrograph_Cross-Polarised.png, Cross-polarized light illumination, sample contrast comes from rotation of polarized light through the sample
Image:Paper_Micrograph_Dark.png, Dark field illumination, sample contrast comes from light scattered by the sample
Image:Paper_Micrograph_Phase.png, Phase contrast illumination, sample contrast comes from interference of different path lengths of light through the sample

Bright field

Bright field microscopy is the simplest of all the light microscopy techniques. Sample illumination is via transmitted white light, i.e. illuminated from below and observed from above. Limitations include low contrast of most biological samples and low apparent resolution due to the blur of out-of-focus material. The simplicity of the technique and the minimal sample preparation required are significant advantages.

Oblique illumination

The use of oblique (from the side) illumination gives the image a three-dimensional appearance and can highlight otherwise invisible features. A more recent technique based on this method is ''Hoffmann's modulation contrast'', a system found on inverted microscopes for use in cell culture. Oblique illumination enhances contrast even in clear specimens, however, because light enters off-axis, the position of an object will appear to shift as the focus is changed. This limitation makes techniques like optical sectioning or accurate measurement on the z-axis impossible.

Dark field

Dark field microscopy is a technique for improving the contrast of unstained, transparent specimens. Dark field illumination uses a carefully aligned light source to minimize the quantity of directly transmitted (unscattered) light entering the image plane, collecting only the light scattered by the sample. Dark field can dramatically improve image contrast – especially of transparent objects – while requiring little equipment setup or sample preparation. However, the technique suffers from low light intensity in the final image of many biological samples and continues to be affected by low apparent resolution.

''Rheinberg illumination'' is a variant of dark field illumination in which transparent, colored filters are inserted just before the condenser so that light rays at high aperture are differently colored than those at low aperture (i.e., the background to the specimen may be blue while the object appears self-luminous red). Other color combinations are possible, but their effectiveness is quite variable.

Dispersion staining

Dispersion staining is an optical technique that results in a colored image of a colorless object. This is an optical staining technique and requires no stains or dyes to produce a color effect. There are five different microscope configurations used in the broader technique of dispersion staining. They include brightfield Becke line, oblique, darkfield, phase contrast, and objective stop dispersion staining.

Phase contrast

: ''In electron microscopy: Phase-contrast imaging''
More sophisticated techniques will show proportional differences in optical density. Phase contrast is a widely used technique that shows differences in refractive index as difference in contrast. It was developed by the Dutch physicist Frits Zernik