Glossary of Terms

A Glossary of Microscope Terms

This is a glossary of commonly used microscopy terms. See also our brief History of the Microscope.

Abbe Condenser: A lens that is specially designed to mount under the stage and which typically, moves in a vertical direction. An adjustable iris controls the diameter of the beam of light entering the lens system. Both by changing the size of this iris and by moving the lens toward or away from the stage, the diameter and focal point of the cone of light that goes through the specimen can be controlled. Abbe condensers are useful at magnifications above 400X where the condenser lens has a numerical aperture equal to or greater than the N.A. of the objective lens being used.

Achromatic Lens: An objective lens which partially corrects for chromatic and spherical aberration. The refraction index and curvature of lens elements create a separation of color. Achromatic lenses correct chromatically for two wavelengths of light (red and blue), and spherically for one wavelength (green).

Aperture Planes: A conjugate set of planes at which light is controlled, and at which the lightsource (lamp filament) would be in focus.

Anisotropy: Properties dependent on direction.

Arm: The part of the microscope that connects the eyepiece tube to the base. Articulated Arm: Part of a boom microscope stand, an articulated arm has one or more joints to enable a greater variety of movement of the microscope head and, as a result, more versatile range of viewing options.

Base: A microscope is typically composed of a head or body and a base. The base is the support mechanism.

Binocular Microscope: A microscope which uses two oculars (eyepieces). In high-power, compound microscopes, the two oculars produce a monoscopic image, while in stereo microscopes, the image is stereoscopic.

Birefringence: Refractive indices of material dependent on polarization and propagation of light. Optically anisotropic.

Body: Often referred to as the head, the body is the upper part of a microscope including, eyepieces and objectives. Most modern microscopes are modular in the sense that the same body can be used with different bases and vice versa.

Boom Stand (Universal Boom Stand): A microscope base that incorporates an adjustable arm or boom and enables the body to be aligned in a variety of different positions. Used in commercial inspection applications.

Camera Adapter: An adapter kit designed to enable a camera to fit on to the trinocular port of a microscope (23mm or 30mm port diameter). The camera connects to a step ring (or T-Mount) and then to the camera adapter.

Chromatic Aberration: The separation of colors caused by refraction. When light enters a lens, different wavelengths (colors) of light are refracted (bent) by different amounts, resulting in these rays focusing at different points when exiting the lens.

Clamp Base: A clamp that replaces the traditional base on the bottom of a boom microscope and enables the pole to be clamped on to the side of a work bench or table.

C-Mount: This is threaded-mounting standard, adapted from 16mm cinematic cameras, and now used as a standard for CCTV cameras and microscope photo-ports. The thread has a 1-inch diameter, width 32 threads per inch (TPI). The flange focal distance (distance from flange to image-plane) is 17.52mm.

Coarse Focus: Adjustments made to focus with a high ratio of mechanical adjustment compared to optical displacement.

Coaxial Focus: A focusing system with both the coarse and fine focusing knobs mounted on the same axis. The coarse focus is typically the larger, outside knob and vice versa. On some coaxial systems, the fine adjustment is calibrated, allowing differential measurements to be recorded.

Comparison Microscope: A microscope that enables side-by-side viewing of two different specimens. The microscope has two sets of objectives with a single set of eyepieces (monocular or binocular), often used in forensic science.

Compound Microscope: Originally used to describe a microscope with more than one objective lens, a compound microscope is now generally understood to be a high power microscope with multiple, selectable objective lens of varied magnifications. See stereo/low power.

Condenser: A lens that concentrates the light on a specimen and increases the resolution. Found in or below the stage on compound microscopes, only. Contrast Plate: Found only on stereo microscopes, one side is white and one black. Either side can be used depending on your specimen. Cover Slip: A thin, square piece of glass or plastic placed over the specimen on a microscope slide. It flattens out liquid samples and helps single plane focusing.

Conjugate Planes: A set of planes within the optical system which are simultaneously in focus.

Conoscopic: Viewing the aperture plane (plane in which light is controlled/modified).

CRI (Color Rendering Index): A scale from 0 to 100, representing a light-source's ability to render colors accurately.

Darkfield Microscopy: a technique used to enhance the contrast in unstained specimens. It works on the principle of illuminating the sample with light that will not be collected by the objective lens, so not form part of the image. This produces the classic appearance of a dark, almost black, background with bright objects on it.

Darkfield Plate: A circular iris that sits on the base of the microscope above the light source and reflects the light horizontally to the specimen, thereby achieving lateral illumination. Digital Microscope: A microscope with a built in digital camera that enables direct feed to a PC, TV or printer.

Diascopic: Transmitted light, which passes through the subject

DIN (Deutsches Institut für Normung): the German Institute for Standardization is an international standards organization that determines the "standard" for a wide variety of different types of technology. The DIN standard is commonly used in reference to objective lenses, describing the parfocal distance (45mm) and optical tube-length (150mm) or mechanical tube-length (160mm).

Dissecting Microscope: Typically interchangeable with stereo microscope, a dissecting microscope is a stereo microscope used in laboratory work.

Doublet (Lenses): Two separate lenses, shaped and spaced in a manner to more-efficiently achieve magnification while better-controlling optical aberrations. A doublet is typically comprised of a convex, crown-glass lens, mated with a concave, flint-glass lens. The lenses can be cemented together, or use oil or air as a spacing medium to achive aberrational corrections.

Dual-View: A monocular microscope that has a second, vertical viewing port. Often used by teachers. It can also be used for photographic applications.

Electron Microscope: A type of microscope that uses electrons rather than light to create an image of the target. It has much higher magnification or resolving power than a normal light microscope, up to two million times, allowing it to see smaller objects and details.

Episcopic: Reflected light, which passes through the objective, reflects off of the subject back at the objective.

Eyepiece: Otherwise referred to as an ocular, the eyepiece is the lens nearest to your eye. Total magnification of a microscope is determined by the sum of the eyepiece magnification multiplied by that of the objective lens.

Eyepiece Tube: The tube in which the eyepiece lens is situated.

Field Planes: A conjugate set of planes in the optical system which include the field diaphragm, specimen plane, intermediate image plane, and the eye.

Fine Focus: Adjustments made to focus with a low ratio of mechanical adjustment compared to optical displacement.

Field of View (FOV): In microscopy, the viewable portion of the specimen plane, as seen through the optical system. Typically measured by dividing the eyepiece's aperture by the total magnification, the FOV can be further restricted by the objective lens' aperture size, and the microscope's tube size.

Fluor Lens: A lens with improved correction for spherical aberration, as compared to achromatic lenses. Fluor lenses are so named, because they initially used fluorite elements.

Focus: (noun)The state in which the focal point of an optical system coincides with an object to be observed.
(verb) Changing the distance between a lens and a subject along the optical axis until the lens's focal point coincides with the portion of the subject to be observed. In an upright microscope, this is typically done by adjusting the vertical position of the stage

Gem/Jeweler's Microscope: A stereo microscope designed for viewing gems and jewelry, typically incorporating an inclined pole, powerful zoom, darkfield plate and intense, variable lighting.

Halogens: A group of non-metallic elements consisting of iodine, astatine, bromine, fluorine, and chlorine. Iodine and bromine are used in halogen incandescent lamps to reduce oxidation, resulting in longer life for the filament.

Head: The part of a microscope used for observation, to which the eyepieces are attached. The head comes in several structural formats, which can include prisms and mirrors for redirecting light to multiple eyepieces at varying angles. Stereo microscope heads are typically fully-contained, compound microscopes, including both objective and ocular lenses.

Illumination System: The light source on light microscopes, typically mounted under the stage, except on inverted microscopes.

Immersion Oil: A special oil used with the 100X objective in order to concentrate the light and increase the resolution of the image. A drop of oil is placed on the cover slip and the objective is lowered until it touches the oil. There are two primary types of immersion oil: Type A and Type B; Type B is more viscous.

Incandescent (lamp): A type of lamp which uses a heated element, such as tungsten, to produce light. A black-body element, called the filament, is heated -- most-often by electricty -- to produce a bright glow. Due to the nature of the reaction created by the heating, the lamp can produce a wide, even light-spectrum, resulting in a perfect CRI of 100.

Interpupillary Distance: The distance between the two eyepieces, usually adjustable to fit individual users.

Inverted Microscope: A microscope designed with the objectives under the stage and the light source above. Used for viewing larger or irregular specimens, often in containers.

Iris Diaphragm: Found on high power microscopes under the stage, the diaphragm is, typically, a five hole-disc with each hole having a different diameter. It is used to vary the light that passes through the stage opening and helps to adjust both the contrast and resolution of a specimen. It is particularly useful at higher powers.

Isotropy: Properties same in all directions.

JIS (Japanese Industrial Standards): A Japanese standards organization. The JIS standard is commonly used in reference to objective lenses, describing the parfocal distance (36mm) and tube length (170mm).

Jeweler's Clip: A special clip that attaches to the stage and is designed to hold precious stones and jewelry for easier viewing.

Koehler Illumination: A method of illumination named after August Koehler, the man who invented it. It is also known as double diaphragm illumination because it employs both a field and an aperture iris diaphragm to control the illumination. If the light path is adjusted properly, it is possible to enjoy the advantages of an evenly-illuminated field, a bright image without glare and minimum heating of the specimen.

Light Microscopes: Any microscope that uses a source of light to create an image of the specimen and, essentially includes all compound and stereo microscopes.

Magnification: The observed increase in size of an image relative to the size of the original image or specimen. In a compound microscope, the total, effective magnification is equal to the magnification-power of the objective lens multiplied by the magnification-power of the ocular lens: MA = Mo x Me. In some cases, although highly unlikely, a tube lens can produce magnification, changing the equation to: MA = Mo x Me x Mt.

Mechanical Stage: A flat mechanism that sits on top of the stage and allows the viewer to move a specimen small distances - a task that is otherwise difficult at higher magnifications. The mechanical stage will typically move in both the X and Y axis, controlled by coaxial knobs. The stage may also have scales which measure the axial movement of the specimen.

Monocular Microscope: A compound microscope with a single eyepiece. Simple and traditional in design, monocular microscopes have been mostly-replaced by binocular microscopes, which cause less eye-fatigue.

Nosepiece: On a compound microscope, it's the mechanism to which objective lenses are attached. The nosepiece will typically accommodate three or more lenses, and can easily be rotated for efficiency. On an upright microscope, the nosepiece would be situated below the head, and above the stage.

Numerical Aperture (N.A): A measure of the diameter of the aperture compared to the focal length of a lens and ultimately, of the resolving power of a microscope. N.A. is equal to the index of refraction of the medium in which the object is placed multiplied by the sine of the angle made with the axis by the most oblique ray entering the instrument, the resolving power increasing as the product increases.

Objective Lens: The initial lens in a compound optical system, so called because of its proximity to the "object" being observed. The counterpart is the ocular lens (eyepiece), which is responsible for focusing the image on the observer's eye.

Orthogonal: Relative to right angles

Orthoscopic: Viewing the field (focal) plane – a plane in which the image is in focus

Parcentered: When changing objectives, the image of the specimen stays centered. Most compound microscopes are parcentered.

Parfocal: When changing objectives, the image of the specimen stays in focus without needing to adjust the focusing knobs.

Phase Contrast: A contrast enhancing technique developed by Frits Zernike in 1953 for which he won the Nobel Prize in Physics. The technique shifts the light phase wavelength, thereby causing the light deviated by the specimen to appear dark on a light background. It is useful for viewing transparent specimens such as living tissue cells.

Plan (Planar) Lens: An objective or ocular lens utilizing geometric-correction to reduce field-curvature, "flattening" the viewfield. While simple achromats produce a curved image with a 65%-flat viewfield, leaving 35% of the field with less-than-optimal focus, planars can provide a 95%-flat viewfield, resulting in far-superior clarity.

Plan Fluor Lens: A lens which corrects for field-curvature, as well as chromatic and spherical aberration. Also called semi-apochromatic, these lenses produce the flat viewfield of plan lenses, correct for the same chromatic aberration as achromatic lenses, and have enhanced correction for spherical aberration (2-3 colors). They also produce a brighter image, making them better suited for applications such as fluorescence microscopy.

Portable Microscope: A cordless or field microscope with a light source independent of 110/220V. Typically, includes a rechargeable LED light source so that it can be used in the field where 110/220V electric supply is unavailable.

Pointer: A piece of high tensile wire that sits in the eyepiece and enables a viewer to point at a specific area of a specimen.

Pole Stand: A microscope stand composed of a base with a single vertical pole (or post). Typically, the body can move up and down as well as rotate around the pole.

Rack and Pinion Focusing Mechanism: A metal rack and pinion used in better quality microscopes for focusing purposes and moving mechanical stages.

Rack Stop: A safety feature that prevents the viewer from allowing the objective lens to accidentally hit the stage and damage the specimen or slide.

Resolution: The ability of a lens to distinguish the fine details of the specimens being viewed. Reticle: A small glass circle, etched by laser with fine measurements and placed within the eyepiece in order to enable actual measurements of the specimen to be taken.

Revolving Nosepiece: A nosepiece which can hold multiple objective lenses, allowing the easy exchange of lenses without repeated removal or installation.

Ring Light: An extraneous light source that connects to the microscope and emits a ring of light for enhanced lighting. Ring lights are LED, fluorescent, halogen or fiber optic and are typically used on boom microscopes.

RMS (Royal Microscopical Society): a British standards organization. RMS is often used in reference to objective lenses, more specifically the mount-threading. The RMS standard for objective lenses is a 20.32mm diameter mount, and 36TPI (threads-per-inch) using a 55° Whitworth thread. Most of our compound microscopes use the RMS thread, as do Olympus and Zeiss.

Siedentopf Head: A head design where the interpupillary adjustment is achieved by twisting the eyepieces in a vertical arc like binoculars. Semi-Plan Objectives: Improve the clarity and resolution of an image compared to chromatic lens, by partially "flattening" the image of the specimen.

Slide: A flat, rectangular, glass plate on which a specimen may be placed.

Slip Clutch: A mechanical device on the focusing knob that allows the knob to "slip" if a viewer keeps turning the knob beyond its range of travel. Prevents damage from occurring to the focusing system.

Spherical Aberration: The separation of colors caused by the shape of a refractive medium. Light passing through a spherical lens encounters varying amounts of refraction dependent on the distance from the central axis. Light encounters greater refraction at the lens' edges, compared to the center. This results in light rays focusing at different points.

Stage: The platform on which slides and specimens are placed for viewing.

Stage Clips: Clips that are attached to the stage and retain the slide.

Stereo Microscope: A relatively low-power microscope with two independent optical-pathways. Using two objective lenses, each paired to its respective eyepiece, this microscope produces a stereoscopic image, with the perception of depth.

Sub-Stage: Microscope components situated below the stage, often including a transmitted illumination system.

T-Mount: A threaded-mounting standard used with photographic lenses. The t-mount can be employed in non-proprietary lens-production, and coupled with a proprietary mounting-adapter for cross-brand compatibility.

Tension Adjustment: An adjustment setting, most-commonly associated with focus, which can increase or decrease resistance to motion. Although partially subjective, tension adjustment may be necessary when increasing or decreasing the load associated with the adjuster.

Triplet (Lenses): Like a doublet, a triplet uses multple individual lenses, grouped together to achieve magnification with less distortion and optical aberrations associated with simple lenses. The triplet can employ varying lens combinations, such as two concave elements with a double-convex element between them. The elements can be cemented together, or use an intermediary medium, such as oil or air.

Turret: A rotating mechanism, such as a nosepiece or annulus-turret, which simplifies the exchange between components (ie. objective lenses, annuli, filters).

Widefield Eyepiece: A larger-diameter eyepiece which can create a larger viewfield. Microscopes with larger tubes, and subsequently larger intermediate images, allow a wider field to be viewed.