Glossary

A

Absentee layer

A layer of dielectric material, transparent in the transmission region of the filter with a phase thickness of 180°.

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 angle of the cone within which light is coupled into the optic (also called acceptance cone and cone angle). This is closely related to f/#.

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 filtering.

Angle of Incidence (AOI)

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.

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.

B

Backscatter

The portion of scattered light that returns in a number of angles distributed about a direction opposite to that of propagation.

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 a 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 version would include a partially transmitting mirror on the glass.

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 multicomponent 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. It is used to measure wavefront distortion in optical parts.

Broadband AR coating

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

C

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”. A single unit in a Fabry-Perot bandpass filter design. It consists of two quarter-wave reflectors (alternating high-index and low-index quarter-waves) separated by a dielectric spacer. As the number of cavities increase, the depth of the blocking outside the passband increases and the shape of the passband becomes increasingly rectangular.

Center wavelength (CWL)

The arithmetic center of the passband of a bandpass filter, based on the FWHM. 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.

Clear Aperture (CA)

The central, useable area of a filter through which light will be transmitted.

Coefficient of thermal expansion (CTE)

A material property that describes volume change as a function of temperature.

Collimation

The process by which a convergent or divergent beam 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. This is essentially a beamsplitter working in reverse.

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 50% peak T when transitioning to transmission from blocking. The cut-off is the wavelength of 50% peak T when transitioning to blocking.

D

DB

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

DC

Dichroics provide wide regions of both transmission and reflection

Detector

A device that provides an electric output that is a useful measure of the radiation that is incident on the device.

Dichroic filter

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

Dielectric

An evaporative material (typically a non-conducting inorganic compound) that forms smooth, amorphous films of a given refractive index that can be used in thin-film filter designs. It typically exhibits high transmission and very low absorption in the desired wavelength range.

Dielectric Coating

High-reflectance or low-reflectance coating composed of alternating layers of dielectric materials with high and low indices of refraction. Generally produced via electron beam gun, resistive heating or sputtered evaporation (deposition).

Dynamic Range

Ratio of the maximum amount of power that can be detected to the minimum amount of power detected above the noise floor

E

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. It is calculated from the thickness and refractive indices of all the layers in the multi-layer stack.

Electron-beam (EB) gun evaporation

A thin-film deposition process whereby a focused electron-beam is rastered across the evaporant to force sublimation. Multisectioned crucibles can be used to apply many materials in one coating process.

Electromagnetic spectrum

The total range of wavelengths extending practically from zero to infinity, including the visible portion of the spectrum known as light.

Emission spectrum

Light intensity versus wavelength that originates from atomic or molecular relaxation from an excited state.

Epi-illumination

Wide-field illumination that impinges on the sample from the viewing direction (through the objective in microscopy).

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 sublimation under high-vacuum conditions.

Excimer Laser

A rare-gas halide or rare-gas metal vapor laser emitting in the ultraviolet (126 to 588 nm), including ArCl, ArF, KrCl, KrF, XeCl and XeF.

F

Fabry-Perot cavity

Two quarter-wave reflectors (alternating high-index and low-index quarter-waves) separated by a dielectric spacer. As the number of cavities increase, the depth of the blocking outside the passband increases and the shape of the passband becomes increasingly rectangular.

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 characterization of cells or particles suspended in solution and singularly passed through a detection chamber (flow cell) for analysis.

Fluorescence

Light emission from a molecular excited electronic state as it relaxes to the ground state.

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.

H

Half Power Points

Points on both sides of the passband curve of a filter, with a value 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).

I

Image quality

The quality of an optic to transmit an unaltered wavefront.

Incident intensity (I₀)

The light intensity, that impinges on a filter.

Index of refraction (n)

A parameter that indicates the relative speed of the light in that medium. Index of refraction is wavelength dependent and is comprised of a real component (n) and an imaginary component (k).

Infrared (IR)

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

Intensity crosstalk

Intensity crosstalk occurs between channels and is a result of insufficient blocking, where light from neighboring channels leak through and are detected along with the signal of interest. As a rule of thumb, the intensity crosstalk must be at least 20 dB below the target signal level.

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 properties of optical surfaces. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers.

Ion-assisted deposition (IAD)

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

L

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 Law)

The incident ray, the normal to the refracting surface, and the refracted ray all lie in a single plane. 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, usually in a small wavelength range.

M

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.

Micron (µm)

1µm = 1000nm. 10⁻⁶ meters.

Multilayer coating

Coating made up of several layers of materials with alternating high and low refractive indices.

N

Nanometer (nm)

Unit of length used to measure wavelength of light. One nanometer is equal to 1 x 10⁻⁹ meters.

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 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.

O

OG and RG

Longpass color absorption glasses – OG glasses absorb blue light; RG glasses absorb blue and green light.

OEM

Original equipment manufacturer.

Optical Density (OD)

Units of blocking or absorption measurements. Conversion: -log¹⁰T = OD. For example, 1% transmission is 0.01 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 occurs to transmit or reflect desired wavelengths.

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.

P

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 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 photodetector made of semiconducting material. It converts photon flux into electricity.

Photon

A quantized unit of electromagnetic radiation. 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

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 at right angles to the direction of propagation. The polarization direction in the beam can be 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 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.

R

Raman Effect (Raman Scattering)

When light hits a surface, some fraction of it scatters in random directions, usually at the same energy. A small percentage of photons (about 1 in a million) exhibit a frequency shift in the scattered energy equal to a vibrational frequency in the molecule. This phenomenon is called Raman scattering.

RB

Rejection band filters reflect a 15 to 40 nm bandwidth more than 99.9%.

Reflection (R)

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

Refraction

The deflection of incident rays as they pass from a medium having one 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.

S

Signal to Noise ratio (S/N)

The ratio of detected intensity within the passband to that outside the passband but within the free spectral range.

Slope

The rate of transition from attenuation (5% of peak transmission) to transmission (80% of peak transmission).

SP: Shortpass

filters transmit wavelengths shorter than the cut-off and reflect 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

An instrument capable of measuring the fluorescence spectrum as a function of excitation wavelength.

Stray Light

Unwanted light that reaches the detector in the optical system

Stokes shift

The difference in wavelength (or energy) between excitation and emission wavelengths due to non-radiative decay in the sample

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.

Surface Coatings

The surface deposition of metallic or metallic oxide materials on a given substrate. These coatings tend to be durable, so a protective coverslip is not needed.

T

TB

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

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 layer of a substance deposited on a substrate in a vacuum by sputtering or sublimation. Thin films exhibit interference effects that can be used to design interference filters.

Transmission

The percent of light that passes through a filter at a given wavelength. Expressed as either percent (95%) or a fraction of 1 (0.85).

U

Ultra-Violet (UV)

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

UV grade fused silica

Silicon dioxide that 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.

V

Visible (VIS)

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

W

Wavefront

A surface of constant phase in a series of electromagnetic waves. The wavefront is perpendicular to the propagation direction of the ray.

Wavefront distortion

The degree of disruption of an optical wavefront, measured with interference in an interferometer or using a Shack-Hartman wavefront sensor.

Wavelength (λ)

The length of a period of a wave with a given energy. Energy and wavelength are inversely proportional.

WB

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