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Absentee layer: A layer of dielectric material, transparent in the transmission region of the filter, due to phase thickness of 180°.

Absolute Blocking: The ratio of the largest peak outside the passband to the peak within the passband; expressed as an area or %T.

Absolute refractive index: The refractive index of a medium in relation to that of a vacuum.

Absorption curve, absorption spectrum: The fraction of incident radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material.

Acceptance Angle: The maximum angle within which light will be accepted by an element, such as a detector or waveguide. In the latter, it is quantified as half the vertex angle of the cone within which optical power may be coupled into bound modes of a fiber (also called acceptance cone).

AF: ALPHA Filter designs are manufactured using Omega’s proprietary technology resulting in extremely steep edges, precise edge placement, and theoretical attenuation >OD 10.

Analytical quality (AQ) optics: Standard quality optics typically polished to a surface finish or quality ranging from 60/40 to 80/50 scratch/dig and a surface figure of Lambda/4 to 3-5 waves depending on the optic type. This quality of optics is mainly for low power laser or light source applications.

Angle of Incidence: The angle formed by an incident ray of light and an imaginary line perpendicular to the plane of the component’s surface. When the ray is said to be “normal” to the surface, the angle is 0°.

Angle of Reflection: The angle formed between the normal to a surface and the reflected ray. This angle lies in a common plane with the angle of incidence and is equal to it.

Angle of Refraction: The angle formed between a refracted ray and the normal to the surface. This angle lies in a common plane with the angle of incidence.

Angstrom (Å): Unit of length used to measure wavelengths of light 1/10th of a nanometer (nm). One Angstrom is equal to 1 x 10⁻¹⁰ meters.

Analytical quality (AQ) optics: Standard quality optics typically polished to a surface finish or quality ranging from 60/40 to 80/50 scratch & dig, and a surface figure of Lambda (λ) 4 to 3-5 waves depending on the optic type.

ANSI: American National Standard Institute.

Anti-reflective coating (AR): An optical thin-film interference coating designed to minimize reflection that occurs when light travels from one medium into another, typically air and glass.


Backscatter: The portion of scattered light that returns in a generally opposite direction to that of propagation.

Bandpass: The range (or band) of wavelengths passed by a wavelength-selective optic.

Bandpass filter: Transmits a band of color, the center of which is the center wavelength (CWL). The width of the band is indicated by the full width at half maximum transmission (FWHM), also known as the half band width (HBW). It attenuates the light of wavelengths both longer and shorter than the passband.

Bandwidth (HBW, FWHM): Width of the passband: specifically, the difference between the two wavelengths at which the transmittance is half the peak value.

Beam diameter: 1. Calculated distance between two exactly opposed points on a beam at a chosen fraction of peak power (typically 1/e2). 2. The diameter of a circular aperture that will pass a specified percentage (usually 90) of the total beam energy.

Beam Divergence: Increases in the diameter of an initially collimated beam, as measured in milliradians (mrad) at specified points; i.e., where irradiance is a given fraction (often 1/e2) of peak irradiance.

Beam Splitter: An optical device for dividing a beam into two or more separate beams. A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces. The cemented face of one prism is coated, before cementing, with a metallic or dielectric layer having the desired reflecting properties, both in the percentage of reflection and the desired color. In a color television camera, for example, a three-way beam splitting prism is employed in which multilayer films are deposited on the interfaces to divert red and green light to two vidicons, leaving the blue image to pass through to the third vidicon tube.

BK7 Glass: Borosilicate crown optical glass with low bubble content and high homogeneity (same optical properties throughout material). BK7 is an excellent visible to NIR material for many optical components.

Blocker: The component of a fully assembled filter that provides attenuation of a specific region of the spectrum.

Blocking: Attenuation of light, usually accomplished by reflection or absorption, outside the passband. Blocking requirements are specified by wavelength range and amount of attenuation.

Brewster’s angle: For light incident on a plane boundary between two regions having different refractive indices, the angle of incidence at which the reflectance is zero for light that has its electrical field vector in the plane defined by the direction of propagation and the normal to the surface. For propagation from medium 1 to medium 2, Brewster’s angle is given as arctan (n2/n1).

Broadband Achromatic Twyman-Green (BAT) interferometer: An equal-arm interferometer that uses a white (broadband) light source.

Broadband AR coating: A coating designed to reduce reflectance over a very wide (broad) band of wavelengths.


Calcium Fluoride (CaF2): Calcium Fluoride is a cubic single crystal optical material that exhibits good transmission from VUV to IR wavelengths (~140nm to 8.0µm). CaF2 is a relatively soft material sensitive to mechanical and thermal shock.

Cavity:   Sometimes called “period”. The basic component of a thin-film filter consists of two quarter-wave stack reflectors separated by a solid dielectric spacer. As the reflectivity of each of the quarter wave stack reflectors increases, the FWHM decreases; as the number of cavities increases, the depth of the blocking outside the passband increases and the shape of the passband becomes increasingly rectangular.

CC: Color correcting filters modify the hue of a signal to make it more easily distinguished.

Center wavelength (CWL): The arithmetic center of the passband of a bandpass filter. It is not necessarily the same as the peak wavelength.

Chamfer: A ground beveled edge on an optic. Used to prevent chipping and/or to allow mechanical fit.

Channel Isolation: The ratio of the light in an optical channel that corresponds to the desired spectral range against the contribution due to light at other wavelengths.

Clear Aperture (CA): The central, useable area of a filter through which radiation can be transmitted.

Coefficient of thermal expansion: A numerical representation for the rate at which a material will exhibit dimensional changes as a direct result of changes in temperature.

Collimation: 1. the process of aligning the optical axes of optical systems to the references mechanical axes or surfaces of an instrument. 2. The adjustment of two or more optical axes with respect to each other. 3. The process by which a divergent beam of radiation or particles is converted into a parallel beam.

Combiner: A semitransparent mirror in an optical system that combines two or more output beams into a single coaxial beam.

Complete blocking: Blocking of light into the far IR; a specification of blocking range, not depth.

Cone angle: In a converging or diverging beam of light, the full angle between the extreme off-axis rays.

Crosstalk: The measurable leakage of optical energy from one optical channel to another; also known as optical coupling.

Cut-on or Cut-off slope: A measure of the steepness of the transmittance curve  x 100% where λ80% and λ5% correspond to 80% and 5% to absolute transmittance points.

Cut-on or Cut-off wavelength (λC): The cut-on is the wavelength of transition from attenuation to transmission, along a continuum of increasing wavelength. The cut-off is the wavelength of transition from transmission to reflection. The cut-on is the wavelength of transition from attenuation to transmission generally specified as the point at which the transition slope achieves 50% of peak transmission. The cut-off is the wavelength of transition from transmission to attenuation and again specified as the 50% point of peak transmission.


DB: Dual-band filters have two passbands and two rejection bands.

DC: Dichroics provide wide regions of both transmission and reflection, exhibiting a high degree of polarization along with a somewhat shallow transition slope.

DCXR: Dichroics that provide extended reflection regions.

Detector: 1. a device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.

DF: Discriminating filters feature very steep-sided passbands with especially deep attenuation of energy close to the band.

Dichroic: In reference to thin-film interference coatings, selectively transmitting and/or reflecting light according to its wavelength rather than its plane of polarization.

Dichroic filter: A filter used to selectively transmit light according to its wavelength and not its plane of vibration.

Dielectric: A material with both conductive and insulative electromagnetic properties. A dielectric thin-film material exhibits far more transmission than absorption at the wavelength of interest.

Dielectric Coating: High-reflectance or low-reflectance coating composed of alternating layers of nonconducting inorganic materials with higher and lower indices of refraction than the substrate. Generally produced via electron beam gun, resistive heating or sputtered evaporation (deposition).

Dynamic Range: Dynamic range of an optical system is often defined by the maximum amount of power that can be accepted by the receiver before saturation occurs to the minimum amount of power that is acceptable at the receiver to realize the BER necessary for performance requirements.

DRLP: Dichroic longpass filters transmit a broad range of energy while efficiently reflecting shorter wavelength energy along another channel in the optical system.

DRLPxr: DRLPxr filters are dichroic longpass filters enhanced to reflect an extended range of energy.

Dual Magnetron Reactive Sputtering – A thin film coating method utilizing an energetic plasma in a controlled magnetic field and vacuum environment to precisely deposit alternating layers of high and low refractive index materials yielding a desired spectral response.


Effective index (n*): A dimensionless constant characteristic of a coating design used to calculate the spectral shift observed when it is used at off-normal angles of incidence.

EFLP: Longpass edge filters reflect more than 99.999% of shorter wavelength energy that is very close to the transmitted energy.

Electron-beam (EB) gun evaporation: The process of thin-film deposition whereby electrons boiled off a heated cathode are used to melt the coating material(s). Multisectioned crucibles can be used to apply many materials in one coating process.

Electromagnetic spectrum: The total range of wavelengths, extending from the shortest to the longest wavelength or conversely, that can be generated physically. This range of electromagnetic wavelengths extends practically from zero to infinity and included the visible portion of the spectrum known as light.

Emission spectrum: The relative intensity of radiation versus wavelength. In fluorescence, the radiation results from return from the first singlet excited states to ground level. The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state. Each element’s spectrum is unique.

Epi-illumination: Illumination that impinges on the sample from the viewing direction. In microscopy, the illuminator and the objective are common.

Etalon: Two flat glass plates separated by a parallel spacer, with the inner surfaces of the plates coated with a partially reflecting layer. When the etalon is placed in a beam of monochromatic light, multiple interferences occurs, forming circular fringes in the manner of the Fabry-Perot interferometer.

Evaporated Coating: Precisely controlled thin layers of solid material(s) deposited on a substrate after vaporization under high-vacuum conditions.

Excimer Laser: A rare-gas halide or rare-gas metal vapor laser emitting in the ultraviolet (126 to 588 nm) that operates on electronic transitions of molecules, up to that point diatomic, whose ground state is essentially repulsive. Excitation may be by E-beam or electric discharge. Lasing gases include ArCl, ArF, KrCl, KrF, XeCl and XeF.

Excitation probability spectrum: The relative probability versus wavelength that will result in a fixed intensity of emission energy.


Fabry-Perot cavity: An optical resonator in which feedback is accomplished by two parallel planes.

Fabry-Perot etalon: A non-absorbing, multi reflecting device, similar in design to the Fabry-Perot interferometer, which serves as a multilayer, narrow bandpass filter.

Free spectral range: The range over which the filter attenuates color or energy of light (light at CWL ±1, HBW excluded).

Flow cytometry: The measurement of cell activity on a singular basis. The characterization of cells or particles suspended in solution and singularly passed through a detection chamber (flow cell) for analysis.

Fluorescein isothiocyanate (FITC): A derivative of fluorescein.

Fluorometer: An instrument for the measurement of fluorescence.

Focal length: The distance between the last surface of a lens, or the front surface of a curved mirror, and the focal plane.

Full width at half maximum transmission (FWHM): Defines the width of the passband of a bandpass filter. It is referenced to the points on the cut-on and cut-off edge where the transmission is one-half of the maximum transmission.

Fused silica: Glass consisting of almost pure silicone dioxide (SiO2); also called vitreous silica. Frequently used in optical fibers and windows.

FWHM: Full width at half maximum. The width of the transmitting band of an optical filter, measured at 50% of the peak transmission.


Half Power Points: Points on both sides of the passband curve of a filter, with a vale 50% of the peak transmittance; used to calculate HBW and CWL.

Half Bandwidth (HBW): The wavelength interval of the passband measured at the half power points (50% of peak transmittance). Expressed as half bandwidth (HBW), full width half maximum (FWHM) or half power bandwidth (HPBW).

HT: High temperature filters protect the sample from excessive source radiation.


Image quality: The quality of an optic to transmit an unaltered wavefront.

Incident radiation (I): The radiation, usually polychromatic, that impinges on a filter.

Index of refraction (n): A quantity assigned to a light-transmitting medium that indicates the relative speed of the light in that medium. Phenomena occurring at the interface of two light-transmitting media, such as refraction and reflection, are governed by the physical laws and the indices of refraction of each medium. For example: vacuum = 1.0, glass = 1.5, germanium = 4.0.

Infrared (IR): Light from the region of the spectrum with wavelengths between 750nm (red) and 0.1mm (microwave).

Integrated Blocking: Ratio of the total transmitted radiation (energy) outside the passband, to the total transmitted radiation within the passband. Integrated blocking is influenced by the source output and detector response as functions of wavelength.

Intensity crosstalk: Intensity crosstalk occurs between channels and is a result of non-ideal optical filtering, where light from neighboring channels can leak through and be detected along with the filtered signal of interest. When the leakage level of a neighboring channel is higher than the noise floor that is associated with the channel of interest, it becomes the dominant noise factor in the SNR. As a rule of thumb, the intensity crosstalk of neighboring channels must be at least 20 dB below the target signal level. This type of crosstalk can be dealt with by using a high quality optical filter to eliminate all unwanted signals outside of the target channel bandwidth.

Interference Filter: An optical filter consisting of multiple layers of evaporated coatings on a substrate, whose spectral properties are the result of wavelength interference rather than absorption.

Interferometer: An instrument that employs the interference of light waves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers.

Ion-assisted deposition: A technique for improving the structure density of thin-film coatings by bombarding the growing film with accelerated ions of oxygen and argon. The kinetic energy then dissipates in the film, causing the condensed molecules to rearrange at greater density.


KG: A shortpass color absorption glass that transmits visible light while attenuating both longer and shorter wavelength energy.


Laser Line Filters: These filters pass a limited band centered on the resonance of the laser and attenuate the background plasma and secondary emission that often result in error signals. In the case of diode lasers, these filters can be used to make the light output more monochromatic.

Law of refraction; Snell’s: The incident ray, the normal to the refracting surface at the point of incidence of the ray at the surface, and the refracted ray all lie in a single plane. The ratio of the sine of the angle between the normal and the incident ray to the sine of the angle between the normal and refracted ray is a constant. If the indices of refraction on either side of a refracting surface are N and N’, and the angles that a ray makes with the surface normal and I and r, then Snell’s law states that N sin I + N’ sin r.

Light-emitting diode (LED): A semiconductor device which emits incoherent light.


Magnesium fluoride (MgF2): Magnesium Fluoride is a birefringent crystal optical material that exhibits high transmission from VUV to IR wavelengths (~110nm to 6.0µm). MgF2 is a relatively hard material resistant to mechanical and thermal shock. MgF2 is typically oriented along the optical axis to avoid birefringent effects and provide maximum transmission.

Micron (µm): 1µm = 1000nm. 10⁻⁶ meters.

Multilayer coating: Coating made up of several layers of materials with alternating high and low refractive indices. Varying combinations will produce a variety of coating properties.


Nanometer (nm): Unit of length used to measure wavelength of light. One nanometer is equal to 1 x 10⁻⁹ meters.

Narrowband AR coating: An AR coating designed to provide increased transmittance and reduced reflectance over a very restricted (narrow) band of wavelengths.

NB: Narrow-band filters isolate monochromatic signals from strong polychromatic illumination.

Near Infrared (NIR): Light from the region of the electromagnetic spectrum with wavelengths between (approximately) 750 nm and 3.0 µ.

Neutral density filter: A light filter that decreases (attenuates) the intensity of light without altering the relative spectral distribution.

Nominal wavelength: An approximate or target wavelength that is characteristic of an optical device (e.g., filter, laser).

Normal incidence: Light striking a surface at an angle perpendicular to the surface.


OG and RG: Longpass color absorption glasses absorb more than 99.999% of shorter wavelength energy that is very close to the transmitted energy. OG glasses absorb blue light; RG glasses absorb blue and green light.

OEM: Original equipment manufacturer.

Optical Density (OD): Units measuring transmission usually in blocking regions. Conversion: -log¹⁰T = OD. For example, 1% transmission is .01 absolute, so -log¹⁰ (0.01) = OD 2.0.

Optical interference filter: Light, color, or energy-selecting device, created by depositing multiple layers of dielectric materials such that constructive/destructive interference reflects all light within the free spectral range except for the passband.

Optical window: Typically flat polished glass or fused silica materials. Generally used for view ports or sealing and or protecting other components within an optical assembly, instrument or laser.

Optimized blocking: To conserve the most energy in the transmission band by controlling only the out-of-band region of detector sensitivity.


Parallelism: The angular wedge difference between two opposing optical surfaces; usually measured as arc minutes or arc seconds.

Passband: The range of wavelengths transmitted by an optical filter.

Peak Transmission (Tpk): The maximum percentage transmission within the passband.

Peak Wavelength: The wavelength of maximum transmittance within the passband; differs from the central wavelength only in filters exhibiting asymmetrical band shapes.

Phase: The portion of a periodic function, such as a wave, which has elapsed and is measured from some fixed origin. If the time for one period is expressed as 360° along a time axis, the phase position is called the phase angle.

Plane of polarization: When light is incident on a thin-film coating, the component of the electric vector parallel to the plane of incidence (P-plane) may react differently than the perpendicular component (S-plane). The plane of incidence is defined by the direction of the incident and reflected beams: at normal incidence the planes of polarization are undefined. Linearly polarized light can be P-plane, S-plane, or a combination, depending on the orientation of the thin-film relative to the polarization axis.

Photodiode: A two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. Photodiodes are used for the detection of optical power and for the conversion of optical power to electrical power.

Photon: A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hѵ, h being Planck’s constant and ѵ, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hѵ/c, c being the velocity of light.

Polarization: With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam can vary at random (un-polarized beam), can remain constant (plane-polarized beam), or can have two coherent plane-polarized elements whose polarization direction make a right angle. In the latter case, depending on the amplitude of the two waves and their relative phase, the combined electric vector traces out an ellipse and the wave is said to be elliptically polarized. Elliptical and plane polarizations can be converted into each other by means of birefringent optical systems.

Polarizer: An optical device capable of transforming un-polarized or natural light into polarized light, usually by selective transmission of polarized rays.

Protected Coatings: The process by which two or more substrates, coated with thin film depositions, are assembled together using an index-matching optical epoxy.

QMAX or QuantaMax: Surface coated single substrate designs with steep edges, very high transmission and no registration shift.


Raman Effect: When light is transmitted through matter, part of the light is scattered in random directions. A small part of the scattered light has frequencies removed from the frequency of the incident beam by quantities equal to vibration frequencies of the material scatting system. This small part is called Raman scattering. If the initial beam is sufficiently intense and monochromatic, a threshold can be reached beyond which light at the Raman frequencies is amplified, builds up strongly, and generally exhibits the characteristics of stimulated emission. This is called the stimulated or coherent Raman Effect. A device illustrating the stimulated Raman Effect is sometimes called a Raman laser.

RB: Rejection band series filters reflect a 15 to 40 nm bandwidth more than 99.9%. The average transmission outside the stop band is 80% with the exception of higher and lower harmonics where relatively height reflectivity occurs. Special rejection band filters which reflect more than one spectral band, or are used at non-normal angles of incidence, are also available.

Reflection (R): The return of light from a surface with no change in its wavelength(s).

Refraction: The bending of oblique incident rays as they pass from a medium having on refractive index into a medium with a different refractive index.

Rejection ratio: The ratio of the maximum transmittance outside the passband to the total transmittance within the passband.

Signal to Noise ratio (S/N): The system ratio of the integrated energy within the passband envelope to the energy outside this envelope and within the free spectral range.

Slope: The rate of transition from attenuation (defined as 5% of peak transmission) to transmission (defined as 80% of peak transmission). Slope = (lambda 0.80 - lambda 0.05) divided by lambda 0.05.

SP: Shortpass filters transmit wavelengths shorter than the cut-off and reflect a range of wavelengths longer the cut-off.

Spacer layer: A layer of material which has an optical thickness corresponding to an integral-half of the center wavelength of the band.

Spectrofluorometer: A dual monochromator instrument capable of measuring the fluorescence spectrum as a function of an excitation wavelength.

Stray Light: Unwanted energy transmitted through the filter.

Stokes shift: The lower frequency of electromagnetic energy emitted as a result of absorbed energy.

Substrate: The underlying material to which an optical coating is applied.

Surface quality: Allowable cosmetic flaws in an optical surface by comparison to reference standards of quality; usually made up of two types of standards defining long defects (such as scratches) and round defects (such as digs & pits).

Synthetic fused silica: A pure Si02 glass.

System Speed: The f/#, which is the ratio of the focal length to the aperture, of an optical system at the location of a filter.

Stray Light: Unwanted energy transmitted through the filter.

Surface Coatings: The surface deposition of metallic or metallic oxide materials on a given substrate.


TB: Triple-band filters have three passband and three rejection bands.

3RD Millennium: 3RD Millennium Filter designs are manufactured using proprietary ALPHA technology and Omega’s patented hermetic assembly.

Temperature Effects: The performance of an interference filter shifts with temperature changes due to the expansion and contraction of the coating materials.

Thin film: A thick layer of a substance deposited on an insulating base in a vacuum by a microelectronic process. Thin films are most commonly used for antireflection, achromatic beamsplitters, color filters, narrow passband filters, semitransparent mirrors, heat control filters, high reflectivity mirrors, polarizer’s and reflection filters.

Transmission: The fraction of energy incident upon the filter at any particular wavelength that passes through the filter. Expressed as either percent (95%) or a fraction of 1 (0.85).

Transmittance (T): The guaranteed minimum value of the peak transmittance of the filter (not necessarily occurring at the centre wavelength).


Ultra-Violet (UV): Light from the region of the electromagnetic spectrum with wavelengths between 15 nm (X-ray) and 400 nm (the blue end of the visible spectrum), approximately.

UV grade fused silica: Synthetic form of silicon dioxide made for SiCl₄, which transmits light over a very broad range of wavelengths from ~ 170 nm to 2.5 µm. UV grade fused silica has excellent thermal stability and is used as a UV to NIR material for many optical components.


Visible (VIS): Light from the region of the electromagnetic spectrum with wavelengths between 400 nm (blue) and 750 nm (red).


Wavefront: In considering a field of electromagnetic energy emanating from a source, the wavefront is a surface of constant phase.

Wavefront distortion: The degree of disruption of an optical wavefront, measured by viewing the interference fringes of a two-arm interferometer with the component at test in one arm and a known reference in the other.

Wavelength (λ): The physical distance that light travels in one wave cycle. All electromagnetic energy is transmitted (or travels) in waves.

WB: Wide band filters combine rectangular band shapes with broad regions of transmission.