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Glossary

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A
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.
ACHROMATIC LENSES
Ideal for applications requiring color correction
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).
ANTI REFLECTION 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.
ASPHERIC LENSES
Designed to correct spherical aberration
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.
B
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.
BNC
Term commonly associated with a type of connector prevalent in industry for joining cables to receivers, transmitters or other cables.
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.
C
C/CS-MOUNTS
wo 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.
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.
CCD
Charge Coupled Device. A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor (MOS), surface storage and information transfer technologies.
CCTV LENS
Closed Circuit Television Lens. Term commonly applied to a family of compound lenses which offer exceptionally high resolution, short focal length and color imaging at fast lens apertures, such as required in the television industry.
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 mage 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.
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.
CYLINDRICAL LENS
A lens with at least one surface shaped like a portion of a cylinder. Positive and negative cylindrical lenses (depending on the shape of the curved surface) are used in anamorphic systems to accentuate features in one axis, such as expanding points of light into lines. Applications include astigmatism correction in the human eye and astigmatism production in rangefinders or fire-control equipment.
D
DAMAGE THRESHOLD
The maximum energy density to which an optical surface may be subjected without failure.
DECENTRATION
The failure of one or more lens surfaces to align their centers of curvature with the geometric axis of a lens system.
DENSITY
The logarithm of the degree of opacity of a translucent medium; expressed as D = log (1/Trans).
DEPTH OF FIELD
The distance along the optical axis through which an object can be located and clearly defined when the lens is in focus.
DEPTH OF FOCUS
The distance along the optical axis through which an image can be clearly focused.
DEVIATION
The angle between the paths of a ray of light before and after passing through one or more lenses.
DIELECTRIC COATINGS
High-reflectance or low-reflectance coatings made up of alternating layers of material with higher and lower indices of refraction than the substrate.
DIFFRACTION
The sidewise spread of light as it passes the edge of an object or emerges from a small aperture; causes halos or blurring of the image.
DIFFRACTION LIMITED
Describes an optical system in which the quality of the image is determined only by the effects of diffraction and not by lens aberrations.
DIN
Abbreviation for Deutsche Industrie Norm and describes a large family of product standards. For film emulsions, the DIN System is the logarithmic method of determining emulsion speeds developed by the German standards organization Deutscher Normenausschuss. The emulsion speed is doubled for each increase of three in the DIN speed value.
DISPERSION
The separation of light into its component colors, as a prism disperses white light into a color band, or a rainbow effect.
DISTORTION
Variations in magnification from the center to the edge of an image, making straight lines seem to curve. Barrel, or negative, distortion causes a square grid to appear barrel-shaped; pincushion, or positive, distortion increases in proportion to the distance from the center of the image.
DIVERGING
The bending of light rays away from each other, achieved with a negative (concave) lens.
DOUBLE-CONCAVE LENS
Causes the light traveling through the lens to diverge
DOUBLE-CONVEX LENS
Causes light to focus to a point
E
EDGING
Grinding, or finishing, the edge of an optical element or lens.
EFFECTIVE FOCAL LENGTH
See Equivalent Focal Length.
ELECTROFORM
To form shaped objects by electrodeposition on a mold.
ELLIPTICAL
Aspherical, as in a lens whose surface is a section of an ellipse rather than of a circle.
ENTRANCE PUPIL
The image of the aperture stop as viewed through the object side of the lens.
EQUIVALENT FOCAL LENGTH (EFL)
The focal length of an infinitely thin lens having the same paraxial imaging properties as a thick lens or multiple-element system.
ERECT IMAGE
An image whose spatial orientation is the same as that of the object; both image and object appear “right side up.”
EXTINCTION RATIO
The ratio of the intensity of a plane-polarized beam that is transmitted through a polarizer whose polarizing axis is parallel to the beam’s plane, versus the intensity when the polarizer’s axis is perpendicular to the beam’s plane.
F
F-NUMBER
A measure of the ability of a lens to gather light. Also called its “speed”. The ratio of the focal length of the lens to its effective aperture.
FIBER OPTICS
The transmission of radiant energy through transparent fibers of glass, plastic or fused silica.
FIELD CURVATURE
An aberration in which the edges of a field seem to be out of focus when the center is focused clearly.
FIELD OF VIEW
The maximum visible space seen through an optical instrument or lens.
FILAR EYEPIECE
A measuring eyepiece containing a screw-micrometer-driven crosshair commonly used to measure image size.
FIRE POLISHED
The process whereby glass is raised to its melting point and formed by molding with a highly polished metal surface.
FLINT GLASS
An optical glass with higher dispersion and higher refractive index than crown glass; a heavy, brilliant glass, softer than crown glass.
FOCAL LENGTH
See Equivalent Focal Length.
FRINGES
Interference bands, such as Newton’s Rings, which are formed when light is reflected from two adjacent polished surfaces, placed together with an air space between them. Used to determine the fit of a lens surface to a test glass.
FRONT FOCAL LENGTH
The distance from the front focal point of an optical system to the first surface.
FUSED QUARTZ
Crystal quartz melted at high temperature to make an amorphous, non-birefringent glass of low refractive index.
FWHM
Full Width, Half Maximum. The bandwidth of an optical instrument as measured at the half-power points.
G
GALVANOMETER
An instrument for detecting and measuring a small electric current by movements of a magnetic needle or of a coil in a magnetic field.
GAUSSIAN OPTICS
Optical characteristics limited to infinitesimally small pencils of light; also called paraxial or first-order optics.
GEOMETRIC OPTICS
That branch of optics dealing with the tracing of ray paths through optical systems.
H
HALF ANGLE
Term commonly associated with scan and all other lenses. Typically, half the angular subtense of the object. See Input Scan Angle.
HARTING DOUBLET
An achromatic doublet made of a bi-convex crown element cemented to a meniscus flint element, with the crown facing the long conjugate.
HIGH-EFFICIENCY COATING
Specialized coating applied to optics to improve transmission or reflection
HOMOGENEITY
The state in which all volume components of a substance are identical in optical properties and composition.
HUYGENIAN EYEPIECE
An ocular consisting of two plano-convex lenses which are formed from similar glass and separated by a space equal to the sum of their focal lengths. This eyepiece is free of lateral chromatic aberration, but because the image plane falls between the two elements it is not suitable for applications involving crosshairs
HYBRID
Anything formed out of heterogenous elements.
I
IMAGE CIRCLE
The circular image field over which image quality is acceptable; can be defined in terms of its angular subtense. Alternately known as circle of coverage .
IMAGE CIRCLE
The circular image field over which image quality is acceptable; can be defined in terms of its angular subtense.
IMAGE INVERSION
Change in the orientation of an image in one meridian.
IMAGE PLANE
The plane perpendicular to the optical axis at the image point.
IMAGE TRANSPOSITION
The flipping of an image’s orientation, such as inversion of an image’s orientation in one meridian or the reversion of an image’s orientation in two meridians.
INDEX OF REFRACTION
The ratio of the speed of light in air to its velocity in another medium; determines how much light bends as it passes through a lens, e.g., high-index flint glass bends light more than low-index crown glass does.
INFRARED
The portion of the spectrum whose wavelengths are invisible to the human eye (range = .76 microns and higher).
INPUT SCAN ANGLE ()
Input scan angle, also known as “half angle,” is half the total angular field of the scan lens. Twice this angle in radians, when multiplied by the calibrated focal length of the scan lens, results in the scan length.
INTERFERENCE FILTER
A filter which controls the spectral composition of transmitted energy by interference. Such filters, typically constructed of thin alternating layers of metals and dielectrics, are also known as narrowband or broadband bandpass filters.
INTERFEROMETER
An instrument that uses the interference of light waves to measure the accuracy of optical surfaces.
INTEROCULAR DISTANCE
The distance between the pupils of the eyes when viewing objects at a distance; normal distance is 62mm.
IRIS DIAPHRAGM
A mechanical device capable of varying the effective diameter of a lens.
J
K
KNOOP HARDNESS
A measure of hardness determined by the depth of penetration of a diamond stylus under a specified load. Similar to a Rockwell hardness test.
L
LASER DIODE
A laser which uses a forward biased semiconductor junction as the active medium. Also known as injection laser diode.
LATERAL COLOR
A chromatic aberration resulting in image size variation as a function of wavelength. Also known as chromatic difference of magnification.
LIGHT RAY
Not really a “ray” but the path of a point of light on a wavefront, indicating the direction the light is traveling.
LONG PASS
Interference filter type which efficiently passes radiation whose wavelengths are longer than a specific wavelength, but not shorter.
LONGITUDINAL COLOR
The longitudinal variation of focus (or image position) with wavelength; often referred to as axial chromatic aberration.
M
MAGNESIUM FLUORIDE
Material used as antireflection coating for lenses because of its low refractive index.
MAGNIFICATION
The enlargement of an object by an optical instrument; ratio between the size of the image and the actual size of the object.
MENISCUS
Describes a lens having one convex and one concave surface.
MICRO-OPTICS
A term referring to small (less than 2mm in size) lenses, beamsplitters, prisms, cylinders or other optical components commonly found in endoscopes or microsocopes. Micro-Optics are also used to focus light in semiconductor laser and fiber optic applications.
MICROSCOPE EYEPIECE
An eyepiece located at the near end of the microscope tube. Often a simple Huygens eyepiece, though other varieties (negative eyepieces, flat field projection eyepieces) are common, depending on application.
MICROSCOPE OBJECTIVE
The lens located at the object end of a microscope tube. Many types of objectives are used in microscopy; simple achromats and color-corrected apochromats are popular choices.
MODULATION TRANSFER FUNCTION (MTF)
Describes the modulation of an image as the frequency increases; ratio of modulation between image and object. Also called sine wave response.
MULTI-ELEMENT SYSTEM
An assembly of single and/or compound lenses optimized to provide certain optical characteristics.
MULTI-LAYER COATING
Coating composed of several layers of material with alternating high-low refractive indices; various combinations produce a variety of coating properties.
N
NANOSECOND
A billionth part of a second.
NARROWBAND COATING
A coating designed to provide transmittance (or reflectance) over a very restricted band of wavelengths.
NEUTRAL DENSITY
As applied to thin films, a coating which appears gray to the eye and has a flat absorption curve throughout the visible spectrum. Neutral density filters decrease the intensity of light without changing the relative spectral distribution of energy.
NEWTON’S RINGS
Used to measure the fit of a lens surface against the surface of a test glass. The rings result when two adjacent polished surfaces are placed together with an air space between them and the light beams they reflect interfere.
NUMERICAL APERTURE
Describes the angle in a cone of light emitted by the condenser and accepted by the objective of a microscope; the index of refraction of the medium in which the image lies multiplied by the sine of the half angle of the cone of light.
O
OBJECT-TO-IMAGE DISTANCE
Also known as the total conjugate distance or track length. Can be finite or infinite depending on application.
OBJECTIVE
The optical element which receives light from the object and forms the first or primary image in telescopes, microscopes and other optical systems.
OBLIQUE RAY
A ray of light that is neither perpendicular nor parallel, but inclined.
OPTICAL AXIS
A line passing through the centers of curvature of a lens or series of lenses in an optical system.
OPTICAL FLAT
A piece of glass with one or both surfaces polished flat. Also known as a test plate, test glass or reference flat.
OPTICAL INTERFERENCE
The interaction of two light waves on the total intensity of light.
ORTHOGONAL
Mutually perpendicular.
P
PARAXIAL IMAGE PLANE
Image plane located by using first-order geometric optics. See Gaussian Optics.
PEAK WAVELENGTH
For bandpass filters, the discrete wavelength which has the maximum transmission value in the passband region.
PERCENT POSITION ERROR
Scan linearity can be defined as a percentage of the expected (design) scan height using the percent position error technique. The difference between the expected (design) and observed scan height at any scan angle divided by the expected design height yields this measure of linearity once converted to a percentage.
PERCENT VELOCITY ERROR
The percent velocity error technique involves calculating the percentage difference between the velocity of an image spot at a specific field angle and the velocity at the optical axis of the lens. See percent position error.
PINHOLE
Generally a small, sharp-edge hole without a lens which can function as an aperture or eye lens.
PLANE OF INCIDENCE
The plane that is defined by the incident and reflected rays.
PLANETARY POLISHER
A polishing machine used in the production of plano parallel elements where both surfaces are polished simultaneously.
PLANO ELEMENTS
Lenses or mirrors with perfectly flat surfaces.
PLANO-CONCAVE
A lens with one flat (plano) surface and the other curved inward.
PLANO-CONCAVE LENS
Causes the light traveling through the lens to diverge
PLANO-CONVEX
A lens with one flat (plano) surface and the other curved outward.
PLANO-CONVEX LENS
Causes light to focus to a point
POISSON’S RATIO
The ratio of the transverse contraction of a bar of material to the elongation per unit length.
POLARIZATION OF LIGHT
The process of affecting light so that its waves vibrate in one plane only; reflection, double refraction, selective absorption and scattering are all ways to polarize light.
POLARIZED LIGHT
Light that vibrates in only one plane.
POLYCHROMATIC ABERRATIONS
The separation of an image into planes of distinct color, caused by the variation of the index of refraction of glass, and the focal length of a lens, with the wavelength of light; in a given plane, all colors but one are unfocused.
POLYHEDRAL
In the shape of a solid formed by plane faces, as in a prism.
POTENTIOMENTER
An instrument used to measure electromotive forces.
POWER LOSS
In applications involving diffraction limited lenses, power loss in the spot is an exponential function of the square of the truncation ratio (Dt/Db): PL= e-2*(Dt/Db)²?.
PRIMARY REFLECTIONS
The principal, intended reflections at optical surfaces, as differentiated from secondary, usually unintended or unwanted reflections occurring in an optical system.
PRINCIPAL PLANES
Imaginary planes at right angles to the optical axis of a lens; the intersection of a plane and the axis is a principal point.
PRISM
A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted.
PULSE MODULATION
The process of periodically or intermittently varying the amplitude of a pulse of light.
Q
Q-SWITCHED
In an optical resonator, the higher the reflectivity of its surfaces, the higher the Q. A Q-switch rapidly changes the Q in the optical resonator of a laser to prevent lasing until a high level of optical gain and energy storage has been reached in the lasing medium; a giant pulse is generated when the Q is rapidly increased.
QUARTER WAVE OPTICAL THICKNESS
Common thin-film term. The QWOT (Quarter Wave Optical Thickness) is the wavelength such that the optical thickness (index ‘n’* physical thickness ‘d’) of a coating evaporant layer is ¼? wavelength. (n*d=/4)
R
RADIANT FLUX
The measurement of the time rate of flow of radiant energy, expressed in watts.
REAL IMAGE
Light rays reproduce an object, called an image, by gathering a beam of light diverging from an object point and transforming it into a beam converging toward or diverging from another point. If the beam is converging, it produces a real image.
REFERENCE FLAT
An optical flat used as a test glass.
REFLECTION
The return of light from a surface with no change in wavelength.
REFRACTION
The change in direction of a ray of light as it passes through two media through which light travels at different speeds.
REFRACTIVE INDEX
The ratio between the speed of light through air to the speed of light through another medium; the ratio determines how much a ray of light will bend as it passes through a given medium.
RESOLUTION
The ability of a lens to image the points, lines and surfaces of an object so they are perceived as discrete entities.
RETICLE
An optical element containing a pattern placed at the image plane of a system. The reticle facilitates system alignment or the measurement of target characteristics.
REVERTED IMAGE
An image in which left and right seem to be reversed.
RING-TYPE AUTO IRIS
A type of auto iris which utilizes a rotating magnet to move the iris vanes.
ROCKWELL HARDNESS
Resistance of a substance to penetration by a pyramidal stylus pressed in under a specific load; See also Knoop hardness.
RS 232 INTERFACE
A term for electronic data exchange cables common in industry.
S
SAG FORMULA
“Sag” is an abbreviation for “sagitta,” the Latin word for “arrow,” and refers to the height of a curve from the chord to the highest point.
SAGITTAL FOCUS
The focus of rays lying in the sagittal plane, which is the plane perpendicular to the meridional plane (See Tangential Meridian).
SCAN LENGTH (SL)
The image height of the scan lens. Mathematically, scan length can be defined as the product of the calibrated focal length (f) and twice the field angle (2* in radians) in object space: SL = 2*(Field Coverage) = f*2().
SCAN LENS
A multi-component objective which is the heart of a graphic arts image recording, printing or engraving system. The balancing of wide angular field, flat image plane and linear relationship between input scan angle and image height make scan lenses ideal for writing characters on film, laser engraving figures or recording characters and figures from texts. Scan lenses are sometimes also called F- lenses because their image height is proportional to the scan angle () and not the tangent of the angle.
SCAN LINEARITY
The degree to which the performance of a particular scan lens design follows the equation Scan Length = 2*field coverage = f*2(). The scan linearity of a system can be defined using both the percent position error and percent velocity error techniques.
SEIDEL ABERRATIONS
The group of primary aberrations in lenses, including coma, astigmatism, curvature of field, distortion, spherical and chromatic.
SHORT PASS
Interference filter type which efficiently passes radiation whose wavelengths are shorter than a specific wavelength, but not longer.
SHUTTER
A device for controlling the amount of time a light-sensitive medium is exposed to light.
SINGLE-LAYER COATING
A thin coating that reduces or eliminates reflections at an air-glass surface, such as MgF2.
SLIT
An aperture, typically rectangular in shape, whose length is large compared to its width. Apertures, generally small compared to the light source, may have fixed or adjustable shapes through which radiation enters or exits an instrument.
SNELL’S LAW OF REFRACTION
Describes the way a ray of light changes direction at a surface between two media that have different indices of refraction. The angle of incidence of the ray is measured from the normal, or perpendicular, to the surface. A ray moving from a low-index medium to a high-index medium bends toward the normal; from high-index to low, it bends away from the normal.
SOLID STATE SYNCHRONIZATION SYSTEM
As applicable to programmable optical shutters, a system of infrared emitting diodes, infrared sensitive detecting transistor and interrupting vane and associated circuitry. The system provides feedback signals associated with the open/closed status of the shutter to users.
SPATIAL FILTERING
Enhancing an image by increasing or decreasing its spatial frequencies.
SPECIFIC GRAVITY
The dimensionless ratio of the mass of an object to the mass of an equal volume of water at 4°C or other specified temperature.
SPECTROMETRY
Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.
SPHERICAL ABERRATION
An optical defect caused when rays of light passing through the curved surface of a lens near its edge converge at a point closer to the lens than those passing through its center (negative aberration); when the outer zone has a longer focal length than the center, the aberration is said to be positive.
SPOT SIZE
See Blur Circle.
STRIAE
An imperfection in optical glass characterized by streaks of transparent material of a different refractive index than the body.
SUBSTRATE
The underlying material to which an optical coating is applied.
SURFACE CONTOUR
The outline or profile of a surface.
T
TANGENTIAL MERIDIAN
In an optical system of revolution, the tangential, or meridional, plane is defined as the plane containing the optical axis and the specified object point.
TELECENTRIC LENS
A multi-component system whose aperture stop is located at the front focus so that the chief rays are parallel to the optical axis in image space. For a telecentric lens, the exit pupil is at infinity.
TELEPHOTO LENS
A multi-component lens arranged so that the overall length of the compound system is less than or equal to the effective focal length.
TEST GLASS
See Optical Flat.
TOTAL INTERNAL REFLECTION
When the angle of incidence of light striking the boundary surface of a substance exceeds the critical angle, the result is total internal reflection.
TRANSMISSION
To admit the passage of light through a medium; light not reflected back to its source is transmitted through the medium.
TRANSVERSE RAY ERROR
Errors, or departures from ideal, measured in a direction perpendicular to the optical axis.
TRUNCATION RATIO
The dimensionless ratio of the Gaussian beam diameter at the 1/e²? intensity point to the limiting aperture of the lens.
TTL
Type of connector commonly used for joining cables to receivers, transmitters or other cables.
U
ULTRAVIOLET
The range of the electromagnetic spectrum from 10 to 400 nanometers.
V
V-COATING
A narrowband coating for specific laser wavelengths.
VIGNETTING
The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures ofelements within an optical system.
VIRTUAL IMAGE
Light rays reproduce an object, called an image, by gathering a beam of light diverging from a point source and transforming it into a beam converging toward or diverging from another point; if the beam is diverging, it produces a virtual image.
VISIBLE SPECTRUM
That part of the electromagnetic spectrum that the human eye can perceive, between the ultraviolet and the infrared (range = .38 microns to .76 microns).
W
WAVE PHENOMENA
The characteristics of radiant energy.
WAVEFRONT DEFORMATION
Departure of a wavefront from ideal (usually spherical or planar) caused by surface errors or design limitations.
WAVELENGTH
The distance light travels in one wave cycle. Electromagnetic energy travels in waves.
WEDGE
An optical element with its faces inclined toward each other at very small angles, diverting light toward the thicker parts of the element.
X
Y
YOUNG’S MODULUS
Modulus of elasticity; the amount of stress required to produce a unit change in length (strain). Expressed in pounds per square inch (PSI) or dynes per square cm.
Z