Glossary
A NIST Physical Constants Reference Source: Reference SouceAbbe number: or Abbe factor or Abbe constant, the ratio of refractive index to dispersion; Vd = (nd-1)/(nf-nc) where Vd = Abbe number at wavelength, nf = index of refraction at 486 nm, nc = index of refraction at 656 nm, and nd = index of refraction at wavelength.
achromat: a lens designed for broadband applications consisting of two or more lens elements composed of discrete glass materials which is corrected for Chromatic Aberration with respect to two wavelengths (normally blue and red).
achromatic lens: a lens designed for broadband applications consisting of two or more lens elements composed of discrete glass materials which is corrected for Chromatic Aberration with respect to two wavelengths (normally blue and red).
achromatic waveplate: a broadband waveplate designed using two different substrate materials with compensating retardances and dispersions of retardance
angle of incidence (AOI): the angle that an incident ray makes with the normal to the surface
angstrom: equals 10^-10 meters = 0.1 nanometers, a unit of length but not an SI unit
anisotropic: having a physical property that is different when measured in two different directions.
antireflection coating (AR): single layer, V-coat or broadband (BBAR) dielectric coatings to reduce transmission loss, minimize stray light and prevent back reflections
aplanat: A lens designed to minimize two monochromatic (single wavelength) wavefront errors called Spherical Aberration and Coma. Often called “Fraunhofer” lenses.
aplanatic lens: A lens designed to minimize two monochromatic (single wavelength) wavefront errors called Spherical Aberration and Coma. Often called “Fraunhofer” lenses.
apochromatic lens: a multi-element lens which minimizes both Spherical aberration and Chromatic aberration at three wavelengths
attenuator: an optic or electrical device which reduces the intensity of a beam a fixed or adjustable amount via absorption, scattering or reflection
attosecond (as): equals 10^-18 seconds
bandpass filter: a filter that passes frequencies within a certain wavelength range and blocks frequencies outside that range
BBAR: broadband anti-reflection coating (multi-layer dielectric)
beam deviation: the difference in angle of an incoming beam to a transmitted, reflected or refracted beam through an optic, particularly relevant to cubes and dispersive prisms
beam dump: a device to trap or dump unwanted light radiation via scattering or absorption
beam expander: a multi-element lens system used to expand (or in reverse, reduce) beam diameter
beamsplitter cube: two right angle prisms where the hypotenuse of one of the prisms is coated with a semi-reflective polarizer or beamsplitter coating; the two prisms are cemented, bonded or contacted together to form a cube.
beamsplitter plate: A durable all-dielectric mutilayer coating used to either split or combine a laser beam, usually at 45 degrees
bi-axial crystal: a birefringent crystal having two axes with different indices of refraction
bi-concave: a singlet lens with both surfaces concave; R1 may or may not equal R2
bi-convex: a singlet lens with both surfaces convex; R1 may or may not equal R2
birefringence: the splitting of a light wave into two waves by an optically anisotropic medium such as calcite or quartz. Also called double refraction.
birefringent: the index of refraction differs along the fast & slow axes, affecting the phase shift of the orthogonally polarized linear polarization states differently. Some birefringent materials are crystal quartz, mica, calcite, ice, sodium nitrate, lithium niobate, rutile, magnesium fluoride and sapphire
BK7: borosilicate crown glass which is transmissive in the visible and near-infrared wavelength regions
Brewster’s angle: A) the angle of incidence at which the reflectance of the TM-polarized (whose electric-field vector lies in the plane of the direction of propagation and the normal to the surface) light is zero.
B) the angle at which the fresnel reflectivity of P-polarized light drops to zero, B = tan-1 (n2/n1)
broadband: a wide spectral bandwidth
CADB: Chemically Activated Direct Bonding™ - epoxy free optical bond
calcite: (CaCO3) a soft, birefringent crystal with broadband transmission through the visible and near-infrared; used in polarizers
Chromatic aberration: a wavefront error produced by dispersion causing different wavelengths to have different focal points. By using separate glass types (crown & flint) for the positive and negative lens elements, the dispersion of each is compensated for by the other, thereby minimizing the total effect.
circular polarization: When one component of linear electric field is retarded by l/4 with respect to the other orthogonal component; this is done by aligning linearly polarized light midway between the slow and fast axes of a quarter waveplate.
clear aperture (CA): The opening through which light can pass unobstructed.
The central area over which the optical specifications apply
coherent: when light waves have a constant phase relationship over time, like a laser beam. See temporal coherence and spatial coherence.
cold mirror: a filter used to reflect visible and transmit near-infrared energy, for temperature control
collimated light: when each ray within a beam is parallel to every other, neither converging nor diverging
Coma: an off-axis non-symmetric wavefront distortion which increases linearly with field angle or distance from the principal axis
compound waveplate: a zero order waveplate constructed using two multiple order waveplates aligned with their axes crossed and designed so that they differ in thickness by the exact amount necessary for a true zero order waveplate; they can be optically contacted, cemented or air-spaced
concave: curved inward; the edge of a concave lens is thicker than the center
concentricty: the deviation between the optical and mechanical axes of a lens, often measured by maximum edge thickness variation
contacted: the process of adhering or connecting two clean, flat surfaces without the use of epoxy or cement
converging lens: A plano-convex, bi-convex or meniscus lens that converges an incident bundle of rays to a focus. (positive lens)
convex: curved outward, the center of a convex lens is thicker than the edge
critical angle: C(l) = arcsin(1/n) where n is the index of refraction of the glass at wavelength l. The angle of incidence at the interface between a denser and less dense medium at which light is refracted along the interface; when the critical angle is exceeded, the light is totally internally reflected.
crown glass: borosilicate glass used in the visible and near-infrared wavelength regions (BK7)
crystal quartz: a positive uniaxial birefringent single crystal commonly used for waveplates and optical rotators
cylindrical lens: The curve on a cylindrical lens is cylindrical in shape, resulting in a focal line instead of a point. Standard thin lens formulas apply, but in one dimension only
dichroic: the reflection and transmission of light as a function of wavelength. A dichroic mirror reflects one wavelength and transmits another.
dielectric: non-conducting, or non-metallic, as in a dielectric coating
diffraction limited: an optic or optical system where the quality of the optical image produced is at the theoretical limit of diffraction theory
dig: a defect on the surface of an optic as defined in average diameter in 1/100 of a millimeter
diode laser: also called a light-emitting diode (LED) or semiconductor laser, available in wavelengths from the UV through the NIR
dispersing prism: a prism used to separate white light into its component colors, or to separate two laser wavelengths traveling along the same beampath
dispersion: the variation of refractive index with wavelength
diverging lens: A plano-concave, bi-concave or meniscus lens that causes parallel light rays to diverge or spread out; the edge of a negative lens is always thicker than the center. (negative lens)
double-V or dual-V: a dual wavelength multi-layer dielectric anti-reflection coating with a spectral trace resembling two "V" shapes, usually designed for harmonic wavelengths
doublet lens: a multi-element lens having two elements, either cemented or air-spaced
equi-concave: a lens with both surfaces concave and R1 = R2
equi-convex: a lens with both surfaces convex and R1 = R2
etalon: Fabry perot etalon - flat, plane-parallel window with a dielectric partially-reflecting coating applied to both sides. The transmission bandwidth is determined by the finesse. The separation of the pass-bands is determined by the free spectral range (FSR)
excimer lasers: argon fluoride (ARF, 193nm), krypton fluoride (KRF, 248nm), xenon chloride (XECL, 308nm), xenon fluoride (XEF, 351-353nm)
extinction ratio: in a polarizer, the ratio of the optical power in the desired transmitted polarization to that in the blocked polarization
Thin Film Plate Polarizers
extraordinary axis: The principal axis in birefringent materials with the higher index of refraction; called the slow axis because light travels slower along that direction
f-number: f/# = f/D The ratio of the focal length of the lens to its clear aperture (D); defines the speed of a lens or lens system.
Fabry Perot etalon: A flat, plane-parallel window with a dielectric partially-reflecting coating applied to both sides. The transmission bandwidth is determined by the finesse. The separation of the pass-bands is determined by the free spectral range (FSR)
far infrared (FIR): The wavelength region of the electromagnetic spectrum ranging from approximately 8um to 14um
fast axis: The principal axis in birefringent materials with the lower index of refraction; called the fast axis because light travels faster along that direction (ordinary axis)
femtosecond (fs): equals 10^-15 seconds
filter: an optical element, usually flat, used to transmit some wavelength(s) of light while blocking or attenuating others. Examples are bandpass, neutral desity or notch filter.
finesse: the ratio of the free spectral range of an etalon to its full width at half maximum of its transmission peak; determines the transmission bandwidth of a solid etalon.
finesse=(ƒ®ãR)/(1-R) where R is the reflectivity of the surface of the etalon
first order waveplate: Often called true zero order waveplates, they can be made as single plates around 100 microns thick or adhered to a thicker host substrate via optical contacting or optical cement. Manufactured for applications requiring increased bandwidth, field of view or thermal stability
flint glass: used in the visible and near-infrared spectral regions, flint glass is softer than crown glass, more dispersive and usually has a higher refractive index and low Abbe number
fluorescence: The emission of light radiation at a longer wavelengths by a material as a result of the absorption of light at shorter wavelengths
Fraunhofer lens: A lens designed to minimize two monochromatic (single wavelength) wavefront errors called Spherical Aberration and Coma. (aplanat)
free spectral range (FSR): the spacing (usually given in frequency) between transmission peaks of an etalon FSR=c/(2nl) where c is the speed of light, n is the index of refraction of the etalon, and l is the thickness of the etalon.
fresnel reflection: the reflectance in air (n ~ 1.0) off an uncoated glass surface at normal incidence: R = ((n-1)/(n+1))2 where n is the refractive index of the substrate material
Fresnel Rhomb: An alternative to the plate retarder these prisms utilize total internal reflection (TIR) to produce a phase delay between the S and P polarization components which is both spectrally and thermally stable.
fused silica: an amorphous form of silicon dioxide with very high laser damage thresholds as well as high transmittance throughout the UV, visible and near-infrared wavelength regions
FWHM: full width half max of an etalon pass band; equals the free spectral range (FSR) divided by the finesse (F)
galilean telescope: a refracting telescope or beam expander designed using a positive and negative lens, the expansion ratio equals the ratio of their respective focal lengths
gas lasers: argon-ion (488-515nm), copper vapor (511/578nm), helium cadmium (442nm), helium neon (633nm), iodine (1315nm), nitrogen (337nm)
Gimbal mount: a kinematically-correct mount for precision rotation or adjustment of an optic without translation of the center point
half waveplate: the difference in phase introduced between the two polarization components equals a half of a wave; also called a l/2 retardation plate. Half waveplates rotate the polarization of linearly polarized light to twice the angle between the incident plane of polarization and the fast optical axis.
harmonic separator: a dichroic mirror specifically designed for the harmonics of Nd:YAG or other laser sources
homogeneity: the level of sameness in material quality and optical property regardless of the direction or location within the bulk material
hot mirror: a filter used to transmit visible and reflect near-infrared energy, for temperature control
hypotenuse: the surface of a right angle prism (RAP) opposite the right angle, the longest of the 3 polished sides
index of refraction: the ratio of the speed of light in a vacuum (c) to the speed of light in a given material at a specified wavelength. The index in a vacuum = 1.0
infrasil: low OH content fused quartz produced by electrically fusing quartz crystal; sometimes called IR grade fused silica
interferometer: an instrument that uses the interference of lightwaves to measure surface flatness, parallelism or radius. Three common types are Fizeau, Michelson and Twyman-Green.
jitter: random variation in the timing of a signal
joule (J): a unit of energy equaling 1 watt/second or 10^7 ergs
laser: light amplification by stimulated emission of radiation; the laser cavity is a source of monochromatic, coherent, reasonably collimated light from a medium (crystal, glass, liquid, dye or gas), within resonator mirrors and excitation source (light or electrical discharge)
laser line filter: high transmission of a laser line with blocking of all other wavelengths, also called "clean up filters"
Laurent formula: the dispersion equation used for crystal quartz
leaky polarizer: a beamsplitter cube with equal optical pathlengths coated for polarization-sensitive beam sampling
lens: a transmissive optic with one or more elements having at least one curved surface each which is used to converge, diverge or collimate a beam of light
LiDAR: an acronym for Light Detection And Ranging which is a method for detecting distant objects and determining their position, velocity, or other characteristics by analysis of pulsed laser light that is reflected from their surfaces.
linear polarization: when the electric field is always parallel to the x or y-direction, or some angle in between.
linear polarizer: a device that creates linear polarization from an arbitrary input by removing the component orthogonal to the selected state via reflection, refraction or absorption
long wave pass: a filter or dichroic mirror which transmits longer wavelengths and reflects or absorbs a shorter wavelength or wavelength region
meniscus lens: a crescent-shaped singlet lens designed to minimize spherical aberration; one surface is convex and the other concave
meter: length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second
mica: a birefringent crystal used in thin sheets, sometimes cemented between glass substrates and used as waveplates or low-cost optical retarders
micron (um): also called a micro-meter, a micron equals 10^-6 meters = 1000 nanometers
microsecond (us): equals 10^-6 seconds
mid-infrared (MIR): The wavelength region of the electromagnetic spectrum ranging from approximately 3um to 8um
mirror: also called a high-reflector, a flat or curved optical element coated with a dielectric stack or metallic material to reflect light
monochromatic: light consisting of a single wavelength or color, or very narrow band of wavelengths (< 0.1nm)
multiple order waveplate: a single element waveplate with order m > 0, generally considered inexpensive, high damage threshhold retarders for single wavelength applications
nanometer (nm): also called a millimicron, a nanometer = 10^-9 meters = 10 angstroms
nanosecond (ns): equals 10^-9 seconds
near-infrared (NIR): The wavelength region of the electromagnetic spectrum ranging from approximately 700nm to 3000nm
negative lens: a plano-concave, bi-concave or meniscus lens that causes parallel light rays to diverge or spread out; the edge of a negative lens is always thicker than the center.
neutral density filter: an absorptive or metallic-coated glass filter providing equal attenuation across and spectral bandwidth, can be constant, lineraly variable or step variable in design
non-polarizer: plate or cube beamsplitter coated such that (typically) Rs-Rp < 3-5%
normal axis: the axis that is perpendicular to or forms right angles with the surface or plane of incidence
notch filter: maximum blocking of a laser line, for Raman and fluorescence applications
numerical aperture (NA): the sine of the angle made between the marginal ray of a lens and the optic axis with respect to an object or image point, multiplied by the refractive index of the incident media, NA = 1/(2 x f/#) where f/# = f/D
optical contact: the process of adhering, bonding or connecting two clean, flat surfaces without the use of epoxy or cement
optical density: a measure of transmittance through a substrate or coating, OD = log (1/T) where T is transmittance
ordinary axis: the principal axis in birefringent materials with the lower index of refraction; called the fast axis because light travels faster along that direction
output coupler: also called a partial reflector, an all-dielectric multi-layer coating used to split a laser beam, usually at normal incidence (0 degrees)
P-polarization: parallel to the plane of incidence, often called transverse-magnetic (TM) or horizontally polarized because the electric field direction of the wave is in the horizontal plane along the direction of propagation
partial reflector: also called an output coupler, an all-dielectric multi-layer coating used to split a laser beam, usually at normal incidence (0 degrees)
pellicle: a very thin, delicate membrane streched over a support frame or cemented to a ring mount, typically used as a 45deg beamsplitter (coated or uncoated) with no change in optical path length, no 2nd surface reflections and no chromatic aberration
photon: a quantum unit of electromagnetic energy of a single wavelength, direction and polarization which acts as both a wave and a particle of light
Planck’s constant: h=6.626 x 10-27 erg•s
plane polarization: when the electric field is always parallel to the x or y-direction, or some angle in between.
plano-concave: A negative lens with one flat (plano) surface and one concave surface
plano-convex: A positive lens with one flat (plano) surface and one convex surface
polarization: the direction in which the electric field of a light wave oscillates
polarizer: a component which outputs polarized light from input unpolarized light via selective absorption, refraction or transmission of a particular polarized state
polarizing beamsplitter cube: cubes used to split a laser beam into two orthogonally polarized components; P-polarization is transmitted while S-polarization is reflected at 90º. Polarizer cubes utilize a dielectric coating at the internal cemented interface, external surfaces are anti-reflection coated
polymer waveplate: an alternative to crystal quartz or achromatic plate retarders for low energy visible and near-infrared applications, Polymer is fairly impervious to changes in wavelength or angle of incidence
porro prism: a type of reflection prism used to rotate an image by 180 degrees and in the opposite direction from its entrance point
positive lens: A plano-convex, bi-convex or meniscus lens that converges an incident bundle of rays to a focus
prism: an optic with at least 2 polished non-parallel surfaces through which light is reflected or refracted
quarter waveplate: When the difference in phase introduced between the two polarization components equals a quarter of a wave. Also called a l/4 retardation plate
radius of curvature (ROC): the radius of the sphere coincident with the optical surface
reflected wavefront distortion (RWD): the amount of deviation from an ideal surface, in terms of waves or fringes peak-to-valley (p-v) or RMS; twice the surface figure
refractive index: the ratio of the speed of light in a vacuum (c) to the speed of light in a given material. The index in a vacuum = 1.0
retroreflector: a prism which reflects the light back parallel to the incoming beam but in the opposite direction
right angle prism: a 45-45-90 prism which can be used as a retroreflector or a bending prism
S-polarization: perpendicular to the plane of incidence (from senkrecht, the German word for perpendicular), also called transverse-electric (TE) or vertically polarized because the electric field direction of the wave is perpendicular to the horizontal plane along the direction of propagation.
sagitta: the depth of a curve as measured from the chord, sag = R - sqrrt (R^2 - r^2) where R is radius of curvature and r is the half-diameter of a lens or curved substrate
scratch/dig: a measure of visible surface defects as defined by MilSpec: MIL-0-13830, usually denoted as two numbers where the first is the visbility of scratches and the second of digs or small pits
Sellmeier equation: the dispersion equation used for most optical glass types (not crystal quartz)
short wave pass: a filter or dichroic mirror which transmits shorter wavelengths and reflects or absorbs a higher wavelength or wavelength region
slow axis: The principal axis in birefringent materials with the higher index of refraction; called the slow axis because light travels slower along that direction
Snell’s law: determines the angle of refracted light an an interface of non-equal indices for any theta > 0deg such that the ratio of the sines of the angles are constant (n2/n1); n1sin(theta1) = n2sin(theta2)
solid state lasers: ruby (694nm), alexandrite (720-780nm), Erbium:glass (1540nm)
spherical aberration: an axially symmetric wavefront error that occurs when rays from a point on the axis passing through the outer zones of the lens focus at a different distance from the lens than rays passing through the central zone
suprasil: a type of fused silica with high chemical purity, high UV transmittance and minimal fluorescence
surface figure: the deviation from an ideal optical surface, in terms of waves or fringes peak-to-valley (p-v) or RMS
surface flatness: the amount that an optical surface deviates from a perfect plane
surface quality: the level of defects that can be visually observed on the surface of an optic and usually given by MIL-0-13830A where the first number is the visibility of the scratches and the second of digs or small pits. (scratch-dig)
surface roughness: the amount the surface of an optic differs from perfectly smooth, and is usually denoted as a root mean square (rms) value in Angstroms
Telcordia: formerly Bellcore, Telcordia qualification is a highly rigorous standard that ensures telecommunication components and instruments perform as required when field deployed. Qualification includes storage and operating temperature, thermal cycling, vibration and shock
tomography: any of several techniques for making detailed images of a predetermined plane section of a solid object while blurring out the images of other planes
total internal reflection (TIR): The reflection that occurs within a substance when the angle of incidence of light at the boundary surface is greater than the critical angle
transmitted wavefront (TWD): the amount of deviation from an ideal or input beam through an optic, in terms of waves or fringes peak-to-valley (p-v) or RMS
true zero order waveplate: Often called first order waveplates, they can be made as single plates around 100 microns thick or adhered to a thicker host substrate via optical contacting or optical cement; manufactured for applications requiring increased bandwidth, field of view or thermal stability
ultraviolet (UV): The wavelength region of the electromagnetic spectrum ranging from approximately 150nm to 400nm
unpolarized: A ray of light in which the direction of polarization varies randomly
V-coat: a narrowband multi-layer dielectric anti-reflection coating with a spectral trace resembling a "V" shape
visible (VIS): The wavelength region of the electromagnetic spectrum ranging from approximately 400nm to 700nm
waveplate: a birefringent optical device that alters the polarization state of a light wave travelling through it, depending on the thickness of the material and wavelength of light; also called a retardation plate or optical retarder
wedge: the parallelism or angle between the two surfaces of an optical component
zero order waveplate: see compound waveplate or true zero order waveplate