Optical Filters Wiki

Optical filter technology uses its own terms. Below we explain the most frequently used technical terms:

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

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

A Multi-Bandpass filter is a filter that transmits almost 100% in 2 or more wavelength bands and blocks light outside these bands.

Learn how Multi-Bandpass filters can be used to build compact fluorescnece instruments here.

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.

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). A beamsplitter can be a colour beamsplitter or polarization beamsplitter.

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

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.

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

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.

An emission filter, also known as a barrier filter or secondary filter, is an optical filter used in fluorescence microscopy and other fluorescence spectroscopy based techniques to selectively transmit emitted fluorescence from an excited fluorophore while blocking the excitation.

The emission filter enables the detection of the emitted fluorescence signal from the sample without interference from the excitation light. This helps to enhance the contrast and specificity of the fluorescence images obtained from the sample.

Read more about filters for fluorescence here.

An excitation filter is an optical filter used in fluorescence microscopy and other fluorescence spectroscopy based techniques to select the wavelength of light that is used to excite a fluorophore.

Other terms used for an excitation filter are primary filter or exciter filter.

The excitation filter ensures that only the appropriate wavelengths of light are used to excite the fluorophores, which helps in obtaining high-quality and specific fluorescence signals from the sample.

Read more about filters for fluorescence here.

Bandwidth at Tmax/2 of a bandpass filter.

Edge filter transmitting longer wavelengths and blocking shorter wavelengths.

See Band Rejection Filter.

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.

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

Edge filter transmitting shorter wavelengths and blocking longer wavelengths.

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