Glossary of Filter Terms

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A

Absentee layer:

A layer of dielectric material, transparent in the transmission region of the filter, due to a phase thickness of 180 degrees.

Absorption curve, absorption spectrum:

The relative tendency of a material to absorb a specific color or energy of light, typically plotted as intensity, transmission, or optical density versus wavelength.

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 degrees.

Angstrom:

Unit of length, one-tenth of a nanometre (nm). 10^-10 metres.

Anti-reflective coating (AR):

An optical thin-film interference coating designed to minimise reflection that occurs when light travels from one medium into another, typically air and glass. AR coatings are the most common interference coatings, and are used on such items as camera lenses and eyeglasses.

Attenuation:

The suppression of signal before detection in an optical system. Usually, the wavelength or wavelength range is defined along with the degree of attenuation expressed in relative or absolute optical density.

B

Bandpass filter:

Transmits a band of colour, the centre of which is the centre 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.

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.

Broadband Achromatic Twyman-Green (BAT) interferometer:

An equal-arm interferometer that uses a white (broadband) light source.

C

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.

Clear aperture:

The physical area of a filter that performs in an optical system. The region is usually similar, concentric, and smaller than the total region of a filter. Unless otherwise stated, performance criteria apply throughout the clear aperture.

Complete blocking:

Blocking of light into the far IR. It is a specification of blocking range, not depth. Due to a metallic coating, the maximum transmission of a completely blocked bandpass filter is 50-60%.

Cone angle:

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

Cut-on or Cut-off:

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. Both are often given as 50% of peak transmission, but can also be given as 5% or 10% of peak transmission.

D

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.

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.

DRLP:

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

DRLP ext:

DRLP ext filters are dichroic longpass filters enhanced to reflect an extended range of energy.

E

Effective index (neff):

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.

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.

Epi-illumination:

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

Excitation probability spectrum:

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

F

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.

Fluorescein isothiocyanate (FITC):

A derivative of fluorescein.

Fluorometer:

An instrument for the measurement of fluorescence.

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.

H

Half-power bandwidth (HBW or HPBW):

Same as FWHM above.

HT:

High temperature filters protect the sample from excessive source radiation.

I

Image quality:

The quality of an optic to transmit an unaltered wavefront.

Index of refraction (n):

A quantity assigned to a light-transmitting medium that indicates the relative speed of the light in that medium. Phenomena occuring 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).

K

KG:

KG is a shortpass color absorption glass that transmits visible light while attenuating both longer and shorter wavelenth energy.

M

Micron (µ):

1µ = 1000nm. 10^-6 meters.

N

Nanometer (nm):

Unit of length used to measure wavelengths of light. 10^-9 meters.

NB:

narrow-band filters isolate monochromatic signals from strong polychromatic illumination.

O

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: -log10(T) = 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.

Optimized blocking:

The feature of conserving the most energy in the transmission band by controlling only the out-of-band region of detector sensitivity.

P

Passband envelope:

The spectral region centered at the CWL and 2 HBW in width where the interference filter switches from reflection to non-reflection and back to reflection (for the DF series, the absolute values are 0.99999+ to 0.1).

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 degrees 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 either P-plane, S-plane, or a combination, depending on the orientation of the thin-film relative to the polarization axis.

S

Signal-to-noise (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.

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.

Stokes shift:

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

Synthetic fused silica:

A pure SiO2 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.

U

Ultraviolet (UV):

Light from the region of the spectrum with wavelengths between 1nm (X-ray) and 400nm (the blue end of the visible spectrum), approximately.

V

Visible:

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

W

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.

WB:

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