Extended Optical Dictionary

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1. Abbe Number:
   The ratio of refractivity to dispersion in an optical medium. Specifically: (nd-1)/(nF-nC), where n is the index of refraction for the Fraunhofer d, F and C lines, respectively.
   Aberration:
   An optical defect resulting from design or fabrication error, e.g., coma, distortion, curvature of field that prevents the lens from achieving precise focus.
   Achromatic:
   Free of color. Describes an optical system in which chromatic aberration has been corrected at a minimum of two wavelengths.
   Airy Disc:
   A pattern of illumination caused by diffraction at the edge of a circular aperture, consisting of a central core of light surrounded by concentric rings of gradually decreasing intensity.
   Anamorphic:
   Distorted, as in an optical system with different magnification levels or with focal lengths perpendicular to the optical axis.
   Angle of Incidence:
   The angle between a ray of light striking a surface and the normal (a line perpendicular to the surface at that point).
   Antireflection Coating:
   A thin layer of material that, when applied to a lens, increases its transmittance and reduces its reflectance.
   Aperture:
   A hole through which light may pass. The aperture stop is that hole in an optical system limiting the bundle of light able to traverse the system.
   Aspherical:
   Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one or more surfaces are aspherical.
   Astigmatism:
   An aberration in a lens in which the tangential and sagittal (horizontal and vertical) lines are focused at two different points along the optical axis.
   Back Focal Length:
   The distance between the last surface of a lens to its back focal plane.
   Bandpass:
   For a filter or thin-film coating, the wavelength range over which transmission is allowed and possibly maximized.Transmission above or below the bandpass range is restricted by design through absorption and/or reflection.
   Bandwidth:
   Usually described in terms of transmission level, the bandwidth is the spectral range over which an interference filter transmits.
   Beamsplitter:
   An optical device which divides an incident beam into two or more separate and distinct beams. A beamsplitter may be as simple as an uncoated plano-plano piece of glass inserted in a beam at an angle to divert a portion of the beam in a different direction. More complex beamsplitters employ coated and cemented right- angle prisms to separate colors.
   Bending:
   The ratio of the curvatures of a lens's two refracting surfaces.
   Bi-Concave:
   Having two outer surfaces that curve inward.
   Bi-Convex:
   Having two outer surfaces that curve outward.
   Birefringence:
   The separation of a beam of light into two beams (ordinary and extraordinary) as it passes through a doubly refracting material or object.
   Blocking:
   Blocking refers to the filter transmittance outside the bandpass region, and can be thought of as the degree to which undesired wavelengths are prevented from being transmitted. Filters with deep out-of-band blocking significantly enhance the signal-to-noise ratio of the system.
   Blur Circle:
   The image of a point-source object formed by an optical system on its focal surface. The precision of the lens and its state of focus determine the size of the blur.
   Borosilicate Glass:
   An optical glass containing boric oxide, along with silica and other ingredients, having relatively high -value and low index of refraction. Some varieties, such as Corning Pyrex® and Schott Tempax®, are resistant to thermal shock.
   Broadband Coating:
   A multi-layer antireflection coating with reduced reflection over a broad spectral band.
   Catadioptric:
   An optical system containing both reflective and refractive elements.
   Cavity:
   A periodic structure of thin films comprised of two quarter-wave stack reflectors separated by a dielectric spacer.Cavities are the building blocks of bandpass filters.
   C/CS-Mounts:
   Two different but standard video camera mount configurations. The more prevalent of the two types is the standard C-mount, which provides a 17.52mm flange focal distance (FFD). The CS-mount typically provides a 12.5mm FFD.C-mount lenses can be used in CS-mount configurations with a 5mm adapter.
   CCD:
2. Charge Coupled Device. A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor MOS), surface storage and information transfer technologies.
   Center Wavelength:
   For filters and coatings, the average of the wavelength values at the half-power points of the transmission band.
   Centration:
   The degree to which the optical axis of a lens and the mechanical axis of its mounting coincide.
   Chromatic Aberration:
   An optical defect in a lens resulting in different wavelengths of light focusing at different distances from the lens, which can be seen as halos around the image.
   Chromatic Dispersion:
   The dispersion of white light into its constituent colors. The refractive index of blue light is higher than that of red light,resulting in a change of image size from one color to the other.
   Circle of Least Confusion:
   The smallest cross-section of a focused beam of light; the point of best focus for the image.
   Clear Aperture:
   The opening in the mount of an optical system that controls the amount of light incident on a given surface; the entrance pupil of the lens.
   Coefficient of Thermal Expansion:
   A material property defined as the ratio of the change in length per original length (or change in volume per original volume) to the incremental change in temperature from a reference. Typically an order of magnitude higher for metals relative to glasses.
   Collimate:
   To adjust two or more optical axes with respect to each other; to make rays of light parallel.
   Collimated Beam:
   A beam of light in which all of the rays are parallel to each other.
   Color:
   The aspect of a light source that can be described in terms of hue, brightness and saturation; the specific property of objects seen as red, yellow or blue as opposed to black, white or gray.
   Coma:
   An aberration that occurs in a lens when rays emanating from points not on the optical axis do not converge, causing the image of a point to appear comet-shaped.
   Compound Lens:
   Two or more optical glass elements, sometimes cemented together, shaped to cancel out aberrations present in either lens alone.
   Cone Angle:
   The central angle of a cone of rays converging to or diverging from a point. See Numerical Aperture.
   Conjugate:
   Two optical elements joined as a pair.
   Continuous Wave Irradiation:
   Emission of radiant energy (light) in a continuous, rather than pulsed, wave.
   Contrast:
   The difference in light intensity in an object or image; defined as (Imax - Imin)/(Imax + Imin), where Imax and Imin are the maximum and minimum intensities.
   Converging:
   The bending of light rays toward each other, achieved with a positive (convex) lens.
   Cosmetics:
   Describes the surface defects of a lens that are not optically critical and do not impair its function. Usually described in terms of scratch and dig.
   Critical Angle:
   Maximum angle of incidence formed by a ray of light as it passes from a dense to a less dense medium, e.g., from glass to air, where the critical angle is about 42 degrees. When the critical angle is exceeded, all the light reflects back to the denser of the two media.
   Crown Glass:
   A silicate glass containing oxides of sodium and potassium, used in compound lenses and spectacles; harder than flint glass, with low index and low dispersion.
   Crystal Quartz:
   Crystalline form of silicon dioxide; very hard with a low expansion coefficient. Transmits light through the range of 180nm (ultraviolet) to 4.5 micrometers (infrared).
   Curvature:
   Departure from flatness of a surface. Defined as the reciprocal of the radius of curvature.
   

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