Optical Vocabulary

Aberration - The failure of a lens to bring all the rays of light to an exact focus. As in many other fields, there are lots of different aberrations, classified by pundits.

Achromatic Lens - (w/o color) A lens consisting of two or more elements, usually made of crown glass and flint glass. This lens has been corrected for chromatic aberration in that it has the same focal length with respect to two selected colors (red and blue) or wavelengths of light. The resultant image is (nearly) free of extraneous colors. It is also corrected for spherical aberration with respect to one color.

Analyzer - A filter that selects one angle of polarized light. See polarized light.

Angle of incidence - The angular measure between an incoming light ray striking a surface and the normal (a line perpendicular to that surface).

Angle of reflection - The angular measure between a reflected light ray and the normal to the reflecting surface. For any surface, the angle of incidence equals the angle of reflection. 
 

Angle of refraction - The measure between a light ray that has been "bent" by passing through a bounding surface and the normal to this surface.
 

Angstrom - A unit of length, 1 Angstrom = 0.000,000,1 mm. A few thousand of these is a convenient measure of the length of a light wave.

Aperture - ( diaphragm, stop, pinhole, slit) A fixed or variable opening or hole through which light may pass.

Apochromatic lens - Similar to the achromatic lens, the apochromatic lens corrects for three colors (red, blue and green), greatly reducing the blur caused by the colors uncorrected by the achromat lens.In addition the lens is better corrected for spherical aberration.

Astigmatism - The aberration in which the horizontal and vertical lines are focused at different points along the optical axis.

Beam splitter - This is a thin film coating, usually on a piece of glass, that allows light to be divided so that a portion will be transmitted and a portion will be reflected.

Binocular - A microscope head with two eyepieces.

Bright field - A type of illumination in which the light passes through the objective is reflected off the sample and passed through the objective to the eye pieces.

Calibrate - To determine the correct scale intervals for any measuring device or to ascertain the errors of the scale, such as the reticle in an eye piece.

Centration - The accuracy with which the optical axis of the lens coincides with the mechanical axis of the mounting.

Chromatic aberration - A difference in focal points depending on the color or wavelengths of light passing through a optical system.

Condenser - A set of lenses used in a compound microscope to "gather" light rays and focus them on the sample.

Contrast - The degree of difference in tone, brightness, or color from point to point or from highlight to shadow in an object or image.

Corrected lens - A lens system which has been corrected for aberrations to produce a clear, sharp image.

Curvature of field - An aberration in which the surface of the best focus is not a plane, but a curved surface. Edges will appear out of focus while the center is in focus, or vice versa.

Dark Field - An illumination technique that makes the sample appear luminous against a background of little or no light. Used for identifying surface contamination or edge definition.

Depth of field - The distance along the optical axis throughout which the object can be located and imaged with satisfactory clarity.

Depth of focus - The distance along the optical axis throughout which the image formed by a lens is focused clearly.

Distortion - The aberration of a lens which causes the image to appear deformed due to a gradual increase or decrease in magnification from the center to the edge. Straight lines will appear to bend at the edge of the field.

Empty magnification - magnification which increases the size, but not the detail, due to a limitation of the resolving power of the optics.

Eyepiece - (oculars) The final lens system in a microscope that the user looks through.

Eye point - The proper location of the eye when using a visual instrument.

Eye relief - The distance from the eyepiece to the user's eye at the eye point.

Filar eyepiece - An eyepiece with a measuring reticle in it and a movable cross hair for making precise measurements.

Filter - A transparent material user to select a color or (range of wavelengths) of light.

Flatness of field - The appearance of the plane or image being flat.

Focal Length - The distance from a point where an image is formed (the focal point) to a point in or near the lens or lens system. [In the sketch at right, this is labeled FL.]
 
Focal Point - That point where the light rays coming from a distant object converge after passing through a lens, coming to a focus and forming an image. [The point FP in this sketch.] 
 

Head - (observation tube) The top viewing portion of the microscope with eyepieces in it. (see monocular, binocular, trinocular and teaching.)

Illumination - The method of lighting the sample. Types of illumination include bright field, dark field, fiber optics, fluorescent ring light, Nomarski, phase contrast, and transmitted.

Illuminator - In a microscope, the source of light which illuminates the sample to be viewed.

Iris diaphragm - An opaque opening which can be opened or closed, like the iris of the eye.

Koehler illumination - includes a field diaphragm control. This type is generally preferred because an uneven distribution of energy in the source does not result in uneven brightness in the field of view.

Meniscus - A type of lens that is crescent in shape, may be converging or diverging.

Monocular - A microscope head with only one eyepiece.

Nanometer - A unit of measurement equaling 0.000,001 mm. Wavelengths of light are measure in nanometers, except occasionally when they are measured in Angstroms. One nanometer is 10 Angstroms.

Numerical Aperture (N.A.) - A ratio that describes the cone of light emitted by the condenser or accepted by the objective lens. Objectives with a larger N.A. have greater resolving power. (a 100x oil lens with a 1.25 N.A. can resolve smaller objects than a 100x lens with a 0.95 N.A.)

Objective lens - The compound lens system in a microscope or other optical system that first gathers light from the sample and forms the first image. These systems are usually well corrected for aberrations, as any image defect cannot be removed by elements later in the light path, but is only magnified further.

Photomacrography - The photographic recording of images as seen in a microscope.

Photo port - The third port on a trinocular head or attachment to a microscope that allows a camera to be attached.

Polarizer - A filter that polarizes light.

Polarized light - light whose waves are uniformly aligned in one direction.

Prism - A transparent body (made of optical glass) with at least two polished plane faces inclined toward each other from which light is reflected or through which light is refracted.

Real image - An image that can be seen with the eye or can be displayed on a screen.

Reticle - A transparent object, usually glass, with a pattern printed on it that is placed in the eyepiece. The pattern may be a scale, a cross hair or a set of boxes for centering an image for photography.

Stage - The surface of a compound microscope that the sample is placed on for viewing. Types include mechanical and motorized stages.

Teaching head - An accessory that allows more than one microscope head per microscope. Also known as dual observation tubes.

Telecentric - A Telecentric cone of rays emanating from an object point remains perfectly perpendicular to the object plane across the entire field-of-view. [see figure 1] In contrast, a conventional lens has cones which are not perpendicular and thus produce parallax. [see figure 2] In figure 1 all three locations [A,B & C] will measur the same size. However, in figure 2, A, B & C will all appear the same size to the camera, but in reality are all different sizes.
 

Trinocular - A microscope head with two eyepieces and a camera port.

Working distance - The distance from the closest surface of a lens system to the sample while the system is in focus.