Microscope Glossary

Abbe Condenser

This is a lens system found under the stage on all serious compound microscopes. When used in conjunction with the iris, it allows you to change the diameter and focal point of the light entering the slide. In effect, as you move the condenser up and down and change the opening on the iris, the contrast and detail in the specimen you are observing changes. This is must for magnifications over 400x.

A condenser should be used with any objective having a numerical aperture (N.A.) greater than 0.4. The typical Abbe condenser has a numerical aperture (N.A.) of 1.25 and will thus accommodate any objective with an N.A. up to 1.25. When using an objective with an N.A. greater than 1.25, a condenser of higher N.A. is also required.

Aberration

An optical defect

Achromatic Objectives

As light passes through a lens and is focused, different colors of light (wavelengths) come to focus at different distances from the lens. This causes an optical defect known as chromatic aberration which produces color fringing around the specimen. This causes loss of detail and resolution. Achromatic objectives (achromats) bring both red and blue wavelengths to the same focal point, but not the green, thus chromatic aberration is not totally eliminated. Still, achromats are the most cost effective objective design and in better grade lenses, will do an excellent job. For this reason, the achromatic objective is the most common design used in microscope objectives.

Apochromatic Objectives

As light passes through a lens and is focused, different colors of light (wavelengths) come to focus at different distances from the lens. This causes an optical defect known as chromatic aberration which produces color fringing around the specimen. This causes loss of detail and resolution. Unlike achromatic objectives, which bring only the red and blue wavelengths to the same focal point, apochromatic objectives bring all color wavelengths to the same focal point, nearly eliminating chromatic aberration. This makes apochromatic objectives state of the art in terms of performance, but due to the complex lens systems involved, these are also the most expensive objectives to produce.

Arm

This is the "neck" of the microscope, which connects the eyepiece tube to the rest of the microscope.

Aperture

In the simplest sense of the word, the diameter of a lens. The larger the lens diameter, the better the resolution, all else being equal, but in a microscope objective, it is a bit more complicated. See numerical aperture (NA)

Asbestos microscope

A microscope used to do asbestos fiber counts, usually in conjunction with legal issues concerning asbestos.

Articulated Arm

This is the "neck" of the microscope, which connects the eyepiece tube to the rest of the microscope. An articulated neck is hinged, allowing you to adjust the angle of the eyepiece tube for more comfortable viewing.

Base

The bottom of a microscope which supports the microscope, often equipped with levelers.

Bertrand Lens

A small lens inserted into the tube of advanced polarized light microscopes to examine interference patterns produced by the objective.

Binocular Head

A microscope head which offers the viewer two eyepieces. Using two eyes, instead of one, greatly reduces eye fatigue over long observing sessions and also improves viewing comfort.

Body Tube Length (see Tube length)

Bright field Microscope

This is the typical microscope used in a Biology classroom. Illumination is provided below the stage, either through a mirror on inexpensive models or a dedicated light source on standard models. In a bright field microscope, the specimen appears dark against a lighted background. This type of microscope/illumination is most useful for specimens that are opaque or semi-transparent or for transparent specimens that have been stained to reveal detail.

C-mount

This is the standard thread mount used in video camera lenses. A microscope or adapter that features a C-mount allows you to attach a video or camcorder to the microscope.

Chromatic Aberration

As light passes through a lens and is focused, different colors of light (wavelengths) come to focus at different distances from the lens. This causes an optical defect known as chromatic aberration which produces color fringing around the specimen. This causes loss of detail and resolution.

Coarse Focus

Most compound microscopes offer a coarse and fine focus. Coarse focus is used a low powers to bring the specimen rapidly into focus. The use of coarse focus should be restricted to lower magnifications, due to the very small clearance or working distance between the objective and the slide at high magnifications. Although better microscopes have a focus stop to prevent damage to the objective when an objective accidentally touches the slide, it is still possible to damage or crack a slide.

Coaxial Focus

This is a focusing knob within a focusing knob system, typically with a smaller fine focus knob located at the center of the larger coarse focus knob.

Compound microscopes

This is the traditional microscope with a revolving nosepiece (turret) which contains three or more objectives. A compound microscope is used to study very small specimens and requires the specimens to be mounted on a slide. Because the clearance (working distance) between the specimen and objective is very small, this type of microscope is not suitable for studying large specimens, such as rocks, twigs, leaves, insects and so on. These larger specimens are observed with a stereo or dissecting microscope.

Compensated

A compensated eyepiece is designed to work with a specific objective to give an exact magnification. For applications requiring counts, the magnification of the system must be calculated precisely for the sake of count accuracy.

Condenser lens

This is a lens that concentrates light on the specimen. More advanced microscopes use an Abbe condenser (see Abbe condenser). On inexpensive microscopes, the reference to condenser is to a small lens placed in the light hole on the stage. The N.A. of the optical system is only as good as the N.A or the condenser and objective used. Inexpensive microscopes typically use a single condenser lens with a N.A of 0.65, meaning you will not reach achieve an N.A higher than 0.65, even if you use an objective with an N.A. higher than 0.65

Cover Slip

Thin piece of plastic or glass used to cover a specimen once it has been mounted on a slide. Standard thickness is .17mm

Dark field Illumination

A method by which the specimen (transparent or semi-transparent) is seen as a bright object against a dark background. In a dark field microscope, just as in a bright field microscope, a condenser focuses a cone of light on the specimen, but in a dark field microscope, the condenser forms a hollow cone of light, dark in the center. When the tip of the cone strikes the specimen, it therefore scatters light against a dark background, making transparent detail more visible than in a bright field microscope.

Depth of Field

Microscopes magnify not only the width of a specimen, but also the thickness. As magnification increases, it becomes possible to observe the specimen at various levels in this thickness. This means that even on a tiny microorganism, it is possible to use the fine focus to bring different "levels" of the specimen in focus. It also means that a sophisticated condenser, such as an Abbe condenser, may be needed to properly focus the light to reveal all the potential detail at any given level of focus in the specimen.

Diaphragm

A diaphragm is an opening for light. On inexpensive microscopes, the diaphragm is a wheel located just under the stage with holes of varying size, allowing the user to control the amount of light reaching the specimen. On sophisticated microscopes, the light is controlled with an iris style diaphragm, consisting of a set of leaves or overlapping plates, usually as part of a condenser system.

DIN

DIN is a standard for microscope design established in Germany many years ago. DIN stands for "Deutsche Industrie Normen". DIN standards can be used to specify eyepiece diameter and size, objective thread size and microscope tube length (160mm). In theory, this allows any DIN standard accessory to be used in any DIN standard microscope, though it is no guarantee of performance or quality. An accessory labeled DIN, simply means it can be used in a DIN standard microscope. However, this does not cover other microscope accessories.

Diopter Adjustment

On a binocular head, this allows the user to adjust the focus for differences in strength between the right eye and the left. Since most people have one eye stronger than another, nearly all binocular heads offer this adjustment, usually in the form of a focusing knob on one eyepiece.

Dissecting microscope

This is a microscope that allows the user to work on and manipulate the specimen while looking through the microscope and is also known as "stereo" microscopes. A dissecting or stereo microscope differs from a standard compound microscope in a number of important ways. 1) A dissecting microscope produces upright, correct right to left images, instead of upside down, reversed images. This obviously facilitates working with a specimen. 2) A dissecting microscope offers much greater working distance. This allows the microscope to be used with very large specimens, such as rocks, flowers, whole insects and so on. 3) A stereo scope uses two separate optical systems, each with its own eyepiece, giving the observer true, three dimensional image, thus the name "stereo". A conventional compound microscope with a binocular head does not produce a three dimensional image, since it is simply a single optical system with two eyepieces. 4) A dissecting microscope is a much lower magnification instrument, in keeping with the specimens usually observed with it. Magnifications typically run between 10 and 40x.

Doublet lens

This is the standard lens design of an achromat. Apochromats typically use three lens.

Eyepiece

On standard compound microscopes, the DIN standard for eyepiece barrel width is 23.2mm. On most dissecting or stereo microscopes, the eyepiece barrel measures 31.75mm or 1.25".

Fine Focus

Used at high magnification to bring the subject into better focus and used at all magnifications to observe the specimen at different depths.

Field of View

The area visible around the subject. A basic law of optics states that as magnification goes up, images size increases, but field of view decreases.

Flat Field Objectives or Optics

Curvature at the edge of the field is common in many lower cost objectives. While not ordinarily a hindrance for visual work, it can be annoying. For photography, however, curvature of field is a serious issue and will produce a photograph that is out of focus around the edges. Objectives that correct for curvature at the edge of the field and are flat across 70-85% of the field of view are known as Flatfield objectives. Objectives that produce a flat field across the entire field of view are known as Plan objectives. The only difference between Flatfield and Plan objectives is therefore the degree of correction or flatness.

Fluorescent microscope

A microscope designed to study fluorescent materials, primarily within cells, by using ultraviolet light. The fluorescent material can occur naturally or it can be induced. This technique has many advantages, but the primary advantage is its ability to distinguish between living and dead cells and monitor activity within living cells. This microscope has made a tremendous contribution in the field of medical research.

FN

On an eyepiece, a number which indicates the diameter of the baffle, which determines the field of view of the eyepiece.

Infinity

In a standard compound microscope with a finite tube optical system, the image produced by the objective and sent to the eyepiece through the optical tube is a cone of light of a specified length, most typically 160mm, though some finite systems use 170mm. Objectives must be matched to this "tube length" in order to perform properly and every microscope objective will be inscribed with its compatible tube length.

Many microscope designs, however, require the use of accessories such as polarizers, prisms and illuminators between the eyepiece and the objective. If these accessories are placed in the tube of a finite tube microscope, the effective length of the system can change and focusing and aberration problems may result. Infinity tube systems were introduced to handle these accessories correctly. In an infinity tube system, an extra lens is placed in the optical path between the objective and the eyepiece. This tube lens creates an area in the optical where light rays run parallel to each other rather than at an angle. This allows insertion of accessories into the optical path with a minimum of distortion or aberrations. Infinity systems also allow the use of larger objectives with better working distance than standard DIN objectives. However, the placement of the tube lens varies in infinity systems, so the effective or "reference" tube length may change from manufacturer to manufacturer and even from one infinity model to the next. Not only do infinity microscopes require the use of infinity objectives, they may also require a specific brand or model of infinity objective to produce the best results. In fact, some manufacturers intentionally alter the thread size of their infinity objectives to prevent their use in other infinity systems.

Illumination

There are several types of electric lighting for microscopes:

Tungsten illumination

The least expensive electric illumination. Tends to run hotter than other types of illumination and provides less image brightness. Usually found on inexpensive microscopes.

Halogen illumination

The brightest incandescent illumination, but relatively cool, so an excellent illumination for photography and studying live specimens.

Fluorescent illumination

A better choice than tungsten. Fluorescent lighting provides better image brightness, longer bulb life, lower temperature and runs cooler.

LED illumination

Very low energy consumption and virtually unlimited bulb life makes this light source a good choice for cordless/battery powered microscopes.

Interpupillary Adjustment

An adjustment on a binocular microscope that allows the user to set the eyepieces at the correct distance for the distance between his/her eyes.

Jensch head

A binocular head that offers side to side interpupillary adjustment for easier adjustment than a Seidentopf head.

Koehler illumination

Proper alignment of the incident or illuminating light for microscopy requires proper alignment of the condenser lens and illumination system to achieve optimum contrast and resolution. A Koehler illumination system requires a condenser system and bulb that can be adjusted for alignment and one that is fitted with two diaphragms - one near the specimen and one near the lamp. The upper diaphragm on the condenser controls the angle of the cone of light and the lower diaphragm controls the area of the circle of light.

Magnification

On a microscope, total magnification is calculated by multiplying the magnification of the eyepiece by the magnification of the objective. If accessories are used in the tube and/or the head may also affect total magnification.

Mechanical Stage

A stage equipped with a mechanism that holds the slide and allows for micrometer adjustments of the position of the slide, both vertically and horizontally. A must for any serious microscope.

Mirror

A simple and inexpensive lighting system that uses light from an external source (sun, lamp and so on) and reflects it upward to the condenser/specimen via a mirror located below the stage. The mirror angle is adjustable. Most mirrors are two sided, plano-convex (flat, curved) to accommodate various light sources. Still a viable option for microscopes that will be used in areas where there is no power supply, but most often found on beginners microscopes.

Monocular Head

A head that provides a single eyepiece. Although inexpensive, a monocular microscope will produce eye fatigue over long viewing sessions.

Nosepiece

The part of the microscope that holds the turret and objectives.

Numerical Aperture(N.A.)

Aperture for any optical instrument is the diameter of the objective lens. All else being equal, the larger the aperture of an objective lens and the better the optical correction of the objective, the better the resolution (ability to show two closely spaced objects as separate objects, rather than a single object) as well as image brightness - all because a larger objective lens supplies a wider cone of light to the optical system than a smaller objective lens.

In microscope objectives, you must also take into account the medium in which the objective operates, because this also affects the width of the angle of light that is possible for an objective to produce. This relationship between magnification, aperture, medium and degree of optical correction in a microscope objective is expressed as its numerical aperture or N.A. For the sake of performance, then, you should consider not only the magnification of an objective, but also its NA. For two objectives of the same magnification, the objective with the higher NA will offer the best resolution and brightness. Dry objectives (used without immersion), have a theoretical N.A. limit of 1.0 and in practice, it is rare to achieve an N.A of 0.95. For applications requiring a higher N.A., you must move into a water or oil objective, since these respective mediums reduce the amount of bending (refraction) as light enters and leaves the slide and allow for a wider angle or cone of light to enter the objective. The highest N.A objectives used in a microscope, then, are objectives that combine immersion with sophisticated optical correction.

Objective Lens

The heart of a microscope optical system. Objectives are threaded onto the nosepiece of the microscope, which typically holds 3-4 objectives. To change magnification, you rotate a different eyepiece into position until it clicks. The barrel of each objective is inscribed with valuable information. This typically includes

• magnification (number with X)
• numerical aperture N.A
• the tube length the objective is designed for (160mm, 170mm or infinity symbol for infinity systems)
• special medium if other than dry or air (oil, water and so on)
• special correction if other than achromat (plan, apochromatic)
• thickness of the cover glass to be used if other than standard .17mm

Ocular

Another name for an eyepiece.

Oil Immersion Lens

A high power objective lens (usually 100X) which uses a drop of oil between the lens and slide. Oil has a similar refractive index to glass, so the refraction of light rays and consequent loss of resolution that occurs with a dry objective as light passes between the cover slip and objective is greatly reduced. Because oil objectives have a high N.A., they require a condenser with a similar high N.A, usually of 1.25.

Par centered

A microscope with par centered objectives will keep the specimen in the field of view as you rotate the objectives and change magnification. All quality microscopes use par centered objectives.

Par focal

A microscope with par focal objectives will only require minor refocusing with the fine focus as you rotate the objectives and change magnification

Plan achromat or objective

Curvature at the edge of the field is common in many lower cost objectives. While not ordinarily a hindrance for visual work, it can be annoying. For photography, however, curvature of field is a serious issue and will produce a photograph that is out of focus around the edges. Objectives that correct for curvature at the edge of the field and are flat across 70-85% of the field of view are known as Flatfield objectives. Objectives that produce a flat field across the entire field of view are known as Plan objectives. The only difference between Flatfield and Plan objectives is therefore the degree of correction or flatness.

Phase Contrast

Phase contrast is a technique used to reveal detail in transparent specimens without resorting to the use stains which can kill the specimen. This makes it a useful tool for the study of living cells and, as such, phase contrast is an integral tool for research in medicine and biology.

Phase contrast uses the principle of light called phase shift. As light passes through a specimen, it is slowed down slightly or "shifted in phase" as it encounters various structures in the specimen compared to light that does not encounter structures. Structures that produce a great deal of phase shift, as found in colored and opaque specimens are easily visible and can be studied effectively with a bright field microscope.

The structures in a transparent specimen, however, do not produce enough phase shift to be visible to the eye. A phase contrast microscope employs phase plates in the objective and condenser to amplify this phase shift so that it can be detected by the human eye. Minute details in the structure of the transparent specimen then become visible.

Phase contrast is only useful for specimens that are transparent, colorless and difficult to see against their background. These phase objects include protozoan, cell organelles and other difficult to study structures. A bright field microscope can be converted to a phase microscope with the addition of phase objectives and a phase condenser.

The phase contrast microscope was developed early in the twentieth century by Frits Zernike, an achievement which earned a Nobel Prize.

Polarized light microscope

A polarizer is a filter that acts somewhat as a Venetian blind, allowing only light vibrating in one plane to pass to pass through. This polarized light can be put to very good use in a microscope since subjects exposed to polarized light under a microscope can reveal very distinctive features and patterns. A polarized microscope incorporates both a polarizer and an analyzer. Also included is a rotating stage equipped with special plates inserted in the light path. By measuring the angles of brightness and analyzing the color that a specimen produces and then comparing the results to an appropriate chart, you can identify the sample.

Polarized light microscopes have a wide range of applications. A typical use is in geology for rock and mineral identification, but they also find wide use for other mineral and geological applications, such as asbestos counts. Polarized light microscopes are used in medicine to study crystals in urine and in cells.

Rack and Pinion

A gearing system that features a pinion gear and a rack (plate with teeth) and that is known for its smooth, even movement. Used on the focusing systems of all quality microscopes.

Rack Stop or Safety Rack Stop

A built in clutch or slip device that prevents over travel of the objective when it comes too close to the slide. Prevents damage to the objective and breakage of the slide.

Refractive Index

The Refractive Index is the ratio of the speed of light in a vacuum to the speed of light through any given medium, such as air, water or oil. In a microscope, the R.I. of the medium is an important factor when determining the numerical aperture, N.A., of an objective. Since the refractive index of air is 1.0, in order to achieve an N.A. greater than 1.0, a different medium must be used.

Resolution

This is the ability to separate or show two closely spaced objects as two, individual objects instead of one, single object. Resolution is a function of magnification, optical correction and the medium and is reflected in the Numerical Aperture (N.A.) of the objective.

Reticule

A reticule or grid in any eyepiece used for measuring or counting.

Ring Light

An advanced lighting system used in stereoscopes to provide maximum illumination, but, just as importantly, even and glare free illumination. A ring light gets its name from the fact that it a circle of light around the nosepiece of the microscope, rather than a single light source. Ring lights are most commonly fiber optics assemblies, but new, less expensive models employ a circle of LED lights. Ring lights are an essential tool for photography with a stereo microscope because it is impossible to completely eliminate glare with standard overhead lighting systems.

RMS thread

This was one of the first standards established for the threads on objectives set by the Royal Microscopal Society (RMS) in England in the late 1800s and this standard is still used for most objective threads, today, though some manufacturers have begun to deviate from this to prevent the use of inappropriate objectives in their systems.

Seidentopf head

This is a binocular head that allows for interpupillary distance (distance between the observer's eyes) adjustment by pivoting the oculars on a central hinge, much as with a conventional binocular.

Tube Length

In a standard compound microscope with a finite tube optical system, the image produced by the objective and sent to the eyepiece through the optical tube is a cone of light of a specified length, most typically 160mm, though some finite systems use 170mm. This distance is referred to as the "tube length" or "optical tube length". Objectives must be matched to this "tube length" in order to perform properly and every microscope objective will be inscribed with its compatible tube length or sometimes simply stamped DIN, which uses a 160mm tube length. Infinity microscopes use a different tube optical system and objectives for infinity systems are stamped with an infinity symbol.

Spherical Aberration

This is an optical defect that occurs when using a curved or spherical lens. In short, it is blurring of the image caused by light rays coming to focus at one distance from the center of the lens and another distance when striking the edge of the lens. It can be corrected by using non-spherical (aspherical) lenses or by using a combination of lens, as in an achromat or apochromat.

Stage

The part of the microscope that holds the specimen.

Stage plates

On a stereo microscope, the stage plate is a piece of glass or metal on which the specimen is placed. Better models have interchangeable stage plates of different colors to enhance contrast as needed.

Stage Clips

On inexpensive microscopes, these are metal clips used to keep the slide in place. Usually spring loaded.

Swing Arm

This is an extended arm that allows the microscope to be swung out over a large specimen. Most commonly used on stereo microscopes for industrial and medical applications.

Stereo microscope

This is a microscope that allows the user to work on and manipulate the specimen while looking through the microscope and is also known as "stereo" microscopes. A dissecting or stereo microscope differs from a standard compound microscope in a number of important ways. 1) A dissecting microscope produces upright, correct right to left images, instead of upside down, reversed images. This obviously facilitates working with a specimen. 2) A dissecting microscope offers much greater working distance. This allows the microscope to be used with very large specimens, such as rocks, flowers, whole insects and so on. 3) A stereo scope uses two separate optical systems, each with its own eyepiece, giving the observer true, three dimensional image, thus the name "stereo". A conventional compound microscope with a binocular head does not produce a three dimensional image, since it is simply a single optical system with two eyepieces. 4) A dissecting microscope is a much lower magnification instrument, in keeping with the specimens usually observed with it. Magnifications typically run between 10 and 40x.

T-mount

A camera adapter that allows the attachment of SLR cameras - cameras with removable lenses. Requires the addition of a t-ring for a specific brand and model of SLR camera.

Trinocular Head

A head with three tubes, usually two for eyepieces and a third for the attachment of a camera.

Turret

This is the section on the nosepiece that holds the objectives and turns, allowing you to switch objectives.

Widefield eyepiece

Field of view is basically a function of magnification, but is also determined by the eyepiece design. There is no standard for what constitutes a wide field eyepiece, but in general, it is an eyepiece which offers a wider field of view than a conventional eyepiece.

Working Distance

This is the distance between the specimen or cover slip and the objective lens. On compound microscopes that use slides, it is measured in millimeters, but to accommodate the larger specimens used with stereoscopes, a working distance of many centimeters is not uncommon.