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An 1879 Carl Zeiss Jena Optical microscope.
An 1879 Carl Zeiss Jena Optical microscope. Year 1879 ( MDCCCLXXIX) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common

The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. A microscope ( Greek: ( micron) = small + ( skopein) = to look or see is an instrument for viewing objects that are A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging Optical microscopes are the oldest and simplest of the microscopes.

There are non-optical microscopes, which require chemical or ion staining of non-living samples, and can magnify exponentially greater than the optical microscope. See: scanning electron microscope, transmission electron microscope. The scanning electron microscope ( SEM) is a type of Electron microscope that images the sample surface by scanning it with a high-energy beam of Electrons

Contents

Optical configurations

Old light microscope
Old light microscope

There are two basic configurations of optical microscope in use, the simple (one lens) and compound (many lenses).

Simple optical microscope

A simple microscope is a microscope that uses only one lens for magnification, and is the original light microscope. Van Leeuwenhoek's microscopes consisted of a small, single convex lens mounted on a brass plate, with a screw mechanism to hold the sample or specimen to be examined. Antonie Philips van Leeuwenhoek (October 24 1632 &ndash August 30 1723 was a Dutch tradesman and Scientist from Delft, the Netherlands A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging Demonstrations by British microscopist have produced surprisingly detailed images from such basic instruments. Though now considered primitive, the use of a single, convex lens for viewing is still found in simple magnification devices, such as the magnifying glass, and the loupe. magnifying glass (called a hand lens in laboratory contexts is a convex lens which is used to produce a magnified Image of an object A loupe (pronounced loop) is a type of Magnification device used to see things one is looking at more closely Light microscope are able to view specimens in colour, an important advantage when compared with electron microscopes, especially for forensic analysis.

History of the microscope

See also: timeline of microscope technology
Compound microscope made by John Cuff in 1750
Compound microscope made by John Cuff in 1750
The oldest published image known to have been made with a microscope: bees by Francesco Stelluti, 1630
The oldest published image known to have been made with a microscope: bees by Francesco Stelluti, 1630[1]

It is difficult to say who invented the compound microscope. Timeline of Microscope Technology 1021 - The properties of Magnifying glass are first clearly described by the Dutch spectacle-makers Hans Janssen and his son Zacharias Janssen are often said to have invented the first compound microscope in 1590, but this was a declaration made by Zacharias Janssen himself during the mid 1600s. The Dutch people ( Dutch:) are the dominant Ethnic group of the Netherlands. Sacharias Jansen (c 1585 - c 1632 was a Dutch spectacle-maker from Middelburg credited with inventing or contributing advances towards the invention of the The date is unlikely, as it has been shown that Zacharias Janssen actually was born around 1590. Another favourite for the title of 'inventor of the microscope' was Galileo Galilei. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher He developed an occhiolino or compound microscope with a convex and a concave lens in 1609. Galileo's microscope was celebrated in the Accademia dei Lincei in 1624 and was the first such device to be given the name "microscope" a year latter by fellow Lincean Giovanni Faber. The Accademia dei Lincei, (literally the " Academy of the Lynxes" but also known as the Lincean Academy) is an Italian science academy located Giovanni Faber or Johann Faber (1574&ndash1629 was a German papal doctor Botanist and art collector originally from Bamberg in Bavaria, Faber coined the name from the Greek words μικρόν (micron) meaning "small", and σκοπεῖν (skopein) "meaning to look at", a name meant to be analogus with "telescope", another word coined by the Linceans[2]. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation.

Christiaan Huygens, another Dutchman, developed a simple 2-lens ocular system in the late 1600s that was achromatically corrected, and therefore a huge step forward in microscope development. Christiaan Huygens (ˈhaɪgənz in English ˈhœyɣəns in Dutch) ( April 14, 1629 &ndash July 8, 1695) was a Dutch An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and Spherical aberration. The Huygens ocular is still being produced to this day, but suffers from a small field size, and other minor problems.

Anton van Leeuwenhoek (1632-1723) is generally credited with bringing the microscope to the attention of biologists, even though simple magnifying lenses were already being produced in the 1500s. Antonie Philips van Leeuwenhoek (October 24 1632 &ndash August 30 1723 was a Dutch tradesman and Scientist from Delft, the Netherlands Van Leeuwenhoek's home-made microscopes were very small simple instruments, with a single, yet strong lens. They were awkward in use, but enabled van Leeuwenhoek to see detailed images. It took about 150 years of optical development before the compound microscope was able to provide the same quality image as van Leeuwenhoek's simple microscopes, due to timely difficulties of configuring multiple lenses. Still, despite widespread claims, van Leeuwenhoek is not the inventor of the microscope.

The components of the microscope

Basic optical microscope elements(1990's)
Basic optical microscope elements(1990's)
Basic optical microscope elements(1900's): 1. ocular lens, or eyepiece 2. objective turret 3. objective lenses 4. coarse adjustment knob 5. fine adjustment knob 6. object holder or stage 7. mirror 8. diaphragm and condenser
Basic optical microscope elements(1900's):

1. ocular lens, or eyepiece
2. objective turret
3. objective lenses
4. An objective in Optics is the lens or Mirror in a Microscope, Telescope, camera or other optical instrument coarse adjustment knob
5. fine adjustment knob
6. object holder or stage
7. mirror
8. A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. diaphragm and condenser

All optical microscopes share the same basic components:

The whole of the optical assembly is attached to a rigid arm which in turn is attached to a robust U shaped foot to provide the necessary rigidity. The arm is usually able to pivot on its joint with the foot to allow the viewing angle to be adjusted. Mounted on the arm are controls for focusing, typically a large knurled wheel to adjust coarse focus, together with a smaller knurled wheel to control fine focus.

Updated microscopes may have many more features, including transmission illumination, phase contrast microscopy and differential interference contrast microscopy, and digital cameras. Phase contrast microscopy is an Optical microscopy Illumination technique in which small Phase shifts in the light passing through a transparent specimen Differential interference contrast microscopy ( DIC) also known as Nomarski Interference Contrast ( NIC) or Nomarski microscopy, is an Optical

On a standard compound optical microscope, there are three objective lenses: a scanning lens (4×), low power lens (10×)and high power lens (40×). Advanced microscopes often have a fourth objective lens, called an oil immersion lens. In Light microscopy, oil immersion is a technique used to increase the resolution of a Microscope. To use this lens, a drop of immersion oil is placed on top of the cover slip, and the lens is very carefully lowered until the front objective element is immersed in the oil film. Such immersion lenses are designed so that the refractive index of the oil and of the cover slip are closely matched so that the light is transmitted from the specimen to the outer face of the objective lens with minimal refraction. An oil immersion lens usually has a power of 100×.

The actual power or magnification of an optical microscope is the product of the powers of the ocular (eyepiece), usually about 10×, and the objective lens being used. Magnification is the process of enlarging something only in appearance not in physical size For the device for looking through a camera see Viewfinder. An eyepiece, or ocular lens, is a type of lens that is attached

Compound optical microscopes can produce a magnified image of a specimen up to 1000× and, at high magnifications, are used to study thin specimens as they have a very limited depth of field. In Optics, particularly as it relates to Film and Photography, the depth of field (DOF is the portion of a scene that appears sharp in the image

How a microscope works

Optical path in a typical microscope
Optical path in a typical microscope

The optical components of a modern microscope are very complex and for a microscope to work well, the whole optical path has to be very accurately set up and controlled. Despite this, the basic optical principles of a microscope are quite simple.

The objective lens is, at its simplest, a very high powered magnifying glass i. e. a lens with a very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus about 160 mm inside the microscope tube. This creates an enlarged image of the subject. This image is inverted and can be seen by removing the eyepiece and placing a piece of tracing paper over the end of the tube. By carefully focusing a brightly lit specimen, a highly enlarged image can be seen. It is this real image that is viewed by the eyepiece lens that provides further enlargement. In Optics, a real image is a representation of an actual object (source formed by rays of Light passing through the Image.

In most microscopes, the eyepiece is a compound lens, with one component lens near the front and one near the back of the eyepiece tube. This forms an air-separated couplet. In many designs, the virtual image comes to a focus between the two lenses of the eyepiece, the first lens bringing the real image to a focus and the second lens enabling the eye to focus on the virtual image. In Optics, a virtual image is an image in which the outgoing rays from a point on the object never actually intersect at a point

In all microscopes the image is viewed with the eyes focused at infinity (mind that the position of the eye in the above figure is determined by the eye's focus). Headaches and tired eyes after using a microscope are usually signs that the eye is being forced to focus at a close distance rather than at infinity.

Stereo microscope

See also: Comparison Microscope
Stereo microscope
Stereo microscope

The stereo or dissecting microscope is designed differently from the diagrams above, and serves a different purpose. A comparison microscope, sometimes known also as Stereomicroscope or dissecting microscope is a device used to analyze side-by-side specimens It uses two separate optical paths with two objectives and two eyepieces to provide slightly different viewing angles to the left and right eyes. In this way it produces a three-dimensional visualization of the sample being examined. Stereoscopy, stereoscopic imaging or 3-D (three-dimensional imaging is any technique capable of recording three-dimensional visual [3]

The stereo microscope is often used to study the surfaces of solid specimens or to carry out close work such as sorting, dissection, microsurgery, watch-making, small circuit board manufacture or inspection, and the like. Microsurgery is a general term for Surgery requiring an operating Microscope.

Unlike compound microscopes, illumination in a stereo microscope most often uses reflected (episcopic) illumination rather than transmitted (diascopic) illumination, that is, light reflected from the surface of an object rather than light transmitted through an object. Reflection is the change in direction of a Wave front at an interface between two different media so that the wave front returns into the medium from which Use of reflected light from the object allows examination of specimens that would be too thick or otherwise opaque for compound microscopy. However, stereo microscopes are also capable of transmitted light illumination as well, typically by having a bulb or mirror beneath a transparent stage underneath the object, though unlike a compound microscope, transmitted illumination is not focused through a condenser in most systems. [4] Stereoscopes with specially-equipped illuminators can be used for dark field microscopy, using either reflected or transmitted light. Dark field microscopy (dark ground microscopy describes microscopy methods in both light and electron microscopy which exclude the unscattered beam from the image [5]

Scientist using a stereo microscope outfitted with a digital imaging pick-up
Scientist using a stereo microscope outfitted with a digital imaging pick-up

Great working distance and depth of field here are important qualities for this type of microscope. Both qualities are inversely correlated with resolution: the higher the resolution (i. e. the shorter the distance at which two adjacent points can be distinguished as separate), the smaller the depth of field and working distance. A stereo microscope has a useful magnification up to 100×. The resolution is maximally in the order of an average 10× objective in a compound microscope, and often much lower.

There are two types of magnification systems in stereo microscopes. One is fixed magnification in which primary magnification is achieved by a paired set of objective lenses with a set degree of magnification. An objective in Optics is the lens or Mirror in a Microscope, Telescope, camera or other optical instrument The other is zoom or pancratic magnification, which are capable of a continuously variable degree of magnification across a set range. Zoom systems can achieve further magnification through the use of auxiliary objectives that increase total magnification by a set factor. Also, total magnification in both fixed and zoom systems can be varied by changing eyepieces. [3]

The stereo microscope should not be confused with a compound microscope equipped with binocular eyepieces. In such a microscope both eyes see the same image, but the binocular eyepieces provide greater viewing comfort. However, the image in such a microscope is no different from that obtained with a single monocular eyepiece.

Digital display with stereo microscopes

Recently various video dual CCD camera pickups have been fitted to stereo microscopes, allowing the images to be displayed on a high resolution LCD monitor. Software converts the two images to an integrated Anachrome 3D image, for viewing with plastic red/cyan glasses, or to the cross converged process for clear glasses and somewhat better color accuracy. The results are viewable by a group wearing the glasses. These files may recorded as well.

Special designs

Main article: Microscopy

Other types of optical microscope include:

An old pocket microscope
An old pocket microscope

Limitations of light microscopes

At very high magnifications with transmitted light, point objects are seen as fuzzy discs surrounded by diffraction rings. A fluorescence microscope (colloquially synonymous with epifluorescent microscope) is a light Microscope used to study properties of organic or inorganic substances Phase contrast microscopy is an Optical microscopy Illumination technique in which small Phase shifts in the light passing through a transparent specimen Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle These are called Airy disks. The Airy disk (or Airy disc) is a phenomenon in Optics. Owing to the wave nature of light, light passing through an Aperture is diffracted The limit of resolution (Resolving power of a microscope) is therefore taken as the ability to distinguish between two closely spaced Airy disks (or, in other words the ability of the microscope to reveal adjacent structural detail as distinct and separate). It is these impacts of diffraction that limit the ability to resolve fine details. The extent of and magnitude of the diffraction patterns are affected by both by the wavelength of light (λ), the refractive materials used to manufacture the objective lens and the numerical aperture (NA or AN) of the objective lens. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 In Optics, the numerical aperture ( NA) of an optical system is a Dimensionless number that characterizes the range of angles over which the system can accept There is therefore a finite limit beyond which it is impossible to resolve separate points in the objective field. Assuming that optical aberrations in the whole optical set-up are negligible, the resolution d, is given by:

d = \frac { \lambda } { 2 A_N }

Usually, a λ of 550 nm is assumed, corresponding to green light. Green is a Color, the perception of which is evoked by light having a spectrum dominated by energy with a Wavelength of roughly 520–570- nm. With air as medium, the highest practical AN is 0. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five 95, and with oil, up to 1. 5. In practice the lowest value of d obtainable is around 0. 2 micrometres or 200 nanometers. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a

A modern microscope with a mercury bulb for fluorescence microscopy. The microscope has a digital camera, and is attached to a computer.
A modern microscope with a mercury bulb for fluorescence microscopy. A fluorescence microscope (colloquially synonymous with epifluorescent microscope) is a light Microscope used to study properties of organic or inorganic substances The microscope has a digital camera, and is attached to a computer. Many compact digital still cameras can record Sound and moving Video as well as still Photograph. A computer is a Machine that manipulates data according to a list of instructions.

Other optical microscope designs (e. g. Stimulated Emission Depletion Microscopy) can offer an improved resolution when observing self-luminous particles, which is not covered by Abbe's diffraction limit for the compound microscope. Stimulated Emission Depletion microscopy or STED microscopy, is a technique that uses the non-linear de-excitation of fluorescent dyes to overcome the resolution limit imposed by Abbe's theory (by Ernst Karl Abbe) is based on the fact that a non-self-luminous particle is illuminated by an extraneous source. Ernst Karl Abbe ( January 23, 1840 &ndash January 14, 1905) was a German Physicist and professor at the University For Ernst Abbe's work in light microscopy, see the Molecular Expressions web site at http://micro.magnet.fsu.edu/optics/timeline/people/abbe.html.

Alternatives to optical microscopy

In order to overcome the limitations set by the diffraction limit of visible light other microscopes have been designed which use other waves.

The use of electrons and x-rays in place of light allows much higher resolution - the wavelength of the radiation is shorter so the diffraction limit is lower. The scanning electron microscope ( SEM) is a type of Electron microscope that images the sample surface by scanning it with a high-energy beam of Electrons An X-ray microscope uses Electromagnetic radiation in the soft X-ray band to produce images of very small objects The atomic force microscope (AFM or scanning force microscope (SFM is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions To make the short-wavelength probe non-destructive, the atomic beam imaging system (atomic nanoscope) is proposed and widely discussed in the literature, but it is not yet competitive with conventional imaging systems. The atomic de Broglie microscope (also atomic nanoscope, neutral beam microscope, or scanning helium microscope when Helium is used as the

STM and AFM are scanning probe techniques using a small probe which is scanned over the sample surface. Resolution in these cases is limited by the size of the probe; micromachining techniques can produce probes with tip radii of 5-10nm.

See also

References

  1. ^ "The Lying stones of Marrakech", by Stephen Jay Gould, 2000
  2. ^ brunelleschi.imss.fi.it "Il microscopio di Galileo"
  3. ^ a b "Introduction to Stereomicroscopy" by Paul E. Köhler illumination is a type of specimen illumination used in transmitted- or reflected-light microscopy An objective in Optics is the lens or Mirror in a Microscope, Telescope, camera or other optical instrument A microscope slide was originally a 'slider' made of ivory or bone containing specimens held between disks of transparent mica Nothnagle, William Chambers, and Michael W. Davidson, Nikon MicroscopyU. ( also known as Nikon or Nikon Corp, is a Multinational corporation headquartered in Tokyo Japan specializing in Optics
  4. ^ "Illumination for Stereomicroscopy: Reflected (Episcopic) Light" by Paul E. Nothnagle, William Chambers, Thomas J. Fellers, and Michael W. Davidson , Nikon MicroscopyU.
  5. ^ "Illumination for Stereomicroscopy: Darkfield Illumination" by William Chambers, Thomas J. Fellers, and Michael W. Davidson , Nikon MicroscopyU.

External links

Headline text

http://www.microscopyu.com/articles/optics/opticshome.html

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