Abbe Condenser

A condenser is a sub stage lens that focuses light on the specimen. The Abbe condenser is the most widely used condenser system in use on microscopes, today. It is typically used with an adjustable iris, in effect allowing the operator to change the diameter and focal point of the light entering the slide. By moving the condenser up and down and changing the opening on the iris, the contrast and detail in the specimen can be precisely set for best image quality. All microscopes at magnifications over 400x will require an Abbe condenser or the equivalent.

The potential N.A, (numerical aperture) of a system is only as good as its weakest link. Any objective with an N.A. of 0.4 or more will require a condenser and oil and water objectives will require a condenser in keeping with their higher N.A. The typical Abbe condenser has a numerical aperture (N.A.) of 1.25 and will thus allow the use of objectives with an N.A. up to 1.25. Higher N.A. objectives will require a higher N.A. condenser system.

Aberration

An optical flaw, usually associated with a particular lens design, though it can also be produced through improper lens grinding.

Achromatic Objectives

Different colors of light (wavelengths) come to a different focus point when they emerge from a lens. The color fringe around the specimen thus produced is known as chromatic aberration. This optical defect causes loss of detail and resolution. Achromatic objectives or achromats focus two wavelengths (red and blue), but not the green, thus reducing chromatic aberration, but not eliminating it. However, for most applications, achromats are suitable and if well made, will do an excellent job at a relatively affordable price. The achromat, for this reason, remains the most common type of objective in use, today.

Apochromatic Objectives

Different colors of light (wavelengths) come to a different focus point when they emerge from a lens. The color fringe around the specimen thus produced is known as chromatic aberration. This optical defect causes loss of detail and resolution. Achromatic objectives or achromats focus two wavelengths (red and blue), but not the green, thus reducing chromatic aberration, but not eliminating it. To bring all wavelengths to the same focus point requires an apochromatic objective, often just referred to as an APO. These high performance objectives employ complex lens systems and are thus expensive, but for serious research they remain the design of choice.

Arm

The section of the microscope body that connects the eyepiece tube to the rest of the body.

Aperture

This is simply the diameter or width of a lens. All else equal, the greater the aperture, the higher the resolution. In a microscope, however, there are other factors that must be considered. See numerical aperture (NA)

Asbestos microscope

This is typically a specialized version of a polarized light microscope modified to asbestos counts for legal work. Requires the use of a compensated eyepiece.

Articulated Arm

An articulated arm is a hinged arm, which allows the user to set the angle of the eyepiece tube for the sake of comfort.

Base

The part of the microscope that comes in contact with the table or other surface used to support it.

Bertrand Lens

This is a small lens used in the tube of a polarized light microscope and is used to study interference patterns for the sake of identification and analysis.

Binocular Head

Monocular head microscopes (microscopes offering only one ocular) produce eyestrain and fatigue over extended viewing sessions. For this reason, all serious microscopes use a binocular head offering two eyepieces. Fatigue and eyestrain are greatly reduced.

Bright field Microscope

This is the most common microscope system used in classrooms and for many applications, a bright field is adequate. In a bright field microscope, objects appear dark against a bright background. Illumination is sub stage via a mirror on inexpensive models or more typically, a built in light source using a bulb of various types. Best used for opaque, semi-transparent or transparent specimens that have been stained.

C-mount

The standard 25.4mm thread size used on camcorders and video cameras with interchangeable lenses. A C-mount equipped microscope has a tube or adapter that allows photography with a camcorder.

Chromatic Aberration

Different colors of light (wavelengths) come to a different focus point when they emerge from a lens. The color fringe around the specimen thus produced is known as chromatic aberration. This optical defect causes loss of detail and resolution. Chromatic aberration is partially corrected with an achromat objective and fully corrected with an apochromat.

Coarse Focus

This knob is used at low power to bring the specimen to focus quickly. The use of the coarse focus knob should be restricted to lower magnification objectives which offer greater working distances. High magnification objectives have very short working distance and in order to prevent damage to the slide, specimen and possibly the objective, high magnification objectives should only be used with fine focus.

Coaxial Focus

A common arrangement of focusing knobs featuring a knob within a knob, most often with the smaller knob the fine focus.

Compound microscopes

As distinguished from a stereo or dissecting microscope, a microscope with a revolving nosepiece (turret) containing three or more objectives. This is the type of microscope most often associated with the word microscope. The compound light microscope requires the use of slides and is designed to study very small specimens at very high magnification. Working distances do not allow the study of large, whole specimens such as rocks plant parts and so on. These larger specimens require the use of a stereo or dissecting microscope.

Compensated eyepiece

For most visual work, knowing the approximate magnification delivered by a system is sufficient. Applications requiring counts, however, require the operator to know the exact magnification of the system. Compensated eyepieces are designed to work with specific objectives to deliver an exact magnification.

Condenser lens

A condenser is a sub stage lens that focuses light on the specimen. The Abbe condenser is the most widely used condenser system in use on microscopes, today. Inexpensive microscopes, however, may offer a condenser which is nothing more than a small, non-adjustable lens placed above the light source, typically with an N.A. of only 0.65. Since the potential N.A, (numerical aperture) of a system is only as good as its weakest link, this places severe limits on performance. All serious microscopes use a minimum of an Abbe condenser system with an N.A. of 1.25 or more.

Cover Slip

A cover slip is the thin piece of glass or plastic that covers a specimen mounted on a slide. It protects both the objective and the specimen. Standard cover glass thickness is .17mm

Dark field Illumination

A dark field microscope will show a transparent or semi-transparent object as bright object against a dark background. Dark field Illumination is achieved by using a cone of light that is lit only at the tip and is dark in the center, rather than a uniformly lit throughout the cone as in a bright field microscope. In a dark field microscope, only the tip of the light cone touches the specimen with the resultant light scattering against a dark background. This produces detail in transparent and semi-transparent specimens that will not be visible in a bright field microscope.

Depth of Field

Magnification in a microscope is a matter of both width and depth. As magnification increases, both the width of the specimen and its depth are magnified. Thus it becomes possible to observe the specimen at varying levels in its thickness. To do this effectively requires a condenser system that can be focus light at precisely the right level in the specimen.

Diaphragm

In simplest terms, a diaphragm is an opening for light. On a microscope it refers to any opening used to control the amount of light reaching a specimen. On inexpensive models, this may be little more than a wheel with holes of varying size. Most working microscopes, however, use an iris diaphragm with overlapping leaves that can be opened or closed as needed.

DIN

This common reference used in microscope descriptions stands for "Deutsche Industrie Normen", a standard for microscope design established in Germany many years ago.

The DIN standard can be used to specify eyepiece diameter and size, objective thread size and microscope tube length (160mm). In theory, any DIN standard accessory can be used in any DIN standard microscope, though it may not be appropriate to do so, nor is the label of DIN a guarantee of performance or quality.

Diopter Adjustment

Most people have one eye that is stronger than the other. An adjustment that allows a user to adjust a binocular microscope for this discrepancy in eyesight is called a diopter adjustment. This is usually a focusing knob located on one of the eyepieces.

Dissecting microscope

A microscope with high magnification is not always desirable or needed for many applications. A microscope that allows three dimensional, low power observation of a specimen is known as a stereo microscope or alternately, a dissecting microscope, because such a microscope allows a user to manipulate and work on a specimen under the microscope.

A stereo microscope should not be confused with a compound microscope which also offers the user two eyepieces. A stereo microscope has a number of significant differences.

For one, a dissecting or stereo microscope produces upright, correct right to left images, instead of upside down, reversed images as in a compound microscope. This makes it a practical choice for manipulating a specimen. For another, a dissecting microscope offers much greater working distance. A long working distance allows a stereo microscope to be used with whole specimens, including rocks, flowers, gems, coins, whole insects and many other objects too large to be used with a compound microscope. For yet another, a stereo microscope employs two separate optical systems, each with its own eyepiece, giving the observer true, three dimensional image, thus the name "stereo". A compound binocular microscope uses two eyepieces, but not two separate optical systems and cannot, therefore offer three dimensional viewing. Lastly, a dissecting microscope is a much lower magnification instrument, in keeping with the larger specimens usually observed with it. The typical stereo microscope will have magnifications between 10 and 40x, though some may offer as much as 100x.

Doublet lens

A standard design used in an achromat objective. Indicates partial correction of chromatic aberration. Total or near total elimination of chromatic aberration is achieved with an apochromat, typically using three elements.

Eyepiece

Also known as an ocular and typically 23.2mm in barrel diameter in a DIN standard compound microscope. Stereo microscopes use an ocular barrel diameter of 31.75mm or 1.25"

Fine Focus

On a compound microscope, fine focus is used to tune the focus on a par focal model and is also essential to focus at different levels in the specimen at high magnification.

Field of View

Field of view is the extent of area visible around a specimen. Field of view is determined by magnification - as magnification goes up, field of view goes down - and also by eyepiece design. Wide-angle design eyepieces provide a wider field of view at any given magnification than standard field of view eyepieces.

Flat Field Objectives or Optics

Distortion or curvature at the edge of the field is a common optical defect in inexpensive objectives. This does not necessarily limit their usefulness for visual observation, but for photography it is a significant flaw and will produce out of focus images at the edge of the field. Higher grade objectives are available which correct this shortcoming. Flat field objectives are flat across 70-85% of the field of view, while Plan objectives are flat across 90-100% of the field of view. The difference between the Flat field and Plan designation is therefore the degree of correction.

Fluorescent microscope

The discovery that some cell proteins will fluoresce under ultraviolet has led to a many advances in cellular biology. A microscope which uses ultraviolet light to study fluorescent materials within cells, either naturally occurring or induced, is known as a fluorescent light microscope. 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

This refers to the diameter of the baffle in an eyepiece which has a direct effect on the available field of view.

Infinity

Traditionally, compound microscopes have been made with optical tubes - the tube that connects the eyepiece with the objective - to accommodate light paths of fixed focal lengths, most often 160mm (DIN standard), but sometimes 170mm. Objectives must be matched to this stated tube length to perform properly and most objectives will be marked appropriately.

More advanced microscope designs, however, place accessories such as illuminators, polarizers, prisms and so in the optical path between eyepiece and objective. This can effectively change the focal length of the system, causing focusing and aberration problems in finite tube microscopes. Infinity microscope systems were introduced to handle these accessories correctly. Infinity tube systems create an area of parallel light rays in the optical path by introducing an extra lens (tube lens) somewhere in the tube between the eyepiece and the objective. Accessories can then be inserted in the optical path with a minimum of distortion or aberration.

This Infinity design also allows the use of larger objectives with better working distance than standard DIN objectives. It should be noted, however, that placement of the tube lens varies from one infinity system to the next. In other words, there is no standardization in the "reference" tube length from manufacturer to manufacturer. Infinity objectives must therefore be matched to the appropriate infinity system. Indeed, some manufacturers intentionally alter the thread size of their infinity objectives to prevent their use in other infinity systems.

Illumination

In reference to the light sources used in microscopes, there are a variety used.

Tungsten illumination

This is conventional electric light bulb technology and the least expensive electric illumination used in microscopes. Tungsten runs hotter than other types of illumination and may cause damage to the specimen. It also provides less image brightness. For this reason, its use is usually restricted to inexpensive microscopes.

Halogen illumination

The is an excellent upgrade to tungsten. It is both brighter and cooler than tungsten and a much better choice for studying live specimens and also for photography.

Fluorescent illumination

Another excellent alternative to tungsten. Fluorescent offers excellent bulb life, lower temperature and better brightness than tungsten.

LED illumination

LEDs are beginning to see more use in microscope illumination, especially as ring lights for stereo microscopes. Temperatures are very low and bulb life is virtually unlimited.

Interpupillary Adjustment

All binocular microscopes allow the user to set the eyepieces at the correct distances for the width of the users eyes. This is known as the interpupillary adjustment or IP.

Jensch head

As far as interpupillary adjustments, binocular heads come in two styles. A Jensch offers side to side interpupillary adjustment, allowing for easier and more precise adjustment than the a Seidentopf head, which uses pivoting eyepieces to set the IP.

Koehler illumination

To achieve the highest potential contrast in a specimen requires a condenser and illumination system that can be finely adjusted and centered. The best known such system is the Koehler illumination system which features a condenser system and bulb that can be critically aligned and which is also typically fitted with two diaphragms - one near the specimen and one near the lamp. The upper diaphragm controls the angle of the cone of light entering the specimen and the lower diaphragm controls the size of the circle of illumination.

Magnification

Total magnification in a simple microscope is simply calculated by multiplying the magnification of the eyepiece by the magnification of the objective. More advanced microscopes, however, may have a head and/or accessories that affect total magnification and these must be factored in to calculate total magnification.

Mechanical Stage

All serious microscopes use a mechanical stage to move the slide in tiny micrometer increments. This is a must for high magnification scanning. A mechanical stage is also adjustable for different sizes of slides.

Mirror

This is the traditional lighting system used on toy and children's microscopes, though a mirror system is still a viable option for serious microscopes which are used in locations with no power supply. A mirror requires light from an external source such as the sun or lamp and the mirror reflects the light source upward to the condenser/specimen via a mirror located below the stage. The typical mirror is two sided, plano-convex (flat, curved) to accommodate various light sources.

Monocular Head

This is a microscope head which offers only one eyepiece for observation. This configuration does reduce the cost but it also produces more eye strain and fatigue over long observing sessions.

Nosepiece

On a compound microscope, the nosepiece is the area of the microscope that holds the turret and objectives.

Numerical Aperture (N.A.)

In most optical instruments, aperture and optical quality are the main determinants of resolution (ability to show tow closely spaced objects as being separate) and hence performance. In microscopes, due to their higher magnifications and the variety of mediums in which objectives are uses (oil, water, air) other factors must also be considered. Aperture (diameter of the objective) is still important, since a larger objective will deliver more light to the specimen, but the medium also affects the width of the cone of light available for studying the specimen. The relationship between aperture, magnification, medium and degree of optical correction is known as an objective's numerical aperture or N.A. Dry objectives (objectives that are used without oil, water or other special medium on the slide) have a theoretical N.A. limit of 1.0 and in practice, it is rare to achieve an N.A even close to that. Applications requiring a higher N.A. will require the use of an oil or water objective, since these respective mediums reduce the amount of bending (refraction) as light enters and leaves the slide. The highest N.A objectives are objectives that combine immersion with sophisticated optical correction.

Objective Lens

The objective lens is the centerpiece of a microscope optical system. Objectives are threaded onto the turret of the microscope, which typically holds 3-4 objectives. Magnification is increased or decreased as needed by rotating different objectives into position. Each objective barrel is inscribed with useful information. This typically includes 1) magnification (number with X) 2) numerical aperture N.A 3) tube length the objective is designed for (160mm, 170mm or infinity symbol for infinity systems, 4) special medium if other than dry or air (oil, water and so on) 5) optical correction if other than achromat (flat field, plan, apochromatic) and sometimes 6) thickness of the cover glass to be used if other than standard .17mm

Ocular

Alternative name for an eyepiece.

Oil Immersion Lens:

To realize an N.A. above 1.0 requires an objective which uses oil or water between it and the cover slip. Oil has a similar refractive index to glass. By using a drop of oil between the cover slip and objective the refraction of light rays and consequent loss of resolution that occurs with a dry objective is greatly reduced. Oil and water objectives must also be used with a condenser with the same or higher N.A. in order to realize their potential.

Par centered

Par centered objectives allow the specimen to remain in the field of view as you change objectives, provided you have properly centered the specimen. This is a standard feature on all quality microscopes.

Par focal

Par focal objectives require only fine focusing as you change the objectives to bring a specimen into focus. This is a standard feature on all quality microscopes.

Plan achromat or objective

Distortion or curvature at the edge of the field is a common optical defect in inexpensive objectives. This does not necessarily limit their usefulness for visual observation, but for photography it is a significant flaw and will produce out of focus images at the edge of the field. Higher grade objectives are available which correct this shortcoming. Flatfield objectives are flat across 70-85% of the field of view, while Plan objectives are flat across 90-100% of the field of view. The difference between the Flatfield and Plan designation is therefore the degree of correction.

Phase Contrast

In order to study detail in transparent specimens with a bright field microscope, the specimens must be stained. This causes damage to the specimen and may even kill it. The phase contrast microscope reveals detail in transparent specimens without resorting to the use stains. This makes the phase contrast microscope one of the most used tools for observing living cells and as such, it has become one of the most used microscopes in medicine and biology.

As light passes through a specimen, it is slowed down slightly or "phase shifted" as it encounters structures in the specimen. The change in speed is directly related to the transparency of the structure. Structures that produce a great deal of phase shift, as found in colored and opaque specimens, are easily visible in a bright field microscope. The structures in transparent specimens, however, do not produce enough phase shift to be visible in a bright field microscope. By employing a series of phase plates in the objective and condenser, a phase contrast microscope is able to amplify tiny phase shifts to the point of visibility. Minute details in the structure that are invisible in a bright field microscope become visible in a phase contrast microscope.

The phase contrast microscope does have its limitations. It is only useful for specimens that are transparent, colorless and difficult to see against their background. These objects include protozoans, cell organelles and other difficult to study structures and for this reason, these objects are sometimes referred to as phase objects. You can convert a bright field microscope to a phase contrast microscope with the addition of phase objectives and a phase condenser.

Because of its contribution to science, the phase contrast microscope earned a Nobel Prize. It was developed early in the twentieth century by Frits Zernike.

Polarized light microscope

Non-polarized light vibrates in all planes. By adding a polarizer, light can be made to vibrate in only one plane, much the same as light that has passed through a Venetian blind. The polarized light microscope puts this principle to good use because many objects produce distinctive signatures when exposed to polarized light under a microscope. A polarized light microscope differs from a conventional microscope in several ways. It adds both a polarizer and an analyzer and it incorporates a rotating stage with plates that can be inserted in the light path. An observer can then measure the angles of light produced and check the colors against a chart to identify a sample.

The polarized light microscope has many applications, but is perhaps best known in geology for rock and mineral identification (requires the preparation of thin sections), but it is also used for other applications such as asbestos counts and in medicine to study crystals in urine and in cells.

Rack and Pinion

Nearly all microscopes use a rack and pinion gear system for focusing. A rack is nothing more than a plate with a row of angled teeth and a pinion gear rotates perpendicular to this plate to move it up and down. This allows for very precise and smooth movement.

Rack Stop or Safety Rack Stop

This is a feature that may be built into the objective or the nosepiece of the microscope itself. It prevents damage to the slide and/or objective when a microscope is accidentally over focused at higher magnifications and brought into contact with the slide. It allows the objective to retract when too much pressure is brought to bear.

Refractive Index (R. I. )

Light is slowed down when it passes through different mediums, such as air, water oil, glass and so on. The ratio of the speed of light in a vacuum to the speed of light through any given medium is known as its Refractive Index or R.I. In a microscope, this can be used to determine the numerical aperture, N.A., of an objective. For instance, air has a refractive Index of 1.0. For an objective to achieve a higher N.A. than 1.0, it must be used in a different medium such as water or oil.

Resolution

Resolution is the ability to separate two closely spaced objects and show them as separate objects. In a microscope, resolution is a function of magnification, optical correction and the medium used and it is reflected in the Numerical Aperture (N.A.) of the objective.

Reticule

Also called a reticle. This is a grid or scale in an eyepiece used for measuring or counting.

Ring Light

Incidental or side lighting systems used on a typical stereo microscope produce detail robbing glare on the specimen. This reduces the effectiveness of the stereo microscope for both visual and photographic purposes. A ring light system eliminates glare and does a better job of highlighting detail in the specimen. A ring light is simply a ring of light that surrounds the nosepiece on a stero (dissecting) microscope. Ring lights are most commonly expensive fiber optics assemblies, but less expensive LED systems are now available for many models.

RMS thread

One of the first standards for thread size on microscope objectives was established by the Royal Microscopal Society (RMS) in England in the late 1800s. This standard is still used for many objectives, though advanced systems, especially infinity systems, have begun to deviate from this to allow for larger objective designs and also to prevent the use of inappropriate objectives in their systems.

Seidentopf head

As far as interpupillary adjustments, binocular heads come in two styles. A Jensch offers side to side interpupillary adjustment, allowing for easier and more precise adjustment than the a Seidentopf head, which uses pivoting eyepieces to set the IP, much the same as folding the barrels on a binocular.

Tube Length

Traditionally, compound microscopes have been made with optical tubes - the tube that connects the eyepiece with the objective - to accommodate light paths of fixed focal lengths, most often 160mm (DIN standard), but sometimes 170mm. Objectives must be matched to this stated tube length to perform properly and most objectives will be marked appropriately.

More advanced microscope designs, however, place accessories such as illuminators, polarizers, prisms and so in the optical path between eyepiece and objective. This can effectively change the focal length of the system, causing focusing and aberration problems in finite tube microscopes. Infinity microscope systems were introduced to handle these accessories correctly. Infinity tube systems create an area of parallel light rays in the optical path by introducing an extra lens (tube lens) somewhere in the tube between the eyepiece and the objective. Accessories can then be inserted in the optical path with a minimum of distortion or aberration.

This Infinity design also allows the use of larger objectives with better working distance than standard DIN objectives. It should be noted, however, that placement of the tube lens varies from one infinity system to the next. In other words, there is no standardization in the "reference" tube length from manufacturer to manufacturer. Infinity objectives must therefore be matched to the appropriate infinity system. Indeed, some manufacturers intentionally alter the thread size of their infinity objectives to prevent their use in other infinity systems.

Spherical Aberration

Spherical aberration is an optical defect that occurs anytime a curved or spherical lens is used to bend light. When light refracted from the center of the lens comes to focus at a different point than light refracted from the edge of the lens, blurring of the image (spherical aberration) occurs. This defect can be reduced or eliminated by using non-spherical (aspherical) lenses or by using a combination of lens, as in an achromat or apochromat.

Stage

The stage is the plate or platform on a microscope that holds the specimen.

Stage plates

Stereo microscopes typically offer a variety of stage plates - a piece of glass or metal on which the specimen is placed. By interchanging stage plates of different colors or transparency, detail in the specimen can be enhanced.

Stage Clips

Metal clips, usually spring-loaded, used to keep the slide in place on inexpensive microscopes.

Swing Arm

An extended or boom arm used with stereo microscopes, most often for industrial applications. Allows the microscope to be extended over an assembly line or specimens too large to fit on a standard stage.

Stereo microscope

A microscope with high magnification is not always desirable or needed for many applications. A microscope that allows three dimensional, low power observation of a specimen is known as a stereo microscope or alternately, a dissecting microscope, because such a microscope allows a user to manipulate and work on a specimen under the microscope.

A stereo microscope should not be confused with a compound microscope which also offers the user two eyepieces. A stereo microscope has a number of significant differences.

For one, a dissecting or stereo microscope produces upright, correct right to left images, instead of upside down, reversed images as in a compound microscope. This makes it a practical choice for manipulating a specimen. For another, a dissecting microscope offers much greater working distance. A long working distance allows a stereo microscope to be used with whole specimens, including rocks, flowers, gems, coins, whole insects and many other objects too large to be used with a compound microscope. For yet another, a stereo microscope employs two separate optical systems, each with its own eyepiece, giving the observer true, three dimensional image, thus the name "stereo". A compound binocular microscope uses two eyepieces, but not two separate optical systems and cannot, therefore offer three dimensional viewing. Lastly, a dissecting microscope is a much lower magnification instrument, in keeping with the larger specimens usually observed with it. The typical stereo microscope will have magnifications between 10 and 40x, though some may offer as much as 100x.

T-mount

An adapter found on compound and stereo microscopes that allows the attachment of an SLR camera (camera with a removable lens). Also requires the addition of a t-ring for a specific brand and model of SLR camera.

Trinocular Head

A three-tube microscope head, most often with two eyepieces and a third for the attachment of a camera, but sometimes offering three eyepieces for teaching.

Turret

The section on the nosepiece that holds the objectives and rotates.

Wide field eyepiece

Field of view is determined both by magnification and eyepiece design. Currently, there is no standard that defines a wide field eyepiece, but in general, it is an eyepiece which offers a wider field of view than a conventional eyepiece.

Working Distance

The distance between the specimen and the outer objective lens. Working distance may measure only a few millimeters on a high magnification compound microscope objective or it may measure many centimeters on a stereo microscope.