Filter technology uses its own terms. Below we explain the most frequently used technical terms.

Angular Shift

The shift of a filter’s nominal design wavelength due to an increase of the angle of incidence (AOI) from perpendicular AOI. The shift is always directed towards shorter wavelengths. It is possible to use the following equation to estimate the angle shift of the spectral curve:

λ(θ) = λ0 [1-(sinθ/n*)2]½  where n* = [nL x nH]½

λ0: nominal design wavelength

θ:  AOI

nL: refractive index of low index material

nH: refractive index of high index material

Bandpass Filter (BPF)

Optical filter, allowing a specific spectral band to pass. Wavelengths outside the bandpass are rejected.

Band Rejection Filter

Filter, rejecting a specific band of wavelengths.


Wavelength difference between the two half-maximum points of a bandpass filter. See FWHM.


Optical component, combining two beams into one beam.

Beamsplitter (BSP)

Optical component, dividing the incident light into two parts. The transmitted and the reflected light is separated by tilting the splitting surface. The beamsplitter is designed to work at a specific AOI (e.g. 45º).

Blocking Filter

Filter designed to reject the sideband transmission of a primary filter.

Colour Beamsplitter

Beamsplitter separating different colours of light at non-normal incidence.


A spurious signal in fluorescence detecting systems arising from spectral areas where both the excitation and emission filter transmit.

Dielectric Mirror

All dielectric coating offering a high reflection in a certain wavelength range.

Edge Filter

Filter, offering a steep transition between high and low reflection. Short wave pass (SWP) and long wave pass (LWP) filters are edge filters. They offer a high transmission in a certain spectral area near the edge.

Full Width at Half Maximum (FWHM)

Bandwidth at Tmax/2 of a bandpass filter.

Long Wave Pass Filter (LWP)

Edge filter transmitting longer wavelengths and blocking shorter wavelengths.

Notch Filter

See Band Rejection Filter.

Optical Density (OD)

A logarithmic measure to specify the wavelength dependent blocking performance of a filter (see also our presentation).


Spectral band of high transmission.


Small holes and imperfections in the coating.


The state of polarisation is determined by the motion of the electrical field vector related to the lightwave. The electrical field vector is described by two components: P and S. Light polarised parallel to the plane of incidence is P-polarised. Light polarised perpendicular to the plane of incidence is S-polarised.

Polarising beamsplitter (PBS)

Beamsplitter separating S- and P- polarised light in a certain wavelength range.

Short Wave Pass Filter (SWP)

Edge filter transmitting shorter wavelengths and blocking longer wavelengths.

Transmission (T)

The ratio of radiation power transmitted by a filter to the incident radiation power. Often expressed in %.