Glossary

The wavelengths of analytical filters are selected to match the atomic emission lines of individual elements, minimising interference between neighbouring lines. They can be used for analytical applications and as narrowband filters.

Anti-reflective coatings reduce the reflection of light at glass surfaces. For uncoated glass with a refractive index of approximately 1.5, around 8% of the light is reflected at the two surfaces combined. A suitable anti-reflective coating can reduce this loss to less than 1%.

A bandpass filter transmits light within a defined wavelength range, known as the passband. Blocking regions on either side suppress unwanted radiation, allowing a selected spectral range to pass.

Beamsplitters are partially transmitting mirrors that divide a light beam into reflected and transmitted components. They are available with different splitting ratios. Achromatic beamsplitters have a splitting ratio that varies only slightly with wavelength across the visible spectrum.

A cavity is an optical resonator: an arrangement of reflective layers or mirrors between which light is reflected back and forth. In interference filters, this structure contributes to spectral selection.

The center wavelength describes the spectral midpoint of a bandpass filter. Like the mean wavelength, it is generally determined from the two half-maximum points. The wavelength of maximum transmission, known as the peak wavelength, does not necessarily coincide with this midpoint.

The clear aperture is the optically usable opening of a system or component. It limits the size of the light beam. For a circular opening, it is specified by the diameter Ø.

Cold-light mirrors, also known as cold mirrors, consist of multiple dielectric layers. They reflect visible light while transmitting most infrared radiation, allowing visible light and thermal radiation to be separated.

Colored glass filters are absorption filters made from colored glass. They absorb light to varying degrees in certain wavelength ranges while transmitting most of the light in others.

Colored glass filters select specific spectral ranges through absorption in the glass. Colored glass IR blocking filters absorb infrared radiation and allow most visible light to pass through.

Colored glass longpass filters transmit light above a specific cutoff wavelength and absorb shorter wavelengths in the glass.

Fluorescence filters are used in applications such as fluorescence microscopy. They select the required excitation and emission bands and suppress unwanted wavelengths. High-quality bandpass filters play an important role in these systems.

In front-surface mirrors, the reflective coating is on the front of the glass. Light is reflected without first passing through the glass, avoiding unwanted secondary images. Aluminium provides a cost-effective reflective material, especially in the visible spectrum. Dielectric overcoats protect the aluminium and can enhance reflectivity.

The full width at half maximum describes the spectral width of a passband. It is the difference between the two wavelengths at which transmission reaches 50% of its maximum value. FWHM is usually specified in nanometres.

Heat-reflecting filters transmit most visible light and reflect infrared thermal radiation. Depending on the design, filters with a Borofloat substrate can withstand temperatures of up to 450 °C.

Interference filters transmit light within selected wavelength ranges. They operate through interference in thin layers deposited on a substrate. Spectral components that are not transmitted are predominantly reflected.

Laser line filters are matched to the wavelengths of commonly used lasers. They suppress unwanted plasma and secondary emissions that can distort measurement signals.

For a bandpass filter, the mean wavelength denotes the midpoint of the passband. It is calculated from the two wavelengths at which transmission reaches 50% of its maximum value: MWL = (λ₁ + λ₂) / 2.

Metal-coated neutral density filters attenuate light through a combination of absorption and reflection. A controlled coating of an Inconel metal alloy is deposited on a transparent optical component. This coating provides largely uniform attenuation over a broad spectral range.

Neutral density filters reduce the intensity of a light beam as uniformly as possible over a broad spectral range. Attenuation is achieved by absorption or by a combination of absorption and reflection.

Transmission describes the fraction of incident light that passes through a filter. It can be specified at a particular wavelength or as a curve across an entire spectrum.

The wavelength λ (lambda) is the shortest distance between two points of a wave that have the same phase. For optical filters, it is commonly specified in nanometres (1 nm = 10⁻⁹ m) or micrometres (1 µm = 10⁻⁶ m).