Interference filter

Interference filter and interference levels are optical devices that use the effect of interference to light frequency-dependent, that is, depending on the color of visible light to filter or mirror. The name of the component as a filter or mirror depends on whether one uses the transmitted or reflected light. Generally, these devices are constructed as a dielectric, thin layers on a support (for example, Bragg mirror ). There are also devices in the form of Fabry-Perot interferometers.

Such a filter or mirror is for light of different wavelengths, different angles of incidence and partially different polarization a different reflectance and transmittance.

Determining properties

The main features are:

• The spectral transmittance is the ratio of transmitted radiant power to incident radiant power. This can also be expressed as the optical density that is associated with the degree of transmission of the formula.
• The spectral reflectance is the ratio of reflected radiation power to incident radiation power.
• The spectral absorption factor, the ratio of the component into another form of energy (e.g. heat) unreacted radiation power to the incident radiation power.
• The spectral dispersion is the sum of the non-directional ( diffuse ) spectral transmittance and reflectance.

Each with the wavelength, the angle of incidence AOI ( engl. angle of incidence ) and the polarization state of the incident light. For transmission and reflectance, a distinction between the directed and the diffuse component. For the spectral description of the specular component is based on, while the sum of the diffuse components yield the spectral scattering. In special cases, the change in the phase relationship between s -and p- polarized portion of the incident radiation through the device plays a role.

Subdivision

The subdivision of interference filters and mirrors may be made with regard to the materials used and in terms of spectral properties. In terms of material selection, there are basically two different shapes. Use filters and mirrors of the first group of semi-permeable, that is very thin, metallic layers (usually two layers, which are separated by a spacer layer, similar to a Fabry -Perot interferometer ). The second group is based on the interference in a stack of usually a plurality of dielectric layers of different materials.

In terms of their spectral properties, a distinction between the following filters:

• Bandpass filter: Has a high transmittance for a given wavelength band, while shorter and longer wavelengths are reflected or absorbed (eg color filter wheels for projectors ).
• Band-stop filter: Has a low transmittance for a given wavelength range, while shorter and longer wavelengths to pass through (for example filters for fluorescence microscopy ).
• Long-pass filter: Has a high transmittance for long wavelengths and low transmittance for short wavelengths (eg cold light reflectors for halogen lamps).
• Short- Pass Filter: Has a high transmittance for short wavelengths and low transmittance for long wavelengths (eg infrared blocking filter for digital cameras).
• A polarizing beam splitter: has a high transmittance for a polarization (typically p- polarization) and a low transmissivity for the orthogonal polarization (typically s-polarization ).

Filters, which are having a different transmission or reflectance for two wavelength ranges also referred to as dichroic filters and interference levels. Components for three wavelength regions are referred trichroitische.

Interference filter may be both narrow-band filters, so-called line filter, as well as wide-band band-pass filter.

Construction

In the classical sense are interference filters and mirrors are not tunable Fabry -Perot interferometer, and for example, consist of a thick support layer (glass ) on which a partly transparent metallic mirror layer (eg: silver, aluminum ) is deposited, followed by a thin dielectric, transparent layer, and a second mirror layer ( multilayer interference filter). Due to the thickness of the dielectric layer sets to which wavelengths are filtered. The transmittance of the mirror to affect the performance of the component layers ( with thin layer mirror is the maximum of the transmitted frequency band width and the intensity of high, this results in a low quality of the filter ).

Operation

To explain the operation of an interference filter or mirror, in the following, a simple system of a thin dielectric layer on a substrate is described.

If a " light beam " in the component, so the light beam in accordance with the Fresnel formulas for each ( optical ) interface part transmits (T1, T2, ...) and reflected (R1, R2, ...). There is a splitting of the rays incident on the surface. The transmitted, refracted rays are again partially reflected at the bottom of the layer and, in turn, meet on the surface. When taking place there after re- reflection refraction leaves a portion of the beams (R1 ), the thin layer, the other part is reflected and learns in the course in the layer multiple reflections. This leads to many parallel exiting beams of the same frequency on both sides of the component.

The interference at thin layers is preceded by a beam splitter. Therefore it is referred to as amplitude graduation; as opposed to interference by diffraction as in the double slit experiment, is spoken in the wavefront division.

To make it easier to illustrate the operation, light reflection is initially provided, i.e., the multiple reflections is negligible. It is sufficient to interference of two partial waves to look at, for example, R0 and R1. The two parallel beams are now complemented by a converging lens (eg the eye ) brought to interference. Due to the different path lengths of the waves in the thin layer, they have, after reflection on a path difference.

Wherein the film thickness, the refractive index of the thin layer and the retardation may be additionally generated by the reflection is.

Through the path difference it comes to extinction ( destructive interference ) or reinforcement ( constructive interference ) of radiation of certain wavelengths. Extinction and amplification of certain wavelengths depending on the chosen layer thickness of the filter and the incident angle of the rays.

Thus, it may lead to complete constructive and / or destructive interference, the following conditions must be met:

• The interfering beams must be close to each other parallel and coherent. This condition is given for the partial beam (T1) and (T2 ) and the part beams ( R1) and (R2).
• The amplitudes of the partial beams must be equal.
• The phase shift must

• (2n - 1 ) · 180 ° (where n = 1,2,3,4, ...) for destructive interference

• N x 360 ° (where n = 0,1,2,3, ...) for constructive interference

Applications

Filter

In the following is listed a number of filters, whose effect is based on interference effects:

• A dielectric filter: filter without metal, but purely from dielectric layers of certain thicknesses and alternating refractive index.
• (Also called compensation layer or anti-reflection layer) - destructive interference of the reflected rays on optical components: Anti -reflective coating. Enhanced transmission through constructive interference of certain wavelengths.

Besides the described interference filters, there are other optical components, which are used or observed interference. This includes, among other things, the Lummer- Gehrcke plate in the light multiple times in a plane-parallel plate reflected ( near the critical angle of total reflection) is in this case grazing emerges and interferes.

Mirror

Dichroic mirrors

Dichroic mirrors are for example ( three- CCD cameras ) used in larger video cameras to split the incident light in the RGB color space, including two such mirrors are used with reflection in various wavelength ranges ( see also CCD sensor).

Dichroic mirrors are used in fluorescence microscopy in order to couple the incident light from a Epifluoreszenzkondensor stimulating light into the beam path of the lens can, without obstructing the passage of the fluorescence emission. For the observation of multiply stained samples also polychroitische mirror can be used that have multiple reflective or transmissive spectral ranges.

Besides its application as spectrally be selected beam splitter dichroic mirror can be used as a beam combiner in order to couple, for example, a plurality of lasers having different wavelengths into a common beam path ( see diode ).

Dichroic mirrors reflect as opposed to reflection at metal surfaces the light of a wavelength with very low losses and are therefore often used in laser technology. Due to the low loss reflection less power is deposited in the mirror during intense laser beams; dichroic mirrors are therefore also at very high laser powers, where metal mirrors would be damaged usable.

In a dichroic dielectric mirrors for laser applications, the reflectance as a function of wavelength can be adjusted almost arbitrarily and very exactly by a suitable choice of the number of layers, thickness and refractive index of the dielectrics used, which constitutes an essential tool for wavelength-dependent coupling of laser beams.

For reflex sights dichroic mirrors are used to project the red laser light from the target point in the eye of the shooter.

Cold light and heat levels

Cold light and heat levels are special, opposite in their effects, dichroic mirrors. A heat mirror (german hot mirror) is characterized by a high transmittance in the visible and high reflectance (low transmittance ) in the infrared. A dichroic mirror ( engl. cold mirror) acts on the other hand exactly opposite, it reflects visible light is good and lets infrared light (heat radiation) happen, for example, for use in dichroic lamps. Infrared radiation, i.e., heat radiation from the lamp passes through the reflector and there is less heating of the illuminated object as with metallic reflectors. This type of light source is also called cold light source.

Pros and Cons

• It can be almost any transmission spectra and reflection spectra are produced. For a steep slope at a particular wavelength, there is often no alternative.
• Angular dependence of the incident beam: the frequency band to be filtered, is influenced by the angle of incidence. This angle-dependent effect of the filter can be utilized for fine adjustment of the filter to wavelengths. The frequency band is shifting towards shorter wavelengths. If it is not parallel to the incident beam so is that it reduces the quality of the filter.
• Temperature dependency: With porous layers may thus affect changes in temperature, the atmospheric water content to a small extent, the refractive indices of the layers, and the spectral characteristics.
• Low absorption coefficient: interference filters absorb typically very little of the incident radiation power and heat up accordingly only weakly on. In contrast, the effect of classical color filter based on the absorption of whole spectral regions, which can lead to a substantial heating of the filter, for example in lighting ( color filters in front halogen spotlights ).
• Based on interference dielectric mirrors achieve a higher reflectivity than metallic mirror and have high damage thresholds are suitable for pulsed high-power laser.
• Interference filters are fade-resistant.
• Interference filters are more expensive than conventional color filter.
• Some layer materials with good optical properties are relatively scratch resistant.
• Thickness, brittle layers or high temperatures in the coating are incompatible with flexible substrates.

Standardize

For the specification of optical interference filters, there is the ISO standard ISO 9211 ( Optics and photonics - Optical coatings ). This consists of the parts

• Part 1: Definitions
• Part 2: Optical properties
• Part 3: Environmental durability
• Part 4: Specific test methods.

The description of the filtering properties of spectacle lenses is in the separate standard EN ISO 13666 Ophthalmic optics - Spectacle lenses - Vocabulary (ISO 13666:1998 ) standardized. The standard is valid in Germany as DIN standard DIN EN ISO 13666.