Microscope Glossary of Terms

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 on microscopes today. It is typically used with an adjustable iris, 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 the best image quality. All microscopes at magnifications over 400x will require an Abbe condenser or the equivalent.

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

Aberration: An optical flaw that is 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 an additional focus point when they emerge from a lens. The color fringe around the specimen this produces is known as chromatic aberration. This optical defect causes a loss of detail and resolution. Achromatic objectives or achromats focus on two wavelengths (red and blue), 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. For this reason, the achromat remains the most common type of objective in use today.

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, other factors must be considered. See numerical aperture (NA)

Apochromatic Objectives: Bringing all wavelengths to the same focus point requires an apochromatic objective, often called an APO. These high-performance objectives employ complex lens systems and are thus expensive, but they remain the design of choice for serious research.

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

Articulated Arm: An articulated arm is a hinged arm that allows the user to set the angle of the eyepiece tube for comfort.

Asbestos Microscope: An asbestos microscope is typically a specialized version of a polarized light microscope modified to asbestos counts for legal work. Asbestos Microscopes require the use of a compensated eyepiece.

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 a polarized light microscope tube to study interference patterns for 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 significantly reduced.

Bright-Field Microscope: This is the most common microscope system used in classrooms, and a brightfield is adequate for many applications. In a brightfield 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 is 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 another focus point when they emerge from a lens. The color fringe around the specimen this produces is known as chromatic aberration. This optical defect causes a loss of detail and resolution. Chromatic aberration is partially corrected with an achromat objective and fully restored with an apochromatic.

Coarse Focus: This knob is used at low power to bring the specimen into focus quickly. The coarse focus knob should be restricted to lower magnification objectives with greater working distances. High-magnification objectives have very short working distances. High-magnification objectives should only be used with fine focus to prevent damage to the slide, specimen, and possibly the objective.

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

Compensated Eyepiece: Knowing the approximate magnification a system delivers is sufficient for most visual work. Applications requiring counts, however, need the operator to know the exact magnification of the system. Compensated eyepieces are designed to work with specific objectives to deliver an actual magnification.

Compound Microscopes: Unlike a stereo or dissecting microscope, a microscope with a revolving nosepiece (turret) contains three or more objectives. This is the type of microscope most often associated with the word microscope. The compound light microscope requires slides and is designed to study microscopic specimens at high magnifications. Working distances do not allow the study of large, whole samples such as rocks, plant parts, etc. These larger specimens require the use of a stereo or dissecting microscope.

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 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 NA (numerical aperture) of only 0.65. Since a system's potential NA is only as good as its weakest link, this severely limits performance. All serious microscopes use a minimum of an Abbe condenser system with an NA of 1.25 or more.

Cover Slip: A cover slip is a thin piece of glass or plastic that covers a specimen mounted on a slide. It protects both the objective and the sample. The standard cover glass thickness is .17mm

Darkfield Illumination: A darkfield microscope will show a transparent or semi-transparent object as a bright object against a dark background. Darkfield Illumination is achieved using a cone of light lit only at the tip and dark in the center rather than uniformly lit throughout the cone as in a brightfield microscope. In a darkfield microscope, only the end of the light cone touches the specimen, with the resultant light scattering against a dark background. This produces detail in transparent and semi-transparent samples that will not be visible in a brightfield 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 sample at varying levels of its thickness. To do this effectively requires a condenser system that can 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 nothing more than a wheel with holes of varying sizes. Most working microscopes, however, use an iris diaphragm with overlapping leaves that can be opened or closed as needed.

DIN - Deutsche Industrie Normen: This standard 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 specify eyepiece diameter and size, objective thread size, and microscope tube length (160mm). Theoretically, any DIN standard accessory can be used in any DIN standard microscope. However, 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 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 enables a user to manipulate and work on a specimen under the microscope.

A stereo microscope should be distinct from a compound microscope, offering the user two eyepieces. A stereo microscope has several 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 a much greater working distance. A long working distance allows a stereo microscope to be used with whole specimens, including rocks, flowers, gems, coins, real insects, and many other objects too large to be used with a compound microscope.

A stereo microscope employs two separate optical systems, each with its eyepiece giving the observer an actual three-dimensional image, thus the name "stereo." A compound binocular microscope uses two eyepieces but not two separate optical systems and cannot offer three-dimensional viewing. Lastly, a dissecting microscope is a much lower magnification instrument due to 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 is used in an achromat objective. Indicates partial correction of chromatic aberration. Total or near-total elimination of chromatic aberration is achieved with an apochromatic, 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 1.25in.

Field of View (FOV): Field of view is the extent of the area visible around a specimen. The field of view is determined by magnification - as magnification goes up, the 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 the standard field of view eyepieces.

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

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. Still, 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%. Therefore, the difference between the flat-field and plan designation is the degree of correction.

Fluorescent Microscope: The discovery that some cell proteins will fluoresce under ultraviolet light has led to many advances in cellular biology. A microscope that 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 benefit is its ability to distinguish between living and dead cells and monitor activity within living cells. This microscope has made a tremendous contribution to the field of medical research.

Field Number (FN): This refers to the diameter of the baffle in an eyepiece which directly affects the available field of view.

Illumination: About the light sources used in microscopes; there is 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: This is an excellent upgrade to tungsten. It is brighter and cooler than tungsten and a much better choice for studying live specimens and 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.

Infinity: Traditionally, compound microscopes have been made with optical tubes that connect the eyepiece intending 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 on in the optical path between the eyepiece and the objective. This can change the system's focal length, 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 minimal distortion or aberration.

This Infinity design allows larger objectives with better working distance than standard DIN objectives. It should be noted, however, that the 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.

Interpupillary Adjustment: All binocular microscopes allow the user to set the eyepieces at the correct distances for the width of the user's 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 Seidentopf head, which uses pivoting eyepieces to set the IP.

Koehler Illumination: Achieving 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 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 calculated by multiplying the eyepiece's magnification by the objective's magnification. 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 toys and children's microscopes. However, a mirror system is still a viable option for serious microscopes used in locations with no power supply. A mirror requires light from an external source, such as the sun or a 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 that offers only one eyepiece for observation. This configuration reduces the cost but 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 (NA): In most optical instruments, aperture and optical quality are the main determinants of resolution (ability to show two closely spaced objects as separate) and performance. In microscopes, due to their higher magnifications and the variety of mediums in which objectives are used (oil, water, air), other factors must also be considered. The aperture (diameter of the objective) is still important since a larger objective will deliver more light to the specimen. Still, 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 NA. Dry objectives (objectives that are used without oil, water, or other special medium on the slide) have a theoretical NA limit of 1.0, and in practice, it is rare to achieve an NA even close to that. Applications requiring a higher NA will require an oil or water objective since these respective mediums reduce bending (refraction) as light enters and leaves the slide. The highest N 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) and 2) numerical aperture 3) the tube length the objective is designed for (160mm, 170mm, or infinity symbol for infinity systems, 4) a 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.

Oil Immersion Lens: An NA above 1.0 requires an objective that 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 the objective, the refraction of light rays and consequent loss of resolution that occurs with a dry objective is greatly reduced. To realize their potential, oil and water objectives must also be used with a condenser with the same or higher NA.

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.

Phase Contrast: The specimens must be stained to study detail in transparent specimens with a brightfield microscope. 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 of stains. This makes the phase contrast microscope one of the most used tools for observing living cells, and 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 brightfield microscope. However, the structures in transparent specimens do not produce enough phase shifts to be visible in a brightfield microscope. A phase contrast microscope can amplify tiny phase shifts to the point of visibility by employing a series of phase plates in the objective and condenser. Minute details in the structure that are invisible in a brightfield microscope become visible in a phase contrast microscope.

However, the phase contrast microscope does have its limitations. It is only useful for transparent, colorless specimens and is difficult to see against their background. These objects include protozoans, cell organelles, and other difficult-to-study structures, so these objects are sometimes referred to as phase objects. You can convert a brightfield microscope to a phase contrast microscope by adding 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.

Plan Achromat or Objective: A plan achromat objective lens offers both flat-field and chromatic abberation correction, offering sharper, clearer images than basic achromatic objectives.

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 the 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 a polarizer and an analyzer, and it incorporates a rotating stage with plates that can be inserted into 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). Still, it is also used for other applications, such as asbestos counts and in medicine, to study crystals in the urine and 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 (RI): Light is slowed down when it passes through different mediums, such as air, water, oil, glass, etc. 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 RI. In a microscope, this can be used to determine an objective's numerical aperture (NA). For instance, air has a refractive Index of 1.0. For an objective to achieve a higher NA 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 objective's Numerical Aperture (NA).

Reticle: This is a grid or scale in an eyepiece for measuring or counting.

Ring Light: Incidental or side lighting systems used on a typical stereo microscope produce a 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 a ring of light that surrounds the nosepiece on a stereo (dissecting) microscope. Ring lights are the most commonly used expensive fiber optic 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. However, advanced systems, especially infinity systems, have begun to deviate from this to allow for larger objective designs and prevent 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 Seidentopf head, which uses pivoting eyepieces to set the IP, much the same as folding the barrels on a binocular.

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 using non-spherical (aspherical) lenses or a combination of lenses, as in an achromat or apochromat.

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

Stage Clips: Metal clips, usually spring-loaded, that are used to keep the slide in place on inexpensive microscopes.

Stage Plates: Stereo microscopes typically offer a variety of stage plates - a piece of glass or metal on which the specimen is placed. Interchanging stage plates of different colors or transparency can enhance the specimen's detail.

Stereo Microscope: Another term for dissecting microscope. Stereo microscopes provide three-dimensional views of the specimen (see full definition above for Dissecting Microscope.)

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

T-Mount: An adapter found on compound and stereo microscopes allows the attachment of an SLR camera (camera with a removable lens). It also requires the addition of a T-ring for a specific brand and model of SLR camera.

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

Tube Length: The distance between the objective lens mounting surface and the eyepiece opening.

Turret: The section on the nosepiece holds the objectives and rotates.

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

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